Astronomer Anna Frebel is part of a team that reported the discovery this week of a star that is almost as old as the universe – using techniques reminiscent of traditional archaeology. Lisa Grossman caught up with her at the Massachusetts Institute of Technology to quiz her about how that works.
You call yourself a stellar archaeologist. What does that mean?
Real archaeologists dig in the dirt, in the desert – we dig in the sky. We try to find the oldest stars that are still there.
What do you mean by "still there"?
They haven't died yet. Their light is a few thousand years old because they are a few thousand light years away – we are looking at stars located in our Milky Way galaxy. This is a very different approach to looking at really distant galaxies whose light has travelled for billions of years – once that light arrives at a detector, it is really, really, old. So the stars in those galaxies may already be dead. That's one way of looking into the past, but in our case we simply look for stars that are already very old – and that's why we call it stellar archaeology.
So what is the deal with this star you have found, is it the oldest we have ever seen?
It is definitely a superlative, but not in terms of age because, although we can estimate it, we cannot actually measure the age. It is a superlative in the sense that this star is the first we have found that we are very sure is of the second generation of stars in the universe. That is, of course, absolutely fantastic. This guy is nearly as old as the universe – it's just amazing.
Stars have generations?
After the Big Bang, we started off with a universe that was just made from hydrogen and helium, the two lightest elements. There were trace amounts of lithium, but we don't need to worry about this. Clouds of hydrogen and helium collapsed to form the first stars. All the other elements that exist and that we are made of – which are all heavier than hydrogen and helium – were created in stars and supernova explosions. These provide enough energy to fuse the nuclei of small elements and forge larger atoms.
So when a star explodes in a supernova, it creates another generation of stars?
Yes. And we look for stars with the least amount of elements heavier than hydrogen and helium because these are least likely to have gone through several generations of star birth. They are closest to the earliest stars.
How can you tell what elements are in a star from so far away?
We use telescopes to take what we call high-resolution spectra – meaning that we split the light up into a rainbow and throughout that rainbow there are colours missing. They have been eaten away by different elements absorbing pieces of light in the stellar atmosphere. From measuring and detecting how much light has been absorbed at which positions in the spectrum, we get a clue as to which kinds of elements are present in the star and how much of them. A picture may be worth 1000 words, but a spectrum is worth 1000 pictures.
How does the new star you found compare with, say, the sun?
The sun is pretty metal-rich – a whole 1.4 per cent. Our guy has ten million times less than the sun – it is chemically the most pristine star ever found. That is why it is so special, and that's what makes it a second-generation star.
Do you know approximately how old "your guy" is?
Ages in astronomy are some of the hardest things to judge, and for individual stars there is no way of determining age unless there is a clock inside the star. Now that sounds a little silly at first, but we have actually found a few old stars containing radioactive elements like thorium and uranium, which decay over time at specific known rates and so can essentially serve as clocks. From comparisons with how much thorium and uranium was produced by the previous supernova, you can get an age-range. We have found that these kinds of chemically primitive stars that we are fishing out, they are something like 13 billion years old. Our new star is likely on this order.
Can your star tell us anything about the history of the universe?
The calcium, magnesium and carbon and all these atoms that we see – in very small amounts – in our star must have been produced by a single, first generation star. It then died as a gigantic supernova explosion and all these newly created elements that it had made were ejected. Then our star formed.
So your star carries information about the first generation parent star?
Yes. And, actually, it looks like that first star did not explode with the fullest power it could have. It probably tried, but it sort of failed. We can tell this because we don't see any iron in our guy. Iron is heavier than the other elements in the first generation star like carbon, so it would have been in the core. The supernova probably went off, but there wasn't enough power to yank out all the iron from the core and it just expelled its outer layers – this is what we see, in our guy.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
