How Low-Mass Stars Die


When we talk about star death, we’re not really talking about death. We’re talking about the end of a functioning star. Astronomers tend to personify cosmic objects like stars, saying that they are born and die, when it’s more like they transition into something new.

With stars in particular, there’s two main courses their “life cycles,” such that they are, can take: one for massive stars and one for low-mass stars.

We can further subdivide low-mass star “deaths” into those of red dwarfs—like our nearest stellar neighbor, Proxima Centauri—and those of medium-mass stars, like the sun.

But before we dive into the final stages of these stellar life cycles, let’s review what kinds of stars we’re talking about here…

A red dwarf is a very tiny star, about as tiny as stars get. The “low-mass star” depicted in the image above is a red dwarf. It’s much smaller than the sun, but bigger than a brown dwarf, a not-quite-star we’ve talked about in a much earlier post. And as you can see above, brown dwarfs are comparable in size to Jupiter.

Specifically, red dwarfs can be found in the bottom right region of the H-R diagram.

See those red-colored stars down at the very bottom—Wolf 359, Proxima Centauri, and DX Cancri? Those are all red dwarfs.

By the way, Star Trek fans might recognize Wolf 359 as the star system where a very famous battle took place…Locutus, anyone? 😉 Yup, that was a red dwarf star.

So, as we can see on the H-R diagram, red dwarf stars are just the lower end of the main sequence of stellar evolution. What makes them different from other stars?

The answer is their interiors—and the answer is the same for every other type of star, too.

Here’s a diagram showing the differences between the interiors of stars of different masses.

Okay, I realize the red dwarf on the far right is a little too tiny to make out…let’s try that again.

The difference between a red dwarf star and basically any other star in existence is that there is no radiative zone. The entire interior of the star convects.

So…what does that mean for its life cycle?

Well, these stars are the coolest stars capable of igniting hydrogen fusion once they form out of their giant molecular cloud. As we’ve explored in previous posts, a star’s next task is to balance the weight of its own mass with the internal pressure of energy production by hydrogen fusion.

Stars along the main sequence—including red dwarfs—maintain what we call hydrostatic equilibrium—essentially their version of homeostasis—for as long as they remain on the main sequence.

The pressure-temperature thermostat works to balance the outward force of radiation pressure from energy production with the inward force of gravity. Essentially, if energy production in the star’s core goes into overdrive, the ensuing radiation pressure forces the star’s layers to expand…which results in them cooling off, contracting, and going back to normal.

Red dwarf stars have a particularly good lot in life. They’re very low-mass, meaning they don’t have a whole lot of weight to support. And they’re not limited to just the hydrogen fuel in their cores because their entire interior is convective. The stuff on the outside that’s too cool to fuse eventually gets mixed with the stuff in the core.

Which means that red dwarf stars can burn away for…well, ages. In fact, stellar models predict they can “live” as long as 4 trillion years.

Guys…that’s longer than the current age of the universe.

That means that astronomers can’t know for certain what happens when red dwarfs die. Stellar models predict that they will eventually contract into white dwarfs, but according to our models, there shouldn’t actually be any observational evidence of that to see yet.

Moving on…what happens to a more familiar group of stars, the medium-mass stars? That is, stars like the sun?

Stars like our own sun have a convective zone, but it lies close to the surface, and a radiative zone lies between it and the core. So even though these stars have plenty of mass for fuel, they can’t use all of it because it isn’t mixed, and they can only fuse hydrogen in their cores.

For the majority of a medium-mass star’s life cycle, the pressure-temperature thermostat works to balance its interior, just like it works in a red dwarf. But there is limited hydrogen fuel in the star’s core, and eventually the core will begin to contract under its own weight.

The cores of low-mass stars can get hot enough to ignite helium fusion, the next stage in stellar evolution. But after that, it’s a dead end. The core will continue to contract under the star’s weight, but it will never get hot enough to ignite carbon fusion. So what happens next?

You may have heard of planetary nebulae and white dwarfs before. But there’s something else that happens before that…

Meet the superwind.

These winds are similar to the solar wind that the sun currently experiences, but much, much stronger. The solar wind doesn’t carry much of the sun’s mass away. While it is a tiny bit less massive than it was when it formed, that wind isn’t making much of a difference.

Giant stars, however, are different. (And by giant stars, I mean post-main-sequence giants, not massive stars. Giant stars are post-main-sequence medium-mass stars.)

These giant stars lose a ton of mass to their superwind. In fact, they can shed an entire solar mass—that is, the mass of the sun—in only 100,000 years. People…our sun will one day evolve into a giant star. And when it does, it can literally lose its entire mass over the course of 100,000 years.

That’s a pretty short time in terms of a billion-plus-year life cycle.

But why do giant stars lose mass like that?

Well, it really all comes down to their size.

You might remember from when I talked about the inverse square law ages ago that gravity gets weaker the farther you get from its source. Essentially, what this image shows is that the same force has to be spread over a greater surface area each time you increase the distance from the force.

You can think of this diagram as a representation of the inside of a star, with gravity strongest at the center and weakest at d=3. At the surface (d=3), the same strength of gravity is spread over nine times the surface area of the star.

(If the surface were at d=4, it would be spread over sixteen times the surface area. The surface area that the force is spread over is just the square of the distance.)

Let’s assume that this diagram depicts a very small star, and one unit of distance is equal to one solar radius (R, the unit of measurement for stellar radii). This star isn’t going to lose a whole lot of mass to the superwind.

But imagine if the star had a radius of, say, 300 R. Now the star’s gravity is spread over 90,000 square units of the star’s surface instead of one. Gravity is going to be a lot weaker there…and it’s going to be easier for mass to drift away.

Add to that the radiation pressure from the interior, which still burns hot and is unable to stabilize without a working pressure-temperature thermostat, and you’ve got a recipe for a superwind.

The superwind complicates the matter of stellar evolution. Astronomers would love to be able to say that stars in a certain mass range evolve in one way and those in another mass range evolve in another way, but they can’t. By the time they’ve evolved through their giant or supergiant phase, they might have lost enough mass to behave like a different class of stars.

For that reason, when modeling stellar evolution, astronomers have to take into account the beginning and ending mass of the star.

Well, now we’ve covered how stars are born, how they evolve, and how they reach their end…just about everything except what actually happens to them. And that, I will begin to cover in my next post!

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