How Big Can a Star Get?

High mass stars in the Large Magellanic Cloud
Astronomers using the Hubble Space Telescope identified nine monster stars with masses more than 100 times the Sun's mass. They lie in the star cluster R136 in the nearby Large Magellanic Cloud. NASA/ESA/STScI

The universe is filled with a huge range of star varieties. Some are large and hot, others are smaller and cooler. When astronomers first began to classify stars, they used mass as a way to differentiate between them. Our Sun, for example, is classified as a lower-mass yellow dwarf. Yet, it is also the standard by which we qualify other stars' masses, hence the term "solar mass". Truly massive stars are many the mass of the Sun. Others, much smaller than the Sun, might only have half a solar mass (or less).

Finding the Most Massive Stars

The physics of stars suggests that they can only get so big and massive and so astronomers work to understand the masses of stars to get a handle on their upper and lower mass limits. But, the question is, how big and massive can a star be? Astronomers search for examples of "extreme" stars at both ends of the mass "distribution" or collection of stars that exist. The most massive star found so far is called "R136a1", and it comes in at 315 solar masses.

It seems that the R136 region, which is a star-making cloud in the nearby Large Magellanic Cloud, is bristling with new stars. The LMC, which is a satellite galaxy of our Milky Way, has long been of interest to astronomers studying starbirth. It's bristling with hot, new stars, and there are at least 9 in the region R136 region that have more than 100 solar masses. Many more have at least 50 times the mass of the Sun. Not only are these stars massive, but they're also extremely hot and bright. Most outshine the Sun. They also give off huge amounts of ultraviolet light, which is common in hot, young stars. In studies using Hubble Space Telescope, astronomers looked at these stars and also noticed that some of them eject huge amounts of material, as well. In some cases, they lose the equivalent of an Earth's mass of material each month, at a speed that approaches 1 percent of the speed of light. Those are some incredibly active stars!

The existence of such massive stars piques questions about how they formed and details about the process of starbirth. The fact that they exist in such high numbers in a small region of a galaxy tells astronomers that their birth cloud had to be very rich in the ingredients that make stars. In particular, they are hydrogen-rich.

High Mass Means a Short Life

Though these stars are the most massive in the nearby galaxy (there only a few of that mass in our own galaxy), their mass also means that they live shorter lives than less-massive stars. The reason is simple: to keep up their lavish mass, these stars need to consume an incredible amount of stellar fuel in their cores. Since each star is born with a set amount of mass, this means that they go through through fuel pretty quickly. For example, the Sun will exhaust its hydrogen fuel about 10 billion years after it was born (about five billion years from now). A very low-mass star would go through its fuel much more slowly and could live for billions of years after the Sun is gone. A very high-mass star, like those found in R136, goes through its fuel in tens of millions of years. That's an incredibly short time.

Massive Stars Die Massive Deaths

When a high-mass star dies, it does so in a very catastrophic, cataclysmic manner: it explodes as a supernova. It's not just a supernova, it's a massive one—a hypernova. We know that one will take place when the star Eta Carinae ultimately dies. Such an explosion happens when the star runs out of fuel in its core and begins to fuse iron. It takes more energy to fuse iron than the star has, so the fusion process stops. The outer layers of the star collapse in on the core and then rebound out, flinging themselves out to space. What's left of the star compresses to become a white dwarf, or more likely a black hole.

The stars in R136 are running on borrowed time. Soon enough, they will start to explode, lighting up the galaxy and spreading the chemical elements cooked up in its core out to space. That "star stuff" will become the next generation of stars, and possibly even planets with life onboard.

Studying stars like these gives astronomers huge insights into how stars form, live their lives, and ultimately die. The high-mass stars are like cosmic labs, revealing stellar life at the extreme end of the family of stars.