What is the interstellar medium? Links What is the Interstellar Medium? Simply put, the interstellar medium is the material which fills the space between the stars. Many people imagine outer space to be a complete vacuum, devoid of any material. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust . In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust. Interstellar Gas: Approximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 75% is in the form of hydrogen (either molecular or atomic), with the remaining 25% as helium. The interstellar gas consists partly of neutral atoms and molecules , as well as charged particles, such as ions and electrons . This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter.

(For comparison, the air we breathe has a density of approximately 30,000,000,000,000,000,000 molecules per cubic centimeter.) Even though the interstellar gas is very dilute, the amount of matter adds up over the vast distances between the stars. The interstellar gas is typically found in two forms: Cold clouds of neutral atomic or molecular hydrogen; and Hot ionized hydrogen near hot young stars. The cold clouds of neutral or molecular hydrogen are the birthplace of new stars if they become gravitationally unstable and collapse. The neutral and molecular forms emit radiation in the radio band of the electromagnetic spectrum . The ionized hydrogen is produced when large amounts of ultraviolet radiation are released by hot newly-formed stars. This radiation ionizes the surrounding clouds of gas. Visible light is emitted when electrons recombine with the ionized hydrogen, which is seen as beautiful red colors of emission nebulae. Examples of emission nebulae are the Trifid Nebula or the Orion Nebula (seen in this photograph). Interstellar Dust: Interstellar dust is not like the dust that you might find under your bed; it is made of very different substances.

These dust particles are extremely small, just a fraction of a micron across, which happens to be approximately the wavelength of blue light waves. The particles are irregularly shaped, and are composed of silicates, carbon, ice, and/or iron compounds. When light from other stars passes through the dust, a few things can happen. If the dust is thick enough, the light will be completely blocked, leading to dark areas. These dark clouds are known as dark nebulae. The Horsehead Nebula, seen to the left, is an example of this. Light passing through a dust cloud may not be completely blocked, although all wavelengths of light passing through will be dimmed somewhat. This phenomenon is known as extinction . The extinction is caused by the light being scattered off of the dust particles out of our line of sight, preventing the light from reaching us. The amount that the light is dimmed depends upon a few factors, including the thickness a nd density of the dust cloud, as well as the wavelength (color) of the light. Because of the size of the dust particles, scattering of blue light is favored.

Therefore, less of the blue light reaches us, which means that the light that reaches us is more red than it would have been without the interstellar dust. This effect is known as interstellar reddening . (Note that this is not the same thing as redshift , which is due to the effects of relative movement between a light source and its receiver.) This process is similar to those that make the sun red at sunset. (To see an explanation of extinction and interstellar reddening that is more mathematical, please visit this site .) In turn, a dust cloud that is illuminated by star light, when viewed from the side, appears blue, as in the close-up of the "Egg Nebula" seen at right. This is similar to the blue sky we see, which is produced by sunlight scattered by the Earth's atmosphere. Aside from passing through, or being blocked from passing through, interstellar dust, light may also be reflected from the clouds of dust. This is seen as a reflection nebula, as is seen in the lower left corner of the Horsehead Nebula image as a bright spot. A reflection nebula is a region of dusty gas surrounding a star where the dust reflects the starlight, making it visible to us.

The picture at left was taken by the Hubble Space Telescope, and is of NGC 1999, a nebula in the constellation Orion. (You can read the press release that accompanies the picture on the HST site .) 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. Interstellar Dust Clouds - Only Limited By Your Imagination slide 2 of 5 What Are They Made Of? These dust clouds are vast collections of particles and molecules made up of carbon, silicates, ice (which could be water, carbon dioxide or ammonia ices), and even iron. They may even contain organic compounds such as formaldehyde, acetylene, acetaldehyde and methanol.

The content of these clouds has been determined through the use of radio telescopes, which can detect the emission spectra of certain molecules and atoms. Reflected light in the visible and UV can also be used to identify elements in the clouds. The source of the material that makes up these clouds comes in part from nova and supernova that occurred eons ago. These factories generate heavier elements as part of the fusion process as the star ages, and depending on the star’s mass, it can push off its outer layers in an elegant planetary nebula or blast the star’s contents into the heavens in a supernova. Over time (and the universe has been around a long time) these particles can move about, forming clouds that can ultimately be the birthplace of new stars. Even a star as modest as our Sun contributes particles to the interstellar medium through the solar wind and coronal mass ejections it generates. slide 3 of 5 Dust Everywhere! If you look at images of galaxies you often can see dark lanes of interstellar dust and gas that often have stars forming near them. This is beautifully illustrated in the image of the galaxy NGC-4921 shown below on the left.

