PostHeaderIcon Polaris: A Cepheid Variable

Polaris, the North Star, the 49th-brightest star in the sky, is a Cepheid variable star. Cepheid variables proved that galaxies are huge and far away instead of small and close by.

Why Stars Are Big:

A star stays star-sized the same way a balloon stays balloon-sized: competing forces. In the case of a Helium balloon the Helium is pushing out, trying to escape from the balloon. If it weren’t for the air around the balloon, it would explode – allowing the Helium to escape. The air (the atmosphere) is pushing back on the balloon, trying to flatten it.

In the case of a star, the energy from the fusion reaction at the core of the star is pushing out, keeping the star big, and gravity is pushing in, trying to collapse the star into a neutron star or black hole. As long as the fusion keeps going, the star stays “inflated.”


Unfortunately, Cepheids have finished fusing all of their Hydrogen into Helium. The next step is to fuse Helium into Carbon, Nitrogen, and Oxygen – but that reaction is hotter and takes more energy to start – energy the star doesn’t have. Since there’s no longer fusion happening in the star, gravity begins to win. The star starts to collapse: squishing all the atoms in the star closer together.

But wait! Squishing releases heat and energy! Suddenly, the star has enough energy to fuse some Helium into other elements. Fusion begins. The energy from fusion pushes out on the star, and the star expands again – losing energy and heat. Sadly, there is no longer enough energy to keep fusing Helium so fusion stops. The star begins to collapse again – releasing heat, energy, and fusion! This pushes out – heat is lost – fusion stops – the star squishes – heat is gained – fusion starts – the star expands … etc, etc, etc.

What Does This Have to Do with Galaxies?

In Cepheids, the speed of the pulsing is related directly to how bright the star is (a period-luminosity relationship). So, if you know how fast a star is pulsing, you know how bright the star really is (not how bright it looks in the sky). Then, by measuring how bright the star looks you can figure out how far away it must be. It’s like a flashlight: if you hold the flashlight and shine it in your face it’s very bright; but if you have a friend hold the flashlight at the other end of a football field and shine the flashlight in your face, you can look right at it: it looks dim.

In 1912, Henrietta Leavitt measured a bunch of Cepheids in the Small Magellanic Cloud and realized that they were too far away to be within our galaxy. Astronomers realized this meant that some things they thought were small might be large and very far away: galaxies.

There are some problems with measuring the distance to galaxies this way, but it got us started.

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