Artwork depicting the first stars in the Universe lighting up the gas clouds from which they were born. Credit: NAOJ
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Artwork depicting the first stars in the Universe lighting up the gas clouds from which they were born. Credit: NAOJ 

When were the *very* first stars born? Earlier than we can see, even with Hubble

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Jun 16, 2020

When you look up at the night sky, you can see thousands of stars. The fact that they're there at all implies they had a beginning, and we know stars can exist for billions of years. We see them die, and we see them being born. It's not a huge leap of logic to presume that, given the finite age of the Universe, there must have been a time when the very first stars were born.

When was that? New, incredibly deep observations using Hubble Space Telescope show that these stars existed even earlier in the life of the Universe than we can see, because their light can't be seen.

Light moves very quickly, but still at a finite speed. That means that the farther away we look in the Universe the earlier in its history we see. That helps us understand what happened early on, but a problem here is that objects at that distance are faint, even for Hubble. A team of astronomers wanted to look at very distant (and therefore appearing very young) galaxies, so they employed a trick the Universe has helpfully made handy: Gravitational lensing.

The gravity from big objects like galaxies and clusters of galaxies bends the space around them, and light follows that curvature, that warping of space. Acting like a lens made of gravity, this can amplify the size and brightness of even more distant objects, making them visible. So this team pointed Hubble at a series of huge distant clusters in order to see what's behind them, so far away their light is otherwise dimmed to invisibility.

The goal was, in part, to look at the colors of these galaxies. Young, massive, stars are blue, and are stupendously luminous. These stars don't live long, so galaxies actively forming stars appear blue; if the galaxies don't make stars these massive stars die rapidly and the galaxy color shifts to being redder. So studying the colors of galaxies tells us what kinds of stars are in them and what the galaxies themselves are doing.

The cluster MACS J0416 is over 4 billion light years from Earth and contains thousands of galaxies. Credit: NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)

From theoretical calculations, we think the very first stars were extremely massive, perhaps many hundreds of times the Sun's mass (!!) and extraordinarily hot and luminous, blasting out most of their light in the ultraviolet. So part of the research these astronomers did was to look for those stars via their color.

By carefully removing the light from the foreground galaxies, they found quite a few galaxies whose light was amplified from 10 up to 100 times by the gravity of the clusters, many of which are so far away we see them as they were when they were less than a billion years old.

And when they looked for that UV light from the Very First Stars what they found was… nothing. The colors of these galaxies indicate that the expected ultraviolet emission just isn't there, including from galaxies that were just 500 million years old at the time.

From various observations, it's thought that the first stars may have been born as early as 200 million years after the Universe itself got its start. These new observations show that, at least in these galaxies, by 300 million years later they were gone.

Artwork depicting the first stars in the Universe lighting up the gas clouds from which they were born. Credit: NAOJ

 

In one sense that's not surprising; stars that massive don't live long before exploding as titanic supernovae, perhaps a million years or two. But this new work shows that even as a population they weren't around long, which means galaxies stopped making them relatively rapidly.

Perhaps this shouldn't be surprising either. Those very first stars were made entirely of hydrogen and helium (and just a soupçon of lithium), because that's all there was in the Universe back then; those were the only elements that came out of the Big Bang. These first stars fused these light elements into heavier ones in their cores to generate energy, and when they exploded they scattered the heavy elements into space around them. The next generation of stars born after these were seeded with these elements (like iron, silicon, oxygen, and so on). The physics is a bit complicated, but it's hard to make extremely massive stars with those elements in them; they help keep the huge amounts of energy stars produce inside the stars, which makes them very hot and… temperamental. They tend to tear themselves apart.

So there was only a short window of time in the early Universe those stars even existed at all, and what these results show is that it's unlikely Hubble will see them. So how can we see them? Perhaps James Webb Space Telescope can spot them; its huge mirror gives it a lot more sensitivity than Hubble and the wavelength of light it sees may be more sensitive to these ridiculously distant objects. It may be possible to look for other things affected by these stars like the gas around them, thus spotting them indirectly.

Astronomers are still working on it. What a breakthrough it will be when the light from these stars is finally seen! Imagine: The very first stars in the Universe, their light traveling for over 13 billion years, struggling against dust, gas, and even the expansion of the Universe itself to reach us here, where they will fall into our telescopes and enlighten us on conditions when the cosmos was but an infant.

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