The enlarged image on the right clearly shows the dark lanes of dust as they meander through a fog of stars. Along the edge of the dust cloud you can see very hot young stars that formed. slide 4 of 5 Images Beyond The Imagination One last image of an interstellar dust cloud that tickles the imagination and is perfect for Halloween is the Witch Head nebula (IC2118), which is also in the constellation of Orion. We see this cloud of dust by the reflected light coming from the hot, blue super giant star, Rigel. The nebula has a strong bluish color because of the fact that the particles that make up the cloud are on the order of fractions of a micron in size and they selectively reflect the short wavelengths of light – blue light, better than the longer wavelengths. It also helps that the light coming from Rigel has a strong blue component because of its high temperature. DOLORES GENDE ASTRONOMY: 11. The Interstellar Medium: Birthplace of Stars After studying this chapter you should be able to: 1. Summarize the composition and physical properties of the interstellar medium. 2.

Describe the characteristics of emission nebulae, and explain their significance in the life cycle of stars. 3. Discuss the properties of dark interstellar dust clouds. 4. Specify some of the radio techniques used to probe the nature of interstellar matter. 5. Summarize the sequence of events leading to the formation of a star like our Sun. 6. Describe some of the observational evidence supporting the modern theory of star formation. 7. Explain how the formation of a star is affected by its mass. ASTRONOMY CHAPTER 10 Flashcards What is another name for an emission nebulae? How is it produced? Definition AKA HII region. Produced UV radiation from nearby hot stars ionizes gas Term What do the red, blue and violet Balmer lines blend together to produce? Definition the pink-red color of ionized hydrogen Term What color does reflection nebulae appear? why? Definition blue, because blue light is scattered by larger angles than red light Term How is reflection nebulae produced? Definition by gas and dust illuminated by a star that is not hot enough to ionize the gas Term What is a dark nebulae?

Definition a nebula that absorbs the light from stars behind it Term What color does interstellar clouds make background stars appear? Definition What does HI (hydrogen-one) consist of? Definition cold clouds of neutral hydrogen Term what does HII (hydrogen-two) consist of? Definition hot clouds with ionized hydrogen Term What else does the interstellar medium contain, other than atoms and ions? What does this do? Definition molecules, which stores energies in rotation and vibration Term What do transitions between energy levels typically emit? Definition radio wavelengths Term What are the two easiest molecules to observe in space? What is the most common molecule in space? Definition CO (carbon monoxide) and OH (hydroxyl); most common is hydrogen Term What do we use to trace hydrogen in space? Why? Definition CO (carbon monoxide) because H and CO exist together Term True or false: Molecules are easily destroyed by UV radiation from hot stars? Definition Stardust Stardust Stardust The Horsehead Nebula, a dense cloud of dust and gas in Orion. Dust is a relative newcomer to the Universe. It joined the gases only after the first stars of the cosmos had matured and their more massive members had exploded.

It is these �supernovae� that seed the interstellar medium with the heavier elements of which dust is composed. Along with the introduction of dust came large aggregates of dust, which are the asteroids, moons, and rocky earth-like planets. But most of the dust still swirls and eddies along with the gases as tiny particles, in great clouds called nebulae. Large collections of these nebulae orbit along with stars around common central points of gravity. These collectives are of course the galaxies. The dustiest area of the Milky Way Galaxy is along the galactic plane, in which our solar system is embedded. Away from the plane, up above and below, there appear to be isolated clouds of gas and dust scattered about. These high latitude dust bunnies are difficult to detect. They tend to be too cold to be glowing with visible light. They are typically only 15 to 17 degrees above absolute zero, which is very cold, but it is still warmer than the background radiation of space, which is about 2.7 degrees Kelvin. The Trifid Nebula Hydrogen glowing red, dust reflecting blue. Galactic clouds of dust and gas are fun targets for amateur telescopes.

