We still use Cepheids as the basis of the extragalactic distance scale. If you can measure the brightness change of a Cepheid in another galaxy, you can find the distance to that galaxy! That's just what Edwin Hubble and his team did back in the early 20th century, proving that galaxies were very, very far away, something that was hotly debated up until then. Cepheids are massive, luminous stars, so bright they can be seen in other galaxies. Credit: Rogelio Bernal Andreo Photo:Īnd that's the key. Ursa Minor would be just another overlooked constellation if it didn’t hold the top of the sky within its borders. Stars dim with distance by a well-understood law, so if you have the apparent and absolute brightness, boom. If you then measure its apparent brightness from Earth, you can get its distance. That means if you can measure its period you can get its luminosity. They're critically important to astrophysics, because it was discovered (by astronomer Henrietta Swan Leavitt in 1908) that the time it took a Cepheid variable to cycle through its brightness was related to its absolute luminosity, how much energy it gives off. Stars that do this are called Cepheid variables, after the first one discovered, Delta Cephei. The mechanism is a little complex, and has to do with helium in the star's atmosphere absorbing energy, but the important bit is that it gets brighter when it gets bigger, and dims as it shrinks. The brightness varies because, in part, Polaris physically enlarges and shrinks in size. The change isn't huge, less than 20%, and it goes from dimmer to brighter and back again every four days or so. Polaris is a variable star, changing its brightness up and down over time. So Polaris is important because of where it is… but it turns out it's important because of what it is, too. Its position is why we call it Polaris, after all. If you can spot this star, you can find north, making it a pretty important cosmic landmark. As it happens, a fair-to-middlin' bright star is very near this spot: Alpha Ursae Minoris. Because the Earth spins, the sky appears to spin around this point (or the Celestial South Pole if you live south of the Equator). This is defined by the spinning Earth: If you stand exactly on the Earth's North Pole, the Celestial pole is straight up, directly above you. Polaris is celebrated because it happens to lie near the Celestial North Pole. But now we do!Īnd here come my favorite words to write: OK, let me explain. Of course we know how far away it is!Įxcept until recently we didn't. It's the 48th brightest star in the night sky, and it's the North Star. Like, for example, the distance to Polaris. Sometimes I find out that something that should be obvious, something we should've known about a long time ago, is still not so well known.
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