Cool Cosmos Interstellar Dust and Gas We often think of the vast areas of space between the stars as being completely empty. However, this is not really true. Much of the space between the stars is filled with atomic and molecular gas (primarily hydrogen and helium) and tiny pieces of solid particles or dust (composed mainly of carbon, silicon and oxygen). In some places this interstellar material is very dense, forming nebulas. In other regions the gas and dust density is very low. The image to the right shows an infrared view of the gas and dust in our galaxy along the plane of our Milky Way galaxy. W. Waller and F. Varosi (GSFC), IRAS, SkyView, NASA Most of this interstellar gas and dust originates from the death of stars which either exploded (supernova) or blew off their outer layers, returning their material to interstellar space. From this material, new stars are formed. Often, the gas and dust between the stars can be detected only in the infrared. Dust grains absorb visible and ultraviolet light which causes them to heat up and radiate in the infrared.

Also, by using infrared detectors, astronomers can penetrate the often invisible interstellar gas and dust clouds and gain much information about their composition and structure. Below are two infrared images taken by NICMOS of material being ejected into space by dying stars. Credits (Left Image): Rodger Thompson, Marcia Rieke, Glenn Schneider, Dean Hines (University of Arizona); Raghvendra Sahai (Jet Propulsion Laboratory); NICMOS Instrument Definition Team, and NASA/AURA/STScI Credits (Right Image): Credits: William B. Latter (Spitzer Science Center/Caltech) and NASA/AURA/STScI A surprise discovery from the IRAS mission was that space is filled with faint wisps of dust which cannot be seen in visible light. This has been given the name "infrared cirrus" because it resembles the cirrus clouds in the Earth's atmosphere. Infrared cirrus is very cold (15-30 K or -433 to -406 F) and can only be detected in the infrared. Its temperature is due to dust grains being slightly heated by starlight.

IRAS image of cirrus at the south celestial pole and ISO-LWS spectra of infrared cirrus Infrared astronomy has also resulted in the detection of several types of complex interstellar molecules. Astronomers using ISO discovered emission lines from interstellar water vapor in a variety of sources including star forming regions, planetary nebulae and near formed stars. ISO also discovered for the first time hydrogen cyanide ice molecules in a dusty cloud surrounding a newly forming star. Many of the most important spectral lines produced by interstellar gas falls within infrared wavelengths. The Spitzer Space Telescope will conduct spectroscopic studies of interstellar dust and gas. Among these research investigations will be the important study of water, ices and organic molecules. Spitzer observations at near-infrared wavelengths will map the inner regions of our Milky Way, yielding important new information that is otherwise obscured by heavy concentrations of dust at visible wavelengths. Solar System Formation: Gas & Dust to Solar Nebula to Sun with Planets - Windows to the Universe What will happen to the Solar System? Solar System Formation Scientists believe that the solar system was formed when a cloud of gas and dust in space was disturbed, maybe by the explosion of a nearby star (called a supernova ).

This explosion made waves in space which squeezed the cloud of gas and dust. Squeezing made the cloud start to collapse, as gravity pulled the gas and dust together, forming a solar nebula. Just like a dancer that spins faster as she pulls in her arms, the cloud began to spin as it collapsed. Eventually, the cloud grew hotter and denser in the center, with a disk of gas and dust surrounding it that was hot in the center but cool at the edges. As the disk got thinner and thinner, particles began to stick together and form clumps. Some clumps got bigger, as particles and small clumps stuck to them, eventually forming planets or moons . Near the center of the cloud, where planets like Earth formed, only rocky material could stand the great heat. Icy matter settled in the outer regions of the disk along with rocky material, where the giant planets like Jupiter formed. As the cloud continued to fall in, the center eventually got so hot that it became a star, the Sun, and blew most of the gas and dust of the new solar system with a strong stellar wind .

By studying meteorites, which are thought to be left over from this early phase of the solar system, scientists have found that the solar system is about 4600 million years old! Shop Windows to the Universe Science Store! The Fall 2009 issue of The Earth Scientist , which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earthï¿½s ancient atmosphere, is available in our online store . Windows to the Universe Community News How the Stars Are Born How the Stars Are Born GO From a cloud of dust and gas to a Sun-like star Sep 21, 2006 12:24 GMT · By Vlad Tarko · Share: Once Newton's theory of gravitation was in place, people started creating models of how the solar system must have been born. The most renowned such theory is the one invented by Kant and Laplace in the second half of the 18th century and which still holds to this day, greatly improved by new discoveries regarding the process by which a star is ignited. The theory states that particles of dust and gas accumulate due to the gravitational attraction; most of this material ends up in a single body - the future star.

Although, initially, the particles can move in any directions, eventually, as they collide or fly by each other, one general rotational motion gradually takes hold. This is called a protoplanetary disks or "proplyd". Most of this material gets packed into a single body that becomes so dense that it ignites: atoms (especially hydrogen atoms) get smashed into each other and transformed into other atoms (such as helium) and light. This process is called fusion and it is the process by which the star produces the light we see. The star does not collapse any more because the fusion process creates a reverse force that balances the gravitation. A star similar in size to our Sun continues to fuse its hydrogen atoms for about 10 billion years. Sometimes, instead of just one star, two or even three stars appear. The rest of the material, the debris that didn't got caught in the star itself (or stars), amasses in the form of the planets. Today, this theory is no longer just a story, as astronomers have observed in various parts of the sky various "stellar nurseries" called dark nebulae. We are able to see these clouds of dust where stars get formed because they are illuminated from behind by other stars.

Here are three of the most spectacular discoveries made by Hubble Space Telescope. The NGC 346 nebula in the Small Magellanic Cloud The Magellanic Clouds are two small satellite galaxies of our galaxy, the Milky Way. This image illustrates three stages of star formation: The darker region above the ridge B contains yet unborn stars. The bright blue stars that can be seen in the region A, below the ridge B, are very young, massive and hot. They are only about 3 to 5 million years old. They emit large amounts of ultraviolet light that is destroying the cloud that has been their stellar nursery and creates the ridge B. This ridge is formed from denser gas and dust and is "eroded" by the outflow of energy from the nearby, hot, young stars. The beautiful pink color in the image is caused by the energized gas of the nursery: the ultraviolet light heats the gas and makes it glow. Thus, we are witnessing here a dark cloud of dust and gas that spawns stars, and as the stars are born the entire nebula is lit up and gradually dispersed. As time passes, the entire cloud will be heated. Without any cold gas, star formation will cease.

