Variable stars

It is not very well known that more than half the stars in the night sky vary in brightness…

It is strange to think that so many stars vary in brightness, but of all the stars that do, most only vary by a small amount. Often this is almost undetectable with the human eye. Even our own Sun is a variable star over its 11-year cycle. As the number of sunspots increase and decrease so does the light output.

However, there are some stars which have a huge change in brightness, going from a moderately bright star in our skies to only being detectable in medium to large telescopes at other times.

What causes this odd behaviour? There are several reasons for this and there are also several types of variable star. We can therefore classify many variable stars into groups. Some of these stars vary in how quickly they change their brightness. Some can change in a matter of days, while others can take years, decades or even centuries to complete a cycle.

One type of variable star is known as an ‘eclipsing binary’. This is where there are two stars orbiting around each other and from our perspective here in our Solar System they line up so that one star seems to pass in front of the other. Often there is a larger, brighter star being orbited by a smaller, dimmer one, and when the smaller passes in front of the larger, the amount of light reaching us appears to dip. Then, as the smaller star seems to disappear behind the larger, the amount of light dips again but not by so much. When both stars are visible, the ‘star’ is at maximum brightness. The most famous of this type of eclipsing binary star is Algol in the constellation of Perseus. These stars are too close together to be seen individually from Earth-based telescopes, but we know there are definitely two stars in the system.

There are several other reasons why the brightness of some stars fluctuates. Some stars actually vary in size, they pulsate like a balloon being filled with air and then let down again. Most well known of this type are the Cepheid variables. They swell and shrink very regularly, so regularly in fact, they can be used as a distance measuring device. In 1912, an astronomer by the name of Henrietta Leavitt worked out that by measuring how bright the stars appeared compared to their rate of variability meant it would be possible to figure out the distance to them. Edwin Hubble used this to work out the distance to the Andromeda Galaxy.

Variable stars can be put into two basic groups, short period and long period, with a third group of irregular and semi-regular variables, which as their name suggests, have no pattern to the variation of their light output. One type of star in this category is the ‘Mira Variable’. This type is named after the star Mira or Omicron Ceti.

It is a cool, red supergiant which has large pulsations that increase and decrease its brightness. It does have a rough period of around 332 days during which time it undergoes a dramatic drop in brightness to well below naked eye visibility. It will eventually become a white dwarf star. There are also stars similar to R Coronae Borealis, which appear to fade quite markedly at odd intervals and then climb back up to their original brightness. This is due to the buildup of carbon dust in the star’s outer atmosphere. As the dust is dispersed, the star regains its brightness.

Another type of variable star is the ‘Gamma Cassiopeiae’ class, which fluctuate their light output irregularly due to the star throwing off material around its equator because they rotate very quickly.

These are just a selection of the different types of stars whose apparent brightness as seen from Earth can vary. Observing variable stars is a fascinating and very popular area for those interested in stargazing.

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