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 in the universe. Some live long and prosper while others are born on the fast track. They 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. You've probably seen a few when you looked at the night sky. 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

What Makes a Blue Supergiant Star What it Is? 

Blue supergiants are born massive; they have at least ten times the mass of the Sun. The most massive ones have the mass of a hundred Suns. Something 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 causes 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. Let's dig a little bit into the science of such objects. To understand them, we need to take a look at the physics of how stars work: astrophysics. It tells us that stars spend the vast majority of their lives in  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.

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. 

Edited and updated by Carolyn Collins Petersen.