Three of the universe’s oldest stars have just been identified — and they’re in our cosmic backyard

Three undergraduate students in an astronomy class at MIT, led by their professor, discovered something extraordinary on the edge of our own galaxy, the Milky Way (so, in cosmic scales, right next door). They’re three of the oldest stars ever — almost as old as the Big Bang, which occurred about 13.8 billion years ago.

These incredible findings, as well as the method used to find them, could help astronomers discover more just like them. MIT Professor Anna Frebel, who co-authored the study with Mohammad Mardini at Jordan’s Zarqa University and MIT’s Hillary Andales, Ananda Santos and Casey Fienberg, joined GBH’s All Things Considered host Arun Rath to discuss these cosmic findings. What follows is a lightly edited transcript.

Arun Rath: Before we get into the details of this, I’m wondering what it’s like for you, as an astrophysicist, to be one of the first humans observing this. That’s got to be an extraordinary moment when that happens.

Anna Frebel: Yeah, absolutely. I mean, I’ve been searching for the oldest stars for my entire career, so I’m kind of used to looking for old stuff and occasionally finding it. But this was certainly a rare find because these guys are so close by.

This is the result of a new search technique where we can really say these three stars are amongst the oldest ones ever found and amongst the very first ones that have formed. Not the very, very first stars — they were big and massive and puffy. These ones are small stars that formed as part of the second generation of stars in the universe, but we’re pretty sure that they’re all part of that second generation. It’s super exciting.

Rath: Stars that old, where are they in their particular life spans? Are they towards the end of their life span as stars?

Frebel: It depends. These ones probably have lifetimes of about 15 billion years. Astronomers like to think in billions of years. Again, for reference, the universe is 13.8 billion years old, and [the stars] are maybe 13.5 billion years old or so.

So they’re certainly in the second half of their lives, but they still have a few billion years to go.

Rath: Wow. You know, all of this — cosmic scales, time and distance — just makes my head spin, but in a really exciting way. Talk about this field, stellar archeology, which sounds like one of the coolest fields around. This was the class you were teaching, right?

Frebel: Yes. I wanted to bring research into the classroom, and every student pair got a star that had never been studied before but had the potential to be quite old.

The concept of stellar archeology is basically the study of the chemical composition of stars [that are] not too far away from us, actually. If they happen to be old, that composition is exactly the same composition as the gas cloud from which these stars formed because these stars have just been sitting there doing nothing until we observe them some 13 billion years later. We can study the early phase of the universe by looking at old stars shining in our Milky Way today.

Rath: Tell us about the analysis. Is it from analyzing the light, or something more complex that gives you a sense of what these stars are composed of?

Frebel: Well, both, actually. We do analyze the light with spectroscopy, [which is] like sending the light through the prism, and it gets dispersed into all the rainbow colors. And then, we do some analysis on that.

This is a bit more of a complex analysis procedure, but it reveals all the content of the stars — so which elements are present, and how much — because, as it turns out, stars have different compositions.

The sun is quite rich in all the elements from the periodic table. Our old stars have much less of all the elements in them because they formed very early on in the universe.

Rath: These stars are on the edge of our galaxy, the Milky Way. Are they part of our galaxy?

Frebel: They’re actually not exactly at the edge. I would say they’re in the outer part.

We live in a spiral disk galaxy with the solar system two-thirds on the way out. Above and below the disk, the disk is enveloped by a bunch of stars. I always sort of say jokingly, “That’s the junkyard of the galaxy,” because lots of old stars are there that actually came from other little dwarf galaxies that the Milky Way has eaten over the course of its life.

Every galaxy grows, and certainly, the Milky Way has done the same by eating smaller neighbors. That’s a pretty cannibalistic process. And then, we put our food in our stomachs, and the Milky Way puts the stars from these other galaxies into this outer part.

The three stars that we uncovered actually do show a signature motion signature — the way they move about the galaxy — that indicates that they’ve come from a small dwarf galaxy a long, long, long time ago.

Rath: Wow. Eating stars and planets like a Hindu god — it’s kind of an amazing way to think about. The age of these stars, at 13.5 billion years old, [are they] older than our galaxy?

Frebel: That’s a good question. You know, our galaxy was a proto-baby galaxy once. It was just a bigger blob than the other blobs around it. Because of gravity, the Milky Way just gobbled up all its smaller neighbors. That’s what made it survive and turn into what we observe today. It’s a little bit unclear when the ‘day zero’ was for the Milky Way, but the proto-blob was certainly present at that time.

Rath: How important are — I mean, it’s super cool just thinking about it, contemplating it — but how important are these findings for astronomy?

Frebel: When you meet someone new, you want to know what their name is, how old they are, maybe where they live and what they do, right? We do the same with all the astronomical objects in the sky, including our Milky Way, including the universe itself.

So, figuring out or mapping out what all lives in the universe — what is big? What is small? What is young? What is old? — that’s all part of us trying to understand the universe that we live in. That means if we now know or have a few more ways to figure out [that there are] really old stars that formed in small systems that got absorbed by the Milky Way at very early times.It gives additional clues for how galaxy formation started, and we have a way to directly study that today, so it’s really fantastic. It certainly helps us to understand that, as well as the origin of the elements.

We are all made from star stuff. The scientific version of that is that elements are synthesized in various processes, in stars and in supernova explosions.

That’s actually much easier to study in the early universe because the universe was much less messy back then. These old stars also help us to uncover the elemental signatures of these very early processes. We can then learn much better where all the elements come from.