Chapter 19 Star Formation

Chapter 19Star Formation

1Few issues in astronomy are more basic than knowing how stars form. Here, in this visible-infrared combined image captured with the new wide-field camera on the Hubble Space Telescope, we see a highly detailed view of the largest stellar nursery in our local galactic neighborhood. Called R136 and located about 170,000 light-years away, this region of about 100 light-years across harbors a rich collection of hundreds of young blue stars amid reddish gaseous nebulosity abundant in hydrogen from which they formed, probably as recently as a few million years ago. (STScI)

19.1Star-Forming Regions Competition in Star Formation19.2The Formation of Stars Like the Sun19.3Stars of Other Masses19.4Observations of Cloud Fragments and Protostars Observations of Brown Dwarfs19.5Shock Waves and Star Formation19.6Star Clusters Eta CarinaeUnits of Chapter 19Star formation is ongoing. Star-forming regions are seen in our galaxy as well as others.19.1 Star-Forming Regions

3Figure 19-1. Extragalactic Star Formation The giant star-forming region at the right, called NGC 604, is roughly 500 pc across. It is found in the nearby galaxy M33, displayed at the left on the much larger scale of 40,000 pc across. (R. Gendler; NASA)

Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core.When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion

5Figure 19-2. Atomic Motions The motions of a few atoms within an interstellar cloud are influenced by gravity so slightly that the atoms paths are hardly changed (a) before, (b) during, and (c) after an accidental, random encounter.

More Precisely 19-1: Competition in Star FormationRotation can also interfere with gravitational collapse, as can magnetism. Clouds may very well contract in a distorted way.

Stars go through a number of stages in the process of forming from an interstellar cloud19.2 The Formation of StarsLike the Sun

Stage 1:Interstellar cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces.

8Figure 19-3. Cloud Fragmentation As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces themselves continue to fall inward and fragment, eventually forming many tens or hundreds of individual stars.

Stage 2:Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation.

Stage 3: The interior of the fragment has begun heating and is about 10,000 K.Stage 4:The core of the cloud is now a protostar and makes its first appearance on the HR diagram.

10Figure 19-4. Protostar on the HR Diagram The red arrow indicates the approximate evolutionary track followed by an interstellar cloud fragment before reaching the end of the KelvinHelmholtz contraction phase as a stage-4 protostar. The boldface numbers on this and subsequent HR plots refer to the prestellar evolutionary stages listed in Table 19.1 and described in the text. Planetary formation has begun, but the protostar is still not in equilibriumall heating comes from the gravitational collapse.

11Figure 19-5. Interstellar Cloud Evolution Artists conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table 19.1. (Not drawn to scale.) Shown are a stage-1 interstellar cloud; a stage-2 fragment; a smaller, hotter stage-4 fragment with jets; and a stage-5 protostar. The duration of each stage, in years, is also indicated.

The last stages can be followed on the HR diagram:The protostars luminosity decreases even as its temperature rises because it is becoming more compact.

12Figure 19-6. Newborn Star on the HR Diagram The changes in a protostars observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6, often called the Hayashi track. At stage 7, the newborn star has arrived on the main sequence.

At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star.The star continues to contract and increase in temperature until it is in equilibrium. This is stage 7: The star has reached the main sequence and will remain there as long as it has hydrogen to fuse.This HR diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun. The shape of the paths is similar, but they wind up in different places on the main sequence. 19.3 Stars of Other Masses

14Figure 19-7. Prestellar Evolutionary Tracks Some pre-main-sequence evolutionary paths for stars more massive and less massive than our Sun.

The main sequence is a band, rather than a line, because stars of the same mass can have different compositions.Most important: Stars do not move along the main sequence! Once they reach it, they are in equilibrium and do not move until their fuel begins to run out.Some fragments are too small for fusion ever to begin. They gradually cool off and become dark clinkers.A protostar must have 0.08 the mass of the Sun (which is 80 times the mass of Jupiter) in order to become dense and hot enough that fusion can begin.If the mass of the failed star is about 12 Jupiter masses or more, it is luminous when first formed, and is called a brown dwarf.Emission nebulae are heated by the formation of stars nearby. In these images, we see the parent cloud in stage 1, contracting fragments between stages 1 and 2, and a new star in stage 6 or 7. The new star is the one heating the nebula.19.4 Observations of Cloud Fragments and Protostars