The clouds that glow with a soft red light are called emission nebulae because they are emitting light of their own. They are mostly hydrogen gas - warmed by young stars born within them. Other clouds, the reflection nebulae, act like mirrors. These are mostly dust, scattering the bluish light from nearby stars because the dust particles are similar in size to the wavelength of blue light. Then there are the dark nebulae. Dark cold clouds don�t glow much at all, and aren�t scattering a detectable amount of light. Thus they are the most elusive to find and observe. Some of them can be found silhouetted by brighter clouds that lay in the distance behind them. The most famous backlit dark cloud is the Horse Head Nebula in Orion, near the easternmost star of his belt. Amateur astronomers who swear to having seen the Horse Head are sometimes said to be members of the liars club. This is because seeing it is so difficult that, except under the most perfect conditions, it is impossible.

So much is required to see the faint ghostly image; large telescopes (18 or more inches diameter), a transparent sky, very still air, a high quality eyepiece, a nebula filter (O3), good dark adapted eyes, and not least - an experienced observer. Detecting faint dark dust clouds doesn�t have to be left to the liar�s club. The discrepancy between a stars intrinsic color and its apparent color, i.e. the degree of reddening, can be calculated, thus betraying the presence and density of an intervening dust cloud. But at high galactic latitudes the clouds are small and widely scattered, making the calculation for hundreds of thousands of stars across the vast expanse of sky a daunting task. Dust clouds cause a certain amount of extinction of starlight that passes through them, especially at the higher (blue) wavelengths. Put another way, interstellar dust particles extinguish light more efficiently at short wavelengths than at long wavelengths. Having attenuated the light more in the blue region than the red, we say that the cloud has �reddened� the starlight.

Stated succinctly, the extinction coefficient (Ql) goes as the radius of the dust grains and inversely as the wavelength of the incident light, Ql ~ r/l. There are exceptions at certain wavelengths due to the atomic composition of the dust and energy resonance, but the mean result is that the color (surface temperature) of stars can appear redder (cooler) than their actual value. The discrepancy can be detected by considering the absorption/emission lines in the star�s spectrum. Understanding interstellar dust is important in many fields of astronomy. Accounting for the degree of stellar reddening caused by dust is crucial in astronomical fields that rely on photometry, such as cosmology and other extra-galactic sciences. These disciplines involve mathematical models that are very sensitive to the small discrepancies in the intensity of starlight that is caused by dust. How is a star born? - Scientific American Scientific American How is a star born? Share on Facebook Richard Brill, an associate professor at Honolulu Community College, offers the following answer: Richard Brill A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion.

All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin. And once the fusion reactions begin, they exert an outward pressure. As long as the inward force of gravity and the outward force generated by the fusion reactions are equal, the star remains stable. Clouds of gas are common in our galaxy and in other galaxies like ours. These clouds are called nebulae. A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. The majority of the gas in nebulae consists of molecules of hydrogen and helium--but most nebulae also contain atoms of other elements, as well as some surprisingly complex organic molecules. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova. The source of the organic molecules is still a mystery. Image: Hubble Space Telescope STAR BIRTHS are started when the interstellar matter in gas clouds, such as the Eagle Nebula shown here, compresses and fuses. Irregularities in the density of the gas causes a net gravitational force that pulls the gas molecules closer together.

Some astronomers think that a gravitational or magnetic disturbance causes the nebula to collapse. As the gases collect, they lose potential energy, which results in an increase in temperature. As the collapse continues, the temperature increases. The collapsing cloud separates into many smaller clouds, each of which may eventually become a star. The core of the cloud collapses faster than the outer parts, and the cloud begins to rotate faster and faster to conserve angular momentum. When the core reaches a temperature of about 2,000 degrees Kelvin, the molecules of hydrogen gas break apart into hydrogen atoms. Eventually the core reaches a temperature of 10,000 degrees Kelvin, and it begins to look like a star when fusion reactions begin. When it has collapsed to about 30 times the size of our sun, it becomes a protostar. When the pressure and temperature in the core become great enough to sustain nuclear fusion, the outward pressure acts against the gravitational force. At this stage the core is about the size of our sun. The remaining dust envelope surrounding the star heats up and glows brightly in the infrared part of the spectrum. At this point the visible light from the new star cannot penetrate the envelope.

