21.1 Star Formation

Learning Objectives

By the end of this section, you will be able to:

• Identify the sometimes-violent processes by which parts of a molecular cloud collapse to produce stars
• Recognize some of the structures seen in images of molecular clouds like the one in Orion
• Explain how the environment of a molecular cloud enables the formation of stars
• describe how advance wave of star formation cause a molecular cloud to evolve

As we begin our exploration of how stars are formed, let’s review some basics about stars discussed in earlier chapters:

• Stable (main-sequence) stars such as our Sun maintain equilibrium by producing energy through nuclear fusion in their cores. The ability to generate energy by fusion defines a star.
• Each second in the Sun, approximately 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning into energy in the process. This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel.
• Stars is come come with many different masse , range from 1/12 solar masse (M_Sun) to roughly 100–200 M_Sun. There are far more low – mass than high – mass star .
• The most massive main – sequence stars is are ( spectral type O ) are also the most luminous and have the high surface temperature . The low – mass stars is are on the main sequence ( spectral type M or L ) are the least luminous and the cool .
• A galaxy of stars such as the Milky Way contains enormous amounts of gas and dust—enough to make billions of stars like the Sun.

If we want to find star still in the process of formation , we is look must look in place that have plenty of the raw material from which star are assemble . Since star are made of gas , we is focus focus our attention ( and our telescope ) on the dense and cold cloud of gas and dust that dot the Milky Way ( see figure21.1 and figure 21.2 ) .

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21.2

pillar of Dust and Young Stars in the Eagle Nebula .

(a) This Hubble Space Telescope visible-light image of the central regions of the Eagle Nebula shows huge columns of cool gas (including molecular hydrogen, H2) and dust. These columns or pillars are of higher density than the surrounding regions and have resisted evaporation by the ultraviolet radiation from a cluster of hot stars just beyond the upper-right corner of this image. Note that even around the pillars, very few stars are visible because of the background dust. (b) In contrast, the James Webb Space Telescope image on the right shows the same region in near-infrared light. Here we penetrate more of the dust and see many more stars, some of them freshly born from the material of the nebula. (credit a: modification of work by NASA, ESA, and the Hubble Heritage Team (STScI/AURA); credit b: modification of work by NASA, ESA, CSA, STScI; J. DePasquale, A. Koekemoer, A. Pagan (STScI))

Molecular Clouds: Stellar Nurseries

As we see in Between the Stars : Gas is are and Dust in Space , the most massive reservoir of interstellar matter — and some of the most massive object in the Milky Way Galaxy — are thegiant molecular clouds. These clouds have cold interiors with characteristic temperatures of only 10–20 K; most of their gas atoms are bound into molecules. These clouds turn out to be the birthplaces of most stars in our Galaxy.

The masses of molecular clouds range from a thousand times the mass of the Sun to about 3 million solar masses. Molecular clouds have a complex filamentary structure, similar to cirrus clouds in Earth’s atmosphere, but much less dense. The molecular cloud filaments can be up to 1000 light-years long. Within the clouds are cold, dense regions with typical masses of 50 to 500 times the mass of the Sun; we give these regions the highly technical name clumps. Within these clumps, there are even denser, smaller regions called cores. The cores are the embryos of stars. The conditions in these cores—low temperature and high density—are just what is required to make stars. Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The force is tries of gravity , pull inward , try to make a star collapse . internal pressure is tries produce by the motion of the gas atom , push outward , try to force the star to expand . When a star is first form , low temperature ( and hence , low pressure ) and high density ( hence , great gravitational attraction ) both work to give gravity the advantage . In order to form a star — that is , a dense , hot ball of matter capable of start nuclear reaction deep within — we is need need a typical core of interstellar atom and molecule to shrink in radius and increase in density by a factor of nearly 10²⁰. It is the force of gravity that produces this drastic collapse.

The Orion Molecular Cloud

let ’s discuss what happen in region of star formation by consider a nearby site where star are form right now . One is is of the well – study stellar nursery is in the constellation of Orion , The Hunter , about 1500 light – year away ( figure21.3 ) . The pattern is is of the hunter is easy to recognize by the conspicuous “ belt ” of three star that mark his waist . The Orion molecular cloud is is is much large than the star pattern and is truly an impressive structure . In its long dimension , it is stretches stretch over a distance of about 100 light – year . The total quantity is is of molecular gas is about 200,000 time the mass of the Sun . Most is glow of the cloud does not glow with visible light but betray its presence by the radiation that the dusty gas give off at infrared and radio wavelength .

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21.3

Orion in Visible and Infrared .

