W Cephei | Universe Today

Written by John Carl Villanueva

W Cephei is one of the more popular variable stars. That is, stars that change their brightness. The reason why W Cephei is a variable star is because it is actually a system of two stars that, most of the time, appear as one. Such a system is called a binary system.

To be more specific, W Cephei is an eclipsing binary system. The term 'eclipsing binary' comes from the eclipsing behavior of the two stars. Since their orbital plane lies in the line of sight of an observer here on Earth, when one star is nearer to us than the other, the former obstructs the latter from our view. This eclipsing behavior also gives rise to the varying brightness of W Cephei.

This particular binary system is popular because W Cephei A, the larger of the two stars, is among the largest in the Universe. In fact, it is second only to VY Canis Majoris among all stars in our galaxy, the Milky Way. If we consider all known stars in the Universe, W Cephei A comes third, after WOH G64.

VY Canis Majoris has an estimated size of up to 2100 Solar radii. WOH G64 has about 2000 Solar radii, while VV Cephei has about 1900. All three are hypergiant stars, i.e., stars with very large masses, high luminosity, and also high rate of mass loss.

VV Cephei B, on the other hand, is a blue star in the main sequence. Main sequence stars are called as such due to their positions in plots of stellar color against brightness. The main sequence is the continuous, distinctive band of stars in this kind of plot.

You can find W Cephei within the constellation Cepheus, which is approximately 2,400 light years from Earth. Through the naked eye, you can spot two other big stars accompanying W Cephei in Cepheus. There's Mu Cephei or Herschel's Garnet Star and HR 8164.

Both W Cephei A and B orbit about a common center of mass. On the average, they are separated by a distance of about 25 AU. However, due to their orbit's high eccentricity, their distance can vary between a minimum of 17 AU and a maximum of 34 AU.

We've got a Youtube video here in Universe Today that features W Cephei A. It's in the article "What is the Biggest Star In the Universe?" Want to learn more about the constellation Cepheus instead? Click on that link.

There's more about it at NASA. Here are a couple of sources there:
What is a Binary Star System?
Why are binary stars interesting?

Here are two episodes at Astronomy Cast that you might want to check out as well:
Where Do Baby Stars Come From?
Where Do Stars Go When they Die?

Filed under: Astronomy