1. Chapter 19 Star Formation
Few 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)

3. 19.1 Star-Forming Regions
Star formation is ongoing. Star-forming regions are seen in our galaxy as well as others.
Figure 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)

4. 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.

5. 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 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.

6. 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.

7. 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

8. 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 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.

9. 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.

10. 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 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 and described in the text.

11. 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 Interstellar Cloud Evolution Artist’s conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table (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.

12. 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 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.

13. Hertzsprung-Russell Diagram

14. Hertzsrpug-Russell Diagram
Shows the relationship between the absolute magnitude and temperature of stars
Main sequence stars are linear with the hottest stars being the brightest and the dimmest being the coolest. The hottest are also the most massive and the coolest are the least massive
Red giants are massive, bright but cooler stars
White dwarfs are small and faint, although not all are white

15. Look at Figure five on pg. 704 in the Earth Science book
To what group of stars does the sun belong?
How are absolute magnitude and temperature related in the main sequence?
What types of stars have high absolute magnitude but low temperatures?
What type of stars have low absolute magnitudes and medium temperatures?

16. 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.

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

18. 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. 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.

20. 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 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 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)

21. 19.4 Observations of Cloud Fragments and Protostars
The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars
Figure 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), 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)

22. 19.4 Observations of Cloud Fragments and Protostars
These are two protostars in the Orion Nebula, at around stage 5 in their development
Figure 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)

23. 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 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 (SST)

24. 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 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 Herbig–Haro 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)

25. 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.

26. 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 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.

27. 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

28. 19.5 Shock Waves and Star Formation
This region may very well be several generations of star formation
Figure 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)

29. 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 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)

30. 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 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 H–R diagram for all the stars of this well-known open cluster. (AURA)

31. 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 Globular Cluster (a) The globular cluster Omega Centauri is approximately 5000 pc from Earth and spans some 40 pc in diameter. (b) A H–R diagram of some of its stars. ( J. Lodriguss)

32. 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.

33. 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 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)

34. 19.6 Star Clusters
This image shows such a star-forming region in the Orion Nebula
Figure 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)

35. 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.

36. 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

37. 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

38. 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