They all relate to Cepheid variables! Before you tune out because it sounds like a hard concept to understand, bear with me. I’ll start with a story from Greek mythology. (Notice how a lot of the constellations have stories from myths?)


King Cepheus 

According to Greek mythology, a man named Cepheus was king of Ethiopia, a country in Africa. He had a really snobby wife named Cassiopeia who thought that she and her daughter, Andromeda, were the two most beautiful women in the world. She would brag to others that they were even more beautiful than the sea-nymphs, the Nereids – a number of beautiful nymphs that would accompany the sea god Poseidon. In fact, Poseidon’s wife was a Nereid.


When Poseidon heard that Cassiopeia said she was prettier than his wife, he sent a sea monster to kill Cepheus, Cassiopeia, and Andromeda. Cepheus and Cassiopeia thought that sacrificing Andromeda would save them, so they chained her to a cliff for the sea monster to eat. At the last minute, a Greek hero named Perseus – who loved Andromeda – flew in on his winged horse Pegasus and saved her from the sea monster.


Cepheus and Cassiopeia are next to each other in the sky. Cepheus looks like a house – a square with a triangle on top (or bottom, depending on where in the sky you see him).


Cassiopeia looks like a W or an M, depending on which season it is. Andromeda, a constellation and a galaxy, is joined with Pegasus, Perseus’ horse. Click on this link, and you can see all them in the winter sky.


Cepheid variables

So now that you know about King Cepheus, let’s talk about the types of stars named after him – Cepheid variables. What are they, and why are they named after him?


Think about stars in the sky – some are bright, some are dim. Well, some stars actually change how bright they shine! Since the star’s brightness isn’t constant, scientists call them variable stars. Cepheids are a special kind of variable star, because they pulse in and out, changing size, and they change in a regular pattern over time. (Think about our day as an example. In the daytime, it’s bright out because the sun is shining. At night it’s dark. Then it starts all over again. This is a regular pattern that changes over time – like a Cepheid variable.)


The first variable star like this was found in the Cepheus constellation, so it was named a Cepheid variable. Here’s a picture of a delta Cepheid.


Click here to see a picture of a Cepheid variable star pulsing in and out.


Light my candle 

So now you know what a Cepheid variable is – a special kind of star whose brightness changes regularly over time. Why are they important?


If you see a star and you want to know how far away it is, there are a number of different ways to measure the distance. Since we don’t have rulers that long, we have to use indirect methods of calculating distance. In 1912, a scientist named Henrietta Swan Leavitt – a lady almost as pretty and intelligent as Ms. Nufer – discovered a new calculation. She saw that some stars let out energy at the same rate (its luminosity), were the same brightness, and pulsed in and out at the same rate (how fast the star pulsed in and out is the time period, or period).


Ms. Leavitt found that a Cepheid that took three days to pulse in and out had a luminosity – or the amount of energy it gave off – about 800 times more than the sun. That star, in other words, was 800 times brighter than our sun!

In another example, she found that a star that took thirty days to pulse in and out was about 10,000 times brighter than the sun. (It gave off about 10,000 times the amount of energy.)

So, she knew how fast the stars pulsed in and out, and she knew how bright they were. Since we know how fast light travels, she could then calculate the distance to the star! Then, if she found another star one day that was the same brightness, she could guess how long its period is, or how long it takes to pulse in and out. In the same way, if she knew how long it took for a star to pulse in and out, she could guess how bright it would be.

The ability to use a standard relationship like that is known in astronomy as a standard candle. Think about a standard you use – if you know one of Ms. Nufer’s classes lasts 46 minutes, how many classes could you fit in 138 minutes? Well, 138 minutes divided by 46 minutes per class equals three classes. You used the standard of 46 minutes per class to figure that out, just like scientists use the standard candle, or standard of how bright a star is to how long it takes to pulse in and out, to measure distance.

The North Star

One last thing – why did I include Minnesota in my question above, including a king and a candle? Well, Minnesota is called the North Star State, and the North Star – Polaris – is a type of Cepheid variable!


Hopefully now you understand more about Cepheid variables, and why they’re so important in astronomy.