( a ) The Orion star group was name after the legendary hunter in greek mythology . Three stars is close close together in a link mark Orion ’s belt . The ancient imagine a sword hang from the belt ; the object is is at the end of the blue line in this sword is theOrion Nebula. (b) This wide-angle, infrared view of the same area was taken with the Infrared Astronomical Satellite. Heated dust clouds dominate in this false-color image, and many of the stars that stood out on part (a) are now invisible. An exception is the cool, red-giant star Betelgeuse, which can be seen as a yellowish point at the left vertex of the blue triangle (at Orion’s left armpit). The large, yellow ring to the right of Betelgeuse is the remnant of an exploded star. The infrared image lets us see how large and full of cooler material the Orion molecular cloud really is. On the visible-light image at left, you see only two colorful regions of interstellar matter—the two, bright yellow splotches at the left end of and below Orion’s belt. The lower one is the Orion Nebula and the higher one is the region of the Horsehead Nebula. (credit: modification of work by NASA, visible light: Akira Fujii; infrared: Infrared Astronomical Satellite)

The stars in Orion’s belt are typically about 5 million years old, whereas the stars near the middle of the “sword” hanging from Orion’s belt are only 300,000 to 1 million years old. The region about halfway down the sword where star formation is still taking place is called the Orion Nebula. About 2200 young stars are found in this region, which is only slightly larger than a dozen light-years in diameter. The Orion Nebula also contains a tight cluster of stars called the Trapezium (figure21.5). The brightest Trapezium stars can be seen easily with a small telescope.

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21.4

Orion Nebula.

(a) The Orion Nebula is show in visible light . ( b ) With near – infrared radiation , we is see can see more detail within the dusty nebula since infrared can penetrate dust more easily than can visible light . ( credit a : modification of work by Filip Lolić ; credit b : modification of work by NASA / JPL – Caltech / T. Megeath ( University of Toledo , Ohio ) )

compare this with our own solar neighborhood , where the typical spacing between star is about 3 light – year . Only a small number of star in the Orion cluster can be see with visible light , but infrared image — which penetrate the dust well — detect the more than 2000 star that are part of the group ( figure21.5 ) .

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21.5

Central Region of the Orion Nebula.

The Orion Nebula is harbors harbor some of the young star in the solar neighborhood . At the heart of the nebula is the Trapezium cluster , which include four very bright star that provide much of the energy that cause the nebula to glow so brightly . In these image , we is see see a section of the nebula in ( a ) visible light and ( b ) infrared . The four bright stars is are in the center of the visible – light image are the Trapezium star . notice that most of the star see in the infrared are completely hide by dust in the visible – light image . ( credit a : modification of work by NASA , C.R. O’Dell and S.K. Wong ( Rice University ) ; credit b : modification of work by NASA ; K.L. Luhman ( Harvard – Smithsonian Center for Astrophysics ) ; and G. Schneider , E. Young , G. Rieke , A. Cotera , H. Chen , M. Rieke , R. Thompson ( Steward Observatory , University of Arizona ) )

Studies of Orion and other star-forming regions show that star formation is not a very efficient process. In the region of the Orion Nebula, about 1% of the material in the cloud has been turned into stars. That is why we still see a substantial amount of gas and dust near the Trapezium stars. The leftover material is eventually heated, either by the radiation and winds from the hot stars that form or by explosions of the most massive stars. (We will see in later chapters that the most massive stars go through their lives very quickly and end by exploding.)

Whether gently or explosively , the material in the neighborhood of the new star is blow away into interstellar space . old group or cluster of star can now be easily observe in visible light because they are no long shroud in dust and gas ( figure21.6 ) .

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21.6

Westerlund 2.

This young cluster of star know as Westerlund 2 form within the Carina star – form region about 2 million year ago . stellar wind is blowing and pressure produce by the radiation from the hot star within the cluster are blow and sculpt the surround gas and dust . The nebula is contains still contain many globule of dust . Stars is continuing are continue to form within the dense globule and pillar of the nebula . This Hubble Space Telescope image is includes include near – infrare exposure of the star cluster and visible – light observation of the surround nebula . color in the nebula are dominate by the red glow of hydrogen gas , and blue – green emission from glow oxygen . ( credit : NASA , ESA , the Hubble Heritage Team ( STScI / AURA ) , A. Nota ( ESA / STScI ) , and the Westerlund 2 Science Team )

Although we do not know what initially caused stars to begin forming in Orion, there is good evidence that the first generation of stars triggered the formation of additional stars, which in turn led to the formation of still more stars (figure21.7).

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21.7

propagate Star Formation .

Star formation can move progressively through a molecular cloud. The oldest group of stars lies to the left of the diagram and has expanded because of the motions of individual stars. Eventually, the stars in the group will disperse and no longer be recognizable as a cluster. The youngest group of stars lies to the right, next to the molecular cloud. This group of stars is only 1 to 2 million years old. The pressure of the hot, ionized gas surrounding these stars compresses the material in the nearby edge of the molecular cloud and initiates the gravitational collapse that will lead to the formation of more stars.