This is what happened in the lower portion of the image, C. The stars in that cluster are about the same age as our Sun, around 4.5 billion years old. We know this because we can see their color: most of them are yellow stars. The larger, hotter stars are blue and they exhaust their energy much more quickly. As a result, in the region C only the smaller, Sun-like, stars have remained. Thus, NGC 346 is a picture of star formation and development: we can see the dark clouds where stars are born, their emergence as young stars, and finally their passage into maturity. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). The Eagle nebula This is one of the most famous pictures taken by Hubble. In the first detail view, region A, we can see three columns of dust and gas (mostly hydrogen) illuminated by the young, hot, massive stars on top of the pillars. In the zoomed region these stars are indicated by arrows. The stars' intense radiation heats the surrounding gas, making it glow.

Moreover, this radiation is responsible for "sculpting" the columns through a process called photoevaporation - the light pushes away the feeblest particles and leaves behind only the larger conglomerates, denser gas globules called EGGs (Evaporating Gaseous Globules). Photoevaporation in the Eagle Nebula has cut newly forming stars off from the cloud feeding them. While some of the EGGs are large enough to eventually become stars, others may never make it. In the second detailed view, B, we can see another stellar nursery (see larger image ). The odd-looking tower is 9.5 light-years (or about 92 trillion kilometers) high, more than twice the distance from our Sun to Alpha Centaury (the nearest star). This giant cosmic sculpture is created by the ultraviolet light coming from the newborn stars. The stars at the top of the tower heat the gas creating the wing-like features. They also create a shock front that pushes against the darker cold gas and that will move down across the tower. Insofar it has just started, lighting up the "wings". This intense pressure compresses the gas, making it easier for stars to form.

The threads of material in the center of the tower are also stellar birthing areas. They are roughly the size of our solar system. In this portion of the tower the formation of stars prevents other stars from forming because the light of the newborn stars is pushing away the gaseous material, dispersing it. From the color of the glowing gases scientists can deduce the chemical composition. The blue color at the top reveals the presence of oxygen while the red color in the lower region is from glowing hydrogen. The Orion nebula The Great Nebula in Orion, located in the same spiral arm of our Galaxy as our Sun, 1500 light-years away from us, is one of the most interesting of astronomical nebulae known. The filamentary structure visible in this image is caused by shock waves. The bright cluster of stars in the center, called the Trapezium, is responsible for most of the illumination in the nebula. The five brightest stars in the Trapezium have masses on the order of 15-30 times that of our Sun.

When astronomers used Hubble's near-infrared camera to see though the cloud of dust they uncovered a swarm of newborn brown dwarfs (the smallest type of stars that can barely fuse deuterium and lithium, but not hydrogen). The Orion Nebula also contains many stellar nurseries. Astronomers have observed approximately 700 stars in various stages of formation within the nebula. A close-up view of a small portion of the nebula has revealed the image below of four stars surrounded by gas and dust, which are considered to be systems in the earliest stages of solar system formation. (Credits: C.R. O'Dell (Rice University), and NASA.) These so-called "proplyds" will evolve on to agglomerate into planets. Astronomers have discovered more than 150 such proplyds, an indication that the formation of solar systems is a fairly common phenomenon. CHECK OUT THE GALLERY (4 Images) Star Birth | Cool Cosmos The Big Bang Star Birth Infrared technology allows astronomers to peer into the heart of star-forming regions, where new stars are being created from towering clouds of dust and gas.

While these clouds block visible light, infrared light can pass through all but the most dense regions, and has allowed us to explore the first steps along the path towards building new solar systems. Stellar Building Blocks Dust clouds in Scorpius Stars form from the simplest of building blocks - huge clouds of gas and dust that permeate the Galaxy. It's hard to believe, but everything in our Solar System, from the Sun to the planets, ocean, plants and us, was once a cloud of gas and dust spread out over a large area of space. There is a lot more hydrogen gas in interstellar space than dust. However, whenever we see a dust cloud we know the gas there is especially dense. While these big clouds of dust and gas lay dormant for many millions and perhaps billions of years, eventually some of them are disturbed. This can happen gradually, maybe caused by the approach of one of the Milky Way's spiral arms as it slowly sweeps around the center of the galaxy, or it can be a sudden event, like a nearby supernova explosion that blasts a shockwave through the cloud.

Either way, a small increase in the pressure and density of the cloud forms knots in the gas and dust that eventually collapse under their own gravity, pulling more and more of the surrounding material in, and forming the stellar "seeds" known as protostars. From Protostar to Star Artist's concept of a protostar As the clouds collapse, they start to rotate, and, like a spinning skater pulling in her arms, each of these seed protostars begins to spin faster the more it collapses. The material falling towards the protostar flattens out into a rotating disk of dust and gas encircling the central core. The protostar warms up, as the potential energy of the material falling in is converted into kinetic energy, but it has not yet ignited to form a fully-fledged star. For the next few million years, the protostar's gravity pulls in more material from the surrounding cloud into its disk. That disk transports the gas and dust onto the protostar, causing the protostar to grow. The increase in mass causes the gravitational field of the protostar to increase and so even more material is pulled into the disk.

The addition of more material, in turn, increases the gravitational field even further, pulling in more material, and so on, creating a feedback loop that keeps the whole process going. Protostellar jets around a baby star Jets can be found around many objects in the Universe, not just baby stars. They can be seen around old stars, and even form from the disks of material falling into black holes! As the star is fed material from the surrounding disk, a small amount of it gets caught up in the forming star's spinning magnetic fields and is flung out along its axis of rotation. These protostellar jets are commonly found around most young stars. Where they impact the surrounding gas, they light up, giving astronomers glowing markers that point back to the source infant stars. The density and temperature of the protostar keep climbing higher and higher, until eventually the core grows to about one tenth the size of our Sun, and becomes hot and dense enough for hydrogen nuclei to spontaneously stick together to form helium, in a process called nuclear fusion. At that instant, the core ignites, and the new star is born. Meanwhile, in the disk, clumps of material have been forming, which are the seeds of new planets.

