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Variable Stars

A star is classed as a variable star if it appears to change brightness (as seen from Earth) over time. 

The changes can come from the star itself, as the amount of energy it emits varies. Changes can also come from outside the star. For example, light from the star being blocked by an orbiting companion star.

We have known about variable stars for nearly four hundred years. The first one, Mira, was found in 1638. It pulsed over an 11-month cycle. Just over a century later in 1786, astronomers had found only ten variable stars. Today we know of more than 150,000!

We study most variable stars by measuring their brightness over time. We plot this on a graph known as a light-curve.

 

There are many kinds of variable star, but here are four of the most common types:

Cepheid Variables

An important type of variable star is called a Cepheid. These are yellow, giant stars which pulsate. This pulsing is caused by the star swelling and shrinking in a regular, repeating way. 

Each time the star swells, it gets cooler and fainter. As it shrinks, it gets hotter and brighter. 

A graph showing "Mag" on the y-axis ranging which decreases in number and "Phase" on the x-axis which ranges from 0 to 2.0. There are small black dots that collectively create a line that starts high up and falls before rising again. This repeats another time.
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Light-curve for a Cepheid Variable

However, this is complicated by how much light the star releases at each stage. The star changes its opacity (how see-through it is) during the process, changing the amount of light we see. The change in opacity is due to a layer of helium in the stars' envelope.

We call each cycle of pulsation a 'period'. Cepheids have short periods (days to weeks) which are related to the brightness of the star. This is called the period-luminosity relationship. If we know the period, we can work out the luminosity. This allows astronomers to calculate how far away these stars are. 

In the 1920s, Cepheids were used to prove that there were other galaxies in the Universe. 

RR Lyrae Variables

RR Lyrae stars are like Cepheids but are far dimmer. They are often part of globular clusters. These stars also have a period-luminosity relationship which we can use to work out the distance to the cluster. 

The periods of RR Lyrae stars are much shorter (a few hours to a day or 2). Their brightness varies by just   20 % in some cases to over 500 % in others!

A graph showing "V Magnitude" on the x-axis which decreases from 8.0 to 7.2 and "Phase" on the x-axis which ranges from 0.6 to 0.8. A red line, labelled "RR Lyrae", starts low down before peaking sharply, slowly falling, and then peaking rapidly again.
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Light-curve for a RR Lyrae Variable

The increase in brightness of these stars occurs at a much faster rate than the decrease. This gives the light-curve a distinctive shape and lets us distinguish them from Cepheid variables.

Mira Variables

Mira variable stars are cool giants which are also pulsating. The pulsations of Mira stars are large. They can vary between 5 times and 30,000 times their brightness over many months. 

The variation is produced in the same way as Cepheid variables, but the opacity changes are due to a layer of hydrogen rather than helium.

A graph showing "Magnitude" on the y-axis decreasing form 11 up to 5 and "Date" on the x-axis ranging from 01/01/18 to 07/01/19. Black dots collectively form a line-like structure that starts at 9 magnitude and falls before rising up, falling again, rising once more, and falling for a final time.
This work by Lithopsian is licensed under Creative Commons Attribution Share Alike 4.0 International
Light-curve for a Mira Variable

The rise and fall in brightness is much more balanced in Mira stars. This allows us to distinguish them from Cepheid and RR Lyrae variables.

Eclipsing Binaries

An eclipsing binary is a system which contains two stars. Both stars orbit in a plane in line with our view of them. This means that both stars will block out light from the other at some point during the orbit. 

Each time a star passes in front of the other we see a dip in the light-curve. 

Titled "Eclipsing Binary Stars", there are size graphics side-by-side in the top half of the image. These show a yellow sphere with a smaller red sphere. The red sphere is orbiting the other, starting with being on the right of the yellow sphere, then in front, then on the left, then behind (with a dotted outline indicating this), then to the left, and in front again. A graph in the bottom half of the image shows how the brightness changes over time. It is mainly a flat horizontal line, but dips in places.
This work by NASA is licensed under Creative Commons Zero v1.0 Universal
Light-curve for an Eclipsing Binary

The size of the dip is controlled by the relative sizes of the two stars. A larger star will block out more light, so there will be a greater change in brightness.

Astronomers use the size of the dip to calculate the relative sizes of the two stars. This can be combined with other observations to find the mass and density of both stars. This has taught us a lot about the different types of stars that exist.

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