What Is a Variable Star?

by Carson

The luminosity of stars is determined by its surface temperature and diameter, but do you know that the luminosity of some stars change dramatically over time, possibly getting brighter, and then dimmer, and then brighter again? Today, we’ll explain variable stars in this article.

Types of Variable Stars

Variable stars can mainly be classified into two types.

  1. Intrinsic variables
  2. Extrinsic variables

If a star is an intrinsic variable, it means that internal factors are causing its brightness to change over time. Since the luminosity of a star is determined by its surface temperature and its diameter, one or both of the two determining factors must change for a star to change its actual luminosity.

On the other hand, extrinsic variables aren’t variable stars on their own. Instead, external factors (i.e., factors outside the star) are causing it to change its apparent brightness. They are not of particular interest when studying stars in general, but astronomers must filter out extrinsic variables correctly when coming across a variable star.

The Way that Variable Stars Change its Brightness

How do variable stars change their luminosity? They often do so in a periodic way. Such variable stars are called pulsating variables, and they change their radius periodically (i.e., it contracts and expands and contracts again). Since the radius is one of the factors for luminosity, the brightness of these variable stars will change.

Among pulsating variables, one subcategory stands out: the Cepheid variables. Their periods go side-by-side with their luminosity. Thus, astronomers can measure the brightness of the star by measuring when it goes back to the same brightness, helping them determine the distance to nearby galaxies. Objects with similar properties are standard candles because their distances to Earth can be directly obtained.

Keep in mind that other forms of pulsating variables exist, such as RR Lyrae and RV Tauri stars.

Why Do Stars Pulsate?

Encountering the fact that some stars change their size, it’s very natural to ask why stars pulsate. Therefore, we’ll explain the pulsation cycle of a star very briefly in this section.

Firstly, gravity causes a star’s outer layers to collapse onto its core and shrink in size. After that, the star’s core will become opaque, and it will become harder for radiation to escape from it. As a result, the star’s core heats up and pressure builds up in it, exerting an outward force and causing the star to expand. Once the star expands, radiation can escape from the star more easily, causing the core to cool down and relieve the pressure. This starts the cycle again as gravity is pulling a star into itself at all times.

What About Extrinsic Variables?

Why do some stars appear to be variable stars even when they aren’t? There are two reasons for that. One of them is a rotational factor, which means that as the star rotates, darker regions and brighter regions pass through our line of sight, making the star’s apparent brightness change slightly. Another possibility is that the star’s brightness is eclipsed by something else, like nearby stars or planets.

The Implications of Variable Stars on Habitability

The variability of a star’s luminosity greatly influences the habitability of planets surrounding it for life as we know it. If the star’s luminosity is stable or only changes very subtly, like the Sun’s luminosity, the temperature of planets around it is stable, and it keeps planets in the habitable zone from deviating from it during the star’s period. However, if the star’s luminosity changes dramatically, planets in the habitable zone may leave the area in which liquid water can form on its surface, thus taking away one of the key things that enable life as we know it to survive.


In this article, we explained what variable stars are, and how they change their luminosity or apparent brightness. If you want to learn more about them, please visit the webpages in the references below.

References and Credits

  1. (n.d.). Variable Stars. Retrieved February 1, 2022, from https://astronomy.swin.edu.au/cosmos/v/variable+stars
  2. (2003, January 28). Pulsating Stars. Retrieved February 1, 2022, from https://www.astronomy.ohio-state.edu/ryden.1/ast162_4/notes16.html
  3. (2015, January 29). Types of Variable Stars: Cepheid, Pulsating and Cataclysmic. Retrieved February 1, 2022, from https://www.space.com/15396-variable-stars.html
  4. (n.d.). Pulsating Variable Stars. Retrieved February 1, 2022, from https://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_pulsating.html
  5. (n.d.). Pulsating Stars: Stars that Breathe. Retrieved February 1, 2022, from http://astronomy.swin.edu.au/sao/downloads/HET611-M17A01.pdf

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