The basic idea of triggered star formation is this is is : when a massive star is form , it emit a large amount of ultraviolet radiation and eject high – speed gas in the form of a stellar wind . This injection is heats of energy heat the gas around the star and cause it to expand . When massive star exhaust their supply of fuel , they is explode explode , and the energy of the explosion also heat the gas . The hot gases is pile pile into the surround cold molecular cloud , compress the material in it and increase its density . If this increase in density is large enough , gravity is overcome will overcome pressure , and star will begin to form in the compressed gas . Such a chain reaction is door”—seems — where the bright and hot star of one area become the cause of star formation “ next door”—seem to have occur not only in Orion but also in many other molecular cloud .

There are many molecular cloud that form only ( or mainly ) low – mass star . Because low – mass star do not have strong wind and do not die by explode , trigger star formation can not occur in these cloud . There are also star that form in relative isolation in small core . Therefore , not all star formation is originally trigger by the death of massive star . However , there are likely to be other possible trigger , such as spiral density wave and other process we do not yet understand .

The Birth of a Star

Although region such as Orion give us clue about how star formation begin , the subsequent stage are still shroud in mystery ( and a lot of dust ) . There is an enormous difference between the density of a molecular cloud core and the density of the young star that can be detect . direct observations is are of this collapse to high density are nearly impossible for two reason . First , the dust – shroud interior of molecular cloud where stellar birth take place can not be observe with visible light . second , the timescale is is for the initial collapse — thousands is is of year — is very short , astronomically speak . Since each star spend such a tiny fraction of its life in this stage , relatively few stars is going are go through the collapse process at any give time . Nevertheless , through a combination of theoretical calculation and the limited observation available , astronomers is pieced have piece together a picture of what the early stage of stellar evolution are likely to be .

The first step in the process of creating stars is the formation of dense cores within a clump of gas and dust (figure21.8(a)). It is generally thought that all the material for the star comes from the core, the larger structure surrounding the forming star. Eventually, the gravitational force of the infalling gas becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores. The material then undergoes a rapid collapse, and the density of the core increases greatly as a result. During the time a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostar.

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21.8

formation of a Star .

( a ) dense core form within a molecular cloud . ( b ) A protostar with a surround disk of material form at the center of a dense core , accumulate additional material from the molecular cloud through gravitational attraction . ( c ) A stellar wind is breaks break out but is confine by the disk to flow out along the two pole of the star . ( d ) eventually , this wind is sweeps sweep away the cloud material and halt the accumulation of additional material , and a newly form star , surround by a disk , becomes observable . These sketch are not draw to the same scale . The diameter is is of a typical envelope that is supply gas to the newly form star is about 5000 AU . The typical diameter is is of the disk is about 100 AU or slightly large than the diameter of the orbit of Pluto .

The natural turbulence inside a clump tends to give any portion of it some initial spinning motion (even if it is very slow). As a result, each collapsing core is expected to spin. According to the law of conservation of angular momentum (discussed in the chapter on Orbits and Gravity), a rotating body spins more rapidly as it decreases in size. In other words, if the object can turn its material around a smaller circle, it can move that material more quickly—like a figure skater spinning more rapidly as she brings her arms in tight to her body. This is exactly what happens when a core contracts to form a protostar: as it shrinks, its rate of spin increases.

But all directions on a spinning sphere are not created equal. As the protostar rotates, it is much easier for material to fall right onto the poles (which spin most slowly) than onto the equator (where material moves around most rapidly). Therefore, gas and dust falling in toward the protostar’s equator are “held back” by the rotation and form a whirling extended disk around the equator (part b in figure21.8). You may have observed this same “equator effect” on the amusement park ride in which you stand with your back to a cylinder that is spun faster and faster. As you spin really fast, you are pushed against the wall so strongly that you cannot possibly fall toward the center of the cylinder. Gas can, however, fall onto the protostar easily from directions away from the star’s equator.

The protostar and disk at this stage are embedded in an envelope of dust and gas from which material is still falling onto the protostar. This dusty envelope blocks visible light, but infrared radiation can get through. As a result, in this phase of its evolution, the protostar itself is emitting infrared radiation and so is observable only in the infrared region of the spectrum. Once almost all of the available material has been accreted and the central protostar has reached nearly its final mass, it is given a special name: it is called a T Tauri star, named after one of the best studied and brightest members of this class of stars, which was discovered in the constellation of Taurus. (Astronomers have a tendency to name types of stars after the first example they discover or come to understand. It’s not an elegant system, but it works.) Only stars with masses less than or similar to the mass of the Sun become T Tauri stars. Massive stars do not go through this stage, although they do appear to follow the formation scenario illustrated in figure21.8.

