Variable Stars — What’s The Fuss All About?

Milky-Way.Kiwi
May 10 · 5 min read

I always wondered why people spent so much time and effort observing variable stars when there are amazing galaxies and nebulae to have a look at. With all of the fascinating things in the universe why were astronomers so interested in the variations of stars? Well, recently I’ve come to discover just why these stars are so important and I’m hooked! Now that I think about it, I’m a little ashamed that I didn’t give variable stars the time and effort they deserve, and more so given the rich history in variable star observations and the hugely important part that this has played in our understanding of how stars develop and the scale of the universe.

What continually amazes me about astronomy is that the things we can see in the night sky are influenced by the tiniest interactions between sub atomic particles and the effects that these cause on the massive objects. When you consider that an electron weighs 0.0000000000000000000000000000009109 kg and how it behaves can have an effect on a star that weighs 1989000000000000000000000000000 kg, it is mind boggling.

3D rendered star, not at all how they actually look to the naked eye. If you could see the star in hydrogen alpha it might look a bit like this.

One of the assumptions we often make about the night sky is that the stars are fixed and are always shining the same. This is because when we go outside we cannot easily perceive their movement and we don’t notice any variations in brightness. This changes if you go outside each night and notice the the positions of the stars and the brightness of them. Over time you’ll notice that some points of light wander across the sky — the planets and other stars vary in brightness. If you could live for a few thousand years you’ll also notice the stars themselves move relative to each other and if you could live for a few billion years then you’ll notice that all stars change in brightness over time.

The challenge we have is that we don’t live very long compared to the life cycle of stars so we cannot observe one star throughout its life. What we can do though is examine the entire population of visible stars and when we do this we find that there are useful relationships. One of these is the relationship between luminosity and surface temperature. These two values are easy to measure for stars, luminosity is the amount of energy a star releases and if we know the distance to the star then we can measure the luminosity for that star. The temperature is much easier to measure, it is simply the colour.

If you make a graph with luminosity on the y axis (the up and down one) and the temperature on the x axis (the left and right one) and plot all of the stars we know then we get a nice distribution across the graph.

The Hertzsprung-Russell diagram with some well known stars.

Stars don’t stay in the same place on the above diagram throughout their lives. The diagonal bit in the middle is called main sequence and this is where stars spend most of their lives fusing hydrogen in their cores. But once they run out of hydrogen that’s when they start wandering to other parts of the diagram.

The variable stars are those that have left the main sequence, there’s quite a few types of variable stars so the main one we will deal with are the regular pulsating variables. These stars inhabit a strip on the above diagram call the instability strip.

The instability strip is shown here as the red strip going from the main sequence up and to the right.

To understand what’s going on inside these variable stars we have to pop one open and have a look:

In the centre of this star is a small core of inert carbon surrounded by helium fusing in the rest of the core. Around this is a layer of hydrogen fusing in a shell.

What causes the pulsations of these stars in luminosity is all to do with the mechanism occurring near the border of the convective zone and radiative zones inside the star.

This a labelled cutaway of a variable star showing the thin convective zone and the large radiative zone. The convective zone is largely opaque and the radiative zone is less opaque enabling the easy escape of radiation from the core through the radiative zone.

These stars that have left the main sequence have helium building up in a area between the radiative and convection zones where the temperature is around 40,000 degrees K. This layer acts like an opaque blanket and prevents the free flow of radiation through it to the convective zone for transport to the surface. The helium in the area absorbs radiation through a process called Helium II ionization. This causes the inside of the star to heat up which puts pressure on the ionisation zone to expand.

The brown ring represents the Helium II ionisation zone that is responsible for the driving the pulsations of these sorts of stars.

As this zone expands it becomes transparent again, releasing the energy that had built up behind it. This energy makes it to the surface and the star has a temporary increase in luminosity. As the radiation that has built dissipates then the outer layer shrink again, reducing the luminosity of the star, compressing the helium II ionisation layer which slows the release of radiation from the core, and the process begins again.

This cyclical process continues over and over. The rate of the pulsations depends on the depth of the helium II ionization layer. The deeper it is the slower the rate. The larger stars in the instability strip can pulsate at rates of between 1 and 50 days, whereas the stars closer to the main sequence can be as fast as 1–3 hours.

What is really amazing about these stars is that for many of them the pulsation rate is directly relatable to a luminosity. This means if we measure a star’s pulsation rate we can determine the luminosity and compare that to how much energy from the star arrives here on Earth. This allows us to then calculate the distance to that star. This is exactly what Edwin Hubble did to calculate the distance to the Andromeda Galaxy, which proved that the Andromeda nebula was not part of our galaxy but its own galaxy.

The stars that enable Hubble to do this work are called Cepheid variables and they inhabit the instability strip up near the super-giants. Other stars in this class of variable include RR Lyrae variables that astronomers use to measure distances to globular clusters and Delta Scuti variables that are used to measure the size of the Milky Way.

So these stars are enormously important for our understanding of the universe and continued investigations of them are building our knowledge of how the internals of stars work. The great thing about variables is that amateur astronomers can observe them and contribute to science through their observations.

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