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

There are some truly giant stars out there in the galaxy and they are really weird! They're called "hypergiants" and they dwarf our tiny sun! These are tremendously massive stars, packed with enough mass to make a million stars like our own. They're born through the same process as other stars and shine the same way, but those about the only similarities between hypergiants and their tinier siblings.

 

Defining Hypergiants

So, what IS a hypergiant star? The exact definition is somewhat vague. Yes, they're big. Really big. But, big isn't the only characteristics that intrigues astronomers about these things. They also behave differently from other stars, particularly as they begin to age.\

Hypergiants were first identified separately from other supergiants because they are significantly brighter; that is, they have a larger luminosity than others. And, we can't forget that they're even more massive than the supergiants. In other words, they are bigger and more massive and much brighter than any other known stars. So, what are they? How do they form? How do they die? As astronomers see and study more of these objects, they're starting to come up with answers to these questions. 

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. All stars spend the majority of their lives on 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 different types of star.

That's true for any star. 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.

For the hypergiants, death is a pretty awesome catastrophe. Once these high-mass stars have exhausted their hydrogen, they expand to become much-larger supergiant stars. Things change inside these stars too: they begin fusing helium into carbon and oxygen. This process helps them avoid collapsing in on themselves, but it also heats them up even more.

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 these oddball stars are some of the most massive stars ever measured. 

Some of them are more than a hundred times the mass of our Sun. The largest is more than 265 times its mass, and incredibly bright. Such 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, and are very luminous.

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. 

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 upper layers collide with the core, and then rebound back out. That's what we see when a supernova explodes. In this case, it's going to be a hypernova.  In fact, some theorize that instead of a typical Type II supernova, you get something called a  gamma-ray burst (GRB). It's incredibly strong, blasting surrounding space with stellar debris and 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. 

Edited by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "What are Hypergiant Stars Like?" ThoughtCo, Jul. 2, 2017, thoughtco.com/hypergiant-stars-behemoths-of-the-galaxy-3073593. Millis, John P., Ph.D. (2017, July 2). What are Hypergiant Stars Like? Retrieved from https://www.thoughtco.com/hypergiant-stars-behemoths-of-the-galaxy-3073593 Millis, John P., Ph.D. "What are Hypergiant Stars Like?" ThoughtCo. https://www.thoughtco.com/hypergiant-stars-behemoths-of-the-galaxy-3073593 (accessed May 20, 2018).