Eventually, radiation pressure from the star blows away the envelope and the new star begins its evolution. The properties and lifetime of the new star depend on the amount of gas that remains trapped. A star like our sun has a lifetime of about 10 billion years and is just middle-aged, with another five billion years or so left. Margaret M. Hanson, an assistant physics professor at the University of Cincinnati, gives this response: Stars form from the gravitational collapse of large clouds of interstellar material. In fact, the space between stars is not empty; it is nearly empty, but not entirely. Interstellar matter, that found lying between the stars, is made from gas and dust. Granted, only about 10 percent of the mass in our Milky Way galaxy is made up of interstellar matter. But this material, as tenuous as it is, exerts a gravitational force, and as a result, it will begin to pull itself together. Image: Hubble Space Telescope GALACTIC NURSERY. Many stars are born in the beautiful Orion Nebula. As this accretion continues, the gravity becomes increasingly strong because its strength rises as the mass increases and the distance of the individual atoms decreases.

Eventually this interstellar matter entirely collapses in on itself. The material at the very center is compressed by the infalling material on the outside, pushing down to get to the center. And this compression heats up the center of the collapsing cloud. At some point, the temperature gets so extremely high at the center, it triggers a fusion reaction. All the material that has fallen in then evolves into a hot, bright star. The star will continue to shine as long as there is hydrogen gas to fuse through nuclear reactions, and the gravitational pressure pushing inward keeps the atoms very hot and tightly packed at the center. For a more advanced, elaborate description, with wonderful pictures, see the Web site A Star Is Born, put together by Lee Carkner of the University of Colorado. Mission:Science APA National Aeronautics and Space Administration, Science Mission Directorate. (2009). De Stella Nova: Birth of a Star. Retrieved , from Mission:Science website: MLA Science Mission Directorate. "De Stella Nova: Birth of a Star" Mission:Science. 2009. National Aeronautics and Space Administration. Imagine one night, you stare up at the stars and spy a new light in the sky.

In the southwestern sky, this bright light looks as big as the planet Mars. Within days, it can be seen shining even brighter than Jupiter making it the brightest object in the night sky. You might think it is an airplane. Now imagine this is 400 years ago - before airplanes, satellites, and even before telescopes. What would you think it is? Kepler's "new star" is shown here in the foot of Ophiuchus. Kepler studied the object every clear night until it disappeared and wrote a book about his observations called "De Stella Nova" meaning "The New Star". Kepler's New Star Johannes Kepler saw just that on October 9th, 1604. He saw a light in the sky that grew brighter than Jupiter in just a few days. By January 1605, Kepler saw the light starting to fade, but it was still brighter than some stars. By March 1606, only 18 months since he first saw it, the object was no longer visible with the naked eye. Kepler wrote a book about what he saw entitled "De Stella Nova" or The New Star. New stars are forming in the Eagle Nebula.

The interstellar gas inside these pillars is dense enough to collapse under its own weight, beginning the formation of young stars. A Stellar Nursery Astronomers believe that clouds of gas and dust, called nebulas, are the birthplace of stars. Imagine an enormous cloud of gas and dust many light-years across. Gravity, as it always does, tries to pull the materials together. A few grains of dust collect a few more, then a few more, then more still. Great Nebula in Orion Over time, within dense regions of a nebula, gravity pulls the hydrogen gas together and begins to spin. As the gas spins faster and faster, the gas contracts and builds up heat and pressure. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. With a good pair of binoculars you can see swirls and wreath shapes. The Orion Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. Pervasive red glow is hydrogen gas and the blue tinted dust that reflects the light of newborn stars.

In this image, NASA's Spitzer Space Telescope has pierced thick cosmic dust to reveal this embedded protostar, or embryonic star, in HH46-IR. Protostar - A New Born Star Eventually, enough gas and dust has been collected into a giant ball that, at the center of the ball, the temperature reaches 15,000,000 degrees Celsius. At this temperature, hydrogen atoms are crushed - or fused - together to form helium creating an explosion. This reaction is the same as what happens when a hydrogen bomb explodes and is called nuclear fusion. The cloud is now a 'protostar'. Stars & Planets This Spitzer Space Telescope image shows a disc of planet-forming debris. The gravitational pull of the baby star keeps the explosions from ripping the star apart. Eventually, the star's gravity and explosions equalize and it becomes a main sequence star. At this time, planets begin to from. A Star Is Born The diameters of most normal stars range from one tenth to ten times as much as the solar diameter. The larger, more massive ones are blue or white, and notably hotter than the Sun.

