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Stars that change

How it works

Cause and effect, with real numbers.

  1. First look
  2. How it works
  3. The physics, still to be written
  4. The frontier, still to be written

How it works

Cause and effect, with real numbers

There are two quite different reasons why a star’s light can change. Either the star itself is changing, or something is getting in the way.

The star itself changes Something gets in the way
What happens The star swells and shrinks, or throws off gas and dust A partner star passes in front of it
The pattern A smooth rise and fall Steady light, with sharp dips
How regular From clockwork to quite unpredictable Clockwork
An example to look for Delta Cephei, Mira, Betelgeuse Algol

Four stars you can follow by eye.

Star Kind How long one cycle takes How much it changes
Delta Cephei Pulsing supergiant (a Cepheid) 5.366 days About 2 times
Mira Pulsing red giant About 332 days About 600 times
Algol Two stars that eclipse 2.867 days About 3 times, for a few hours
Betelgeuse Red supergiant Not regular Usually a little; to about a third in early 2020

The figures for how much each changes are in light the eye can see. Those for Mira and Algol were worked out here from the published magnitudes. Each step of one magnitude is a change of about 2.5 times. Mira usually goes from about magnitude 3 to about magnitude 10, which is seven steps, or about 600 times. Algol goes from 2.1 to 3.4, which comes to a little over 3 times.

Why a star pulses. The fusion in the star’s core stays steady all the while. What changes is how easily the heat can get out through the outer layers. When those layers hold heat in, pressure builds beneath them and pushes them outwards. They swell past the point of balance, the heat escapes, the push dies away, and gravity pulls them back in. They fall past the point of balance too, and it all starts again. The star behaves like a weight bouncing on a spring.

A brilliant white star with four spikes of light, wrapped in pale shells and wisps of dust, among smaller blue and orange stars.
A photograph. RS Puppis, a pulsing star about 6,500 light years away, by the Hubble Space Telescope. It brightens and fades by almost five times every 40 days or so. Each brightening travels out through the dust round the star and lights it up shell by shell, which astronomers call a light echo. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration. Acknowledgment: H. Bond (STScI and Penn State University), CC BY 4.0.

Mira. Mira is a red giant about 400 light years away and roughly 700 times as wide as the Sun. Its swings are much bigger than a Cepheid’s and not so regular. Some peaks are a good deal brighter than others. Its surface is cool even for a red giant, between about 1,900 and 2,600 kelvins as it swells and shrinks.

Algol. The two stars of Algol are far too close together to be told apart in a telescope. We happen to see their orbit almost edge on, so each passes in front of the other once in every circuit. When the fainter star covers the brighter one, the light drops for just over 9 hours from start to finish. When the brighter one covers the fainter, the dip is too small to notice by eye. Its fading had been noticed long before anyone understood it. In the 1780s the astronomer John Goodricke gave the right explanation. He was also the one who found the changes in Delta Cephei, in 1784.

Two pictures of a blurred orange disc side by side. On the left it is round and evenly bright. On the right its lower half is much darker.
A photograph. The face of Betelgeuse in January 2019 and December 2019, by the SPHERE instrument on ESO's Very Large Telescope in Chile. The dates are part of the published picture. Very few stars are big enough and close enough to be seen as more than a point. Credit: ESO/M. Montargès et al., CC BY 4.0.

The Great Dimming. Betelgeuse always varies a little. What happened in the winter of 2019 to 2020 was far more than usual, and for a while there was talk that the star might be about to explode. The pictures above settled it. The team that took them found that the star had pushed out a large bubble of gas. Soon afterwards a patch of its surface cooled, and that let the gas turn into solid dust. The dust hid the southern part of the star from us. It was not an early sign of a supernova.

New stars that are not new. Now and then a star flares up where none could be seen before. It used to be called a nova, the Latin for new, and the name has stuck. It is in fact an old pair of stars. One is a white dwarf, and it pulls hydrogen across from its partner. The gas piles up on its surface, growing hotter and denser, until it goes off like a bomb. Only the surface layer is blown away. The white dwarf survives, so the same thing can happen again. Most take between 10,000 and 100,000 years to gather enough gas. A few, such as RS Ophiuchi, manage it every 20 years or so.

Why they matter. For a Cepheid, the time taken over one pulse tells you how luminous the star really is. Compare that with how bright it looks and you have its distance. The lesson on the distance ladder shows how this gave the first distance to another galaxy.

The Sun. The Sun’s output rises and falls by about 0.1 per cent over its 11-year cycle of activity. Against the stars above, that is very steady indeed.

Where this comes from

The words are written for this site. Sums in the text are worked from the figures in these sources.