One of the great quests in astronomy is to find the first stars. These stars lived and died within a few hundred million years of the Big Bang, but for those in very distant parts of the universe, their light may only be reaching us now, having spent 14 billion years crossing the space between. At such a distance, it’s hard to spot a galaxy, let alone individual stars, yet many astronomers think we are getting close, thanks to the sheer size of some of these behemoths. So why did the early universe have stars so much bigger than any that exist today?
Before we answer that, a little background and explanation of terms. There are some truly immense stars today if we are talking about size, rather than mass. Famously, if the center of Betelgeuse was where the Sun is, its outer limits would stretch almost to Jupiter, making its radius nearly 1,000 times that of the Sun, and its volume close to a billion times bigger. Those figures are approximate, Betelgeuse’s constantly changing surface, which resembles a
Moreover, Betelgeuse is just our local supergiant, famous because it is relatively close. There are substantially larger stars, such as
However, while these stars have volumes far greater than the Sun, that is because they have puffed up as they run out of hydrogen near the ends 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 galaxy contain around
Very few stars reach this – in fact, most stars have masses considerably less than the Sun.
So how is it that we are hunting “
The first stars (known as Population III) were formed entirely from hydrogen and helium along with a little lithium, lacking all the heavier elements that exist today, which are the products of previous stellar generations. These heavier elements, which astronomers call metals, usually make up a very small proportion of the stars’ starting mass, but it turns out those small impurities are very important.
The Big Bang is thought to have left behind gas clouds containing 1,000 solar masses or so at points where
When patches of gas like that occur in the modern universe, such as in star formation regions like the
This doesn’t mean all the Population III stars were giants. One paper proposes the minimum was not far above
Even so, it seems the saying “there were giants in those days”, while
Consequently, a 1,000 solar masses star would outshine our Sun by more like 3 million times. That’s still more than enough for a tiny minority of the first stars to have an outsize impact on galaxy formation, and perhaps be visible over billions of light years.