These seeds sweep up material in the disk in a process called accretion, forming the planets of a new solar system. Once the star has started nuclear fusion, the heat and wind from the infant star begin to blast the gas and dust away, creating a cavity in the cloud. As more and more matter gets funneled onto the star from the disk, the star gets larger and larger, causing it to push harder and harder against the cloud and the disk, enlarging the cavity, vaporizing the disk, and halting the growth of planets. Sequence of star formation through a spiral arm Infrared observations of protostars The process of star-formation has traditionally been difficult for astronomers to observe. From the time the knot of dust begins to collapse until the protostar becomes a star, the stellar seed is warming up, but is not hot enough to give out large amounts of visible light. Not only that, but the whole process takes place deep inside dense clouds of dusty gas, so the small amounts of visible light that are given out by the protostar are immediately swallowed up.

Dark cloud hiding a baby star Looking at the clouds from the outside, all we see from Earth is the dark exterior, with no view of the dramatic action taking place in the center. The visible light from a very young star can only escape the cloud once the cavity created by the star grows large enough to poke a hole through the cloud. Then, like a chick breaking out of an egg, the star’s light pours into the galaxy. It can take millions of years for this process to happen, by which time the stars are no longer fledglings, but are fully formed and shining brightly. In order to study the very first steps of star formation, astronomers have, instead, turned to infrared astronomy. The warm protostar, while not hot enough to give out visible light, glows with an abundance of infrared light. Moreover, the wavelength of infrared light allows it to pass straight through the surrounding dusty clouds as if they were barely even there, and so all of this infrared light reaches Earth and allows us to study the very earliest stages of the star formation process.

With our infrared telescopes, we have been able to peer deep into the dusty cocoons and watch the whole process, from the formation of protostars to the final ignition of nuclear fusion in their cores. Baby Stars in Orion's Belt Infrared/visible views of Orion The Orion Nebula is one of the only star-forming nebulas that can be seen with the naked eye. It is sometimes called the "Sword of Orion" since it appears to hang below Orion the Hunter's belt. The Orion Nebula (M42) is our closest massive star-making factory. Found in the sword area of the winter constellation Orion the Hunter, the nebula is 1,450 light-years from Earth. Formed in a cold cloud of gas and dust, the nebula contains about 1,000 young stars, which illuminate the cloud, creating the beautiful swirls of material seen in infrared. Images from infrared telescopes like Spitzer have shown us the dusty side of this bright nebula. Dark patches in the visible light view (right side) glow brilliantly in mid infrared light (left side).

In the center of the Orion Nebula are four monstrously massive stars called the Trapezium, which are about 100,000 times as luminous as the Sun. The heat and wind from these stars is making a cavity in the surrounding cloud, by blasting gas and dust away. Just above this brightly glowing bowl lies the densest area of the cloud, which looks completely dark in the visible light image. Here we see hints of some of the very youngest stars in this area starting to form. In this view from Spitzer, we see infrared clouds in red, numerous protostars in green, and fuzzy green globs which are the result of jets of gas that are shot into the clouds by young stars and protostars. Stars in both images that are not associated with the nebula are seen in blue. Orion in the far infrared Combining infrared light from Spitzer with even longer wavelengths of light from the Herschel telescope reveals more of this dense filament of dust. This rainbow-colored image is showing us how the dust temperatures vary throughout the nebula. The hottest dust appears blue, heated by the light of young stars like the Trapezium.

The coldest areas, which are only tens of degrees above absolute zero, glow faintly only at the very longest wavelengths seen by Herschel, and appear red in the image. No single wavelength of light, or even small slice of the spectrum, can possibly tell the whole story of star formation. Each of these images brings out features not seen in the others, and each palette of color illuminates a different side of the story of how stars are born. The Neighboring Flame Nebula WISE view of Flame Nebula The famous "Horsehead Nebula" is an opaque dark cloud in the shape of a horse's head that can usually be found just below the Flame Nebula. However, since dust becomes transparent in the infrared, the Horsehead has simply faded away in this image! The Flame Nebula is another vast cloud of gas and dust where baby stars are being born.

It too can be found in the constellation of Orion, on the left side of the three distinctive stars making up the "belt." It is being lit up by a star inside 20 times the mass of the Sun, and would be as bright to our eyes as the stars in Orion’s belt if it weren’t for the dust surrounding it, which makes it appear four billion times dimmer than it actually is! As a comparison, two of the three stars that make up Orion’s belt can be seen just to the right of the Flame nebula. This distinctive image comes from the Wide Field Infrarared Survey Explorer (WISE) which surveyed the entire sky in infrared light. An intriguing feature in this image is the bright red arc to the lower right of the nebula. The arc surrounds a multiple star system, 1,070 light-years away in the sword of Orion, called Sigma Orionis. The star system moves through space at the breakneck speed of 50 kilometers per second (113,000 miles per hour).

At that speed, winds from Sigma Orionis crash into the gas and dust outside of it and create a shock wave (called a "bow shock"), where the material in front of the speeding system piles up. The energy from the bow shock heats up the dust in the region and makes it glow in the infrared. Published: 03 August, 2013 If you were to fall feet-first into a black hole, the difference in gravity between your feet and your head would stretch you like spaghetti, right before you were shredded into your individual atoms, starting with your feet and working up. Continue the conversation on Stars | Science Mission Directorate Science Mission Directorate Exoplanets Stars Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may coalesce about them. Consequently, the study of the birth, life, and death of stars is central to the field of astronomy.

Star Formation Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star. Three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs; this would explain why the majority the stars in the Milky Way are paired or in groups of multiple stars. The observations of Eta Carinae's light echo are providing new insight into the behavior of powerful massive stars on the brink of detonation. Credit: NOAO, AURA, NSF, and N. Smith (University of Arizona) As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of this material ends up as part of a star — the remaining dust can become planets, asteroids, or comets or may remain as dust.

In some cases, the cloud may not collapse at a steady pace. In January 2004, an amateur astronomer, James McNeil, discovered a small nebula that appeared unexpectedly near the nebula Messier 78, in the constellation of Orion. When observers around the world pointed their instruments at McNeil's Nebula , they found something interesting — its brightness appears to vary. Observations with NASA's Chandra X-ray Observatory provided a likely explanation: the interaction between the young star's magnetic field and the surrounding gas causes episodic increases in brightness. Main Sequence Stars A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood. Our Sun will stay in this mature phase (on the main sequence as shown in the Hertzsprung-Russell Diagram) for approximately 10 billion years. Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.

As shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of luminosities and colors, and can be classified according to those characteristics. The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K. Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years. On the other hand, the most massive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants.

