What do you call a cloud in space that is composed of dust and gas? | Reference.com What do you call a cloud in space that is composed of dust and gas? A: Quick Answer A cloud in space that is composed of dust and gas is call a nebula. The word "nebula" is derived from the Latin word for "cloud," and nebulae in space are indeed large interstellar clouds made up of dust, hydrogen, helium and plasma. The study of nebulae is of particular interest to astronomers because they indicate much about stellar and planetary formations. There are four types of nebula, characterized by different gaseous and particulate composition. They are: Planetary nebula Some of the largest known and identified nebulae in the Milky Way are: Crab Nebula 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 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.

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 News | A Star is Born News | November 26, 2001 A Star is Born Vast clouds of gas and dust are swirling throughout our Milky Way galaxy. Some of these clouds are stellar nurseries, places where thousands of stars like our Sun are being born right now. These clouds are huge, about 100,000 times the size of our solar system and many thousands of times the mass of our Sun. Life Rises from a Death Spiral So just how does a run-of-the-mill, dark cloud of gas and dust become a star-forming region?

Astronomers know that the cloud needs to be stirred up. They think a nearby exploding star, called a supernova, can trigger this motion. A supernova is the fiery death of a massive star, which causes gas to move up to 10,000 or 20,000 kilometers (approximately 6,000 to 12,000 miles) per second. Once this occurs, the moving gas and dust in the cloud collect more in some areas than others. The gravitational attraction of this slight clump attracts more gas and dust, which then creates stronger gravity, which in turn attracts more gas and dust. Pretty soon, this clump of matter is on its way to becoming a star! These clumps are the very youngest stars. At this point, they are 300 to 500 times the size of our solar system, yet these stars are difficult to see. For one thing, they are still surrounded by a dark cocoon of matter, deep inside the larger cloud of gas and dust. Out of the billions of stars in our galaxy, we currently know of about only 50 stars at this stage of development.

However, as we develop better detectors, we're finding more baby stars. Upcoming missions, such as the JPL-managed Space Infrared Telescope Facility, will aid in understanding this and later stages of star formation. Gravity Moves into Action In the next stage of star development, gravity pulls more and more matter from the surrounding nebula, a concentration of gas and dust, into these clumps. Over a period of 10,000 to 100,000 years, the central `knot' of matter in each mass concentration gets denser and warms up. The clump glows simply from gravitational contraction; energy literally comes from matter falling into the core of the clump. Eventually, the cocoon of matter disperses and the teenage star is more easily seen. After this long birthing process, the toddler star is able to break out of its natal cocoon of matter. This occurs first through the north and south poles, in part because streams of matter move away from the poles of the star at about 100 to 300 kilometers (approximately 50 to 200 miles) per second.

Exactly how and why these jets of matter form is not well-understood, but they are seen in nearly every star in this age range. These jets can create dramatic structures and motions as they literally smash into the rest of the nebula. In fact, many protostars, or forming stars which have not yet begun to shine, are found not from observations of the protostar itself, but inferred from observations of the matter pushed around by the jets. Over a period of about 100,000 to 3,000,000 years, the disk thins out, and the jets slow down and turn off. Sometime in this stage, the protostar stops accumulating significant amounts of matter from the disk and nebula. Although much smaller than when it started out, the system is still pretty large. The disk is about twice as large as the solar system, and the protostar is about twice as large as the Sun. Planets Begin to Form Over the next 40 million years, the protostar contracts and warms up, and the disk steadily thins out and cools off. The disk cools off enough for clumps of matter to form in the disk.