Sirius, in the constellation Canis Major, and Vega, in Lyra, are examples of hot, massive normal stars that are fairly close to the Sun (8.6 light years and about 26 light years away, respectively). They are white, several times more massive than the Sun, and have surface temperatures about 10,000° C. Other, more distant normal stars have temperatures up to about 40,000° C. There are many normal red stars near the Sun, with temperatures of a few thousand degrees and masses much less than that of the Sun. None, however, is bright enough to be seen without a telescope. Next Phase of a Star's Life - The Red Giant Today the Sun is in a state of balance between two forces: gravity, which pulls it inward, and the pressure of the hot gas and outward streaming radiation from the central nuclear furnace. The Sun is about 1.4 million kilometers (865,000 miles) in diameter about 109 times the diameter of the Earth - and has a surface temperature of about 6000° C. The core of the Sun will eventually run out of hydrogen fuel because it has all been fused into helium.

Without hydrogen as fuel, the outward explosions generated from the nuclear fusion no longer counteract the force of gravity. The outer layers of the star begin to collapse and the star builds up heat and pressure again, just as it did when it was born. This newly generated heat temporarily counteracts the force of gravity and the outer layers, still mostly hydrogen, are then pushed outward. The atmosphere of the star expands greatly and it becomes a red giant. At this point, the Sun will become a few hundred times bigger and the surface of the Sun would extend out past the orbit of Mars. As nuclear evolution continues, the Sun may become a variable, pulsating in size and brightness over periods of several months to a year. The visual brightness of such a star may vary by a factor of 100. Could this be what Kepler saw? Perhaps a medium sized star that was far enough away that it could not be seen without a telescope evolved into the red giant stage. Even though the red giant could get 100 times brighter, Kepler's observation showed a star getting gradually dimmer and not varying in brightness. Perhaps a giant explosion could release light that bright? What happens to a star when it dies?

Death of a Medium Sized Star, Like Our Sun All stars evolve the same way up to the red giant stage. But what happens next depends on the initial mass of a star. Throughout the red giant phase of a medium star, the hydrogen gas in the outer shell continues to burn and the temperature of the core continues to rise. At 200,000,000 degrees Celsius the helium atoms in the core fuse to form carbon atoms. This fusion releases energy temporarily. In a sun-sized star, this process may only take a few minutes. The atomic structure of carbon is too to be further compressed by the mass of the surrounding material. When the last of the helium atoms are fused into carbon, the medium size star begins to die. The star will now begin to she its outer layers as a diffuse cloud. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. Eventually, only about 20% of the star's initial mass remains and the star spends the rest of its days cooling and shrinking. Gravity causes the last of the star's matter to collapse inward and compact. This is the white dwarf stage.

At this stage, the star's matter is extremely dense. White drawfs shine with a white hot light. Once all their energy is gone, they no longer emit light. The star had now reached the black dwarf phase in which it will remain forever. Death of a Massive Star In early 2002, in the dim winter constellation Monceros (Moh-NO-ser-os) the Unicorn. The star, named V838 Monocerotis (Moh-NO-ser-u-tis) or V 838 Mon for short, grew very bright then cooled and faded away. Periodically, a bright object appears in a galaxy and remains that way for days to months. It is referred to, erroneously, as a new star or nova (Latin for "new"; plural, novae). It is, in fact, a star that for more than one reason experiences a major flare-up that later dies down leaving the star intact but with loss of material. Star V838 in the Monoceras constellation underwent such a flare-up of a blue star, releasing considerable material as it reached a luminosity of ~600000 times that of the Sun.

What caused this star to become the brightest star in our Milky Way Galaxy for a short time? Could this be what Kepler saw over 400 years ago? A star like our Sun will eventually evolve into a red supergiant as it exhausts the helium in its core. At this stage, it will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which last only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Astronomers think this is what happened to V838 Mon. At that size, we could see much more of the star, which might explain why V838 Mon appeared so much brighter. Images from NASA's Hubble Space Telescope revealed the enlarged size wasn't the size of the star, but instead the dust shell around V838 Mon. The glow was bright enough to light up layers of dust surrounding the star, like a flashlight shining through smoke in a dark room. Every time Hubble astronomers take a picture, the dust ring looks larger than before. But, in the Hubble pictures, the dust really isn't growing. Instead, light from the flash is sweeping through the dust, lighting up different parts.