Stars and Their Fates In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant. If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.

Average Stars Become White Dwarfs For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf . White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers - why didn't they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation. Pressure from fast moving electrons keeps these stars from collapsing. The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! These paradoxical stars are very common - our own Sun will be a white dwarf billions of years from now. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down. This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate as described below.

White Dwarfs May Become Novae If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. Nova is Latin for "new" - novae were once thought to be new stars. Today, we understand that they are in fact, very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter - mostly hydrogen - from the outer layers of that star onto itself, building up its surface layer. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs (those near the 1.4 solar mass limit mentioned above) may accrete so much mass in the manner that they collapse and explode completely, becoming what is known as a supernova. Supernovae Leave Behind Neutron Stars or Black Holes Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova . A supernova is not merely a bigger nova. In a nova, only the star's surface explodes.

In a supernova, the star's core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion - fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions. On average, a supernova explosion occurs about once every hundred years in the typical galaxy.

About 25 to 50 supernovae are discovered each year in other galaxies, but most are too far away to be seen without a telescope. Neutron Stars If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star . Neutron stars are incredibly dense - similar to the density of an atomic nucleus. Because it contains so much mass packed into such a small volume, the gravitation at the surface of a neutron star is immense. Like the White Dwarf stars above, if a neutron star forms in a multiple star system it can accrete gas by stripping it off any nearby companions. The Rossi X-Ray Timing Explorer has captured telltale X-Ray emissions of gas swirling just a few miles from the surface of a neutron star. Neutron stars also have powerful magnetic fields which can accelerate atomic particles around its magnetic poles producing powerful beams of radiation. Those beams sweep around like massive searchlight beams as the star rotates. If such a beam is oriented so that it periodically points toward the Earth, we observe it as regular pulses of radiation that occur whenever the magnetic pole sweeps past the line of sight.

In this case, the neutron star is known as a pulsar. Black Holes If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material - often the outer layers of a companion star - is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion. From the Remains, New Stars Arise The dust and debris left behind by novae and supernovae eventually blend with the surrounding interstellar gas and dust, enriching it with the heavy elements and chemical compounds produced during stellar death. Eventually, those materials are recycled, providing the building blocks for a new generation of stars and accompanying planetary systems. The Outer Planets: A Star is Born A Star is Born Summary: Stars form in cold, dense regions of space called molecular clouds.

When the force of gravity pulling in on the cloud is greater than the strength of internal pressure pushing out, the cloud collapses into a protostar. Sections: WHERE DO STARS FORM? Stars form in the densest regions of the interstellar medium, or ISM, called molecular clouds. The ISM is the name given to the gas and dust that exists between the stars within a galaxy. It is 99% gas and 1% dust, by mass. Properties of ISM gas in our region of the Milky Way Galaxy: 70% hydrogen 2% elements heavier than helium The Orion Nebula (click for more images) Molecular clouds are perfect star-forming regions because the combination of these atoms into molecules is much more likely in very dense regions. This photograph shows the Orion Nebula, an interstellar cloud in which star systems - and possibly planets - are forming. Our own solar system presumably formed as gravity caused the collapse of a similar large cloud of gas. The piece of cloud that formed our Solar System is known as the solar nebula. Click the photo to the left to see more images of the Orion Nebula. WHY DO STARS FORM?

A star forms when a molecular cloud collapses under its own gravity forming a dense core sustained by nuclear fusion. This happens only when the force of gravity pulling in exceeds the outward push of pressure. High-density molecular clouds have stronger forces of gravity pushing in, making it easier to overcome the total pressure within the cloud. SHOW ME THE MATH Once started, the collapse of the solar nebula continues because the force of gravity exerted on the cloud grows stronger as the cloud shrinks in size. The universal law of gravitation is written mathematically: Fg is the force of gravitational attraction, M1 and M2 are the masses of the two objects, and d is the distance between their centers. The symbol G is known as the gravitational constant. Its value is G = 6.67 · 10-11kg·m2/s2. Before its collapse began, the gas that made up the solar nebula was probably spread out over a roughly spherical region a few light-years in diameter. This gas was extremely low in density and extremely cold.

Possible causes for the start of a collapse: Shock wave from a nearby exploding star Collision of 2 molecular clouds The Crab Nebula (click to enlarge) This is a picture of the Crab Nebula taken with the Hubble Space Telescope. The Crab Nebula was created when a star exploded. Our Solar System is thought to have formed from particles expelled from a supernova explosion over 4 billion years ago. As the solar nebula shrinks in size, its density, temperature and shape all undergo dramatic changes. HEATING: The temperature of the solar nebula increases as it collapses. As the cloud shrinks, its gravitational potential energy is converted to the kinetic energy of individual gas particles falling inward. These particles crash into one another, converting their kinetic energy into thermal energy. SPINNING: Like an ice skater pulling in her arms as she spins, the solar nebula rotates faster and faster as it shrinks in radius. This increase in rotation rate represents conservation of angular momentum. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the cloud's shrinkage makes fast rotation inevitable.

Star Formation Star Formation: Readings: Schneider & Arny: Unit 60 Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero). At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form. Since the regions are dense, they are opaque to visible light and are known as dark nebula . Since they don't shine by optical light, we must use IR and radio telescopes to investigate them. Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 104 solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass.

These clumps then form into protostars and the whole process takes about 10 millions years. How do we know this is happening if it takes so long and is hidden from view in dark clouds? Most of these cloud cores have IR sources, evidence of energy from collapsing protostars (potential energy converted to kinetic energy). Also, where we do find young stars (see below) we find them surrounded by clouds of gas, the leftover dark molecular cloud. And they occur in clusters, groups of stars that form from the same cloud core. Protostars: Once a clump has broken free from the other parts of the cloud core, it has its own unique gravity and identity and we call it a protostar. As the protostar forms, loose gas falls into its center. The infalling gas releases kinetic energy in the form of heat and the temperature and pressure in the center of the protostar goes up. As its temperature approaches thousands of degrees, it becomes a IR source. Several candidate protostars have been found by the Hubble Space Telescope in the Orion Nebula . During the initial collapse, the clump is transparent to radiation and the collapse proceeds fairly quickly. As the clump becomes more dense, it becomes opaque.

