Pedagoguery

Since stars are one of the fundamental elements of astronomy, you would think that their formation would be a relatively well understood phenomenon. And, to a certain extent, you would be right. It happens when dense molecular clouds collapse, fragmenting into protostars which accrete gas until they gain enough mass to ignite a hydrogen fusion reaction in their cores. However, this simple scenario belies four very important unanswered questions. Where do molecular clouds come from? What causes their collapse? How do multiple stars forming in close proximity affect each other? And, how do massive stars form? Astronomers are beginning to get the glimmers of answers to all four of these questions.

Interstellar space is full of ultraviolet radiation. This radiation breaks apart hydrogen molecules and ionizes hydrogen atoms. This ionized hydrogen is relatively energetic, or in other words, hot. However, in order to collapse, this heat must be shed. The gas in this state contains about one atom per cubic centimeter of space. The gravitational vicissitudes of the galaxy can cause some gas to collect in denser pockets, and it is here where things get interesting. The gas can cool through the action of ionized carbon monoxide molcules radiating in the infrared. These atoms collide with other particles, absorbing some of their kinetic energy into rotational or vibrational modes of the molecule, which are then radiated away. In this way, the cloud cools. Eventually, it becomes cool enough for hydrogen molecules to form through the inter mediation of dust particles.

This process has increased the density to about 1000 atoms per cubic centimeter, but it is still not enough. The Spitzer Space Telescope has observed clouds so dense that they are opaque to infrared light and having a density of more than 10,000 atoms per cubic centimeter. These clouds are called infrared dark clouds are dense enough, but the link from the standard molecular clouds to infrared dark clouds is still uncertain.

Once you do have a dense cloud containing a core with between 100 and 100,000 solar masses, what triggers gravitational collapse? There are a few different candidates. The first is the old standby of the nearby supernova. The shockwave of the supernova could easily destabilize these cores into collapsing, but there are more common options. Observations of star forming regions in our galaxy have shown that massive stars clear out a bubble within the cloud. The expanding inner edges of the bubble show strong new star formation with many protostars at widely separated points having formed nearly simultaneously. So, it appears that there are many different things that can trigger the collapse of a cloud core.

Once a core starts to collapse, it tends to break apart into multiple protostars. There is abundant evidence that stars form in clusters and any theory of star formation has to explain how they influence each other. Observations of the Christmas Tree Cluster have shown that in many spots, tight clusterings of protostars exist – as many as 10 within a 0.1 light year radius. There are two competing theories of how they interact. The first one posits that some of the protostars will grow faster than the others, accreting most of the available mass. The ones that lose out may well get gravitationally slingshot out of the cluster. There may be a class of sub-stellar objects roaming the galaxy as a result of this.

The alternative theory uses turbulence within the gas, rather than competition between the protostars, to explain the mass differences. Not only can the turbulence within the gas help explain core collapse, it can also help explain the distribution of the masses of the resulting stars. There is actually a fair amount of observational evidence to back this theory up. However, the same observations that show evidence for the turbulence theory at larger scales, also show evidence for competition at smaller scales. The answer is probably a combination of elements.

Finally, we come to the question of massive stars. Current theory predicts that there would be a sharp cutoff at about 20 solar masses. When a star reaches that size, the radiation pressure outwards is greater than the gravitational pull inwards. Unfortunately for this theory, we definitely observe stars more massive than 20 solar masses, so those stars have to be formed somehow.

Computer modeling may help explain how such stars form. The models show that initially, mass accumulation is relatively symmetrical. However, when a protostar grows to about 11 solar masses, the accretion disk starts to become unstable and develops a spiral pattern. When the protostar reaches 17 solar masses, its radiation pushes bubbles outwards, but these are primarily outside the plane of the accretion disk. The instability of the disk creates clumps in which smaller protostars form. One of the protostars could easily start to grow faster than the primary, and soon become nearly the same size. The gas distribution around both stars is quite uneven, and accretion can move in sudden bursts. One particular simulation ended with the primary star attaining a mass of 42 solar masses, and the secondary with 28 or 29. This scenario also explains why most massive stars are found with companions.

As with most astronomical phenomena, star formation appears to be more complicated, and more interesting, than it first appears.

Next time, dark stars.