This SciShow Space video is supported by Fabulous, an app that helps you start building your ideal daily routine.

The first 100 people who click on the link will get 25% off a Fabulous subscription. [♪ INTRO] The Milky Way is filled with stars of all ages, from newborns surrounded by swirling dust to ancient stars that have burned for billions of years. But even though astronomers date stuff in space all the time, from meteorites to the universe itself, dating a star is hard.

It turns out a star’s age can’t be measured directly, but there are a few creative ways to calculate it indirectly. If a star is just one of many bound up in a cluster, there’s a pretty straightforward technique. Since all of its celestial siblings formed around the same time, astronomers can get an approximate age by dating the cluster as a whole.

And to do that, they plot all of the stars on a Hertzsprung-Russell, or H-R, diagram. A star’s position on this diagram is based on its luminosity and surface temperature, which astronomers calculate using a star’s brightness, its distance from us, and the color it appears to be. Dim, cool stars are found in the bottom right, and bright, hot stars are in the top left.

As a star ages, it can be found in different parts of the diagram. And the exact path it takes depends on its mass. But for most of its life, a star hangs out in a diagonal band called the main sequence.

Then, when it runs out of hydrogen fuel, the relationship between brightness and color changes. The star gets redder and brighter overall, and it moves away from the main sequence. This is known as turn off, and by plotting out all the stars in the cluster, you can see a sort of kink separating the stars that are still on the main sequence from those that aren’t.

That’s because the most massive stars in a cluster, the ones further to the left on the H-R diagram, leave the main sequence first. With the highest luminosities, they use up their fuel the fastest. And as time presses onward, stars with lower and lower masses finally run out, too.

So the older the cluster, the further the turn off is to the right on the H-R diagram. After identifying where that turn off is, the astronomers know which stars have just the right mass to be currently running out of hydrogen fuel. And that mass dictates how much time must have passed to reach that point in its life.

So astronomers can cross reference the turn off with their models for stellar evolution to calculate the cluster’s age…and by extension, the age of all its stars. But not all stars live in clusters. Many live alone, like our Sun.

To estimate the ages of these so-called field stars, astronomers can use stellar models that describe how certain properties change over time. For some stars on the main sequence, the older it is, the slower it spins. So after measuring a star’s color and rotation rate, astronomers can use this relationship to calculate its age to an accuracy of five to 20 percent.

But this method, known as gyrochronology, isn’t always reliable. It breaks down for stars that are too young, or don’t fall within a certain mass range. And it depends on how precise the models are in the first place.

The Sun is used to calibrate these models, because it’s the one star whose age we do know, by dating debris left over from our solar system’s formation. But astronomers can’t assume all stars act like our Sun does. So while they continue to test their models, one team decided to watch some supermassive stars change color in real time…kind of.

Stars spend millions if not billions of years on the main sequence, but when they do finally turn off, they pass through a feature of the H-R diagram called the Hertzsprung gap. It's a relatively empty part of the diagram because stars evolve through it super quickly, at least compared to the rest of their lifespan. Exactly how fast this happens depends on the mass of the star.

A star that’s between eight and 18 times the mass of our Sun can cross the gap and transition from bluish white to red in just thousands of years. And that’s quick enough for humanity to collectively take notice. So this team dove into historical records that described the appearance of various stars.

They scoured descriptions from ancient Babylonians, Greeks, Romans, Chinese, Arabs, and First Nations people. And multiple documents suggest that Betelgeuse has transitioned from yellow to red in the last thousand years. After combining that knowledge with Betelgeuse’s measured mass, the team was able to identify the evolutionary path that it took to get to its current position on the H-R diagram.

Then, they worked backwards to calculate its age. That calculation is about 14 million years, with another 1.5 million years to go before Betelgeuse explodes as a supernova. Now, finding a star that made this color transition in recent history is extremely rare, and it only works for stars at a very specific point in their life cycle, so this isn’t an especially practical method for dating stars.

But it is a pretty fascinating one. So far, no single method gives an exact age for any given star. But a combination of approaches can at least offer some insight.

And once you know the ages of stars, you can begin to piece together how stars, planets, and galaxies evolve and change over time. A star’s age can also suggest whether life had time to emerge or evolve in certain planetary systems. So astronomers will have to keep using all the tools available to them, keep refining those tools, and keep seeking out new ones.

Which is just science in action. Just like there’s no one method that’s perfect for dating stars, there’s no one method that’s perfect for self care and habit forming. That’s why this video’s sponsor, Fabulous, has a variety of tools from an on-demand library of coaching sessions to routine-forming apps.

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The first 100 people who click on the link will get 25% off a Fabulous subscription! Thanks for watching and thanks to Fabulous for supporting this SciShow Space video! [♪ OUTRO]