Cosmic dawn: when the stars switched on

Astronomers have pinned down roughly when the first stars and galaxies appeared in the universe...
13 July 2021

Interview with 

Richard Ellis, UCL


The Milky Way galaxy.


The universe may have started with a Big Bang, but the first stars and galaxies didn’t shine until much later during what we call the ‘cosmic dawn’. And up until recently we haven’t had a very good idea of when this cosmic dawn happened. But a team of astronomers think they’ve finally pinned it down, using some measurements that literally look back in time to these first galaxies. Phil Sansom heard from UCL astrophysicist Richard Ellis, who led the study...

Richard - Well, the universe itself is 13.8 billion years old, so that's about 3 times older than the earth. And we're looking back about 97% of the way back to the beginning. So we're looking back to when the universe was in its infancy, only a few hundred million years old.

Phil - That seems extraordinarily long ago to actually be physically looking at.

Richard - Yeah, well, let's try to understand this. So the universe is a very big place and light travels at a fixed speed. So when we use our powerful telescopes we're time travelers, we're looking back in time. So the light rays have taken so long to reach us that we're looking at those sources as they were not today, but as they were in the distant past. So the work that my group at UCL has done is to look at 6 of the most very distant objects, they're seeing when the universe was 550 million years old, so that's about 5% of the present age of the universe. But we studied these objects in detail and determined how old the stars in those galaxies were at that particular time. So we can then backtrack from that moment and say, well, when did those galaxies actually switch on their stars for the very first time.

Phil - These objects that you're looking at, Richard, if they're so far away, how can you work out how actually old the stars within them are? Because the images you get and the readings must be very, very vague and blurry.

Richard - Yeah, these galaxies don't look familiar at all. They're physically little blobs. Just to put it in context, our own galaxy is the Milkyway, and these galaxies are about a 20th of the size of the Milkyway, but they're in their infancy. They're forming stars very, very rapidly. Now to age date, in other words, to figure out how old a group of stars is, astronomers have a well-known trick that has been calibrated and used very successfully for stars in our own Milkyway which looks at a feature of hydrogen in the atmospheres of the stars which tells us how massive the star is, and particularly how old it is. And with Hubble and other facilities, with these 6 galaxies, we've been able to measure this hydrogen feature and determine the age of each of these galaxies individually. They're not all the same age. Some of them are younger than others. And so the impression we're getting is, you know, that cosmic Dawn wasn't, you know, just some sort of Monday morning - it switched on and that was it, you know. It occurred over a period of a hundred million years or so

Phil - Why is this important for us today? Are there sort of ramifications that we can take from this big prolonged switch on?

Richard - Yeah, let's talk about that. Why is this important? Well, firstly, these early stars, these first generation stars, only have 2 chemical elements in them - hydrogen and helium. These are the only chemical elements that are formed soon after the Big Bang. When the universe began all the heavier elements, including the stuff that makes up you and me, was material that was synthesised in stars. So by going back to this moment, we're in a sense looking at the origin of everything we see around us, including our own bodies. God, when did all this happen? When did this begin? And the answer, since you haven't asked me, is 250 to 350 million years after the Big Bang

Phil - We were getting to that! 250 - 350 million years?

Richard - That's right, Now, let me say something else about why this is important and that is astronomy doesn't change breakfast, or you know do very much for our daily lives. But I think it inspires people, particularly young kids to get into science and engineering.


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