What are Hypergiant Stars Like?

eta carinae -- a hypergiant star
Eta Carinae is a hypergiant in the southern hemisphere skies. It's the bright star (left), embedded in a nebula, and it's thought this star will die in a hypernova event within the next million years. European Southern Observatory

The universe is filled with stars of all sizes and types. The biggest ones out there are called "hypergiants", and they dwarf our tiny Sun. Not only that, but some of them can be truly weird.

Hypergiants are tremendously bright and packed with enough material to make a million stars like our own. When they're born, they take up all the available "starbirth" material in the area and live their lives fast and hot. Hypergiants are born through the same process as other stars and shine the same way, but beyond that, they are very, very different from their tinier siblings. 

Learning about Hypergiants

Hypergiant stars were first identified separately from other supergiants because they are significantly brighter; that is, they have a larger luminosity than others. Studies of their light output also show that these stars are losing mass very rapidly. That "mass loss" is one defining characteristic of a hypergiant. The others include their temperatures (very high) and their masses (up to many times the mass of the Sun).

Creation of Hypergiant Stars

All stars form in clouds of gas and dust, no matter what size they end up being. It's a process that takes millions of years, and eventually the star "turns on" when it starts to fuse hydrogen in its core. That's when it moves onto a period of time in its evolution called the main sequence. This term refers to a chart of stellar evolution that astronomers use to understand the life of a star.

All stars spend the majority of their lives on the main sequence, steadily fusing hydrogen. The bigger and more massive a star is, the more quickly it uses up its fuel. Once the hydrogen fuel in any star's core is gone, the star essentially leaves the main sequence and evolves into a different "type". That happens with all stars. The big difference comes at the end of a star's life. And, that's dependent on its mass. Stars like the Sun end their lives as planetary nebulae, and blow their masses out to space in shells of gas and dust.

When we get to hypergiants and their lives, things get really interesting. Their deaths can be pretty awesome catastrophes. Once these high-mass stars have exhausted their hydrogen, they expand to become much-larger supergiant stars. The Sun actually will do the same thing in the future, but on a much smaller scale.

Things change inside these stars, too. The expansion is caused as the star begins to fuse helium into carbon and oxygen. That heats the interior of the star up, which eventually causes the exterior to swell. This process helps them avoid collapsing in on themselves, even as they heat up.

At the supergiant stage, a star oscillates between several states. It will be a red supergiant for a while, and then when it starts to fuse other elements in its core, it can become a blue supergiant. IN between such a star can also appear as a yellow supergiant as it transitions. The different colors are due to the fact that the star is swelling in size to hundreds of times the radius of our Sun in the red supergiant phase, to less than 25 solar radii in the blue supergiant phase.

In these supergiant phases, such stars lose mass quite rapidly and therefore are quite bright. Some supergiants are brighter than expected, and astronomers studied them in more depth. It turns out the hypergiants are some of the most massive stars ever measured and their aging process is much more exaggerated. 

That's the basic idea behind how a hypergiant grows old. The most intense process is suffered by stars that are more than a hundred times the mass of our Sun. The largest is more than 265 times its mass, and incredibly bright. Their brightness and other characteristics led astronomers to give these bloated stars a new classification: hypergiant. They are essentially supergiants (either red, yellow or blue) that have very high mass, and also high mass-loss rates.

Detailing the Final Death Throes of Hypergiants

Because of their high mass and luminosity, hypergiants only live a few million years. That's a pretty short lifespan for a star. By comparison, the Sun will live about 10 billion years. Their short lifespans mean that they go from baby stars to hydrogen-fusion very quickly, they exhaust their hydrogen quite fast, and move into the supergiant phase long before their smaller, less-massive, and ironically, longer-lived stellar siblings (like the Sun).

Eventually, the core of the hypergiant will fuse heavier and heavier elements until the core is mostly iron. At that point, it takes more energy to fuse iron into a heavier element than the core has available. Fusion stops. The temperatures and pressures in the core that held the rest of the star in what's called "hydrostatic equilibrium" (in other words, the outward pressure of the core pushed against the heavy gravity of the layers above it) are no longer enough to keep the rest of the star from collapsing in on itself. That balance is gone, and that means it's catastrophe time in the star.

What happens? It collapses, catastrophically. The collapsing upper layers collide with the core, which is expanding. Everything then rebounds back out. That's what we see when a supernova explodes. In the case of the hypergiant, the catastrophic death isn't just a supernova. It's going to be a hypernova. In fact, some theorize that instead of a typical Type II supernova, something called a gamma-ray burst (GRB) would happen. That's an incredibly strong outburst, blasting surrounding space with incredible amounts of stellar debris and strong radiation. 

What's left behind? The most likely result of such a catastrophic explosion will be either a black hole, or perhaps a neutron star or magnetar, all surrounded by a shell of expanding debris many, many light-years across. That's the ultimate, weird end for a star that lives fast, dies young: it leaves behind a gorgeous scene of destruction.