2-4 The Life and Death of Stars

The Hertzsprung-Russell Diagram

As we have seen already, the Sun generates energy by using nuclear fusion at its core. It squeezes hydrogen into helium, losing a little mass but that gets converted into energy. The energy it gives off every second is called its luminosity.

We have also seen that the temperature of the visible surface of a star is related to its temperature. Cooler stars are reddish-coloured, hotter stars are bluish-white.

In the early 1910s, two astronomers -- Ejnar Hertzsprung in Denmark and Henry Russell in the United States -- noticed that if you drew a graph with luminosity on the vertical axis, and star colour on the horizontal axis, something very interesting happens when you plot individual stars as dots on this graph. Here, we'll show the Sun all alone on this graph.

However, astronomers soon made the link between star colour and surface temperature, so the horizontal axis turned into temperature. But, inconveniently, blue is on the left which means that the temperature axis, left-to-right, goes hot to cool, not what you'd expect.

If you graph a lot of stars as individual dots, you see some clear patterns:

We call this the Hertzsprung-Russell (H-R) Diagram, and there are some clear trends.

There are three main places where stars generally are:

1. A long band that goes diagonally from top-left to bottom-right, called the main sequence

2. A patch in the top-right, called the red giants

3. A patch in the bottom-left, called the white dwarfs

More generally, we can block-out areas of the H-R Diagram to see what stars falling in each of the regions are like:

This means we can see what each of these general groups of stars are like:

• the Main Sequence has stars ranging from hot, bright, blue stars, through the Sun (a fairly average star), and down to cool, dim red stars

• red giants have a cool surface but are very bright

• white dwarfs have a hot surface but are very dim

Individual stars are labeled on the diagram above, and a few are worth noting:

• In the far bottom-right corner is Proxima Centauri, which is the next-closest star to Earth (after the Sun); since it's so cool and dim, you can't even see it from Earth without a telescope

• Sirius is the second-brightest star in our sky (after the Sun), and it's nicely in the Main Sequence

• Rigel and Betelgeuse are both found in the constellation Orion, and while they both have a very high luminosity, one is hot and one is cool

How a Star Dies

The H-R Diagram is a great tool to help us see where stars are, in terms of brightness and temperature. It can also help us see how stars change as they run out of hydrogen fuel at their core.

The Very Smallest Stars

Extremely small stars like Proxima Centauri continually stir themselves inside-out using convection, so all the hydrogen on the outside gets mixed into the core.

This means they will last a very long time -- possibly a trillion years -- and gradually cool and fade to black.

Sun-Sized Stars

If a star is about the size of the Sun, it will have a convective outside and a conductive (sometimes called radiative) core:

All the hydrogen on the outside can't get mixed into the core. So, whatever hydrogen was in the core when the star formed, that's all it can ever use for fusion. This means the hydrogen will run out much faster than it would for very small stars.. and that's a problem.

As the hydrogen in the very centre stops fusing, gravity can squeeze it inwards and compress it more, which makes it hotter, and it can start fusing helium together to make even heavier elements. This process gets repeated until, for a star like the Sun, we end up with carbon and oxygen in the core (this is nowhere near the proper scale but it gives you an idea about the layers):

When each new element starts fusing, there's a burst of energy outwards that blasts the upper layers further out. This has two effects that we can track on the H-R Diagram:

• The star's visible surface gets cooler as it expands, making it redder, so the star moves a bit to the right on the diagram.

• Since the star grows in size, it becomes much brighter, so the star moves up on the diagram.

Now the star is a red giant, which is what Betelgeuse (in Orion) is. But it can't last forever: the outer layers eventually drift away, all fusion stops, and the old core is left behind as a white dwarf star. It's small in size so its luminosity is low, but it's still hot because it will take a while for it to entirely cool down.

So, that's our Sun's eventual fate: swelling up to be a red giant, then gradually fading away as a white dwarf. It will be quite a thing to see, but it's not the most exciting thing ever. Since people tend to like it when things blow up dramatically...

The Largest Stars

The very biggest stars (greater than 8 times the Sun's mass) undergo the same processes, but eventually get up to iron in their fusion process. Some people say this looks like the layers of an onion:

The more-massive the star was initially, the faster this process happens. Stars like the Sun spend about 8-10 billion years on the Main Sequence, fusing hydrogen into helium. But the very biggest stars might only spend one million years there: that might sound like a long time, but in terms of stars that's the blink of an eye!

Since this all happens so quickly, the outer layers don't get much of a chance to expand outwards -- then again, the star is already physically large, so it's very luminous already. When all fusion finally shuts off, the star's outer layers race inwards and bounce off the dense core, shooting outwards in a bright explosion called a supernova.

In the year 1054, astronomers in different parts of the world noticed a bright new star in the sky, which later became a fuzzy patch of light we now call the Crab Nebula. The most accurate observations of this event appear to be from China, although it was also noted by observers in Japan and the Middle East, and possibly Indigenous North American cultures.

In reality, they were watching a supernova explosion, and the material left behind was that new nebula. Here's a simulation of what it might have looked like (forgive the dramatic music):

If this had happened within 50 light-years of us, this would have ended all life on Earth. Thankfully it was in a different neighbourhood of our galaxy, about 6500 LY away -- close enough to see, but far enough away for us not to be obliterated by the huge burst of gamma rays sent outwards by it.

But, what is left behind? It depends on the initial mass of the star.

If the star is between 10 and 25 solar masses, what is left behind is a neutron star. This is a bizarre object: the outer layers smashing into the core squeeze together all the protons and electrons into a giant, hot ball of neutrons about the size of Toronto.

The density of this object is so high, it's hard to comprehend. You can find various ways of describing it, but one popular one is that a tablespoon full (about 15 mL) of this material would be more massive than Mount Everest. The gravitational pull at its surface would be about 200 billion times that of Earth, which would easily flatten you into a pancake. We believe neutron stars also occasionally eject material out of their north and south poles, although we're not quite sure why yet.

If the star is above 25 times the Sun's mass, that's when the real strangeness sets in. The core would get compressed so much, and gravity at it surface would be so strong, that not even light could get out of it. (Light rays are affected by gravity as well, as Einstein showed.) This object is called a black hole, and it doesn't emit any light at all. Once you get too close to a black hole, you can never get out of it.

This might not look like much, but it's the first direct picture we've ever taken of a black hole, and it was put together from a bunch of individual observations in 2019. Here's an artist's conception of what it might look like if an Earth-like planet was orbiting a black hole; it's also shown with an accretion disk of gas and dust that's spinning down into the black hole.

Here's a little more information about black holes:

Practice

The Basics

1. Briefly describe what happens at the end of a star's life when it's (a.) very small, (b.) about the mass of the Sun, (c.) 10-25 times the Sun's mass, and (d.) bigger than 25 solar masses.

2. In what part of the Hertzsprung-Russell Diagram would a star be if it was (a.) cool and bright, (b.) hot and dim, and (c.) hot and bright?

3. What is the Main Sequence?

Extensions

1. Cygnus X-1 was the first-ever black hole candidate that was ever identified. Do a little research to see how the University of Toronto helped in its discovery.

2. Betelgeuse is currently a red giant, but its brightness has been changing in recent years. Based on what you know about how stars change at the end of their lives, can you work out why we're seeing these changes?