17Figure 19-8. Star Formation Phases (a) The M20 region shows observational evidence for three broad phases in the birth of a star. The parent cloud is stage 1 of Table 19.1. The region labeled contracting fragment likely lies between stages 1 and 2. Finally, the emission nebula (M20 itself) results from the formation of one or more massive stars (stages 6 and 7). (b) A closeup (including Hubble inlays) of the area near region B outlines (in drawn ovals) especially dense knots of dusty matter. (c) A Spitzer Telescope infrared image of the same scene reveals those cores thought to be stellar embryos (arrows). (AURA; NASA)

The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars

18Figure 19-9. Orion Nebula, Up Close (a) The constellation Orion, with the region around its famous emission nebula marked by a rectangle. The Orion Nebula is the middle star of Orions sword (see Figure 1.8). (b) Enlargement of the framed region in part (a), suggesting how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars. The three frames at the right show some of the evidence for those protostars: (c) false-color radio image of some intensely emitting molecular sites, (d) nearly real-color visible image of embedded nebular knots thought to harbor protostars, and (e) high-resolution image of one of many young stars surrounded by disks of gas and dust where planets might ultimately form. (Astrostock-Sanford; SST; CfA; NASA)

These are two protostars in the Orion Nebula, at around stage 5 in their development

19Figure 19-10. Protostars (a) An edge-on infrared image of a planetary system-sized dusty disk in the Orion region, showing heat and light emerging from its center. On the basis of its temperature and luminosity, this unnamed source appears to be a low-mass protostar on the Hayashi track (around stage 5) in the HR diagram. (b) An optical, face-on image of a slightly more advanced circumstellar disk surrounding an embedded protostar in Orion. (NASA)

Protostars are believed to have very strong winds, which clear out an area around the star roughly the size of the solar system

20Figure 19-11. Protostellar Wind (a) The nebular disk around a protostar can be the site of intense heating and strong outflows, forming a bipolar jet perpendicular to the disk. (b) As the disk is blown away by the wind, the jets fan out, eventually (c) merging into a spherical wind. In contrast to this art, part (d) is an actual infrared image of a hot young star (at right) whose powerful winds are ripping away the disk (at left) surrounding a Sun-like star (at center). This system is located about 750 pc away in the star-forming cloud IC 1396. (SST)

These two jets are matter being expelled from around an unseen protostar, still obscured by dust

21Figure 19-13. Protostellar Outflow This view of the Orion molecular cloud shows the outflow from a newborn star, still surrounded by nebular gas. The inset shows a pair of jets called HH1 and HH2, formed when matter falling onto another protostar (still obscured by the dusty cloud fragment from which it formed) creates a pair of high-speed gas jets perpendicular to the flattened protostellar disk. The jets are nearly 1 light-year across. Several more HerbigHaro objects can be seen at the top right. One of them, the oddly shaped waterfall at top right, may be due to an earlier outflow from the same protostar responsible for HH1 and HH2. (AURA; NASA)

Discovery 19-1: Observations of Brown DwarfsBrown dwarfs are difficult to observe directly, as they are very dim. These images are of two binary-star systems, each believed to contain a brown dwarf. The difference in luminosity between the star and the brown dwarf is apparent.

Shock waves from nearby star formation can be the trigger needed to start the collapse process in an interstellar cloud19.5 Shock Waves andStar Formation

23Figure 19-14. Generations of Star Formation (a) Star birth and (b) shock waves lead to (c) more star births and more shock waves in a continuous cycle of star formation in many areas of our Galaxy. As in a chain reaction, old stars trigger the formation of new stars ever deeper into an interstellar cloud.

Other triggers: Death of a nearby Sun-like star Supernova Density waves in galactic spiral arms Galaxy collisionsThis region may very well be several generations of star formation

25Figure 19-15. A Wave of Star Formation? A group of star-forming regions in the galaxy NGC 4214, possibly representing several generations in a chain of star formation. (NASA)

Because a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.19.6 Star Clusters

26Figure 19-16. Newborn Cluster The star cluster NGC 3603 and a portion of the larger molecular cloud in which it formed. The cluster contains about 2000 bright stars and lies some 6000 pc from Earth. The field of view shown here spans about 20 light-years. Radiation from the cluster has cleared a cavity in the cloud several light-years across. The inset shows the central area more clearly, including the most massive star in the region (called Sher 25, above and to the left of the cluster), which is already near the end of its lifetime, having ejected part of its outer layers and formed a ring of gas. Many low-mass stars, less massive than the Sun, can also be seen. (ESO; NASA)

This is a young star cluster called the Pleiades. The H-R diagram of its stars is shown. This is an example of an open cluster.

