Pedagoguery

The first generation of stars produced by the universe are fundamentally different from those we see today. The main reason for this is the composition of the universe as compared to its composition today. The big bang nucleosynthesis produced a large quantity of helium, and a trace amount of lithium, but those were the only elements other than hydrogen around. Compare that with today where there are significant proportions of elements like carbon, oxygen, nitrogen, iron, and silicon. While in absolute terms, the abundances of those elements are quite small, they have a disproportionate impact on star formation.

Star formation begins with the gravitational collapse of a gas cloud. However, such a collapse produces heat, so the size of the collapsing cloud must be large enough to overcome the heat-induced motion of the constituent particles. In star formation today, the cloud possesses a good way of cooling itself, because of the presence of carbon monoxide in the cloud. One of the energy modes of the carbon monoxide molecule is easily excited by collision with other atoms and molecules in the surrounding gas. The molecule then emits a photon as it returns to its ground state. The photon is not of an energy that is easily captured by hydrogen, so it typically escapes from the cloud, thus carrying heat energy away. The abundance of carbon monoxide is low in the cloud, but the cooling effect of the carbon monoxide molecules is quite significant. This allows stars in the range of a few tenths to about 10 times the mass of the sun to form.

In the early universe, however, there was no carbon monoxide. As a result, heat could not easily escape a collapsing cloud. There was molecular hydrogen, which is much less efficient at cooling than carbon monoxide is. The result that the minimum mass required to overcome heat pressure was significantly larger – those first stars were anywhere from 30 to 500 times as massive as the sun. The first stars were monsters.

The interstellar medium at this time was electrically neutral, and it had been ever since the recombination era, when the universe cooled to the point where electrons and protons could join to form hydrogen atoms. These new stars quickly changed that. With surface temperatures exceeding 100,000°C, and luminosities millions of times more than the sun, the copious amounts of ultraviolet put out by these stars quickly re-ionized the interstellar medium.

How do such stars age? The short answer is "very quickly". A star's mass is the primary determining factor for its evolution. However, heavy element abundance is also significant. Astronomers have good computer models that give us an indication of how current stars evolve. These models can be checked against observation, because we can see many stars of many different masses and compositions at many different stages in their life cycle. However, there are very few stars around as massive as the first generation. The only candidate we know of is Eta Carinae, which is thought to have a mass of around 100 solar masses, but since it is shrouded in a nebula caused by its own instability, we do not know for sure. We can only project and make educated guesses about how those first stars behaved.

One thing is certain, starts that are as massive as the first stars will all end their lives violently. The smaller ones as supernovae, and the larger ones a hypernovae. Stars with about 140 to 260 solar masses would likely have completely blown themselves apart when they went hypernova, seeding the early universe with heavy elements. Unlike a typical core collapse supernova, when such a massive star starts to fuse oxygen in its core, the photons become so energetic, they spontaneously form matter-antimatter pairs. The particles provide less outward pressure than photons, and without that outward pressure, the star quickly begins to collapse, causing the internal temperature to skyrocket, fusing all of the remaining nuclear fuel. The star explodes, leaving nothing behind.

Stars that are less massive than those will leave black holes behind black holes, and it is probably those black holes that merged to form the progenitors of the supermassive black holes that are found in the center of nearly all galaxies.

Next time, when branes collide.