Everything You Ever Wanted to Know About Supernovas

Podcast Transcript

One of the brightest things in the universe is caused by one of the most powerful explosions in the universe: a supernova.

While they don’t happen very often, they were known to ancient peoples and their appearance would often be recorded because they were such a rare and special event.

Today, astrophysicists have a much better understanding of what supernovas are and how they can help us better understand the rest of the universe.

Learn more about supernovas on this episode of Everything Everywhere Daily.

Supernovas, when they occur in our galaxy, are so bright that they can be seen in the sky, even in the middle of the day. They don’t happen that often, however. Centuries or millennia can pass between a supernova which can be visible with the naked eye on Earth.

Astronomers have detected the remnants of a supernova in the constellation Vela which probably exploded between 10,000 to 20,000 years ago. It would have been strong enough to be seen by humans, but there are no records of it.

There may have been records of a supernova that occurred about 6,500 years ago in ancient Indian texts.

The first confirmed record of a supernova occurred in the year 185. Astronomers noted the bright star which suddenly appeared out of nowhere. The light lasted for about eight months before disappearing. They noted that, unlike comets, this light didn’t move in the sky.

This was recorded by Chinese astronomers, but there is reason to believe that the Romans might have recorded it as well, but the text has since been lost.

The term nova is a Latin term which simply means new.

It is possible that Chinese astronomers observed 20 different supernovas over a period of 2,000 years. The problem is, it is hard to know if ancient sources actually saw a supernova, or if they just saw a comet.

In 1006, what was probably the brightest supernova ever was recorded all over the world. Chinese, Arab, and European sources all made note of the event. Arab astronomer Ali ibn Ridwan said it was a quarter of the brightness of the full moon.

In 1054, the supernova which created the Crab Nebula occurred.

The last supernova in our galaxy which humans could observe without telescopes took place in 1604.

With the advent of telescopes, however, astronomers began to see things they couldn’t explain. Some stars would suddenly become brighter for no reason.

In 1885, a supernova was detected in the Andromeda Galaxy which, for a brief period of time, was brighter than the entire galaxy.

In the 20th century, astrophysicists began to figure out exactly what this phenomenon exactly was.

In 1931, Walter Baade and Fritz Zwicky postulated that supernovas were actually giant explosions, which explained their sudden appearance and disappearance.

In 1938, Baade realized that nebulas, large gas clouds, were the results of supernovas and that the Crab Nebula was the leftovers from the 1054 supernova.

In 1941, the categorization of supernovas was developed when they realized there were different types based on their spectral signature.

So what is happening with supernovas? What causes such an explosion?

This will require a bit of explaining and there are several different concepts involved.

I’ll actually start with a type II supernova. (and just for the record, I’m aware that there are subdivisions under both types I and II supernovas, but I’m just trying to simplifying the concepts here)

A type II supernova occurs when a single large star collapses.

When a star is formed is made out of hydrogen gas. The gas coalesces due to gravity until the pressure becomes so great that the hydrogen atoms actually fuse into helium atoms. The heat from the fusion will cause expansion of the gases, counterbalancing the inward force of gravity.

This will continue for many millions of years until the hydrogen is exhausted. Then the remaining helium will begin to fuse into carbon.

As the lighter elements are exhausted, this process will continue with heavier and heavier elements, but only to a point.

Once a star starts producing iron, it will no longer give off energy by fusing with other atoms.

At this point, the fusion stops as it would require energy to fuse the atoms. Without any fusion taking place, there is no longer any heat that will counteract the gravity pulling everything together.

Once this happens, the star will collapse in a fraction of a second, causing a massive explosion.

What results depends on the size and mass of the star. If it is over 25 times the size of our sun, it might end up as a black hole. If it is about 10 to 25 times greater, it will end up as a neutron star.

The other type of supernova is a Type I supernova. It is the result of a binary star system made up of two stars.

These stars are closer in size to our sun. One of the stars in this system becomes a white dwarf. This is the exhausted core of a star that wasn’t large enough to get to the point where it created iron. A white dwarf is a star so dense, that it is only prevented from collapsing further due to something called electron degeneracy pressure. I’m not going to dive too deep into the concept of electron degeneracy pressure, but basically, the matter is compressed so much, that only quantum effects in the individual atoms are stopping it from collapsing even further.

The extremely dense star then begins stealing matter from its partner star, which would usually, but doesn’t have to be, a red giant.

The white dwarf will begin adding mass and growing. However, there is a hard limit to how much mass a white dwarf star can have. It is exactly 1.44 solar masses.

This is known as the Chandrasekhar limit and it was proposed by the great Indian-American physicist and Nobel prize winner Subrahmanyan Chandrasekhar.

Beyond the Chandrasekhar limit, electron degeneracy pressure can no longer withstand the pull of gravity, and the star will collapse and explode.

So, why are these massive solar explosions so important?

There are two reasons why you should care about supernovas.

The first has to do with the type I supernovas in particular.

One of the major problems in astronomy is measuring distances. Let’s say you are on a dark road and you see a light in the distance. How far away is the light?

The truth is there is no way to know if you don’t know the source of the light. It could be someone with a flashlight not that far away, or it could be a massive searchlight miles away.

Type I supernovas are important because when they explode, they all do so with exactly the same force. Because they only explode once they pass the Chandrasekhar limit, every type I supernova explodes with the exact same mass, and thus have the exact same luminosity.

In astrophysics, this is known as a standard candle. Because these types of supernovas are all the same, by measuring the relative luminosity, astronomers can calculate the distance.

So, type I supernovas are really handy. I’ll go into how the age and size of the universe are measured in a future episode.

The other reason why supernovas are so important is that that is how all of the heavy elements are made.

As I mentioned before, a star can’t create any element heavier than iron via fusion. So how are heavier elements like lead, gold, and uranium created? Where does the energy come from to make these elements?

It comes from one of the only forces in the universe which has enough energy to fuse these heavy elements together: a supernova.

That means that all of the heavy elements on Earth, and all of the elements in your body heavier than Iron, like copper and zinc, at one point came from a supernova. A supernova that took place probably billions of years before our solar system was even formed.

So supernovas not only help us understand the rest of the universe, but they also, in a very literal sense, make up ourselves and our world.

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The Associate producer of Everything Everywhere Daily is Thor Thomson.

Today’s review comes from listener Bob DelGIorno over on Audible.com. He writes:

My new riding companion

Gary is my new riding buddy, as I drive around town with my 11 yr old daughter and 5-month-old son…as a homeschooling dad, this counts as part of our history curriculum! Keep up the fantastic job

Thanks, Bob. While I never planned it, I have had several people tell me how they listen to the show with their kids. Just to let everyone know, I’ll always keep the language clean, and the subject matter will be as family-friendly as history will allow.

Remember, if you leave a review, I might read your review on the show.