Astronomy: Stars & Black Holes (#84)

In the last episode, I went back to space and talked about galaxies and the Milky Way. I talked a little bit about stars because galaxies are made of them! But I said I was going to do a separate episode going more in-depth about stars themselves. So that’s what I’m going to be doing in this episode: I’ll talk about the different types of stars, black holes, and also the most important star in our solar system: the Sun!

If you haven’t listened to my last episode about galaxies, then I’d definitely recommend it. You don’t have to listen to that one first though - I might reference things from it this time, but this one works as its own standalone episode!

Just like last time, I need to thank Charlie McDonnell’s book Fun Science for providing most of the information for this episode. As always, all of the resources for this episode will be in the notes on our website, learnaboutpod.com. Let’s go!

So, what are stars? Stars are huge, luminous, incredibly hot balls of plasma floating in space. Scientists estimate that there are around 70 billion trillion stars in the observable universe. So let’s go through some of the different types.

Early Stars

Protostars

Before stars, we have protostars. A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy released by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

T Tauri stars

Prostars then become T Tauri stars. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion. Stars will remain in this stage for about 100 million years.

Main Sequence stars

The first type of mature stars is main sequence stars. These make up the majority of stars in the universe, including the Sun as well as our nearest neighbours, Sirius and Alpha Centauri A. These types of stars can vary wildly in size, mass and brightness, but they’re all basically doing the same thing: converting hydrogen into helium in their cores, and releasing a huge amount of energy doing it. These stars can grow to more than 100 times the mass of the Sun. There are a few different types of main sequence stars...

Red Dwarf stars 

First up is red dwarf stars, which I mentioned in the last episode. This is the most common type of star in the universe - they make up about three quarters of the stars in the Milky Way. Because they’re smaller, red dwarfs are much cooler and dimmer - approximately 3,200 degrees C compared to the Sun’s 5,400 degrees.

They also give off very little light, which makes them really hard for us to see in the night sky. They are, however, the longest living stars because they burn dimly, meaning they use up less energy and they can have an estimated lifespan of trillions of years.

Yellow Dwarf stars

Next is yellow dwarf stars, which is the category the Sun falls into. Sometimes these are referred to as ‘medium stars’ and they have a typical lifespan of 10 billion years. The sun is currently around 4.5 billion years into its lifespan. If you’re interested in knowing what will happen when the sun reaches the end of its lifespan… carry on listening because I’ll come back to this later.

Orange Dwarf stars

In the last episode, I said red dwarf stars might be more important than we think. I realised when writing this episode that I actually meant orange dwarf stars, which are the final type of main sequence stars. And here’s why. Orange dwarf stars are very interesting to scientists as they’re the most useful in our quest to find life on other planets.

These stars stay stable for around 15 to 30 billion years compared to the sun’s 10 billion, but they’re also brighter and warmer than red dwarfs. Because of their much longer lifespan combined with their increased brightness and warmth, it seems likely that they’d leave a large window in which life on planets orbiting them could flourish.

Not only that, but they also emit less damaging radiation than stars like our sun, which would give DNA a better chance of emerging. (This makes life on earth even more impressive, and makes me more convinced that there must be life elsewhere. If we do one day find alien life, it seems most likely that they’d be orbiting an orange dwarf star.)

Giant and Supergiant Stars

We now move onto the next type: giant and supergiant stars, more specifically, red giant and red supergiant stars. Red giant stars are essentially what our sun will become after a further 5 or so billion years of its life. As a yellow dwarf star dies, it begins to grow, sometimes reaching more than 400 times its original size, and becomes a red giant star.

For red supergiant stars, you have to find a star about 10 times as big as our sun, and wait for it to reach the end of its life cycle. Only then you’ll end up with a supergiant star. These can be thousands of times bigger than our sun, making them the largest known stars in the universe. They don’t tend to stick around for very long, generally burning out in anything from a few thousand to possibly a few million years.

Fading Stars

Brown Dwarf stars

Finally, we have fading stars, and the first we have another colour of dwarf star - brown dwarf stars. These are the failures or the misfits - the stars that never were. Size-wise, they sit somewhere between planets and red dwarfs - too big to be called a planet but not big enough to generate the pressure and heat needed to be categorised as a star either.

White Dwarf stars

On the other end of the spectrum, white dwarf stars represent the final stage in the life cycle of a star, when it’s been reduced down to nothing but its cooling, shrunken core. Eventually white dwarfs will turn into black dwarfs, which are completely cold, dead stars.

Neutron stars

Finally, we have neutron stars. When a massive star collapses, usually after a supernova (which is the explosion of a star), this is typically what you end up with. These are the smallest and densest stars known to exist in the universe, even with a radius of around 11km (7 miles), they can still have a mass of about 2 times that of the sun.

Sidenote: What causes a supernova?

As I said, a supernova is the explosion of a star. It is the largest explosion that takes place in space. Supernovas are often seen in other galaxies, but they’re difficult to see in our own Milky Way galaxy because dust blocks our view.

A supernova happens where there is a change in the core of a star. A change can occur in two different ways, with both resulting in a supernova.