These clumps push the matter in the disk around, smashing into other clumps, and eventually forming planets. This is why planets in the same solar system are all essentially in the same plane and going the same way around the Sun -- it is the same orientation and direction of rotation of the original disk from which the planets formed. Understanding this stage is crucial to understanding exactly how and where planets form in the disk, and many scientists at JPL spend a lot of time thinking about how this might work. After about 50 million years, the star finally stops contracting. The relentless push of gravity has compressed the core, heating it up until the star turns hydrogen into helium in its core. This process releases energy, causing the star to glow brightly. The pressure of all this energy trying to get out of the star stops the contraction caused by gravity. The forming planets still need more time to cool off and even combine into larger planets, more like what we see in our own solar system. Depending on how far away from the parent star the planets have formed, there is a chance that life could begin to form on some of them. Then things can get REALLY interesting ...

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 Interstellar Medium (ISM) | Astronomy 801: Planets, Stars, Galaxies, and the Universe Planets, Stars, Galaxies, and the Universe Print When you observe the night sky, you see the stars as pinpoints of light against a black background. You have also probably been told that outer space is a “vacuum”—that is, that, other than stars and planets, it is very empty.

It is true that space is so empty that it is a more perfect vacuum than we can create in the laboratory on Earth; however, space is not completely empty. Here are some images that show some obvious examples of regions in space that contain some material between the stars: Hubblesite: The Eagle Nebula In all of these images, you see different regions of space that are either filled with what appears to be glowing gas (similar to a neon light), or dark regions that are obscuring the stars behind them. All of these objects are called nebulae, a word which is Latin for "cloud." Taken as a whole, the gas and dust that fills the space between stars is called the interstellar medium. If you look at the images at the links above in some depth, you'll see some or all of the following: Clouds that glow an almost uniform, bright red Dark clouds that appear to block all or most of the light of stars or bright clouds behind them Clouds that glow a blue color very similar to the blue of our sky At this point, let's address the physical processes responsible for the appearance of these various nebulae, and give them more descriptive names.

Emission Nebulae As the name implies, emission nebulae emit emission spectra, not continuous or absorption spectra. If you refer to the lesson on the production of emission lines, what is required for a cloud of gas to emit an emission spectrum is for electrons in the gas to be in energy levels above level 1. As they move down to the ground state, they will emit photons of specific energies, producing emission lines at those specific energies. One point we glossed over in that lesson is: How do the electrons get into the higher energy levels in the first place? In the case of emission nebulae, there is a clue usually quite apparent in the image. For example, in the image below of massive stars in NGC 6357, you see a large region of glowing red emission nebula surrounding a number of massive stars (although there is no way to tell just from looking at the picture that they are massive stars). Figure 5.1: Massive stars in NGC 6357 Credit: APOD The massive stars give off a lot of ultraviolet light, which the hydrogen gas absorbs.

The electrons in the H atoms absorb so much energy that they are actually stripped completely from the H atoms, creating a sea of hydrogen nuclei and free electrons. This is a process called ionization. The free electrons can recombine with the hydrogen nuclei, and then, as they cascade down to the ground state, they emit photons. Because the clouds are primarily made of hydrogen, the spectrum of a typical emission nebula shows strong hydrogen emission lines. The strongest hydrogen line in the visible part of the spectrum is H-alpha at 656.3 nm, which is in the red part of the spectrum. Astronomers refer to neutral hydrogen atoms as HI (pronounced "H-one") and they refer to ionized hydrogen nuclei as HII ("H-two"). For this reason, the glowing red emission nebulae found surrounding massive stars are often referred to as "HII Regions." We will talk about these more later, but there are objects known as planetary nebulae that also give off emission spectra. Here is an example emission spectrum from a planetary nebula taken by Prof.

Robin Ciardullo of Penn State: Figure 5.2: Emission Spectrum Credit:Robin Ciardullo, PSU On the x-axis is wavelength in Angstroms (1 Angstrom is 1/10 of a nanometer, so 656.3 nm = 6563 Angstroms), and on the y-axis is the relative flux (or apparent brightness). You can see that there are several very prominent emission lines—the H-α line at 6563 Angstroms, the H-β line at 4861 Angstroms, a helium line, and several oxygen lines, for example. In this nebula, the oxygen line at 5007 Angstroms is stronger than H-α, so this nebula will appear more green than red. This is seen in the central regions shown in the photo below.

