Some variable stars change in brightness due to internal variations, such as periodic pulsations or stellar eruptions. These variations can be caused by processes such as changes in temperature, size, or composition of the star.
Some stars are simply not stable: their internal layers contract and expand periodically, much like a beating heart. This stellar "breathing" is related to imbalances between internal pressure and gravity, causing regular or irregular pulsations. When the star contracts, its temperature rises and it becomes brighter; conversely, when it expands, it cools down and loses brightness. In some stars called Cepheids, the rhythm of these instabilities is so regular that astronomers use them to measure cosmic distances. Others, more unpredictable, experience sudden changes in brightness with no obvious logic, just because their thermal balance is a bit off inside. These internal mechanisms make variable stars true flickering cosmic candles, fascinating yet capricious.
When two stars orbit each other, forming a binary system, gravity causes them to occasionally pass in front of one another from our point of view. The result is that their apparent brightness changes constantly, depending on their respective positions. Sometimes, one star even siphons off material from its neighbor, creating a hot gas exchange. This transfer often leads to violent brightness variations that can be clearly observed from Earth. These little turbulent stellar duos constantly change, and it is thanks to them that we gain a much better understanding of how gravity shapes the universe.
Stars often have a rather strong magnetic field, and sometimes even quite twisted. Imagine a huge magnet, but on a stellar scale: these magnetic fields create dark areas called stellar spots, which are less luminous and cooler than the rest of the star. As the star rotates, these spots pass in front of us, creating a fluctuating brightness since we see them appear and disappear with each rotation.
In some stars, the magnetic field is particularly turbulent, constantly changing the size and location of the spots. This further affects their brightness as observed from Earth: the larger and more numerous the spots, the dimmer the brightness. At other times, these fields also trigger gigantic eruptions on the star's surface (stellar eruptions), where bursts of super-hot material are ejected, temporarily illuminating the star and causing even more brightness variations.
When a star is observed from Earth, there are times when interstellar dust clouds interpose on its path. These dust clouds function somewhat like a very thin veil placed in front of the star, thereby altering the perceived light. Since dust is never evenly distributed, the movement of these veils causes irregular or periodic variations in the brightness of the observed star. This phenomenon is called interstellar extinction, and it can give stars a slightly reddish tint: it is exactly like how our Sun appears to redden when it is low on the horizon through the particle-laden terrestrial atmosphere. Some variable stars, known as eclipsing variables, owe their regular brightness changes precisely to this veil effect created by the dust and gases surrounding them.
Stars can sometimes abruptly release large amounts of energy: these are stellar outbursts. During these episodes, brightness suddenly increases for a few minutes or hours before slowly returning to normal. The more active and younger a star is, the more frequent this type of event becomes.
There are also stellar winds, which are constant flows of particles escaping from the star. When these energetic particles encounter the surrounding gases, they can cause sudden or cyclic changes in observable brightness. These phenomena explain why some variable stars experience rapid fluctuations in brightness.
The variations in brightness of a star can also be caused by a stellar companion orbiting around it, leading to regular eclipses that are different from those observed during terrestrial eclipses, where the phenomenon is much shorter.
The Kepler space telescope, launched in 2009, has enabled the discovery of around 3,000 previously unknown variable stars, thus revolutionizing our understanding of the stellar life cycle and evolution.
Some variable stars, such as Cepheids, serve as 'standard candles' in astronomy. By using their period and intrinsic brightness, astronomers accurately calculate galactic and extragalactic distances.
In 1596, the star Mira was identified by the astronomer David Fabricius as a variable star, thus becoming the very first star of this type to be documented.
Yes, some variable stars like Mira or Algol can be observed with the naked eye during their periods of high brightness. Their periodic variations even allow amateur astronomers to easily track their evolution from home.
No, a star can become variable at certain specific stages of its stellar evolution. For example, stars like Cepheids become temporarily variable during their giant star phase before entering a more stable phase.
Indeed, the Cepheid variable stars, whose brightness period is directly linked to their intrinsic luminosity, serve as 'standard candles' in astronomy to accurately calculate distances to distant galaxies.
Regular variable stars, such as Cepheids and RR Lyrae, result from well-defined physical processes like the rhythmic pulsations of their outer layers. In contrast, irregular variable stars may experience random variations due to unstable mechanisms or complex interactions such as stellar eruptions or the accretion of material from a companion star.
A clear sky and a simple telescope or even a good pair of binoculars are sufficient to make observations and track the brightness variation of bright variable stars like Algol. For a more detailed analysis, a CCD camera, an appropriate filter, and photometric analysis software will be useful.
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