The Coolest Stars: The Red Supergiants
Updated: Aug 13, 2020
Red supergiant stars are cool in more ways than one, and are at the forefront of massive star research. This article is a short summary of red supergiant stars. I first started researching red supergiants around 8 years ago and they have had me hooked ever since. My research focuses on studying the chemistry of these stars and, more recently, trying to understand how frequently these stars appear as part of binary systems.
What are red supergiants?
Red supergiant stars are the final stage of evolution of the majority of massive stars. The more a star expands, the colder it becomes. Therefore, when a massive stars appears red, it is because it is very cool (like the red, and coolest, part of a flame). These stars are known as the red supergiants.
Any star lucky enough to be more than about 8 times the mass of the Sun is defined as a massive star. Massive stars are important because they are one of the few types of stars that give back to their surrounding environment. Throughout their lives, they enrich their environment by expelling material from their outer atmospheres. Then, when massive stars die, they explode as supernova that eject huge amounts of material and leave behind a black hole or neutron star. This material gets mixed up and used to create future generations of stars and planets, hence the phrase "we are all made of stardust".
One of the most impressive aspects of red supergiant stars is their size. They can be up to 1,500 times the radius of the Sun! If we were foolish enough to replace the Sun with a red supergiant star, it would engulf the four rocky planets and almost reach Jupiter (see the artist's impression of Betelgeuse).
The celebrity red supergiant: Betelgeuse
Betelgeuse is by far the most famous example of a red supergiant star. Betelgeuse is the closest red supergiant star to the Earth and, because of that, it is probably the best studied red supergiant around. Despite that, we still don't know very much about it! The recent dimming of Betelgeuse still isn't well understood and we very much can't predict what will happen next! (Although the answer is likely nothing.)
The life of a massive star
All stars spend most of their lifetime fusing hydrogen to helium in their core. Because massive stars are so hot, they consume their hydrogen fuel much quicker than stars like the Sun. In general, the more massive the star, the shorter its lifetime.
Stars aren't static
Stars are always changing, but generally they do so slowly. For example, the Sun has looked extremely similar to how it does today for around 4.5 billion years and will look pretty similar for roughly the next 4.5 billion years. Massive stars, on the other hand, go through a dizzying array of evolutionary phases and explode at the end of their lives, all within just 50 million years. The red supergiant phase is a small fraction of this evolution, accounting for just 10% of the star's life (or 2 to 5 million years). In astronomical terms, this is the blink of an eye. This relatively rapid evolution means that Betelgeuse and all other massive stars we see in the Milky Way today didn't exist when the dinosaurs roamed the Earth.
An evolutionary sweet spot
Not all massive stars become red supergiant stars. The most massive stars (above roughly 40 times the mass of the Sun) expel material from their atmospheres so quickly and effectively that they never have a chance to become red supergiants. Conversely, the cores of stars with a mass less than around 8 times that of the Sun never get hot enough to explode as supernova. Instead, they shed their outer layers much less explosively to become so-called White Dwarf stars that display impressive "planetary nebula".
Red supergiants are a sweet spot of stellar evolution with masses between these limits. As stars become red supergiants they expand drastically and become much cooler. In the space of a few hundred thousand years, their outer atmospheres go from being around 20,000°C to around 3,000°C and they expand from around 10- to around 1,500-times the radius of the Sun. This huge change is one of the most drastic physical changes in all of stellar astrophysics.
The death of a massive star
Red supergiants and black holes
Most red supergiant stars will end their lives in a dramatic core-collapse supernova that leaves behind a black hole or neutron star. This happens essentially because the core of the star can no longer support its own weight and collapses in on its self (creating the black hole or neutron star) and the outer layers bounce back (creating the supernova explosion). The more massive red supergiants, with masses greater than around 23 times the mass of the Sun, buck this trend and evolve first to a different evolutionary phase before exploding. Others don't explode at all, but collapse straight into a black hole. Predicting what stars look like when they explode is a key, uncertain, aspect of stellar evolution.
Supernova contribute significantly to the periodic table of elements. Massive stars create unique conditions that allow the creation of a huge variety of chemical elements. Many of the elements that make up the periodic table are only produced in massive stars. In short, we are only here because of supernova explosions.
Stars like company
We now know that most stars are born with a close companion star - forming what is known as a binary system - and massive stars are the most prone to companions. Evolving in close proximity to another star can have drastic consequences. If two stars are close enough, they merge, becoming almost a born-again star. Stars that are further apart swap material and interact with each other. Merging and interacting affects stars' lives and how they die profoundly. Observations of red supergiants show that around 50% are the product of two stars merging. The effects of binary interactions on stars are only just beginning to be understood, and this is a major area of current research.
The future for red supergiants
In 50 million years' time, all of the red supergiants we can currently observe will have either exploded or disappeared. In this article, I have attempted to summarise some of the interesting aspects of red supergiant stars and explain why I find these stars so fascinating. Thanks to Betelgeuse, red supergiants are well-known and well-studied stars, but there is still a huge amount to learn about how stars reach the red supergiant phase, how they behave during this phase and how they end their lives. From my perspective, one of the most important questions to answer is how binary evolution affects red supergiant stars, which is a question that we are only now beginning to answer by observing red supergiants in other galaxies.