Figure 5.3: The Ring Nebula (M57) Credit: APOD Dark Nebulae In regions where there are a lot of stars (such as in the APOD of a Giant Molecular Cloud ), or regions with bright emission nebulae (such as in the Hubblesite image of a Bok Globule ), we can see dark nebulae that appear to be blocking the background light sources behind them. These dark nebulae are also interstellar clouds, but unlike the emission nebulae, they are very cold (10 K, as opposed to about 10,000 K for emission nebulae) and very dense. These clouds are called molecular clouds because the conditions in them are just right for the creation of molecular hydrogen. Be aware that two hydrogen atoms that have linked together form the molecule H2 (which is also pronounced "H-two", but should not be confused with HII). Another constituent of these dark clouds is interstellar dust . The dust particles are very tiny grains of different solids, and if you compare their chemical makeup to common materials on Earth, they are similar in many ways to soot and sand.

As photons of light encounter a dark cloud of gas and dust, we observe two effects: first, the light is extinguished—that is, the background light sources appear dimmer than if there was no intervening cloud. This effect is referred to as interstellar extinction. The second effect is referred to as interstellar reddening. As the name suggests, the light from a background source will appear redder than if it had not passed through the cloud. As photons of light pass through the cloud, they get scattered from their original paths. Blue light scatters more than red light, so less blue light and more red light from the background object makes it through the cloud. If you look closely at the APOD image of Barnard 68 , you will see that all the stars visible near the edge of the cloud look red or orange, while those just outside the boundaries of the cloud look blue or white, which is caused by this reddening effect. Again because of the wavelength dependence of scattering by dust, most of the infrared light will actually pass through the cloud. See the image below of Barnard 68 taken through infrared filters. You can see the reddening, but you can also see right through the cloud!

Figure 5.4: Barnard 68 taken through infrared filters. Credit: European Southern Observatory Reflection Nebulae The bright blue regions seen in some of the images above are reflection nebulae, and they are also caused by the scattering of light by dust particles. The light from a star encounters dust particles and reflects. In most cases, the particles are the correct size to scatter blue light more efficiently than red light, so the reflection nebula appears to us in the reflected blue light from the nearby star. Since the light we see is reflected starlight, the spectrum of the reflection nebula is similar to the spectrum of the star's light. More complex nebulae Finally, let's consider some of the images of the regions that include overlapping nebulae of different types. First, look again at the very famous picture of a region of sky called the Eagle Nebula. Figure 5.5: The Eagle Nebula Credit: Hubblesite The dark pillars are part of the edge of a molecular cloud. Some nearby, very bright stars (off the edge of the picture) are illuminating these pillars of gas, and their intense radiation is causing the outer layers of the molecular cloud to evaporate (this process is called photoevaporation).

What astronomers have found is that this process of photoevaporation is revealing that inside the densest knots in the pillars are newly formed, baby stars. If you study carefully some of the other images, for example the panorama of Carina, you can find similar structures to the ones seen in the Eagle Nebula. In fact, we see that all of the youngest stars in the sky live in regions with a lot of interstellar matter. It seems clear that there must be a connection between the stars and the interstellar gas. By observing these regions in many wavelengths of light, astronomers have found that new stars are formed out of the gas and dust in these clouds. The process is not completely understood, but in the following section, we will discuss how stars form. Want to learn more? At Hubblesite, they have "image tours" that take you on in-depth studies of some of their more detailed images. You can tour the Eagle Nebula , the Orion Nebula , and the Carina Nebula . The Eagle Nebula image is one of the most famous Hubble images ever taken, and so PBS has made a video on the making of the Eagle Nebula image .

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.

Question: What is a cloud of gas and dust called in space ?
Answer: NEBULA