3. FORMATION OF STARS AND PLANETS

In 1687, Isaac Newton published one of the greatest scientific books of all time, the Principia Mathematica. In it he explained his famous three laws of motion, and also his law of universal gravitation, which we use today to calculate the orbits of stars and planets. The law of universal gravitation says that all matter attracts all other matter, and Newton recognized a major consequence of that law: clouds of gas will tend to fragment and collapse as a result of their own gravity. This is the basic mechanism responsible for the formation of new stars from interstellar gas clouds.

But three forces counteract gravity and tend to prevent this collapse:

1. Inertia: massive objects resist changes of their motion. Fast-moving objects will not be deflected easily by gravity. For example, the Sun's gravity attracts comets. However, most comets do not fall into the Sun, but fly away again after passing close to the Sun. In this case (and in describing the orbits of planets), we call the effect of inertia centrifugal force. In the case of interstellar gas clouds, the inertia due to turbulent motions of the gas tends to counteract gravity. However, when gravity wins and causes the gas to fragment and collapse, the collapsing clouds invariably have a slight tendency to rotate one way or another. The collapse amplifies this rotation, and so the fragment will develop into a central dense cloud surrounded by a swirling disk of gas that is supported by another manifestation of inertia:
2. centrifugal force: The central cloud will form a star, and planets will form in the disk. Often, centrifugal force will cause the collapsing gas cloud to undergo
3. fission: into two clouds orbiting each other. Then, each of these fragments might collapse further and become a star. This mechanism explains why so many stars are in binary systems.

Centrifugal force causes the outer part of a collapsing interstellar cloud to flatten into a disk, where planets are formed. Source: The Birth of Stars, by NASA.

4. Heat pressure:

   Warm gas tends to expand under its own pressure. If interstellar gas is warm enough, this pressure will prevent gravity from causing fragmentation and collapse. This accounts for the fact that we see star formation occurring only in the very cold dense clouds, never in the relatively warm diffuse interstellar gas. When gas is compressed it becomes warmer. Therefore, if interstellar gas, even dense gas, begins to fragment and collapse due to gravity, the heating due to compression will make the fragments tend to resist further collapse. Without a way to get rid of this heat, the pressure will stop the collapse.

5. Magnetic pressure:

   Interstellar gas is pervaded by a magnetic field, and this magnetic field tends to become stronger when compressed and to counteract gravity. This pressure tends to resist collapse and retard star formation. Interstellar gas can move through magnetic fields, but only very slowly. Therefore, magnetic fields tend to retard star formation, but they cannot stop it. Typically, it takes about 10⁷ (ten million) years for stars to form in a molecular cloud.

THE NEVER-ENDING BATTLE: Most of the action we see in the universe, from planetary motions to star formation to the motion of the universe itself, results from a never-ending battle between the attractive force of gravity and the various forces that counteract gravity. Often, gravity wins -- for example, in star-forming regions. But this doesn't happen immediately. In the case of star formation, some subtle things must occur to help gravity in its ultimate victory. I have already mentioned one: the fission of rotating collapsing clouds.

But for gravity to win, the collapsing gas cloud must get rid of its heat pressure. It does this by converting the heat (due to the random motions of the gas molecules) into radiation, which can escape the cloud. The fast-moving molecules are constantly colliding with each other. Occasionally, the collision will cause one of the molecules to become internally excited, and then the molecules bounce off each other with less energy then they had originally. The excited molecule then emits a photon. The net result is that thermal energy (the random motion of molecules) is converted into radiation energy, cooling the gas and permitting gravitational collapse to proceed.

Interstellar molecules and star formation: Gravity can win over heat pressure only when the density is high and the temperature is very low, i.e., in the dense clouds. But at such low temperatures, most atoms cannot radiate when they collide. Only molecules can become internally excited during a low temperature collision. Therefore, a gas cloud without molecules couldn't radiate enough, and would not be able to collapse to form a star. The molecules are necessary for star formation. The radio and infrared emission lines which we see coming from molecules in star-forming regions are the very photons that are carrying off the heat energy and permitting collapse.

Ultraviolet starlight is deadly to interstellar molecules. When struck by UV photons, the molecules break apart into atoms. (Just as UV radiation from the Sun destroys molecules within your skin cells and leaves toxic fragments that may eventually cause cancer.) Throughout most of interstellar space, there is enough UV starlight to prevent any significant buildup of molecules in the diffuse gas.

The importance of interstellar grains: In the dark clouds, the interstellar dust grains block the UV starlight, permitting the molecules to survive. Moreover, gas atoms will stick to the surfaces of the dust grains and combine with other atoms there to form new molecules, which evaporate from the grains and return to the gas. (This process is called surface catalysis. By the same process, catalytic converters in automobiles convert toxic exhaust gases into CO₂ and water.) Thus, the grains increase the concentration of molecules in the gas, both by facilitating their formation and by preventing their destruction. Astronomers who dislike dark interstellar clouds because they make it hard to see stars should think again: without those dark clouds, there would be no stars! The role of grains in star formation is one of the many subtle processes that determine the ecology of the Milky Way Galaxy.

Self-regulating star formation: When enough stars form in a dense cloud, they will terminate the process of star formation. The UV light from the hot stars will penetrate into the dark clouds, destroying the molecules and heating the gas. That will cause the rest of the gas cloud to disperse. The beautiful optical emission nebulae (e.g., the Eagle Nebula) that you see in star forming regions are bright just because of the dense gas that is being driven away from the dense clouds by the UV light of newborn stars.

Propagating star formation: Gravity is not the only force that can compress interstellar gas. The supernova explosions of massive stars can compress diffuse interstellar gas into dense clouds. The dense clouds can then cool enough that gravity can take over. The result is that the deaths of stars can stimulate the birth of new stars.

As you can see, star formation is the result of a subtle interplay of many processes, some of which are summarized in the table below.

PROTOSTARS

The best way to observe the birth of stars is with telescopes sensitive to far-infrared and submillimeter waves, which are not absorbed by the dark dust clouds that invariably surround the newly-forming stars. This is one of the hottest areas of astronomy because the technology to observe star formation at these wavelengths has only existed for about a decade and its power is increasing fast. But now we can observe star formation in progress, and the picture that emerges is illustrated below.

In star-forming regions, we also see the remarkable phenomenon of protostellar jets, as illustrated below. These jets are narrow streams of gas that are flowing out of protostars at velocities of a few hundred kilometers per second. The jets strike interstellar gas and the impact causes the gas to heat up and emit radiation. The hot spots are called Herbig-Haro objects. Sometimes we see the jets as bipolar outflows, meaning that there are two jets oppositely directed from the protostar, but often we see the jet on only one side because the opposite jet is hidden within a dust cloud. The physical mechanism responsible for these jets is not well-understood. We suspect that magnetic fields play an essential role in collimating the outflows.

As you can see, the subject of star formation is an exciting frontier of astronomical research. Today, we have more questions than answers, but we are learning fast.

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Last modified March 16, 2002
Copyright by Richard McCray