The first type of supernova happens in binary star systems. Binary stars are two stars that orbit the same point. One of the stars, a white dwarf, steals matter from its companion star. Eventually, the white dwarf gobbles up (yep, I’m returning the phrase “gobbles up”) too much matter. Having too much matter causes the star to explode, resulting in a supernova.

The second type of supernova occurs at the end of a single star’s lifetime. As the star runs out of nuclear fuel, some of its mass flows into its core. Eventually, the core is so heavy that it can’t withstand its own gravitational force. The core collapses, which results in a giant supernova.

You do not have to be a scientist, or even have a telescope, to hunt for supernovas. For example, in 2008 a teenager discovered a supernova. Then in January 2011, a 10-year-old girl from Canada discovered a supernova while looking at night sky images on her computer. The images, taken by an amateur astronomer, just happened to include a supernova. With some practice and the right equipment, you could find the next supernova!

Black Holes

Now that we've covered the different types of stars, we’re going to move onto one of the most awesome but terrifying things in the universe: black holes. Technically speaking, black holes are not stars. If you take a large enough star at the end of its life (with a core 3 times the size of the sun), when it collapses in on itself, it forms a black hole.

Black holes are infinitely dense, and their gravitational pull is so powerful that absolutely nothing can escape them. That’s why they’re black - not even light can get away from them. Not even time and space can exist inside one of these things. It’s another one of those things that I just can’t compute in my brain.

The only way it’s possible to observe one is to look around them - because of their powerful gravitational fields, any nearby material is caught up and dragged in (or gobbled up, to go back to our favourite technical term). As soon as any matter reaches what’s known as the black hole’s ‘event horizon’, there’s absolutely no turning back.

A crazy thing about black holes is that they’re so powerful, they literally distort the fabric of the universe around them. If you find a spinning black hole, space itself will be spinning around it. This disc of rotating space around the black hole is known as the ergosphere. If you find yourself in this area of space, it’s impossible to stay still - as space is being dragged around, it’ll carry you along with it.

You don’t even need to be near the event horizon or the ergosphere for it to be deadly. Any matter that’s captured by a rotating black hole rarely just falls directly into it, but travels around the black hole before being consumed. As a result, black holes can have a lot of stuff whirring around them, with the matter nearer the black hole moving faster than the matter on the edges. This results in everything rapidly rubbing together, which generates heat. This stuff gets so hot that it actually starts glowing.

Fun fact: Stars aren’t the only things that can become black holes - any amount of matter could, as long as it was shrunk down to a small enough size. If you shrunk, say, a person and they were able to retain their mass, they’d just be an incredibly dense point, and their gravitational pull would be so strong that they’d start dragging in everything around them!

What will happen at the end of the Sun’s lifespan?

So earlier, I mentioned that I’ll talk about what happens when the sun dies. When the sun goes, the Earth goes with it. But our planet won't go quietly. Rather, when the sun expands into a red giant, it will vaporise the Earth.

If this makes you anxious, just know that we don’t have to worry about this any time soon. The time scale is massive — several billion years from now. To put this into perspective, humans have been around only about 40-thousandth that amount of time; if the age of the Earth were compressed into a 24-hour day, humans would occupy only the last second, at most.

So what happens when the sun goes out? 

The sun was born 4.57 billion years ago, and to burn as bright as it does, it must consume 600 million tonnes of hydrogen, which it converts into helium — a process known as nuclear fusion — every second. However, since there is only a finite amount of hydrogen within the sun, one day it will run out.

Over the next 4 billion years or so, the sun will continue to consume hydrogen while building up helium. As the helium continues to accumulate, the sun’s core will shrink. As a result, nuclear fusion reactions will accelerate, and faster fusion reactions means there is more energy being produced.

So, in roughly 3.5 billion years, the sun will actually shine 40 percent brighter than it does today, which will result in the melting of the poles, the boiling of the oceans, and a complete loss of Earth’s atmosphere. There will likely be no life left on Earth since the planet will become very hot and dry — just like Venus.

Then, about 5.4 billion years from now, the sun will have exhausted all of its hydrogen. The sun’s core will get really hot and dense, and it’ll shrink; however, the outer region of the sun will expand and grow. It could expand as far out as Mercury, Venus, and maybe even Earth — vaporising all three planets. Even if the expanding dying sun doesn’t reach Earth, the sun’s high temperatures will completely burn the planet.

When the sun becomes empty, it will become unstable and begin to pulse. Each pulse will remove more and more of the sun’s mass until all that is left is the cooler core. At this point, the sun will be a white dwarf. It will spend the rest of its life like this, slowly cooling and dimming, until nothing is left.

If you haven’t already guessed by this point, Earth, if it is still around, will be a completely inhospitable planet.

So with that, that’s all I’ve got for this episode… I’m sorry to end on a depressing note! But it’ll be literally billions of years from now, so you don’t need to worry about that. (We just have the climate to worry about first, but I’m not going to get into that.)

If you want more space episodes, listen to my last one, check the index on the site… etc. I’m also currently planning a few bonus episodes for Patreon, including one about alien life.