Escaping IR radiation is trapped, and the temperature and pressure in the center begin to increase. At some point, the pressure stops the infall of more gas into the core and the object becomes stable as a protostar. The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After a few million years, thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed, and its future evolution is now set. T-Tauri Stars: Once a protostar has become a hydrogen-burning star, a strong stellar wind forms, usually along the axis of rotation. Thus, many young stars have a bipolar outflow, a flow of gas out the poles of the star. This is a feature which is easily seen by radio telescopes. This early phase in the life of a star is called the T-Tauri phase. One consequence of this collapse is that young T Tauri stars are usually surrounded by massive, opaque, circumstellar disks.

These disks gradually accrete onto the stellar surface, and thereby radiate energy both from the disk (infrared wavelengths), and from the position where material falls onto the star at (optical and ultraviolet wavelengths). Somehow a fraction of the material accreted onto the star is ejected perpendicular to the disk plane in a highly collimated stellar jet. The circumstellar disk eventually dissipates, probably when planets begin to form. Young stars also have dark spots on their surfaces which are analogous to sunspots but cover a much larger fraction of the surface area of the star. The T-Tauri phase is when a star has: vigorous surface activity (flares, eruptions) strong stellar winds variable and irregular light curves A star in the T-Tauri phase can lose up to 50% of its mass before settling down as a main sequence star, thus we call them pre-main sequence stars. Their location on the HR diagram is shown below: The arrows indicate how the T-Tauri stars will evolve onto the main sequence. They begin their lives as slightly cool stars, then heat up and become bluer and slightly fainter, depending on their initial mass.

Very massive young stars are born so rapidly that they just appear on the main sequence with such a short T-Tauri phase that they are never observed. T-Tauri stars are always found embedded in the clouds of gas from which they were born. One example is the Trapezium cluster of stars in the Orion Nebula. The evolution of young stars is from a cluster of protostars deep in a molecular clouds core, to a cluster of T-Tauri stars whose hot surface and stellar winds heat the surrounding gas to form an HII region (HII, pronounced H-two, means ionized hydrogen). Later the cluster breaks out, the gas is blown away, and the stars evolve as shown below. Often in galaxies we find clusters of young stars near other young stars. This phenomenon is called supernova induced star formation. The very massive stars form first and explode into supernova. This makes shock waves into the molecular cloud, causing nearby gas to compress and form more stars. This allows a type of stellar coherence (young stars are found near other young stars) to build up, and is responsible for the pinwheel patterns we see in galaxies.

Brown Dwarfs: If a protostar forms with a mass less than 0.08 solar masses, its internal temperature never reaches a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf, halfway between a planet (like Jupiter) and a star. A star shines because of the thermonuclear reactions in its core, which release enormous amounts of energy by fusing hydrogen into helium. For the fusion reactions to occur, though, the temperature in the star's core must reach at least three million kelvins. And because core temperature rises with gravitational pressure, the star must have a minimum mass: about 75 times the mass of the planet Jupiter, or about 8 percent of the mass of our sun. A brown dwarf just misses that mark-it is heavier than a gas-giant planet but not quite massive enough to be a star. For decades, brown dwarfs were the "missing link" of celestial bodies: thought to exist but never observed. In 1963 University of Virginia astronomer Shiv Kumar theorized that the same process of gravitational contraction that creates stars from vast clouds of gas and dust would also frequently produce smaller objects.

These hypothesized bodies were called black stars or infrared stars before the name "brown dwarf" was suggested in 1975. The name is a bit misleading; a brown dwarf actually appears red, not brown. In the mid-1980s astronomers began an intensive search for brown dwarfs, but their early efforts were unsuccessful. It was not until 1995 that they found the first indisputable evidence of their existence. That discovery opened the floodgates; since then, researchers have detected dozens of the objects. Now observers and theorists are tackling a host of intriguing questions: How many brown dwarfs are there? What is their range of masses? Is there a continuum of objects all the way down to the mass of Jupiter? And did they all originate in the same way? The halt of the collapse of a brown dwarf during its formation occurs because the core becomes degenerate before the start of fusion. With the onset of degeneracy, the pressure can not increase to the point of ignition of fusion. Brown dwarfs still emit energy, mostly in the IR, due to the potential energy of collapse converted into kinetic energy.

There is enough energy from the collapse to cause the brown dwarf to shine for over 15 million years (called the Kelvin-Helmholtz time). Brown dwarfs are important to astronomy since they may be the most common type of star out there and solve the missing mass problem (see cosmology course next term). Brown dwarfs eventual fade and cool to become black dwarfs. Relative sizes and effective surface temperatures of two recently discovered brown dwarfs -- Teide 1 and Gliese 229B -- compared to a yellow dwarf star (our sun), a red dwarf (Gliese 229A) and the planet Jupiter, reveal the transitional qualities of these objects. Brown dwarfs lack sufficient mass (about 80 Jupiters) required to ignite the fusion of hydrogen in their cores, and thus never become true stars. The smallest true stars (red dwarfs) may have cool atmospheric temperatures (less than 4,000 degrees Kelvin) making it difficult for astronomers to distinguish them from brown dwarfs. Giant planets (such as Jupiter) may be much less massive than brown dwarfs, but are about the same diameter, and may contain many of the same molecules in their atmospheres.

The challenge for astronomers searching for brown dwarfs is to distinguish between these objects at interstellar distances. Neither planets nor stars, brown dwarfs share properties with both kinds of objects: They are formed in molecular clouds much as stars are, but their atmospheres are reminiscent of the giant gaseous planets. Astronomers are beginning to characterize variations among brown dwarfs with the aim of determining their significance among the Galaxy's constituents. In this painting a young brown dwarf is eclipsed by one of its orbiting planets as seen from the surface of the planet's moon. Birth of Stars & Protoplanetary Systems - NASA JWST JWST will be able to see right through and into massive clouds of dust that are opaque to visible-light observatories like Hubble, where stars and planetary systems are being born. Birth of Stars & Protoplanetary Systems Pillars of Creation + The Pillars of Creation in the Eagle Nebula captured in visible light by Hubble. Stellar nurseries are hidden within the towers of dust and gas. Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)/J. Hester, P.

