First stars long sought, now found

Hey there, time traveller!
This article was published 11/08/2015 (3707 days ago), so information in it may no longer be current.

Big telescopes are in the news with yet another cosmic breakthrough. In June, a team of astronomers at the European Southern Observatory discovered a galaxy — called CR7 after Real Madrid soccer star Cristiano Ronaldo.

It is our first look at the first stars. It is a big deal. Here is why:

Theory says the early universe — in its first hours and its first million years — had only three elements: lots of hydrogen, some helium, a trace of lithium. All other elements (astronomers call them metals), were made in stars. Gravity formed stars from gas clouds; this took hundreds of millions of years. Strangely, we have strong confirmation for the first part of this theory, but not the second.

We still see the hydrogen and helium and lithium in their predicted proportions. But until now we have failed to see the first stars. Why have they been hidden?

Hot atoms give off light of precise colours that are like a signature of the emitting elements. So the first stars gave off “hydrogen light” and “helium light.” For example, the strongest light from hydrogen is called the Lyman-alpha line and it is ultraviolet.

Problem: space filled with hydrogen atoms is perfect to absorb this colour. Long before it gets to us the light is gone. Solution: after a while the light has ionized almost all the hydrogen into free protons and electrons, so there are few neutral atoms to absorb more light. In this way, over a few hundred million years after stars first lit up, the universe became transparent to the first stars’ light. But meantime what was happening to the first stars?

Calculations show the first stars were huge, a hundred or even a thousand times more massive than our sun. The next problem is a consequence of this. Their gravity makes the first stars very hot and so they burn up their hydrogen fuel quickly, as in millions rather than billions of years. They then collapse in supernovas that blast their metals into space. We see lots of stars that formed from gas that was “polluted” with the metals. We see them because they formed after the universe became transparent. These stars form differently from the first stars thanks to the metals in them.

So here’s the challenge: Find a bunch of stars that formed from unpolluted hydrogen and helium after other stars made space transparent and before they too burn out and explode. It means looking through a narrow window across almost 13 billion light years of space to a time 800 million years after the universe began. It’s a bit like searching for a needle that long ago was in a haystack for a short time.

The ESO team used several big telescopes to select the CR7 galaxy 13 billion light years away and to gather and decipher data from it. They found that bright patch has light from hydrogen and helium alone — no metals — the long-sought signature of the first stars. It is the brightest Lyman-alpha source ever found at such a distance. Other regions of the same galaxy have stars with metals in them, older stars that, the authors reason, ionized the local hydrogen so the first stars’ “hydrogen light” could set out on its way to us.

They say: “We may be witnessing, for the first time, direct evidence for waves of (first) star formation…”

The James Webb Space Telescope, due for launch in 2018, will be well suited to a search for the first stars. The ESO team jumped the gun. As it points out, JWST should be able to confirm the findings with relative ease or just might point to the only possible alternative — a never-yet-seen black hole forming directly from hydrogen gas. That would be even bigger news.

 

Colin Gillespie is a physicist and author whose most recent book is Time One: Discover How the Universe Began. He writes a weekly web blog Science Seen.