RUTH ANGUS>> 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?
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.
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.
I’m Ruth Angus.
I’m an assistant curator of astrophysics here at the American Museum of Natural History.
I lead a research group and we all study stars and planets in the Milky Way galaxy.
I was a massive science fiction nut when I was younger,
I was really into science fiction novels.
I probably first became interested in space in the first year of undergraduate studies.
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.
Of course, 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.
What we see when we look through a telescope is stars.
We very rarely can see planets, because planets are so much fainter, relative to the star.
For our own solar system, we can actually figure out the age of the planets and the sun
through meteorites.
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.
We assume stars and planets basically formed at the same time,
that the planets are the same age as the stars.
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.
As someone who studies planets,
I’m interested in measuring the ages of stars that are like our sun.
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.
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.
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.
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.
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 sun spots.
And these are caused by magnetic field loops that poke out of the surface.
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,
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.
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.
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,
so I can’t go and visit the space telescope myself.
Or they come from telescopes that are on the ground, but that are very big, very expensive.
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.
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.