Scowen (Arizona State U.) Related Content Inside the Pillars of Creation While this image is spectacular, there are actually stars that Hubble can't see inside those pillars of dust. And that's because the visible light emitted by those stars is being obscured by the dust. But what if we used a telescope sensitive to infrared light to look at this nebula? Near Infrared View The Pillars of Creation in the Eagle Nebula captured in infrared light by Hubble. The light from young stars being formed pierce the clouds of dust and gas in the infrared. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) The next image is another Hubble view, but this time in near-infrared. In the infrared more structure within the dust clouds is revealed and hidden stars have now become apparent. (And if Hubble, which is optimized for visible light, can capture a near-infrared image like this, imagine what JWST, which is optimized for near-infrared and 100x more powerful than Hubble, will do!) Mystic Mountains + Comparison of the Carina Nebula in visible light (left) and infrared (left), both images by Hubble. Credit: NASA/ESA/M.

Livio & Hubble 20th Anniversary Team (STScI) Another nebula, the "Mystic Mountains" of the Carina Nebula, shown in two Hubble images, one in visible light (left) and one in infrared (right). In the infrared image, we can see more stars that just weren't visible before. Why is this? How Do Infrared Cameras Work? We can try a thought experiment. What if you were to put your arm into a garbage bag? Your arm is hidden. Invisible. But what if you looked at your arm and the garbage bag with an infrared camera? Remember that infrared light is essentially heat. And that while your eyes may not be able to pick up the warmth of your arm underneath the cooler plastic of the bag, an infrared camera can. An infrared camera can see right through the bag! Man in visible light. Credit: NASA/IPAC Man in infrared light. Credit: NASA/IPAC And this is how the infrared telescope works as well. It sees the heat or infrared light being emitted by the stars within the cooler dust clouds. Star-Forming Region + ALMA image of the young star HL Tau and its protoplanetary disk.

This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. Credit: ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF) The Dusty Cocoons of Star and Planet Formation JWST's amazing imaging and spectroscopy capabilities will allow us to study stars as they are forming in their dusty cocoons. Additionally, it will be able to image disks of heated material around these young stars, which can indicate the beginnings of planetary systems, and study organic molecules that are important for life to develop. IN DEPTH Key Questions JWST will address several key questions to help us unravel the story of the star and planet formation: How do clouds of gas and dust collapse to form stars? Why do most stars form in groups? Exactly how do planetary systems form? How do stars evolve and release the heavy elements they produce back into space for recycling into new generations of stars and planets? JWST's Role in Answering These Questions + Infrared Spitzer image of a star-forming region.

Credit: NASA/JPL-Caltech/ Harvard-Smithsonian CfA To unravel the birth and early evolution of stars and planets, we need to be able to peer into the hearts of dense and dusty cloud cores where star formation begins. These regions cannot be observed at visible light wavelengths as the dust would make such regions opaque and must be observed at infrared wavelengths. Stars, like our Sun, can be thought of as "basic particles" of the Universe, just as atoms are "basic particles" of matter. Groups of stars make up galaxies, while planets and ultimately life arise around stars. Although stars have been the main topic of astronomy for thousands of years, we have begun to understand them in detail only in recent times through the advent of powerful telescopes and computers. A hundred years ago, scientists did not know that stars are powered by nuclear fusion, and 50 years ago they did not know that stars are continually forming in the Universe. Researchers still do not know the details of how clouds of gas and dust collapse to form stars, or why most stars form in groups, or exactly how planetary systems form. Young stars within a star-forming region interact with each other in complex ways.

The details of how they evolve and release the heavy elements they produce back into space for recycling into new generations of stars and planets remains to be determined through a combination of observation and theory. + The stages of solar system formation. Credit: Shu et al. 1987 The stages of solar system formation are illustrated to the right: starting with a protostar embedded in a gas cloud (upper left of diagram), to an early star with a circumstellar disk (upper right), to a star surrounded by small "planetesimals" which are starting to clump together (lower left) to a solar system like ours today. The continual discovery of new and unusual planetary systems has made scientists re-think their ideas and theories about how planets are formed. Scientists realize that to get a better understanding of how planets form, they need to have more observations of planets around young stars, and more observations of leftover debris around stars, which can come together and form planets. Related Content More Comparison Images Here's is another stunning comparison of visible versus infrared light views of the same object - the gorgeous Horsehead Nebula: Visible Light Horsehead Nebula The Horsehead Nebula in visible light, captured by the Canada-France Hawaii Telescope.

Credit: NASA Infrared Light Horsehead Nebula The Horsehead Nebula in infrared light, captured by the Hubble Space Telescope. Credit: NASA/Space Telescope Science Institute (STScI) Related Video This video shows how JWST will peer inside dusty knots where the youngest stars and planets are forming Interstellar Medium and the Milky Way Dust The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebulae. Extinction Starlight passing through a dust cloud can be affected in a couple of ways. The light can be totally blocked if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the light and the thickness of the dust cloud. All wavelengths of light passing through a dust cloud will be dimmed somewhat. This effect is called extinction. Dark dusk clouds like these "Bok globules" in IC 2948 were once thought to be holes in the sky.

The dark clouds block the light from the emission nebula behind. A dark cloud, Barnard 86, is silhouetted against a starry background. Stars form in the dark clouds. A young cluster, NGC 6520, probably associated with Barnard 86 is seen just to the left of it. The discovery of the dust is relatively recent. In 1930 R.J. Trumpler (lived 1886--1956) plotted the angular diameter of star clusters vs. the distance to the clusters. He derived the distances from inverse square law of brightness: clusters farther away should appear dimmer. IF clusters all have roughly the same linear diameter L, then the angular diameter q should equal a (constant L) / distance. But he found a systematic increase of the linear size of the clusters with distance. This seemed unreasonable! It would mean that nature had put the Sun at a special place where the size of the clusters was the smallest. A more reasonable explanation uses the Copernican principle: the Sun is in a typical spot in the Galaxy.

It is simply that more distant clusters have more stuff between us and cluster so that they appear fainter (farther away) than they really are. Trumpler had shown that there is dust material between the stars! The extinction of starlight is caused by the scattering of the light out of the line of sight, so less light reaches us. Reddening Not all wavelengths are scattered equally. Just as our air scatters the bluer colors in sunlight more efficiently than the redder colors, the amount of extinction by the interstellar dust depends on the wavelength. The amount of extinction is proportional to 1/(wavelength of the light). Bluer wavelengths are scattered more than redder wavelengths. The 1/l behavior of the scattering indicates that the dust size must be about the wavelength of light (on the order of 10-5 centimeters). Less blue light reaches us, so the object appears redder than it should. This effect is called reddening, though perhaps it should be called "de-blueing". If the dust particles were much larger (say, the size of grains of sand), reddening would not be observed.

