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13.4 Star Formation

Astronomers once believed that the stars were fixed and unchanging as they revolved around Earth on the outermost crystalline sphere. These ancient Greek scientists could not conceive of the enormous distance to the stars, or their true sizes and energy outputs. Today, they know that stars continue to be born, evolve, and die. How do we know that stars change and evolve? The universe offers several lines of evidence that stars are constantly being born. First, while the Solar System is only about 4.6 billion years old, the entire Milky Way galaxy — our system of 400 billion stars — is at least 10 billion years old. Thus, the Sun formed much more recently than the oldest stars we can see in the sky. From this, we can infer that the stars around us did not all form at the same time.

Astropedia Image
Pleiades, also known as the Seven Sisters, in infrared light. Click here for original source URL.

Many young, massive stars are grouped into clusters. Clusters are held together by the mutual gravity of all the stars they contain. The stars in these clusters formed at about the same time. Because of tidal forces and the tendency of each star in the cluster to follow its own orbit around the center of our galaxy, most clusters disperse into isolated stars in only a few hundred million years. H-R diagrams of clusters show that few of these clusters are much older than this. The famous Pleiades cluster, for example, is only about 50 million years old. The existence of such clusters shows that star clusters did not all form at the beginning of the history of the galaxy; some continue to form today.

Massive stars evolve fastest. Calculations show that stars of 20 to 100 times the mass of the Sun can last only a few million years in a visible state where they shine by energy from fusion reactions. Since we still see these massive stars shining, we can conclude that they must have formed less than a few million years ago. Thus, star formation has been a continuing process during the whole history of the galaxy, including the last million years. There are stars in the sky younger than the human species.

Astropedia Image
The Eagle Nebula. Click here for original source URL.

The Sun formed when a diffuse interstellar cloud contracted and produced a central star surrounded by a dusty nebula. Our solar system has left us with a set of clues that indicates a rapid collapse from a gas cloud to a star, taking only a few million years. Astronomers have come up with a more general theory of this process that can explain other stars with different masses.

When scientists use the word theory, they usually mean a well-tested body of related ideas, often with a mathematical formulation that can be applied to a variety of cases. The theory must be backed up by observations. It should make predictions that can be verified. Astronomers have made some progress toward a theory of star formation. There are several questions we might want this theory to answer. What causes some clouds to contract and not others? How much of the material in a molecular cloud goes into stars and how much is left over? Why does this process only give stars with a mass range of a factor of a thousand — from one tenth to one hundred times the mass of the Sun? The theory involves the opposing forces of gravity versus thermal pressure.

Gravity pulls all the atoms in a cloud inward. But even at only 10 K the atoms are striking each other at a speed of 0.4 kilometers per second (nearly 1000 mph) and creating an outward pressure that opposes the tendency to collapse. As a cloud contracts, gravity increases since a denser cloud packs more mass into the same amount of space. On the other hand, outward pressure also increases if the gas heats up, since this makes the atoms and molecules move faster. So how does a cloud actually collapse? Some of the heat that keeps a cloud puffed up can escape in the form of photons associated with particular energy transitions in atoms and molecules. In other words, emission lines can help cool the gas. Another trigger to cause the collapse of a cloud can be compression due a nearby stellar explosion.

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Sir Fred Hoyle. Click here for original source URL.

In 1902, the English astrophysicist James Jeans added an important insight into the outline of star formation. He calculated the mass and temperature at which a cloud would begin its gravitational contraction. The calculation is idealized, because it does not consider the effects of magnetic fields or the possibility that the cloud might be rotating. The simple theory predicts that in the denser interstellar medium, masses contracting will be several hundred to a thousand solar masses. This mass is several times larger than the most massive stars and thousands of times larger than the least massive stars. In other words, the simple process of gravitational contraction cannot produce the full range of star masses.

So how do individual stars form? Conditions in the interstellar medium are such that the gas subdivides into enormous concentrations containing enough mass for hundreds of stars. We know of star clusters with this many stars. The new fragments become stars, and the whole mass turns into a star cluster. It is not hard to see why the interstellar gas might begin to contract. The material is not uniform; clots of dust and gas exist and are constantly agitated by the galaxy's rotation, material ejected from the later stages of stellar evolution, and other influences. Naturally, this movement allows some clouds to accumulate enough material for contraction to begin. It has even been suggested that stars might form like an infection spreading through the body, where one region of star formation causes an adjacent region of gas to form stars, and so on. In this way, star formation might spread though an entire galaxy.

The simple idea of a collapsing gas cloud is not the whole story of star formation, however. Star formation in molecular clouds occurs more slowly and less efficiently than the simple theory of gravitational collapse would lead us to expect. The presence of magnetic fields also complicates the process of star formation. Current thinking is that magnetic fields prevent gravitational collapse in molecular clouds. Magnetic fields create a "pressure" that opposes the inward force of gravity. Rotation of the entire cloud can also slow its collapse into stars. Scientists have taken many effects into account to` build a complex theory of star formation. The theory is grounded in observations but it is also informed by computer simulations. As long as the simulations include the correct microphysics (abundance of elements, atomic and molecular energy transitions) and macro physics (gravity, turbulence, shock heating) they can help illuminate a situation far too messy to encapsulate in a few equations. 

Modern theory envisages four stages in the process of star formation in molecular clouds. First, a slowly rotating core forms in a molecular cloud. Second, the core becomes unstable and collapses into a proto star and a surrounding disk, both of which are embedded in an infalling envelope of dust and gas. The collapse phase is "inside-out," in the sense that the material nearest the core collapses first. The third stage is the onset of the first fusion reactions, which initially is the production of deuterium. The energy released from nuclear reactions produces a stellar wind of outflowing material, which opposes the infalling material from farther out. The stellar wind rushes through the paths of least resistance at the rotational poles, leading to jets of material that flows out along the poles. The final stage comes when the wind fans out until it flows in all directions. At this point, a young stellar object has formed, still surrounded by its nebular disk.