Chapter 19

Chapter 19 Star Formation This remarkable image—actually a large mosaic of a billion bits of data stitched together from hundreds of smaller images—shows a classic star-forming region. The Hubble Space Telescope captured this optical view of the Orion Nebula, a stellar nursery lying roughly 1400 light-years from Earth, populated with thousands of young stars that have recently emerged from the loose matter comprising the surrounding nebulosity. The infrared spectrum at bottom was acquired by Europe’s Herschel Space Observatory, another outstanding telescope orbiting Earth. This chapter-opener represents state-of-the art photography and spectroscopy in astronomy today. (STScI; ESA)

Units of Chapter 19 19.1 Star-Forming Regions More Precisely 19-1 Competition in Star Formation 19.2 The Formation of Stars Like the Sun 19.3 Stars of Other Masses 19.4 Observations of Cloud Fragments and Protostars Discovery 19-1 Observations of Brown Dwarfs 19.5 Shock Waves and Star Formation 19.6 Star Clusters Discovery 19-2 Eta Carinae

19.1 Star-Forming Regions Star formation is ongoing. This image shows a group of young stars embedded in the nebula in which they formed. Figure 19-1. Stellar Nursery This combined visible-infrared image captured by the new wide-field camera on the Hubble Space Telescope shows a highly detailed view of the star cluster R136, a huge group of bright young blue stars still embedded in the glowing reddish nebula, called Tarantula, in which they formed a few million years ago. The whole region shown is about 100 light-years across. (NASA/ESA)

19.1 Star-Forming Regions 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.

19.1 Star-Forming Regions When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion. Figure 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 Formation Rotation can also interfere with gravitational collapse, as can magnetism. Clouds may very well contract in a distorted way.

19.2 The Formation of Stars Like the Sun Stars go through a number of stages in the process of forming from an interstellar cloud.

19.2 The Formation of Stars Like 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. Figure 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.

19.2 The Formation of Stars Like the Sun 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.

19.2 The Formation of Stars Like the Sun Stage 4: The core of the cloud is now a protostar and makes its first appearance on the H–R diagram. Figure 19-4. Protostar on the H–R Diagram The red arrow indicates the approximate evolutionary track followed by an interstellar cloud fragment before reaching the end of the Kelvin–Helmholtz contraction phase as a stage-4 protostar. The boldface numbers on this and subsequent H–R plots refer to the prestellar evolutionary stages listed in Table 19.1.

19.2 The Formation of Stars Like the Sun Planetary formation has begun, but the protostar is still not in equilibrium—all heating comes from the gravitational collapse. Figure 19-5. Interstellar Cloud Evolution Artist’s conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table 19.1. (Not drawn to scale.) The duration of each stage, in years, is indicated.

19.2 The Formation of Stars Like the Sun The last stages can be followed on the H–R diagram: The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact. Figure 19-6. Newborn Star on the H–R Diagram The changes in a protostar’s 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.

19.2 The Formation of Stars Like the Sun 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.

19.3 Stars of Other Masses This H–R 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. Figure 19-7. Prestellar Evolutionary Tracks Prestellar evolutionary paths for stars more massive and less massive than our Sun.

19.3 Stars of Other Masses 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.

19.3 Stars of Other Masses 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.

19.4 Observations of Cloud Fragments and Protostars 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. Figure 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)

19.4 Observations of Cloud Fragments and Protostars The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars Figure 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 Orion’s sword (see Figure 1.8). (b) Enlargement of the framed region in part (a), but here shown in the infrared, revealing 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) nearly real-color visible image of embedded nebular “knots” within the Orion Nebula itself, (d) false-color radio image of some intensely emitting molecular sites, and (e) high-resolution image of one of many young protostars surrounded by disks of gas and dust where planets might ultimately form. (P. Sanz/Alamy; SST; CfA; NASA)

19.4 Observations of Cloud Fragments and Protostars These are two protostars in the Orion Nebula, at around stage 5 in their development Figure 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 H–R diagram. (b) An optical, face-on image of a slightly more advanced circumstellar disk surrounding an embedded protostar in Orion. (NASA)

19.4 Observations of Cloud Fragments and Protostars Protostars are believed to have very strong winds, which clear out an area around the star roughly the size of the solar system. Figure 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)

19.4 Observations of Cloud Fragments and Protostars These two jets are matter being expelled from around an unseen protostar, still obscured by dust Figure 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. Several more Herbig–Haro objects can be seen at the top right of the main image—one of them resembling a “waterfall.” (AURA; NASA)

Discovery 19-1: Observations of Brown Dwarfs Brown 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. Discovery 19-1 Figure (NASA)

19.5 Shock Waves and Star Formation Shock waves from nearby star formation can be the trigger needed to start the collapse process in an interstellar cloud. Figure 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.

19.5 Shock Waves and Star Formation Other triggers: Death of a nearby Sun-like star Supernova Density waves in galactic spiral arms Galaxy collisions

19.5 Shock Waves and Star Formation This region may very well be several generations of star formation Figure 19-15. A Wave of Star Formation? A group of star-forming regions in the galaxy NGC 4214 possibly displays several generations in a sequential chain of star formation. (NASA)

19.6 Star Clusters 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. Figure 19-16. Newborn Cluster The star cluster NGC 3603 and part of the larger molecular cloud in which it formed. The cluster contains about 2000 bright stars and lies some 20,000 light-years from Earth. Radiation from its most massive stars has cleared a cavity in the cloud several light-years across. The inset shows the central area more clearly, revealing many small stars less massive than the Sun. (ESO; NASA)

19.6 Star Clusters 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. Figure 19-17. Open Cluster (a) The Pleiades cluster (also known as the Seven Sisters because only six or seven of its stars can be seen with the naked eye) lies about 400 light-years from the Sun. (b) An H–R diagram for all the stars of this well-known open cluster. (AURA)

19.6 Star Clusters This is a globular cluster—note the absence of massive main sequence stars and the heavily populated red giant region. Figure 19-18. Globular Cluster (a) The globular cluster Omega Centauri is approximately 16,000 light-years from Earth and spans some 130 light-years in diameter. (b) A H–R diagram of some of its stars. (P. Seitzer)

19.6 Star Clusters 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 age—those stars have already used up their fuel and have moved off the main sequence.

19.6 Star Clusters 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. Figure 19-19b. Protostellar Collisions In the congested environment of a young cluster, star formation is a competitive and violent process. (b) Large protostars may grow by “stealing” gas from smaller ones, and the extended disks surrounding most protostars can lead to collisions and mergers. This frame from another 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. (I. Bonnell and M. Bate)

19.6 Star Clusters This image shows such a star-forming region in the Orion Nebula Figure 19-20. Young Stars in Orion (a) A short-exposure visible-light image (observed with a filter that is transparent mainly to certain emission lines of oxygen) shows the central regions of the Orion Nebula 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 an extensive star cluster containing stars of many masses, possibly including many brown dwarfs (see also Figures 5.29c and d as well as 19.9). (Lick Observatory; NASA)

Discovery 19-2: Eta Carinae Eta Carinae’s 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 Figure (ESO; NASA)

Summary of Chapter 19 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 protostar

Summary of Chapter 19 (cont.) 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 slowly

Summary 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 spherical