Why were the first stars so huge?

One of the greatest quests in astronomy is finding the first stars. These stars lived and died within a few hundred million years of the Big Bang, but for stars in very remote parts of the universe, their light may only be reaching us now, after traversing the space between them for 14 billion years. At such a distance it is difficult to spot a galaxy, let alone individual stars. Still, many astronomers think we’re getting close, thanks to the sheer size of some of these behemoths. So why did the early universe have stars that were so much bigger than any stars that exist today?

Before we answer that, first some background information and explanation of terms. There are some truly immense stars today if we’re talking size rather than mass. It is known that if the center of Betelgeuse were at the position of the Sun, its outer limits would extend almost to Jupiter, making its radius almost 1000 times greater than that of the Sun, and its volume almost a billion times would be larger. These figures are approximate; Betelgeuse’s ever-changing surface, which resembles a pot coming to a boil, is so difficult to measure. Estimates vary by 30 to 40 percent, but there is no doubt that it is very, very large.

Plus, Betelgeuse is just our local supergiant star, famous for being relatively close. There are considerably larger stars, such as VY Canis Majoris.

Although these stars have a volume much larger than that of the Sun, this is because they have blown up as they run out of hydrogen towards the end of their lives. Mass is a more important measure of a star, and here the range is smaller. The most massive known stars in our Milky Way contain about 125 solar masses. There are also questions about these estimates, because unless they have a companion star, we can only measure the mass fairly indirectly. Nevertheless, it is generally accepted today that there is a limit between 100 and 200 solar masses.

Very few stars achieve this; in fact, most stars have a mass considerably less than that of the Sun.

So why is it that we are hunting for ‘celestial monsters’ believed to have a mass of 5,000 to 10,000 solar masses? Although not yet confirmed, a recent discovery of illuminated helium in the early universe makes the most sense if illuminated by stars with a mass of a thousand times that of the Sun – five to ten times as much as is possible today.

The first stars (known as Population III) were formed entirely of hydrogen and helium along with some lithium, lacking all the heavier elements that exist today and are the product of previous generations of stars. These heavier elements, which astronomers call metals, usually make up a very small fraction of the starting mass of stars, but it turns out that these tiny impurities are very important.

The Big Bang is believed to have left behind gas clouds of about a thousand solar masses at points where dark matter halos peaked. Atomic hydrogen is a very poor heat emitter. When a gas cloud of pure hydrogen collapses, it heats up as its gravitational potential energy turns to heat, eventually reaching the temperatures and pressures at which fusion begins, creating a star. Not everyone agrees, but most astrophysicists think that as long as the gas is a poor heat sink, the entire cloud could condense into a single star, at least sometimes.

Where such pockets of gas exist in the modern universe, such as in star-forming regions such as the Orion Nebula, the gas is mainly hydrogen but contains a mixture of metals, some of which radiate heat much better. That extra radiation causes pieces of gas to fragment long before they become stars, preventing the products from growing too large.

This does not mean that all Population III stars were giants. One paper states that the minimum was not much above 0.8 solar mass, thanks to the gas sometimes fragmenting into smaller parts. That is a lot larger than the current minimum size, where stars such as Proxima Centauri, with a mass of 0.12 solar masses, are common. Nevertheless, this means that most early stars were well within the range we know.

Still, it seems that the saying “there were giants in those days,” while inaccurate for humanity’s prehistory, did apply to stars. A small fraction of early stars were truly enormous. Because the luminosity for main sequence stars increases by more than the third power of the mass, we would expect a star 1000 times more massive than the Sun to be more than a billion times brighter. In fact, this relationship breaks down for stars with more than 55 times the mass of the Sun.

Consequently, a star with a mass of 1,000 solar masses would outnumber our Sun by about 3 million times. That’s still more than enough for a small minority of the first stars to have an outsized impact on galaxy formation, perhaps being visible for billions of light years.

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