Time and more data may shed more light on this mystery but even if we put aside the idea of aliens, the very concept of stars flashing at all, even naturally, is fascinating. We are used to our own star, the Sun, being a reliably steady source of light, and to the naked eye the stars of the night sky appear unchangeable, dimmed only by light pollution and atmospheric twinkling.
Stellar Pulse
In fact when they are studied closely, it turns out that in fact many stars do wax and wane in brightness. For some this is becuase of an object (such as a companion star or a planet) passing in front of it. But the variable stars that I find most intriguing are those that pulsate brighter and dimmer on their own, as if breathing in and out. For example, Mira, the first such star ever discovered, around 400 years ago, takes 11 months to take one breath in and out. At its dimmest level it is less than 1% as bright as its maximum. As you can see in the image above, it is actually in a close binary system with a smaller companion to the left (Mira B, white dwarf). Mira A is a unstable red giant, near the end of its life, and huge amounts of its mass is being dragged away by the gravity of its companion.
Certain types of regularly pulsating stars, known as Cepheids, have played a huge part in astronomy because a relationship has been found between their variation time and brightness, which means that they can be used as distance markers well beyond the Milky Way. (Astronomical distances are very difficult to measure directly, for obvious reasons). It was by observing a Cepheid variable known as V1 in a blurry patch in the sky that famous astronomer Edwin Hubble proved in 1923 that the universe extends beyond our own galaxy – by showing that the blurry patch is in fact another galaxy, the Andromeda (also known as M31). This image shows the location of V1 in that galaxy, and close-ups of how its brightness varies over a few weeks.
Not all stars that vary do so in such a predictable way, however. One of the most famous variable stars is Eta Carinae, which in the 1840s brightened so dramatically that it became the second brightest star in the sky, before fading back over a few decades to its previous level. This event became known as 'The Great Eruption' – the massive star lost around 10% of its mass and produced a cloud of debris named Homunculus Nebula which continues to spread out from the star. But that was not the end of Eta Carinae's activity – amazingly the star managed to survive. There was another major brightening in the 1890s, and erratic changes ever since. Astronomers are far from understanding what is going on here and the clouds of debris surrounding the system also confuse the picture, making it complicated to see what the star is actually doing. But one school of thought holds that another huge eruption may be due very soon – what a spectacular sight that would be!
http://chandra.harvard.edu/edu/formal/variable_stars/bg_info.html
http://www.nasa.gov/mission_pages/hubble/science/star-v1.html
http://www.nasa.gov/mission_pages/hubble/science/eta-carinae.html
https://www.newscientist.com/article/dn28191-citizen-scientists-catch-cloud-of-comets-orbiting-distant-star/
1. NASA/CXC/SAO/M. Karovska et al.
2. See image for credit
3. Credit: ESA/NASA