It is “common knowledge” – and the scare quotes should be a warning – that the Sun is an average star.
But it’s not, and in fact it’s not even close: the Sun is in the top 90th percentile of stars by mass. That’s because more than half of the universe’s stars are small, cool red dwarfs, faint bulbs with half to less than 10 percent of the sun’s mass. The lower limit is around 7 to 8 percent of the Sun’s mass; less than that, and there is not enough pressure in the core to sustain nuclear fusion, which is the most important property of what makes a star a star.
But what about the other end? There are stars far better than our own. Is there an upper limit to how massive a star can be?
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Yes, it is, and we see some stars approaching. If they get also close, however, they produce so much energy that they tear themselves apart. One reason why this “too close” region is not itself the hard limit on stellar mass is because its value has changed over time!
Before we dive into the fun science of all this, let’s remember the reasons why mass is what matters here rather than size or weight. Size is a problem because stars lack well-defined surfaces, and this problem gets worse the bigger a star gets – the biggest ones are so inflated that they just disappear with distance from their respective centers like nebular clouds. Weight will not work because it is only a second-order measure of mass – or rather how strong the gravitational force is on an object with mass. You have the same mass on Earth as you do on the Moon, although you weigh differently because the Moon’s gravity is weaker.
Mass is critical because it dictates the delicate equilibrium that defines a star, a balance between the inward pull of gravity and the outward pressure of light emanating from the star’s core. Gravity is a direct result of mass, but the amount of energy generated in a star’s core also comes from mass. The more massive the star, the more pressure there is in the star’s center and the hotter it gets.
A star’s radiance comes from nuclear fusion—specifically, by squeezing hydrogen atoms together hard enough to make helium (although the process itself is a bit more complicated). This releases energy mainly in the form of gamma rays, which are absorbed by the surrounding material and heat it up. The rate of fusion depends on the star’s core temperature, which depends on, well, its mass. The rate actually depends very strongly on core temperature: in a star like the Sun, the fusion rate scales as the fourth power of temperature, so a small change in temperature greatly affects how quickly the core generates energy.
Higher mass stars use a different fusion process, i.e ridiculous depending on temperature; the fusion rate can scale with temperature to about the 20th power! This coupling is so strong that doubling the temperature of a massive star’s core increases the energy generation rate by a factor of one million.
You may now see why stars can only get so big. If you pile on too much mass, the star’s gravity strengthens, the pressure in the core rises, the temperature increases, and then the fusion rate skyrockets. If too much energy is dumped into the star’s upper layers, they become so hot that they not only expand; they also blow away material, thereby losing mass. This forms a negative feedback loop that limits how massive a star can be. Stars in this crazy state aren’t particularly stable either; the fusion rate can be stormy, and the star undergoes incredibly violent paroxysms.
The theoretical upper limit for stellar mass also depends on other factors, but is probably around 300 times the mass of the Sun. Stars this large are incredibly rare, and only a few with more than 200 solar masses are known. The most massive star we know of is R136a1, a beast in the Large Magellanic Cloud, a satellite galaxy in the Milky Way. That’s about 160,000 light years away – which is fine by me! It blasts out seven million times as much energy as the Sun, so keeping it in another galaxy is a pretty good idea.
R136a1 is part of a star cluster called R136, which was thought to be a single star when it was first discovered. It was a problem because R136 is so luminous that it needs thousands of times the sun’s mass to be so bright. However, observations by the Hubble Space Telescope confirmed that it was indeed a small cluster of stars. However, the brightest member, R136a1, is still a monster: it is estimated to have about 290 times the mass of the Sun – close to the theoretical limit. It is probably only about a million years old and will last another two million before exploding as a supernova.
Because R136a1 is so close to the top of the mass scale, it is unlikely that we will find another star that massive. But that has not always been the case.
Another factor in how massive a star can become is the abundance of heavy elements in the outer layers. Many of these are very good at absorbing the energy coming up from the star’s interior, which makes the star hotter. If the star gets too hot, it blows away the outer layers. So, much like spicy seasoning, even a pinch of heavy elements can have a big effect.
However, in the very young universe, these elements did not exist yet! Early on, matter in the cosmos was almost exclusively hydrogen and helium (with only a soup of heavier elements such as lithium). Massive stars eventually churned out heavier elements later, first boiling those elements up in their cores via fusion and then making more when they inevitably exploded as supernovae, seeding gas clouds for the next generation of stars. Today these elements are relatively common, but that was not the case when the first generation of stars arose. Because of this, the earliest stars can be incredibly massive: some models show that they could have had many thousands of times the mass of the Sun!
These genesis stars all lived and died early in the timeline of the universe’s existence, and their light would have traveled so far to reach us that, despite their enormous luminosity, they would appear very faint if we were to spot them; no confirmed first-generation star has yet been seen (although there is at least one candidate).
Astronomers search diligently for them, of course. Once that’s confirmed, we’ll have to significantly increase our estimate of how big a star can become—perhaps not today, but sometime in the future. And when we do, we’ll have learned another key factor in how stars are born, how they live, and how they die—and how all that depends on what they’re made of and when in the history of the cosmos we see them.
