Welcome back!

I hope all of you are safe and doing well.

There is some momentous space news! On the 30th of May, 2020, astronauts lifted off from the United States for the first time in nine years. More significantly, this is the first time that they were taken to the International Space Station not in a NASA rocket, but in the Falcon 9 rocket built by Elon Musk helmed private company SpaceX. 

Watching the launch shows how far we have come since the launches of the 20th Century, or even the launches in the early 2000’s. Not only are the spacesuits more slim, the crew capsule is streamlined, equipped with touchscreens. The exhaust of the rocket is considerably lower than that of the past. And, perhaps most incredibly, the Stage 1 Booster is now reusable, and even landed back on Earth on a drone ship.

 It truly feels as though we are living in the future. With this launch, it feels as though we are looking at the future of space travel, a revolution in the manners with which we attempted to touch the heavens in the past. We have transcended to a new level, and, truly, the sky feels like it cannot even attempt to be a limit.

There will be time to discuss rocketry, as well as human space travel. That time is not today, however. Today, we must continue our discussion about stars, those incredible balls of ionized gas, those tremendous nuclear reactions scattered throughout space like glitter on tarmac. More specifically, we will be discussing the life cycle of stars, and what happens to them when they die.

So sit back, relax, and enjoy our continued sojourn in the wonderful world of stars!

Last time, we discussed what stars were and how they formed. We discussed the stellar nurseries that gives birth to new stars. We discussed the early stages of a star’s life. But now, we must discuss the later stages. What happens after a star is formed?

Well, it joins what astronomers call the Main Sequence!

The main sequence is the period in the star’s life in which the star has reached a larger size than when it was a protostar, and is now readily converting hydrogen to helium in the process that we discussed last time.

Incidentally, our Sun is in the main sequence. It is currently roughly 75% hydrogen and 24% helium, with the other 1% consisting of trace gases. This is fairly typical for most main sequence stars and is to be expected with a star of that size.

Of course, the main sequence does not exist forever. Our Sun will remain in the main sequence for about 5 billion years. As we will discuss later, the size of a star greatly affects how long it lives.

 So the largest stars, such as blue giants, will only be in the main sequence for about 20 million years. While that may seem a lot, when viewed from an astronomical scale, it’s very short. Comparatively, a very small star, such as a red dwarf, will be in the main sequence for about 100 billion years

Let’s put this ratio into easier terms. A small, red dwarf star will live 5,000 times longer than a blue giant star. Now, let’s add an analogy. Imagine you are frying an egg. The white will set in about 10 seconds on a hot (and I mean HOT) griddle, leaving the yolk nice and runny to dip your bread into. This is just like the lifetime of a blue giant star. By comparison, imagine you’re cooking a turkey overnight for thanksgiving. It’ll be in the oven with relatively low heat for around 13 hours. That’s like the lifetime of a red dwarf star. 10 seconds vs. 13 hours. Quite a big gap when phrased that way!

But what happens after the main sequence? The main sequence will only end when all the hydrogen in the star has been used up. What happens next?

The next stage is called the “Red Giant” stage. When the hydrogen is used up, the outer layers expand, and the stars swell to immense sizes. Here is an interesting diagram showing the evolution of a Sun-like star, with sizes to scale:

Credit: NASA

As you can see, red giants are simply immense. And with this enormous size brings problems. The sun’s radius will expand by 100 to 1,000 times its original size. So – and I realize that I am about to drop a bomb here – life will cease to exist on Earth. Whether the increased size of the Sun will consume the Earth, or stop just shy of doing so, is still within contention. However, it is clear that the Sun coming this close to the Earth will raise temperatures to such a degree that the oceans will boil off and plants will wither and die.

Ok. I said it. However, luckily for us, there is a date to this doomsday: roughly 5 billion years in the future. In numerical form: 5,000,000,000 years. That equates to around 66 million lifetimes. So you, your children, grandchildren, great-grandchildren, great-great-grandchildren, great-great-great-grandchildren, great-great-great-great-grandchildren…and so on, will all be safe. Don’t worry about it for now.

Incidentally, red giants are among the most common stars in our stellar neighborhood. The upper left shoulder of the constellation Orion is one of the most famous red giant stars: Betelgeuse. This star is absolutely immense: if it replaced the Sun, it’s surface would reach Saturn. Here is an image for size comparison:

How Does Our Sun Compare With Other Stars? | NASA Space Place ...

Credit: NASA

And stars this large end their lives in a spectacular way: a supernova.

“But wait!” you say. “Why does a star supernova?”

As I discussed earlier, a star leaves the main sequence when its supply of hydrogen runs out, and it is all fused into helium. This fusion does not stop here, though. Red giant stars are also continually fusing elements, becoming heavier and heavier. They fuse helium into lithium, lithium into beryllium, beryllium into boron, boron into carbon, and so on, until they reach iron. Iron, incidentally, is the heaviest element that a star can fuse. After this critical stage is reached, the star simply cannot fuse any farther, and the core collapses. The outer layers, now unrestrained by the once-substantial gravity of the core, spread outwards in a tremendous explosion that we call a Type 2 supernova (more about supernovas in a later post).

Before you get excited, however, I must reinforce that this cinematic method of departure is only limited to the largest of stars. When the Sun becomes a red giant, it’s size will reach the orbit of the Earth (compared to Betelgeuse reaching the orbit of Saturn). As a result, when the Sun’s core collapses, the outer layers will not have been constrained by so much gravity, and will instead just coagulate around the Sun in what we call a planetary nebula.

But what happens after this?

In stars as massive as Betelgeuse, there are two paths. In one path, the star becomes a neutron star, an incredibly dense core remnant. In the other, it becomes a black hole (you can read my post on black holes to learn more).

In stars as small as our Sun, the collapsed core burns off its layers, and what is left is a white dwarf, a slow burning cool star remnant that will last for hundreds of billions of years until it is finally extinguished into what astronomers call a black dwarf.

Here is a helpful diagram that breaks down everything that I discussed above:

Stellar evolution

Credit: ESA

And that, my friends, is the life cycle of stars! I hope you enjoyed reading and learned something new. As always, keep your eyes peeled for new posts, and make sure to comment any questions you may have.

Clear skies!