Blue Supergiant Stars: Behemoths of the Galaxies

star-forming region R136
The very massive star R136a1 lies in this star-forming region in the Large Magellanic Cloud (a neighbor galaxy to the Milky Way). It is one of many blue supergiants in this region of the sky. NASA/ESA/STScI

There are many different types of stars that astronomers study. Some live long and prosper while others are born on the fast track. Those live relatively short stellar lives and die explosive deaths after only a few tens of millions of years. Blue supergiants are among that second group. They are scattered across the night sky. For example, the bright star Rigel in Orion is one and there are collections of them at the hearts of massive star-forming regions such as the cluster R136 in the Large Magellanic Cloud

Rigel
Rigel, seen at the bottom right, in the constellation Orion the Hunter is a blue supergiant star. Luke Dodd/Science Photo Library/Getty Images

What Makes a Blue Supergiant Star What it Is? 

Blue supergiants are born massive. Think of them as the 800-pound gorillas of the stars. Most have at least ten times the mass of the Sun and many are even more massive behemoths. The most massive ones could make 100 Suns (or more!).

A star that massive needs a lot of fuel to stay bright. For all stars, the primary nuclear fuel is hydrogen. When they run out of hydrogen, they start to use helium in their cores, which causes the star to burn hotter and brighter. The resulting heat and pressure in the core cause the star to swell up. At that point, the star is nearing the end of its life and will soon (on timescales of the universe anyway) experience a supernova event.

A Deeper Look at the Astrophysics of a Blue Supergiant

That's the executive summary of a blue supergiant. Digging a little deeper into the science of such objects reveals a lot more detail. To understand them, it's important to know the physics of how stars work. That's a science called astrophysics. It reveals that stars spend the vast majority of their lives in a period defined as "being on the main sequence". In this phase, stars convert hydrogen into helium in their cores through the nuclear fusion process known as the proton-proton chain. High-mass stars may also employ the carbon-nitrogen-oxygen (CNO) cycle to help drive the reactions.

Once the hydrogen fuel is gone, however, the core of the star will rapidly collapse and heat up. This causes the outer lays of the star to expand outward due to the increased heat generated in the core. For low- and medium-mass stars, that step causes them to evolve into red giants, while high-mass stars become red supergiants.

The constellation Orion and the red supergiant Betelgeuse.
The constellation Orion holds the red supergiant star Betelgeuse (the red star in the upper left part of the constellation. It is due to explode as a supernova -- the end point of massive stars. Rogelio Bernal Andreo, CC By-SA.30

In high-mass stars, the cores begin to fuse helium into carbon and oxygen at a rapid rate. The surface of the star is red, which according to Wien's Law, is a direct result of a low surface temperature. While the core of the star is very hot, the energy is spread out through the star's interior as well as its incredibly large surface area. As a result, the average surface temperature is only 3,500 - 4,500 Kelvin.

As the star fuses heavier and heavier elements in its core, the fusion rate can vary wildly. At this point, the star can contract in on itself during periods of slow fusion, and then become a blue supergiant. It's not uncommon for such stars to oscillate between the red and blue supergiant stages before eventually going supernova.

A Type II supernova event can occur during the red supergiant phase of evolution, but, it can qalso happen when a star evolves to become a blue supergiant. For example, Supernova 1987a in the Large Magellanic Cloud was the death of a blue supergiant.

Properties of Blue Supergiants

While red supergiants are the largest stars, each with a radius between 200 and 800 times the radius of our Sun, blue supergiants are decidedly smaller. Most are less than 25 solar radii. However, they have been found, in many cases, to be some of the most massive in the universe. (It's worth knowing that being massive isn't always the same as being large. Some of the most massive objects in the universe—black holes—are very, very small.) Blue supergiants also have very fast, thin stellar winds blowing away into space. 

The Death of Blue Supergiants

As we mentioned above, supergiants will eventually die as supernovae. When they do, the final stage of their evolution can be as a neutron star (pulsar) or black hole. Supernova explosions also leave behind beautiful clouds of gas and dust, called supernova remnants. The best-known is the Crab Nebula, where a star exploded thousands of years ago. It became visible on Earth in the year 1054 and can still be seen today through a telescope. Although the progenitor star of the Crab may not have been a blue supergiant, it illustrates the fate awaiting such stars as they near the ends of their lives.

Hubble Space Telescope image of the Crab Nebula. NASA

Edited and updated by Carolyn Collins Petersen.