How Do We Measure the Ages of Stars? With Astrophysicist Ruth Angus

[A digital science visualization flies through the stars of the Milky Way Galaxy, revealing its iconic spiral shape.]

RUTH ANGUS, Assistant Curator, Division of Physical Sciences: What did the Milky Way look like when it was first born, when it was first formed? And how has it changed between then and now? 

[High-definition telescope photographs show a cosmos full of gleaming stars and colorful clouds of gas and particles.]

ANGUS: I’m talking about changes over billions of years. 

The only way to answer these questions is to put everything on a timeline. And we do that by measuring the ages of stars. 

[Another visualization moves through space at eye-level with innumerable stars.]

ANGUS: But it is really difficult to measure the age of a star. That is basically the most difficult thing you can try to measure about a star.

[American Museum of Natural History logo appears, followed by the title, “How Do We Measure the Ages of Stars? with Astrophysicist Ruth Angus”]

[Ruth Angus sits in her office, the dome of the Hayden Planetarium visible through a window behind her.]

ANGUS: I’m Ruth Angus. I’m an assistant curator of astrophysics here at the American Museum of Natural History. 

[A group of scientists sit at a large conference table reviewing data, with a portrait of astronomer Galileo Galilei visible on the wall behind them.]

ANGUS: I lead a research group and we all study stars and planets in the Milky Way galaxy.

[A selection of science fiction novels are lined up along a bookshelf, including authors George Orwell, Isaac Asimov, Arthur C. Clarke, Kazuo Ishiguro, Stanislaw Lem, W.E.B. Du Bois, Douglas Adams, and Frank Herbert.]

ANGUS: I was a massive science fiction nut when I was younger, I was really into science fiction novels.

[Ruth CHUCKLES.]

ANGUS: I probably first became interested in space in the first year of undergraduate studies.

[Speaking from her office, Ruth gestures emphatically.]

ANGUS: Because I was so into science fiction, I was like, okay, I’ll study astrophysics, but only if I can study exoplanets, planets outside the solar system.

[Three-dimensional artists’ conceptions of what exoplanets in the TRAPPIST-1 solar system may look like.]

ANGUS: Of course,

[Ruth CHUCKLES again.]

ANGUS: I still somewhat study planets, but I’m more of a stellar astronomer, I study stars more than planets. You have to study stars if you want to understand planets.

[A group of visitors peers into telescopes in the Museum’s Cullman Hall of the Universe.]

ANGUS: What we see when we look through a telescope is stars.

[Views of the night sky seen from Earth, filled with stars.]

ANGUS: We very rarely can see planets, because planets are so much fainter, relative to the star.

[Models of the planets of the solar system hang above viewers’ heads in the Museum’s Scales of the Universe display.]

ANGUS: For our own solar system, we can actually figure out the age of the planets and the sun through meteorites.

[A tour guide at the Museum discusses a large meteorite with visitors.]

ANGUS: We can’t do that for other stars. We have to use models and indirect methods to figure out the ages of those stars and planets.

[An animation shows a star and planet begin to coalesce from a primordial soup of matter.]

ANGUS: We assume stars and planets basically formed at the same time, that the planets are the same age as the stars.

[A series of high-definition photographs of regions of the Milky Way Galaxy.]

ANGUS: As soon as we can date stars, we can place planetary systems on a timeline.

That allows us to build an evolutionary picture of the planetary systems in the galaxy, or of the stars themselves, or of the galaxy itself.

[Ruth speaks from her office.]

ANGUS: So as someone who studies planets, I’m interested in measuring the ages of stars that are like our sun.

[Visualization of an exoplanet orbiting a solar-type (sun-like) star.]

ANGUS: Because most of the planets that we know of orbit stars like our sun. 

But you have to have something that changes in order to date it.

[Seen from Earth, the sun shines brightly and steadily in a blue sky.]

ANGUS: And our sun doesn’t change, so these stars are also pretty unchanging. So that’s why it’s so difficult to measure the age of a star. 

Now, there is something that actually does change over time relatively rapidly, which is the rotation rate of a star.

[Visualization of tumultuous gasses spinning in a torus (donut-like) shape.]

ANGUS: Stars start out spinning with whatever spin they had that they inherited from the cloud of gas that they originally collapsed out of. 

But over time, they start to spin more slowly and more slowly and more slowly, as they lose matter from a wind of particles that streams away from the surface of the star.

[Video of the Sun rotating, created from real NASA photographs of the star’s surface.]  

ANGUS: So our sun, for example, spins once every 26 days-ish, about once a month. But it didn’t always spin at that rate. It used to spin much more quickly than it does today. And in the future, it will spin more slowly than it does today. So if we can measure the rotation rate of a star, we can get a pretty good idea of its age.

So how do we measure the rotation rates of stars?

There’s a really cool technique that we use. If you’ve ever seen a picture of the sun, you might have noticed that the sun sometimes has dark spots on its surface called sunspots.

[Real NASA video clips of sunspots, which appear as black spots surrounded by orange-caramel-colored coronas, moving across the surface of the sun.]

ANGUS: And these are caused by magnetic field loops that poke out of the surface.

[A different form of photography and visualization by NASA shows how the sunspots correspond to loops protruding out of the sun’s magnetic field.]

ANGUS: Those spots aren’t really black, but they often look black. They’re just a little bit less bright than the rest of the surface of the sun.

But as the sun rotates, these spots rotate with the surface. And because these spots are slightly fainter, it means that overall, the total brightness of the sun, as seen by us on Earth, changes. It fluctuates.

So we can apply this same principle to other stars.

[Animation of the space telescope TESS (Transiting Exoplanet Survey Satellite) taking detailed readings of the cosmos.]

ANGUS: We can use telescopes that very precisely measure the total brightness of stars over time, and we can see the brightness of stars going up and down, up and down, as the spots rotate around and around. And we can therefore infer how fast a star is spinning based on the pattern of light that we see.

And then we can use models to convert that spin rate into an age. 

Once I have ages for stars, there are all sorts of things that I can do.

[More detailed photographs of the Milky Way.]

ANGUS: I can look at how the stars themselves change over billions of years. I can look at how planetary systems change over billions of years. And I can look even at how the Milky Way as a whole is changing over billions of years.

The majority of the data that I use come from space telescopes…

[Animations of the Hubble and James Webb space telescopes flying through space.]

ANGUS:…so I can’t go and visit the space telescope myself.

[A NASA ground telescope views the night sky from Earth.]

ANGUS: Or they come from telescopes that are on the ground, but that are very big, very expensive.

[Animation of the James Webb space telescope scanning the sky around it, which fades into the real photograph of the Milky Way taken by the telescope using this technique.]

ANGUS: And so instead of those telescopes taking individual observations of stars, they tend to produce these large data sets that are maybe hundreds of thousands of stars, maybe millions, maybe even billions of stars in these databases.

And we have information like the brightness of these stars, the brightness over time, how the stars change, the temperatures of the stars, the colors of the stars. And that’s the kind of information that I work with.

[A series of photographs of huge, ground-based telescopes scanning the night sky full of gleaming stars.]

ANGUS: We’re sort of detectives putting together a puzzle from a world that we cannot influence. We can’t conduct experiments in the same way that experimental scientists do. We can’t set up the conditions that we need to test our hypothesis. All we can do is observe. 

We just wait for the light to hit our telescopes and then we make inferences about the physics based on that.

[CREDITS.]