If the dust particles were much smaller (say, the size of molecules), the scattering would behave as 1/l4. Trumpler showed that a given spectral type of star becomes increasingly redder with distance. This discovery was further evidence for dust material between the stars. If the Sun is in a typical spot in the Galaxy, then Trumpler's observation means that more distant stars have more dust between us and them. You see the same effect when you observe the orange-red Sun close to the horizon. Objects close to the horizon are seen through more atmosphere than when they are close to zenith. At sunset the blues, greens, and some yellow are scattered out of your line of sight to the Sun and only the long waves of the orange and red light are able to move around the air and dust particles to reach your eyes. At near-infrared (slightly longer than visible light) the dust is transparent. At longer wavelengths you can see the dust itself glowing and you can probe the structure of the dust clouds themselves as well as the stars forming in them (young stars that are hidden from us in the visible band). The Spitzer Space Telescope observes in the infrared and has opened up a new universe to us.

Although it is not the first infrared space telescope, it is the largest infrared space telescope ever launched so it has the greatest light-gathering power and resolution of any infrared telescope. A couple of examples of the power of infrared I use in my class are the movie of the dark globulae IC 1396 and the movie of Herbig-Haro 46/47 on the Spitzer images site that transitions from the visible light image to the near infrared to the mid-infrared. Other movies contrasting the views at visible with infrared of other places in the galaxy as well as other galaxies are available there too. Vocabulary Chapter 18, Chapter Review SUMMARY The interstellar medium occupies the space among the stars. It is made up of cold (less than 100 K) gas, mostly atomic or molecular hydrogen and helium, and dust grains . Interstellar dust is very effective at blocking our view of distant stars, even though the density of the interstellar medium is very low. The spatial distribution of interstellar matter is patchy. The general diminution of starlight by dust is called extinction . In addition, the dust preferentially absorbs short-wavelength radiation, leading to a distinct reddening of light passing interstellar clouds.

Interstellar dust is thought to be composed of silicates, graphite, iron, and dirty ice. Interstellar dust particles are apparently elongated or rodlike. The polarization of starlight provides a means of studying them. A nebula is a general term for any fuzzy bright or dark patch on the sky. Emission nebulae are extended clouds of hot, glowing interstellar gas. Associated with star formation, they result when hot O- and B-type stars heat and ionize their surroundings. Studies of the emission lines produced by excited nebular atoms allow astronomers to measure the nebulas properties. Some excited atomic states take so long to emit a photon that the spectral lines associated with these transitions are never seen in terrestrial laboratories, where collisions always knock the atom into another energy state before it can emit any radiation. When these lines are seen in nebular spectra, they are called forbidden lines. Nebulae are often crossed by dark dust lanes, part of the larger cloud from which they formed. Dark dust clouds are cold, irregularly shaped regions in the interstellar medium that diminish or completely obscure the light from background stars.

Astronomers can learn about these clouds by studying the absorption lines they produce in starlight that passes through them. Another way to observe cold, dark regions of interstellar space is through 21-centimeter radiation . Such radiation is produced whenever the electron in an atom of hydrogen reverses its spin, changing its energy very slightly in the process. This radio radiation is important because it is emitted by all cool atomic hydrogen gas, even if the gas is undetectable by other means. In addition, 21-cm radiation is not appreciably absorbed by the interstellar medium, so radio astronomers making observations at this wavelength can see to great distances. The interstellar medium also contains many cold, dark molecular clouds , which are observed mainly through the radio radiation emitted by the molecules they contain. Dust within these clouds probably both protects the molecules and acts as a catalyst to help them form. As with other interstellar clouds, hydrogen is by far the most common constituent, but molecular hydrogen happens to be very hard to observe. Astronomers usually study these clouds through observations of other tracer molecules that are less common but much easier to detect.

Often, several molecular clouds are found close to one another, forming an enormous molecular cloud complex millions of times more massive than the Sun. SELF-TEST: TRUE OR FALSE? 1. Interstellar matter is quite evenly distributed throughout the Milky Way Galaxy. HINT 2. In the vicinity of the Sun, the amount of mass in the form of interstellar matter is comparable to the amount of mass in the form of stars. HINT 3. There is a lack of heavy elements in interstellar gas because they go into making interstellar dust. HINT 4. The fact that starlight becomes polarized as it passes through the interstellar medium tells us that interstellar dust particles are spherical in shape. HINT 5. A typical region of dark interstellar space has a temperature of about 500 K. HINT 6. An emission nebula is a cloud of dust reflecting the light of a nearby group of stars. HINT 7. Emission nebulae display spectra almost identical to those of the stars embedded in them. HINT 8. Forbidden emission lines can occur in emission nebulae because the density of interstellar gas there is extremely low. HINT 9.

Because of the obscuration of visible light by interstellar dust, we can observe stars only within a few thousand parsecs of Earth in any direction. HINT 10. A typical dark dust cloud is many hundreds of parsecs across. HINT 11. Because of their low temperatures, dark dust clouds radiate mainly in the far infrared part of the electromagnetic spectrum. HINT 12. Most interstellar matter exists in the form of molecular clouds. HINT 13. 21-cm radiation provides astronomers with information on the density, temperature, and internal motions of interstellar gas. HINT 14. 21-cm radiation can pass unimpeded through the entire Milky Way Galaxy. HINT 15. Water, formaldehyde, carbon monoxide, and numerous organic molecules have all been found in molecular clouds. HINT SELF-TEST: FILL IN THE BLANK 1. The interstellar medium is made up of _____ and _____. HINT 2. To scatter a beam of radiation, a particles size must be _____ the wavelength of the radiation. HINT 3. Extinction is the _____ of starlight by interstellar _____. HINT 4.

The density of interstellar matter is very _____. HINT 5. Interstellar gas is composed of 90 percent _____ and 9 percent _____. HINT 6. The temperature of a typical emission nebula is about ____ K. HINT 7. The process in which clouds of cool dense gas (such as in the Eagle Nebula) are eaten away by radiation from nearby hot young stars is called ____. HINT 8. An HII region is another name for _____. HINT 9. Dark dust clouds can have temperatures as low as _____ K. HINT 10. 21-cm radiation is emitted by _____ hydrogen. HINT 11. 21-cm radiation results from a change in the _____ of the electron in a _____. HINT 12. Emissions from molecular clouds are in the _____ part of the electromagnetic spectrum. HINT 13. _____ is thought to play an important role in the formation of molecules in molecular clouds. HINT 14. The most common constituent of molecular clouds is molecular _____. HINT 15. A molecular cloud complex may contain as much as _____ solar masses of gas.