27Figure 19-17. Open Cluster (a) The Pleiades cluster (also known as the Seven Sisters or M45) lies about 120 pc from the Sun. The naked eye can see only six or seven of its brightest stars. (b) An HR diagram for all the stars of this well-known open cluster. (AURA)

This is a globular clusternote the absence of massive main sequence stars and the heavily populated red giant region.

28Figure 19-18. Globular Cluster (a) The globular cluster Omega Centauri is approximately 5000 pc from Earth and spans some 40 pc in diameter. (b) A HR diagram of some of its stars. ( J. Lodriguss)

The differences between the H-R diagrams of open and globular clusters are that the globular clusters are very old, whereas the open clusters are much younger.The absence of massive main sequence stars in the globular cluster is due to its extreme agethose stars have already used up their fuel and have moved off the main sequence.The presence of massive, short-lived O and B stars can profoundly affect their star cluster, as they can blow away dust and gas before it has time to collapse.This is a simulation of such a cluster.

30Figure 19-19. Protostellar Collisions In the congested environment of a young cluster, star formation is a competitive and violent process. Large protostars may grow by stealing gas from smaller ones, and the extended disks surrounding most protostars can lead to collisions and even mergers. The cluster environment can play a crucial role in determining the types of stars that form. This frame from a supercomputer simulation shows a small star cluster emerging from an interstellar cloud that originally contained about 50 solar masses of material, distributed over a volume 1 light-year across. Remnants of the cloud are shown here in red. (M. Bate, I. Bonnell, and V. Bromm)

This image shows such a star-forming region in the Orion Nebula

31Figure 19-20. Star Formation in Orion Some views of the central regions of the Orion Nebula. (a) A short-exposure visible-light image (observed with a filter that is transparent only to certain emission lines of oxygen) shows the nebula itself and four bright O-type stars known as the Trapezium, but few obvious other stars. (b) A Spitzer Space Telescope view of the same part of the nebula shows more clearly the irregular gas and dust amidst many young stars still embedded in that dust (see also Figure 5.28). (Lick Observatory; NASA)

Eta Carinaes mass is 100 times that of the Sun; it is one of the most massive stars known. It suffered a huge explosion about 150 years ago. The last image shows the cloud expanding away from the star.Discovery 19-2: Eta Carinae

When a star first appears on the H-R diagram it is

cool and faint.cool and bright.hot and faint.on the main sequence.Nuclear fusion will start when a proto star's

surface temperatures and densities are high enough to split apart hydrogen nuclei.surface temperatures and densities are high enough to force hydrogen nuclei to fuse together.core temperatures and densities are high enough to split apart hydrogen nuclei.core temperatures and densities are high enough to force hydrogen nuclei to fuse together.The collapse of a forming star is usually halted by

the increasing pressure forces within the object.an explosion.the force of gravity.the generation of light within the object.A proto star generates its energy by

oxidation of carbon.fusion of hydrogen to helium.fission of uranium.conversion of gravitational to thermal energy.none of the above.When a star first appears on the H-R diagram, its actual total brightness, as compared to the sun's current value, is

much greater.about the same.much less.undetectably fainter.Which stars reach the main sequence in the shortest time after their birth

largestreddestdensestmost massiveAs a proto star contracts it moves

down in the H-R diagram.up in the H-R diagram.to the left in the H-R diagram.to the right in the H-R diagram. Stars begin to form when an interstellar cloud begins to contract The cloud fragments as it contracts; fragments continue to collapse and fragment until their density is high enough to prohibit further fragmentation The fragment heats up enough to radiate a significant amount of energy; it is now a protostarSummary of Chapter 19 The protostar continues to collapse; when the core is dense and hot enough, fusion begins The star continues to collapse until the inward force of gravity is balanced by the outward pressure from the core. The star is now on the main sequence More massive stars follow the same process, but more quickly Less massive stars form more slowlySummary of Chapter 19 (cont.) Star formation has been observed near emission nebulae Collapse may be initiated by shock waves One cloud tends to fragment into many stars, forming a cluster Open clusters are relatively young, small, and randomly shaped Globular clusters are old, very large, and sphericalSummary of Chapter 19 (cont.)