Shallow Sky Object of the Month: Caph—Beta Cas—A Star in Transition

Original article appears in GuideStar December, 2014.

Beta Cas

by Bill Pellerin, GuideStar editor

Object: Caph-Beta Cas
Class: Delta Scuti variable star
Magnitude: 2.25
R.A.: 00 h, 9 m, 11 s
Dec: 59 degrees, 08 minutes, 59 seconds
Distance: 55 ly
Constellation: Cassopeia
Spectral: F2
Optics needed: Unaided Eye

Why this object is interesting:
Beta Cas is an old star and like many old stars it is starting to show its age by becoming variable. In the categories of variable stars this one is called a Delta Scuti variable star and it has some common characteristics with Cepheid variable stars. Cepheids are famous because their intrinsic luminosity (power output) is related to their period of variability. Thus, they and the Delta Scuti variable stars allow astronomers to measure the distance to stars and star systems.

If you know the intrinsic brightness of a star it’s easy to determine the distance using the inverse square law. That is, the brightness we see is inversely proportional to the square of the distance.

For Beta Cas, the period of variability is only about 2.5 hours, but the magnitude of the variability is only .06 magnitude, tough to see visually but easy with a photometer.

Beta Cas is ending its hydrogen fusion phase, the initial phase of all stars, and entering its helium fusion phase (the result being carbon and oxygen). Stars, as they move through life also change positions on the Hertzsprung-Russell diagram and as they do that they go through periods of instability (variability) that informs astronomers about what’s going on with the star.

Generally the mechanism is that the star becomes more opaque during the dim part of the cycle, becomes hotter (because the heat is kept in by its opacity). When the star becomes hot enough to burn off the dimming layer, it gets bright again and the cycle begins anew.

It won’t be long (in astronomical time), though, before the star has had enough of this phase and begins helium burning as its primary energy source.

The star is high in the sky now and easy to see at magnitude 2.25.

HAS Online Store

Get Connected!

HAS has begun using RainedOut, a text message service, to communicate late-breaking news about events. Click here to learn more and subscribe!

Night Sky Network