Variable Stars

At the inaugural meeting of the Costa Blanca Astronomy Society Christine Ord gave a very interesting presentation on Variable Stars. Christine pointed out that there are currently 43,000 known variable stars with 14,000 of them in need of further observation and that this was an area in which amateur astronomers can play a big roll.

Turning to the stars themselves it was explained that there are two broad categories of variable star:

  • Intrinsically variable stars. That is those stars which actually undergo changes within themselves which causes their brightness to vary, and
  • Extrinsically variable stars. These are stars where something external to the star itself causes the change in its apparent light output.

Intrinsic Variables

The first detected variable star was Mira which takes eleven months to go from peak brightness to peak brightness and varies by around 6 magnitudes from brightest to dimmest. There are now a large number of known variable stars which have broadly similar features and, as a group, they are known as Mira variables.

The changes in brightness of variable stars are measured by comparing them with nearby stars which burn at a constant brightness. The changes in brightness are then plotted onto a light curve which allows us to compare different variable stars with each other looking for patterns and for changes in the pattern of particular stars as they progress through their lives.

Another important class of variables are the Cepheids which have allowed astronomers to measure distances out into the Universe showing that it is much bigger than was originally thought.

Intrinsic variables like Miras and Cepheids are at a late stage of their development and expand and shrink as a result of the consequences of the nuclear activity at their core. As they expand they get brighter. When they shrink they get dimmer.

Other forms of intrinsic variables include cataclysmic binary stars and symbiotic stars which Christine gave detailed descriptions of.

Extrinsic Variables

These are completely different and there are again a number of sub-categories.

Two of the main groups are:

  • Eclipsing binaries, and
  • Rotating stars
31 Crateris may be an eclipsing binary

Image via Wikipedia

Eclipsing binaries are two stars of different size and brightness in orbit around each other. As the dimmer star passes in front of the brighter the light output drops and then recovers as the dimmer star moves behind the brighter one.

Rotating stars – One example of a rotating variable is a star that has many surface features like star spots that actually block out some of the light we see. As the star rotates the spots will cause the light received to dip when that surface is facing us and increase as it turns away.

Another example of rotating variables are  Pulsars. These  are very dense neutron stars which rotate very quickly at rates varying from once every couple of seconds to several hundred times every second and fire out beams of light and energy in opposite directions. Where we are in line with these beams we see pulses of energy each time the beam sweeps past rather like the light from a lighthouse.

The talk was a good introduction to a fascinating subject.

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