This illusion is called a "light echo." De Stellar Nova or Supernova? In 1941, astronomers used the Mt. Wilson 100-inch telescope and identified a gaseous remnant of Kepler's "new star". It wasn't a star afterall. But what happened? Massive stars burn brighter and perish more dramatically than most. When a star ten times more massive then Sun exhaust the helium in the core, the nuclear burning cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to any heavier element. Without any source of heat to balance the gravity, the iron core collapses until it reaches nuclear densities producing a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into future generations of stars and planets.

Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible. How do you know that our star is not a first generation star? A team of astronomers, led by Ravi Sankrit and William Blair of Johns Hopkins University in Baltimore, Md., are using the Great Observatories to analyze the remains, called Kepler's supernova remnant, in infrared radiation, visible light, and X-rays. The combined image unveils a bubble-shaped shroud of gas and dust that is 14 light-years wide and is expanding at 4 million miles per hour (2,000 kilometers per second). Observations from each telescope highlight distinct features of the supernova remnant, a fast-moving shell of iron-rich material from the exploded star, surrounded by an expanding shock wave that is sweeping up interstellar gas and dust. This broad study of the supernova remnant also may help astronomers identify the type of star that produced the explosion. Supernovas arise from two very different types of stars: low-mass, white-dwarf stars and massive stars.

Of the six known supernovas in our Milky Way over the last 1,000 years, Kepler's supernova is the only one for which astronomers are uncertain of the type of star that exploded. Star Facts: The Basics of Star Names and Stellar Evolution Star Facts: The Basics of Star Names and Stellar Evolution By Charles Q. Choi, Space.com Contributor | December 16, 2014 09:41pm ET MORE Credit: University of Leicester Stars are giant, luminous spheres of plasma. There are billions of them — including our own sun — in the Milky Way Galaxy. And there are billions of galaxies in the universe. So far, we have learned that hundreds also have planets orbiting them. History of observations Since the dawn of recorded civilization, stars played a key role in religion and proved vital to navigation. Astronomy , the study of the heavens, may be the most ancient of the sciences. The invention of the telescope and the discovery of the laws of motion and gravity in the 17th century prompted the realization that stars were just like the sun, all obeying the same laws of physics.

In the 19th century, photography and spectroscopy — the study of the wavelengths of light that objects emit — made it possible to investigate the compositions and motions of stars from afar, leading to the development of astrophysics. In 1937, the first radio telescope was built, enabling astronomers to detect otherwise invisible radiation from stars. In 1990, the first space-based optical telescope, the Hubble Space Telescope , was launched, providing the deepest, most detailed visible-light view of the universe. Star naming Ancient cultures saw patterns in the heavens that resembled people, animals or common objects — constellations that came to represent figures from myth, such as Orion the Hunter, a hero in Greek mythology. Astronomers now often use constellations in the naming of stars. The International Astronomical Union, the world authority for assigning names to celestial objects, officially recognizes 88 constellations . Usually, the brightest star in a constellation has "alpha," the first letter of the Greek alphabet, as part of its scientific name. The second brightest star in a constellation is typically designated "beta," the third brightest "gamma," and so on until all the Greek letters are used, after which numerical designations follow.

A number of stars have possessed names since antiquity — Betelgeuse , for instance, means "the hand (or the armpit) of the giant" in Arabic. It is the brightest star in Orion, and its scientific name is Alpha Orionis. Also, different astronomers over the years have compiled star catalogs that use unique numbering systems. The Henry Draper Catalog, named after a pioneer in astrophotography, provides spectral classification and rough positions for 272,150 stars and has been widely used of by the astronomical community for over half a century. The catalog designates Betelgeuse as HD 39801. Since there are so many stars in the universe, the IAU uses a different system for newfound stars. Most consist of an abbreviation that stands for either the type of star or a catalog that lists information about the star, followed by a group of symbols. For instance, PSR J1302-6350 is a pulsar, thus the PSR. The J reveals that a coordinate system known as J2000 is being used, while the 1302 and 6350 are coordinates similar to the latitude and longitude codes used on Earth.

A young, glittering collection of stars looks like an aerial burst. The cluster is surrounded by clouds of interstellar gas and dust—the raw material for new star formation. The nebula, located 20,000 light-years away in the constellation Carina, contains a central cluster of huge, hot stars, called NGC 3603. Credit: NASA, ESA, R., F. Paresce, E. Young, the WFC3 Science Oversight Committee, and the Hubble Heritage Team Star formation A star develops from a giant, slowly rotating cloud that is made up entirely or almost entirely of hydrogen and helium. Due to its own gravitational pull, the cloud behind to collapse inward, and as it shrinks, it spins more and more quickly, with the outer parts becoming a disk while the innermost parts become a roughly spherical clump. According to NASA, this collapsing material grows hotter and denser, forming a ball-shaped protostar . When the heat and pressure in the protostar reaches about 1.8 million degrees Fahrenheit (1 million degrees Celsius), atomic nuclei that normally repel each other start fusing together, and the star ignites.

