Some time ago I spoke, to a certain point in passing, of a type of variable star: the Cepheids, and its usefulness in being able to know the diameter of the Milky Way. But there are more types of variable stars, so it’s time to take a look at them to understand how they help us understand modern astronomy …
A variable star is nothing more than a star that changes in brightness. That is, if its apparent magnitude changes according to our perspective from Earth, it is a variable star. Changes can take place over years or in a matter of tenths of a second, and can range from a minimal change in magnitude to tens of thousands of times the magnitude of the star. Today we know more than 100,000 variable stars and, in all probability, the list will grow in the coming years. Our own Sun is a variable star since its magnitude changes by one tenth over the 11 years that a solar cycle lasts. We don’t usually define it that way, but it is the closest variable star to our planet.
THE HISTORY OF VARIABLE STARS
Johannes Holwarda.
Credit: Crispijn van de Passe – Rijksmuseum Amsterdam
The first known variable star was named Ómicron Ceti, although later (luckily) it received a name that you may have heard: Look. It was described for the first time in 1596 by David Fabricius (a German astronomer who stands out, among other things, for being one of the first to use a telescope to observe the sky) who took it for a nova (it has nothing to do with the supernovae , it is a nuclear explosion in a white dwarf that makes it brighter). It was Johannes Holwarda who, in 1638, discovered the regular period of Mira. It varies every 11 months, and its discovery served to understand that the stars were not eternal and invariable as the ancient philosophers believed.
In a way, the development of astronomy cannot be understood without the discovery of variable stars and supernovae, as they helped the development of science. In that same seventeenth century, two other variable stars were discovered: Algol and Ji Cygni. By 1850, many variable stars had already been discovered. Just 7 years ago, in 2008, that figure was already 46,000 variable stars in the Milky Way.
THE CHARACTERISTICS OF VARIABLE STARS
In the Trifid Nebula there are several Cepheid variable stars.
Credit: ESO / VVV consortium / D. Minniti
We have already made it clear that a variable star is one that experiences a change in its luminosity in a specific period of time, but there are several reasons why we can see a difference in the luminosity of the star from our planet. For that reason, we say that there are two main categories for variable stars.
Intrinsic variable stars, in which their variation in luminosity is due to changes in the physical properties of the star (which in turn causes us to divide it into three other subgroups): pulsating variable stars , which are all those that suffer some type of pulse (that is, their radius expands and contracts), eruptive variable stars , which are those that experience eruptions on their surface (flares and ejections of matter), and cataclysmic variable stars , which are those that experience some kind of cataclysmic change in their structure (that is, they explode in the form of a nova or supernova).
On the other hand, we have the extrinsic variable stars in which the change in luminosity (from our perspective) is due to external reasons, and that leads us to classify them into two subgroups. On the one hand, we have the eclipsing binary stars , in which the brightness of a star is reduced by the passage of another star, or a planet, in front of it (as is the case of Algol, which is actually a system triple), and on the other hand we have rotating variable stars, which are those in which the star’s rotation causes its luminosity to be affected from our perspective (for example, by having such gigantic sunspots that, when they are oriented to our planet, its brightness seems less).
INTRINSIC VARIABLE STARS
The star RS Puppis is a Cepheid variable.
Credit: NASA / Hubble
If I have managed to explain myself well, until now (with a bit of luck) you will be quite clear about all this fuss. This is where things get complicated, and a lot. It does not reach us with the subgroups. Variable stars are also classified by their type. That is, a pulsating variable star of the Alpha Cygni type is not the same as a pulsating variable star of the Beta Cephei type, nor is it the same as a pulsating variable star of the Delta Cephei type (these, by the way, are what we know as Cepheids, which is what I was referring to in the article on the diameter of the milky way that I mentioned earlier).
If you ever need to understand the particularity of a particular type, you can look up the information about its archetypal star. It may seem very cryptic, but in reality what we are saying when talking about a pulsating variable star of the Alpha Cygni type is that it is a variable star with a behavior very similar to that of the star of the same name (Alpha Cygni, which may sound familiar to you). very cryptic, but if I tell you that we also know her as Deneb, things may change). In this case, Deneb (and all pulsating variable stars of the Alpha Cygni type) vary in apparent magnitude by 0.1 over a period of days or weeks.
In the same way, pulsating variable stars of the Beta Cephei type experience a behavior very similar to that of the star Beta Cephei (which we also know as Alfirk), that is, their magnitude varies very little (around 0.01 to 0, 03 of its apparent magnitude) every few hours (between 2 and 12). And so with all the others. You can check the full list (if you are interested) on Wikipedia .
Mira, the first known variable star and the archetype of variable stars of the Mira type.
Credit: DSS 2 / ESO
Of all of them, it is probably correct to say that the most important / popular are the Cepheids (which are yellow giants that pulse with a very regular period), the RR Lyrae type pulsating variables, which are based on the star RR Lyrae, Lyre constellation, which is, surely, one of the most studied and that we use as a standard candle. That is, it helps us to calculate how far away other objects with a similar luminosity are.
Finally, we have the pulsating variable stars of the Mira type, which you will surely have deduced by now that they have a pulsation period very similar to that of Mira. They are red giants with a minimum and maximum apparent magnitude that changes in complete orders over a period of months or even years (Mira ranges from 2.5 to 10 every 11 months).
The same is true of the eruptive variable star types (among which we also include protostars and unstable stars such as yellow hypergiants). That is, they are also classified, in almost all cases, following the pattern of an archetypal star (except for the ones I mentioned above). Lastly, cataclysmic variable stars are simpler. They are basically supernovae, novae, and dwarf novae.
EXTRINSIC VARIABLE STARS
Animation of the star Algol.
Credit: Dr. Fabien Baron, Dept. of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090
The story with extrinsic variable stars is practically identical. We also classify them according to their type, which follows that of an archetypal star, except in the case of ellipsoidal variable stars, which are binary stars so close that they are not spherical and, therefore, their apparent brightness seems to vary depending on which side they are pointing. to our planet at every moment. Of all these, the most important type is probably the Algol-type variable stars.
These are eclipsing stars that have eclipses in very specific periods, while the rest of the time they have a constant luminosity. The Beta Lyrae variables may be worth mentioning as well because of how bloody they are. They are binary stars so close together (their archetype is the star Beta Lyrae, which we also call Sheliak) that the light they emit constantly varies, making it almost impossible to know when each eclipse begins and ends.
ARE INDISPENSABLE
Although this article is not an exhaustive guide to all types of variable stars, it is necessary to keep in mind that variable stars are very important in today’s astronomy, especially because they allow us to calculate the distances to certain objects and also determine the distances. masses, radii, temperatures and luminosity of the stars, in what phase of their evolution they are and a long etcetera …