HINT REVIEW AND DISCUSSION Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website. The number of squares preceding each problem indicates the approximate level of difficulty. 1. The average density of interstellar gas within the Local Bubble is much lower than the value mentioned in the textin fact, it is roughly 103 hydrogen atoms/m3. Given that the mass of a hydrogen atom is 1.7 10-27 kg, calculate the total mass of interstellar matter contained within a Bubble volume equal in size to planet Earth. HINT 2. Assuming the same average density as in the previous question, calculate the total mass of interstellar hydrogen contained within a cylinder of cross-sectional area 1 m2, extending from Earth to Alpha Centauri. HINT 3. Given the average density of interstellar matter stated in Section 18.1, calculate how large a volume of space would have to be compressed to make a cubic meter of gas equal in density to air on Earth (1.2 kg/m3). HINT 4.

Assuming a density of 3000 kg/m3, estimate the mass of the dust particle illustrated in Figure 18.3 (a). HINT 5. A beam of light shining through a dense molecular cloud is diminished in intensity by a factor of two for every 5 pc it travels. By how many magnitudes is the light from a background star dimmed, if the total thickness of the cloud is 60 pc? HINT 6. Interstellar extinction is sometimes measured in magnitudes per kiloparsec (1 kpc = 1000 pc). Light from a star 1500 pc away is observed to be diminished in intensity by a factor of 20 over and above the effect of the inverse-square law. What is the average interstellar extinction along the line of sight, in mag/kpc? HINT 7. Spectroscopic observations of a certain star reveal it to be a B2II giant, with absolute magnitude 6. ( Secs. 17.4, 17.7 ) The stars apparent magnitude is 14. Neglecting the effects of interstellar extinction, calculate the distance to the star.

If the stars distance is known (by other means) to be 5000 pc, calculate the average extinction along the line of sight, in mag/kpc. ( More Precisely 17-1 ) HINT 8. A star of apparent magnitude 10 lies 500 pc from Earth. If interstellar absorption results in an average extinction of 2 mag/kpc, calculate the stars absolute magnitude and luminosity. HINT 9. A star of known absolute magnitude 25 has apparent magnitude 10. If interstellar absorption results in an average extinction of 2 mag/kpc, calculate the stars distance. (Note: This problem does not have an algebraic solution. You wil have to solve it by numerical meansessentially trial and error on a calculator.) HINT 10. To carry enough energy to ionize a hydrogen atom, a photon must have a wavelength of less than 9.12 10-8 m (91.2 nm). Using Wiens law, calculate the temperature a star must have for the peak wavelength of its blackbody curve to equal this value. ( Sec.

3.4 ) HINT 11. Estimate the escape speeds near the edges of the four emission nebulae listed in Table 18.1, and compare them with the average speeds of hydrogen nuclei in those nebulae. ( More Precisely 8-1 ) Do you think it is possible that the nebulae are held together by their own gravity? HINT 12. What would the mass of M8 have to be in order for its escape speed to equal its average molecular speed? HINT 13. If a group of interstellar clouds along the line of sight have radial velocities in the range 75 km/s (receding) to 50 km/s (approaching), calculate the range of frequencies and wavelengths over which the 21.1-cm (1420 MHz) line of hydrogen will be observed. ( Sec. 3.5 ) HINT 14. Calculate the radius of a spherical molecular cloud whose total mass equals the mass of the Sun. Assume a cloud density of 1012 hydrogen atoms per cubic meter. HINT 15.

A cloud of atomic hydrogen has a radius of 1 pc and an average density 106 hydrogen atoms per cubic meter. Collisions between atoms ensure that, at any instant, 3/4 of all atoms are in the upper (parallel spin) state, as discussed in Section 18.4. The transition producing the 21-cm line is very unlikelythe probability that any given atom in the upper state will make the transition during any given second is about 3 10-15 (compare with 108 for the Ha transition). Use these figures, together with the Planck formula for the energy of the photon emitted in the transition, to estimate the total radio luminosity of the cloud. ( Sec. 4.2 ) HINT COLLABORATIVE EXERCISES 1. Exploring Density. The interstellar medium has a very low density of about 1000 per cubic kilometer. Estimate the population density of students in the tallest dormitory on campus using units of students per cubic feet and compare to the population density of the classroom. Explain your reasoning.

RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 18 on the Companion Website for Astronomy Today 4/e. 1. Access the "Messier Catalog" pages and determine which objects constitute the majority of objects M1 through M10 and which objects for M90 through M100. 2. Access the "Nebulae: Fuzzy Patches In Space" page and define the four types of nebulae. PROJECTS 1. The constellation Orion the Hunter is prominent in the evening sky of winter. Its most noticeable feature is a short, straight row of three medium-bright stars: the famous belt of Orion. A line of stars extends from the eastmost star of the belt, toward the south. This line represents Orions sword. Towards the bottom of the sword is the skys most famous emission nebula, M42, the Orion Nebula. Observe the Orion Nebula with your eye, with binoculars, and with a telescope. What is its color? How can you account for this? With the telescope, try to find the Trapezium, a grouping of four stars in the center of M42.

These are hot, young stars; their energy causes the Orion Nebula to glow. 2. Observe the Milky Way on a dark, very clear night. Is it a continuous band of light across the sky or is it mottled? The parts of the Milky Way that appear missing are actually dark dust clouds that are relatively near the Sun. Identify the constellations in which you see these clouds. Make a sketch and compare with a star atlas. Find other small clouds in the atlas and try to find them with your eye or with binoculars. SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text. 1. One of the most interesting nebula is the Great Orion Nebula, M42. Use SkyChart III to locate Orion, and determine what time of the year it is overhead at a convenient time for viewing. While the nebula is visible to the unaided eye, it is much better with binoculars, and only gets more interesting as you use larger and larger telescopes to observe it.

In addition to the Practice Problems and Destinations modules, the Companion Website at provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites.

Question: What name is given to visible clouds of dust and gas in space?
Answer: Nebulae