Is Betelgeuse shrinking?

Image of Betelgeuse's disk and extended atmosphere taken by the Very Large Telescope. Credit: ESO/P. Kervella, Source.

I read that Betelgeuse is shrinking. Is it true, and will there be any consequences?

Betelgeuse is a star very often studied by astronomers.

Before answering why Betelgeuse is shrinking, let’s explain how to keep a star in a stable state. To do that, two forces must be in equilibrium: gravity, which pulls inward and tries to squeeze matter, and pressure (gas pressure and radiative pressure), which pushes outward. If gravity wins - the star shrinks, if pressure dominates - the star diffuses into space.

Betelgeuse's asymmetric pulsation, as seen in the ultraviolet by the Hubble space telescope. Credit: Andrea Dupree, Ronald Gilliland, CfA, STScI, NASA, ESA.

No star is in a perfect equilibrium state as stars are dynamic objects. Even our Sun is constantly trying to balance pressure and gravity, and it alters its radius periodically. However, the scale of those changes is so small, that they do not endanger life on Earth in any way. A branch of astrophysics devoted to study these oscillations, called helioseismology.

Betelgeuse is a special type of star – it is a pulsating star. Pulsating star vary their radii on short timescales, like hours, days, months or years, more or less regularly. Their atmospheres are unstable, hence maintaining the pressure-gravity balance results in changes in the star’s brightness and radius.

How does it happen?

A schematic pulsation cycle, described below.

Take a look at the figure above, where the schematic pulsation cycle is presented. Let’s start with a phase when a star has built a layer (orange-dotted circle) below which the pressure is too low to balance gravity (1). As a result, the layer is compressed and becomes more opaque (2). Now, the energy produced inside the star cannot get through this less transparent layer, and the temperature inside it goes up, and so does pressure (3). When it is high enough to overwhelm gravity, the star expands (4) and all energy that could not get through is released (5). The star can finally cool down. At the same time pressure below the layer drops (6) to the level where gravity takes the lead (1), so the star begins to contract again and the cycle restarts.

As for Betelgeuse, its pulsations are not regular. It is classified as a long period, semi-regular variable star.

What is the destiny of Betelgeuse? Well, in the near future, after every shrinking period, it will begin to swell and then shrink again in a new pulsation cycle. But its final fate is to explode as a supernova. It is hard to predict when it is going to happen exactly, as Betelgeuse is not an easy object to study in order to make theoretical predictions. We need to wait for it at least for about 100,000 years, maybe longer. Betelgeuse is at a safe distance from Earth and – despite some non-scientific speculations back in 2012 that the explosion of Betelgeuse would bring us the end of the world predicted by Mayans – this supernova event will have only one consequence for people on Earth: it will give us a stunning show on the sky, with Betelgeuse as bright as the Moon!

Dr. Monika Adamow
UT Austin