FEATURE ARTICLE
The Formation of Star Clusters
Clouds in the summer sky provide clues about the organization of star populations
Bruce Elmegreen, Yuri Efremov
Making Bound Clusters
Even though all star clusters may form by the same mechanism—involving the gravitational collapse of dense cloud cores that are part of an overall hierarchical structure set up by turbulence—there is still a difference between bound and unbound clusters that results from a difference in the efficiency of conversion of gas into stars. If a high fraction of the gas goes into stars, then the mass that remains in the star cluster when the residual gas gets dispersed will be high enough to bind the cluster together gravitationally. If a low fraction goes into the stars, then the final stellar mass will be too low to keep the cluster bound for the relative speeds of the stars.
Star birth is a violent event, and the efficiency of star formation depends on how readily the young stars destroy the cloud in which they formed. When the gas finally collapses into a star, it drags in magnetic and turbulent energy, often forming a dense orbiting disk around the star. This disk stretches and shears the gas even more, pumping more energy into magnetism and turbulence. As a result, the stellar collapse builds up enormous pressures that get released in the form of flares, winds and jets. Some of these bursts probably resemble solar storms, but they are much more violent in a young star. A violent and irregular wind like this can disrupt the low-density gas around a star. Young, massive stars are also extremely bright and hot, and this radiation can heat the surrounding gas, causing enormous amounts of it to move away. As a result of all this activity, young massive stars push away the residual cloud around them, limiting the amount of gas available for new stars in that region.
There seem to be two situations where star birth is not quite so disruptive, and then the efficiency can become high enough to allow a bound cluster to form. One arises for rather small clouds, where few massive stars get a chance to form. The Pleiades cluster, for example, probably formed in a small cloud because its total stellar mass is less than 1,000 suns. The other situation arises for clouds of any mass in extremely high-pressure environments. This is where globular clusters form.
An important component of this idea is the observation that massive stars are extremely rare. Stars that are massive enough to disrupt the surrounding cloud are not likely to appear in a cluster consisting of a few hundred stars. Perhaps only one in 1,000 stars has sufficient mass to be destructive. Small clouds, which form only a few hundred stars, can often survive the entire birthing process without getting destroyed, producing a small bound cluster.
In fact, all bound clusters in normal galaxy disks have total stellar masses less than several thousand suns. This is precisely the threshold dividing small clouds that are not likely to form massive O and B stars from large clouds that are likely to form such stars. All larger star groups either have or once had massive stars. These stars always lead to rapid cloud disruption in normal galaxy disks, and this causes the stars to disperse in the form of short-lived OB associations.
The formation of globular clusters, which are both massive and bound, requires a different environment. Globular clusters are so massive (100,000 suns or more) that they must certainly have contained massive OB stars when they formed. The relative fraction of high- and low-mass stars seems to be about constant everywhere, so it would be unlikely to have differed much in the early universe when the halo globulars formed. Indeed, the young globulars found in interacting galaxies contain massive stars. Many of these globulars are so young that the massive stars have not yet had time to evolve and explode as supernovae.
If massive stars are so destructive, what holds a cloud together as it forms a globular cluster? The key to this problem seems to be the way in which massive stars destroy their clouds. They do this primarily by heating the gas to a rather uniform 10,000 degrees Celsius, the temperature at which hydrogen (the main constituent) becomes ionized. During this ionization, the single electron in each hydrogen atom gets removed by an energetic photon. The electron typically comes off the atom very quickly, and the energy of motion from many such electrons rapidly heats the cloud, causing it to expand and disperse as an ionized nebula. The Orion nebula (M42) around the O stars in the young Orion cluster is a prime example. Once most of the hydrogen atoms in the region have lost their electrons, the stars can only heat the gas further after the atoms have recombined with their electrons. Then the gas and the stellar photons reach an equilibrium where photon heating and collisional excitation of atoms balance atomic cooling from the emission of low-energy recombination photons and other photons.
Gas at 10,000 degrees has a random thermal motion with a speed of about 10 kilometers per second. In normal galaxy disks, this motion greatly exceeds the escape speed of a star-forming cloud, so the ionized gas easily disperses the dense, cold gas. At very high pressures, however, all star-forming clouds of a given mass will have much higher velocity dispersions when they are in equilibrium with higher surface pressures.
If the pressure of the environment exceeds 1,000 times the local pressure in the interstellar medium near the sun, then clouds that are massive enough to make a globular cluster will have an escape speed greater than the thermal speed of ionized gas. In that case, the ionized nebulae formed by embedded stars will not readily disrupt the clouds, and star formation will proceed up to a high efficiency, even with massive stars. The result is a dense and massive bound cluster.
We can determine the pressure needed to produce a globular cluster in two ways. First, the clusters themselves tell us most of what we want to know, because they have an effective pressure today that is produced by the motions of their stars. This pressure is essentially GM2/R4 (where G is the gravitational constant, M is the cluster mass and R is the cluster radius). When we calculate these values for globulars in the Milky Way, we find that the pressures are 1,000 to 10,000 times that of the local interstellar medium. This pressure of stellar motion is a remnant of the pressure of the gas in which the stars formed.
Second, we can infer the pressure in the galactic halos at the time when the old globular clusters formed. This pressure must have been very large because all of the galaxy's gas had to have a very high velocity dispersion—comparable to the speed observed in today's galaxy rotations (200 kilometers per second)—in order to be in the halo in the first place. This is because a spheroidal gas cloud, like a young galaxy, has nearly isotropic random velocities, so the material can be carried against gravity to equal distances in all directions. This is very different from the present-day galaxy disk, which is thin because the gas velocities are low perpendicular to the disk, and much larger, because of rotation, parallel to the disk. With such fast random motions in the young galaxy, all of the clouds that formed by turbulent compression also had to be at very high pressures, easily exceeding 1,000 or 10,000 times the local interstellar pressure. The origin of this high pressure in the clouds was essentially the crashing and mixing of high-speed turbulent flows; the pressure inside and outside each cloud would equilibrate. Thus the globular clusters that formed in the young halos of galaxies were forged at extremely high pressures.
In colliding galaxies the pressure is also high because the interaction causes gas to accrete into the central regions, where it ends up with an extremely high column density (a high mass per unit area in the disk). This accretion is driven by the loss of angular momentum through torques exerted on the gas by strong gravitational forces from spiral arms that are generated by the interaction. Because the pressure in any self-gravitating disk is essentially the gravitational constant G multiplied by the square of the mass column density, the pressure in these regions is also extremely high, again about 1,000 to 10,000 times the local value.
The young globulars in the Large Magellanic Cloud probably formed because this galaxy is interacting with the Small Magellanic Cloud (another satellite galaxy of the Milky Way). Every 100 million years or so the two Magellanic galaxies come close to each other, causing strong tidal forces to agitate the interstellar gas inside each galaxy. This increases the local pressure and causes the formation of young globular clusters. As it happens, the distribution of the ages for the observed globulars is consistent with the estimated collision rate between the two galaxies.
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