Milwaukee Astronomical Society

Stargazing

The Constellation Cepheus


  1. The reddest star in the sky is μ (Mu) Cephei, "The Garnet Star" in the constellation Cepheus. {Trace out Cepheus.} Cepheus is a king, and Cassiopeia is his queen.

  2. Cepheus is sort of a house-shaped constellation, and μ (Mu) Cephei is located halfway between the two stars at the bottom of the house. It is a red super-giant, 1,500 times the size of the sun. It was considered the largest star known to man until just recently when 3 other stars (which you can't see without a big telescope) were measured about the same size but just barely edge it out. Placed where our sun is, the surface of μ Cephei would extend out past Jupiter. To really appreciate how red this star is, it sometimes helps to use the binoculars (or the telescope).

  3. Another star in Cepheus is of crucial importance to astronomy -- δ (Delta) Cephei. {Locate δ Cephei.} This star is a "variable", meaning the star's brightness varies over time -- in this case it varies between that of ζ (zeta) Cephei and ε (epsilon) Cephei over a period of five days. How bright is it now -- as bright as ζ, e, or in between? We will assess again each night that we can during the week.

  4. This star was the first of its type to be discovered, hence these variable stars are called 'Cepheid' stars. Their discovery, as it happens, rocked the astronomy world. So what's the big deal?

    Cepheid stars have gotten to just the right mass to be unstable - so the whole star is pulsating, the surface of the star is actually rising and falling, with a rhythm that is so precise you could set your watch to it. It was discovered in 1912 that this rhythm depends directly on the true brightness of the star -- the brighter the star, the longer the time between peaks.

    Now when a star is closer to us, it seems brighter. When it's farther away it seems dimmer. So if we know the true brightness of the star, and we measure its apparent brightness, we can figure out the distance of the star. If the star is part of a cluster or a galaxy, this tells us the distance to that entire body of stars. This has been used to find the distances to globular clusters, other galaxies and even our distance from the center of our own galaxy -- 28,000 light years.

    In 1924, Edwin Hubble (yes, the telescope is named after him) used Cepheids to measure the distance to the Andromeda 'nebula' (2.3 million lightyears) and proved that it is not another solar system in formation but an 'island universe', another galaxy like our own. This was an extraordinary declaration about the structure of the universe back in 1924. Our whole system of measurement of the universe is built upon the Cepheids as our basic yardstick.

  5. Then on top of all that, if we look in the telescope we can see that bright yellow δ Cephei has a beautiful blue companion -- so this is both a variable star and a binary! The yellow star is the variable component, and once was a blue star like its companion. It is now expanding out as it goes through the end-of-life process, and is passing through an unstable phase as it does so. The pair is about a thousand light-years from us, based on the Cepheid period-brightness relationship.

    Yep so that's how we figured out how far we are from the center of our galaxy. We're looking in the wrong direction to see the center of our galaxy. For that we need to swing around to the constellation Sagittarius -- to the south -- which is our next stop on the tour.


 
Back to Virgo Go to Summer Index On to Sagittarius

Questions

Your questions and comments regarding the Stargazing section are welcome. You can e-mail the author, Randy Culp for inquiries, suggestions, new ideas or just to chat.
Updated 12 June 2019