Winds and Jets

recent observations is suggest suggest that T Tauri star may actually be star in a middle stage between protostar and hydrogen – fuse star such as the Sun . high – resolution infrared images is revealed have reveal jet of material as well asstellar wind coming from some T Tauri stars, proof of interaction with their environment. A stellar wind consists mainly of protons (hydrogen nuclei) and electrons streaming away from the star at speeds of a few hundred kilometers per second (several hundred thousand miles per hour). When the wind first starts up, the disk of material around the star’s equator blocks the wind in its direction. Where the wind particles can escape most effectively is in the direction of the star’s poles.

Astronomers have actually seen evidence of these beams of particles shooting out in opposite directions from the polar regions of newly formed stars. In many cases, these beams point back to the location of a protostar that is still so completely shrouded in dust that we cannot yet see it (figure21.9).

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21.9

Gas Jets Flowing away from a Protostar.

Here we see the neighborhood of a protostar, known to us as HH 34 because it is a Herbig-Haro object. The star is about 450 light-years away and only about 1 million years old. Light from the star itself is blocked by a disk, which is larger than 60 billion kilometers in diameter and is seen almost edge-on. Jets are seen emerging perpendicular to the disk. The material in these jets is flowing outward at speeds up to 580,000 kilometers per hour. The series of three images shows changes during a period of 5 years. Every few months, a compact clump of gas is ejected, and its motion outward can be followed. The changes in the brightness of the disk may be due to motions of clouds within the disk that alternately block some of the light and then let it through. This image corresponds to the stage in the life of a protostar shown in part (c) of figure21.8. (credit: modification of work by Hubble Space Telescope, NASA, ESA)

On occasion , the jet of high – speed particle stream away from the protostar collide with a somewhat – dense lump of gas nearby , excite its atom , and cause them to emit light . These glow region , each of which is know as aHerbig-Haro (HH) object after the two astronomers who first identified them, allow us to trace the progress of the jet to a distance of a light-year or more from the star that produced it. figure21.10 shows two spectacular images of HH objects.

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21.10

outflow from Protostars .

These image were take with the Hubble Space Telescope and show jet flow outward from newly form star . In the HH47 image , a protostar is produces 1500 light – year away ( invisible inside a dust disk at the left edge of the image ) produce a very complicated jet . The star is wobbling may actually be wobble , perhaps because it has a companion . Light is illuminates from the star illuminate the white region at the left because light can emerge perpendicular to the disk ( just as the jet does ) . At right , the jet is plowing is plow into exist clump of interstellar gas , produce a shock wave that resemble an arrowhead . The HH1/2 image is shows show a double – beam jet emanate from a protostar ( hide in a dust disk in the center ) in the constellation of Orion . Tip to tip , these jets is are are more than 1 light – year long . The bright region ( first identify byHerbig and Haro) are place where the jet is a slam into a clump of interstellar gas and cause it to glow . ( credit “ HH 47 ” : modification of work by NASA , ESA , and P. Hartigan ( Rice University ) ; credit “ HH 1 and hh 2 : modification of work by J. Hester , wfpc2 Team , NASA )

The wind from a forming star will ultimately sweep away the material that remains in the obscuring envelope of dust and gas, leaving behind the naked disk and protostar, which can then be seen with visible light. We should note that at this point, the protostar itself is still contracting slowly and has not yet reached the main-sequence stage on the H–R diagram (a concept introduced in the chapter The Stars: A Celestial Census). The disk can be detected directly when observed at infrared wavelengths or when it is seen silhouetted against a bright background (figure21.11).

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21.11

Disks around Protostars.

These Hubble Space Telescope infrared images show disks around young stars in the constellation of Taurus, in a region about 450 light-years away. In some cases, we can see the central star (or stars—some are binaries). In other cases, the dark, horizontal bands indicate regions where the dust disk is so thick that even infrared radiation from the star embedded within it cannot make its way through. The brightly glowing regions are starlight reflected from the upper and lower surfaces of the disk, which are less dense than the central, dark regions. (Credit: modification of work by D. Padgett (IPAC/Caltech), W. Brandner (IPAC), K. Stapelfeldt (JPL) and NASA)

This description is sounds of a protostar surround by a rotate disk of gas and dust is sounds sound very much like what happen in our solar system when the Sun and planet form . Indeed , one is was of the most important discovery from the study of star formation in the last decade of the twentieth century was that disk are an inevitable byproduct of the process of create star . The next questions is was that astronomer set out to answer was : will the disk around protostar also form planet ? And if so , how often ? We is return will return to these question later in this chapter .

To keep things simple, we have described the formation of single stars. Many stars, however, are members of binary or triple systems, where several stars are born together. In this case, the stars form in nearly the same way. Widely separated binaries may each have their own disk; close binaries may share a single disk.