Nuclear fusion converts a small amount of the mass of these atoms into extraordinary amounts of energy — for instance, 1 gram of mass converted entirely to energy would be equal to an explosion of roughly 22,000 tons of TNT. Evolution of stars The life cycles of stars follow patterns based mostly on their initial mass. These include intermediate-mass stars such as the sun, with half to eight times the mass of the sun, high-mass stars that are more than eight solar masses, and low-mass stars a tenth to half a solar mass in size. The greater a star's mass, the shorter its lifespan generally is. Objects smaller than a tenth of a solar mass do not have enough gravitational pull to ignite nuclear fusion — some might become failed stars known as brown dwarfs . An intermediate-mass star begins with a cloud that takes about 100,000 years to collapse into a protostar with a surface temperature of about 6,750 F (3,725 C). After hydrogen fusion starts, the result is a T-Tauri star , a variable star that fluctuates in brightness.

This star continues to collapse for roughly 10 million years until its expansion due to energy generated by nuclear fusion is balanced by its contraction from gravity, after which point it becomes a main-sequence star that gets all its energy from hydrogen fusion in its core. The greater the mass of such a star, the more quickly it will use its hydrogen fuel and the shorter it stays on the main sequence. After all the hydrogen in the core is fused into helium, the star changes rapidly — without nuclear radiation to resist it, gravity immediately crushes matter down into the star's core, quickly heating the star. This causes the star's outer layers to expand enormously and to cool and glow red as they do so, rendering the star a red giant . Helium starts fusing together in the core, and once the helium is gone, the core contracts and becomes hotter, once more expanding the star but making it bluer and brighter than before, blowing away its outermost layers. After the expanding shells of gas fade, the remaining core is left, a white dwarf that consists mostly of carbon and oxygen with an initial temperature of roughly 180,000 degrees F (100,000 degrees C).

Since white dwarves have no fuel left for fusion, they grow cooler and cooler over billions of years to become black dwarves too faint to detect. (Our sun should leave the main sequence in about 5 billion years.) A high-mass star forms and dies quickly. These stars form from protostars in just 10,000 to 100,000 years. While on the main sequence, they are hot and blue, some 1,000 to 1 million times as luminous as the sun and are roughly 10 times wider. When they leave the main sequence, they become a bright red supergiant, and eventually become hot enough to fuse carbon into heavier elements. After some 10,000 years of such fusion, the result is an iron core roughly 3,800 miles wide (6,000 km), and since any more fusion would consume energy instead of liberating it, the star is doomed, as its nuclear radiation can no longer resist the force of gravity. When a star reaches a mass of more than 1.4 solar masses, electron pressure cannot support the core against further collapse, according to NASA. The result is a supernova.

Gravity causes the core to collapse, making the core temperature rise to nearly 18 billion degrees F (10 billion degrees C), breaking the iron down into neutrons and neutrinos. In about one second, the core shrinks to about six miles (10 km) wide and rebounds just like a rubber ball that has been squeezed, sending a shock wave through the star that causes fusion to occur in the outlying layers. The star then explodes in a so-called Type II supernova. If the remaining stellar core was less than roughly three solar masses large, it becomes a neutron star made up nearly entirely of neutrons, and rotating neutron stars that beam out detectable radio pulses are known as pulsars. If the stellar core was larger than about three solar masses, no known force can support it against its own gravitational pull, and it collapses to form a black hole . A low-mass star uses hydrogen fuel so sluggishly that they can shine as main-sequence stars for 100 billion to 1 trillion years — since the universe is only about 13.7 billion years old , according to NASA, this means no low-mass star has ever died.

Still, astronomers calculate these stars, known as red dwarfs , will never fuse anything but hydrogen, which means they will never become red giants. Instead, they should eventually just cool to become white dwarfs and then black dwarves. Constellations ancient and modern grace the skies year round. Let's see what you know about the star patterns that appear overhead every night.

Question: What are the clouds of interstellar dust, said to be the birthplace of stars?
Answer: Nebulae