Starquakes, starshakes, and asteroseismology

Staring up at the night sky, you might have thought that all of those stars are constant - they are always the same brightness. However, if you measure the brightness of any star over some period of time, it will change! We call these special time series measurements of stellar brightness light curves.

Example light curve for a variable star observed by NASA's Kepler space telescope.

With increasingly sensitive telescopes, we’ve noticed that most (if not all) stars are actually variable at some level, like what we see in the plot above. With modern instruments, we have seen . But what causes a stars brightness to change? As it turns out, there are a lot of reasons why a star’s brightness would change! Let’s go through a few of them here.

Starquakes

Much like how the Earth has earthquakes, stars have starquakes - and much like how seismologists here on Earth use Earthquakes to study the interior of our planet, asteroseismologists (like me) use starquakes to study the interiors of stars!

Asteroseismology is formally defined as the study of stellar interiors through the analysis of starquakes, or stellar pulsations. In the past 20 years, asteroseismology has been the engine behind a revolution in stellar physics, and has driven methodological develops, improvements in theoretical models, and provided updates to our understanding how how stars live and die.

Luckily for us astronomers, there are more than a dozen different types of pulsating stars that allow us to study stars at all masses and ages. These stars have starquakes that last anywhere from a few seconds to several days, depending on the type and size of star involved.

Binary stars

As I have a whole other post on this, I’ll just point you there.

Rotation

Back in 1610, Galileo used his telescopes to star at the Sun (which was, and still is, ill advised). When he did so, he noticed faculae or dark spots on the Sun. After staring at the Sun for a month in a row, he even noticed that the spots disappeared and reappeared over ~30 days. Today, we’ve observed that tons of other stars have spots. More importantly, when we look at the light curves of stars with spots, we see that the overall brightness changes with the spots. This gives us a direct way to measure the rotation rates of stars!

Stochastic signals

Stars have turbulent surfaces (often caused by winds or convection) that cause random flickers. When we observe a star’s brightness, the flicker produces a quasi-random brightness variations which appear as colored or correlated noise. In the past couple of decades, we’ve begun to realize how the amplitude and timescale of the flicker is related to different properties of the star.

Supernovae

Although stars live for an incomprehensibly long time, they do eventually die. Dying stars are responsible for the most energetic phenomena in the Universe - Supernovae explosions. While there are a variety of different flavors of supernovae, we’ll just cover the standard core-collapse supernova channel here.

Stars that are initially about 8 times more massive than our Sun, formally known as massive stars (great naming scheme), will experience a very different life cycle than stars less massive than that limit. Whereas stars similar to our Sun are not massive enough to fuse elements heavier than Carbon, massive stars are hot and dense enough to fuse up to Iron in their cores. At the point where they finally run out of fusable nuclear fuel (e.g. Hydrogen, Helium, Carbon, Oxygen, Neon, Silicon) and have an inert Iron / Nickel core, the star has an energy crisis. It is no longer energetically favourable to fuse Iron (instead it would require energy to split Iron), and so the star no longer has an internal energy source and it begins to collapse in on itself. This takes of order ~minutes, at which point the infalling layers will rebound on the Iron core and explode outwards, generating a thermonuclear supernova explosion, serving as a bright candle in the dark night sky for all to see.