High Mass Stars: Crash Course Astronomy #31

Summary

This script explores the life and death of massive stars, contrasting them with our Sun. It delves into the fusion processes that create heavier elements up to iron, which paradoxically triggers a catastrophic core collapse. The ensuing supernova explosion not only marks the star's end but also seeds the universe with the heavy elements necessary for life, highlighting the cosmic cycle of creation and destruction.

Takeaways

• 🌟 Stars are in a constant struggle between gravity pulling them in and internal heat trying to expand them.
• 🌕 For stars like our Sun, the balance between these forces changes in their later years, leading to an expansion and then a shedding of outer layers.
• 🔥 In stars, fusion of atomic nuclei releases energy and creates heavier elements, with each step requiring higher temperatures and pressures.
• 🌌 Lower-mass stars like the Sun stop fusing elements at carbon due to insufficient core temperatures.
• 💥 High-mass stars can reach temperatures over 500 million degrees Celsius, allowing for the fusion of carbon into neon, magnesium, and sodium.
• 🌀 The life cycle of a massive star involves alternating between red and blue supergiants as the core switches between different fusion reactions.
• 🌐 The outer appearance of a massive star changes dramatically, becoming extremely luminous and visible over vast distances, like Betelgeuse and VY Canis Majoris.
• ⏳ Massive stars have shorter lifespans due to their hotter cores and faster fusion rates, using up their fuel much quicker than lower-mass stars.
• 💣 The fusion of silicon into iron is particularly problematic as it consumes energy rather than producing it, leading to a core collapse.
• 🌌 The core collapse of a massive star can result in the formation of either a neutron star or a black hole, depending on the star's mass.
• 💥 The core collapse triggers a supernova, one of the most violent events in the universe, where the star explodes and releases a tremendous amount of energy and heavy elements into space.
• 🌌 Supernovae play a critical role in the creation and distribution of heavy elements throughout the universe, contributing to the formation of future stars and planets, including the elements found in living organisms.

Q & A

What is the primary struggle that stars experience throughout their life?

Stars are in a constant struggle between gravity trying to collapse them and their internal heat trying to inflate them. This struggle is typically at an uneasy truce for most of a star's life.

How does a star like the Sun end its life?

A star like the Sun ends its life by expanding briefly and then blowing away its outer layers, leaving behind the gravitationally compressed core. It ends with a 'whimper,' which is a dramatic but less explosive event compared to more massive stars.

What is the process that occurs in the core of a star where atomic nuclei can fuse?

In the core of a star, under high pressure and temperature, atomic nuclei can get squeezed together and fuse, releasing energy and creating heavier elements. This process is known as nuclear fusion.

Why do lower-mass stars like the Sun stop at carbon in their fusion process?

Lower-mass stars like the Sun stop at carbon because once carbon builds up in the core, the star's fate is sealed and it can no longer sustain the temperatures and pressures required to fuse heavier elements.

What happens when a star has more than about 8 times the Sun’s mass and reaches temperatures in excess of 500 million degrees Celsius?

When a star has more than about 8 times the Sun’s mass and reaches such high temperatures, carbon will fuse, creating neon, magnesium, and some sodium, continuing the process of creating heavier elements.

What is the significance of the fusion of silicon in a star's life?

The fusion of silicon is significant because it leads to the creation of iron. However, iron fusion is problematic as it consumes energy rather than releasing it, which accelerates the core's collapse and can lead to a supernova explosion.

What is a red supergiant and how does it differ from a regular red giant?

A red supergiant is a star that has swelled up significantly more than a regular red giant due to the immense energy generated after hydrogen fusion stops in its core. They are incredibly huge and luminous, much larger than a typical red giant.

Why do massive stars have shorter lifespans despite having more fuel?

Massive stars have shorter lifespans because their cores are much hotter and fuse elements at much higher rates, which causes them to run out of fuel more quickly compared to lower mass stars.

What happens to the core of a star during a supernova explosion?

During a supernova explosion, the core of the star collapses, either forming a neutron star if the star is less than about 20 times the Sun’s mass, or a black hole if the star is more massive.

How do supernovae contribute to the creation of heavy elements in the Universe?

Supernovae contribute to the creation of heavy elements through a process called explosive nucleosynthesis, where the extreme heat and compression during the explosion forge new elements, scattering them into space.

Are we on Earth at risk from a nearby supernova explosion?

No, we are not at risk from a nearby supernova explosion. Even though supernovae are incredibly violent, space is vast, and a supernova would have to be at least as close as 100 light years from us before we start feeling any real effects.

Outlines

00:00

🌟 Stellar Struggle and Evolution

This paragraph discusses the life cycle of stars, focusing on the balance between gravitational collapse and internal heat. It explains how stars like the Sun end their lives quietly, while more massive stars explode in supernovae. The process of nuclear fusion within stars is detailed, describing how heavier elements are created through fusion at increasing temperatures and pressures. The paragraph also explains the limitations of lower-mass stars like the Sun, which stop fusing elements at carbon, and the progression of fusion in more massive stars, leading to the creation of iron and the subsequent catastrophic collapse.

05:03

💥 The Iron Catastrophe and Supernova Birth

This section delves into the critical role of iron in the life-ending process of massive stars. It describes how iron fusion consumes rather than releases energy, leading to a core collapse. The collapse results in an outer layer crash at near light speed and the potential formation of either a neutron star or a black hole, depending on the star's mass. The paragraph explains the violent aftermath of the core's sudden halt, including the shock wave and neutrino burst that cause a supernova explosion. It highlights the supernova's role in scattering heavy elements throughout the universe, contributing to the creation of new stars and planets.

10:06

🌌 The Cosmic Significance of Supernovas

The final paragraph emphasizes the importance of supernovae in the universe's elemental composition. It describes how supernovae create and disperse heavy elements, such as calcium, phosphorus, and nickel, which are integral to the formation of planets and life itself. The paragraph also reassures that we are not in danger from nearby supernovae, and it reflects on the personal connection each of us has to the stars, as elements within us were forged in the heart of long-dead stars. The script concludes with credits and an invitation to explore more educational content from PBS Digital Studios.

Keywords

💡Gravity

Gravity is the force that attracts two bodies towards each other, proportional to their mass and the distance between them. In the context of the video, gravity is the force that constantly tries to collapse stars, counteracted by the internal heat that tries to inflate them. The script discusses how gravity plays a crucial role in the life cycle of stars, especially during their twilight years when it leads to the formation of a white dwarf, neutron star, or black hole.

💡Fusion

Fusion is the process where atomic nuclei combine to form a heavier nucleus, releasing energy in the process. The script explains how stars like the Sun undergo fusion, starting with hydrogen fusing into helium and progressing to heavier elements like carbon, neon, and silicon, depending on the star's mass. Fusion is central to the video's theme as it drives the life cycle of stars and the creation of heavier elements.

💡Supernova

A supernova is a powerful and luminous stellar explosion that occurs at the end of a massive star's life cycle. The script describes how the accumulation of iron in a star's core leads to a catastrophic collapse and explosion, scattering heavy elements throughout the universe. Supernovae are significant as they are responsible for the creation and distribution of many of the heavy elements necessary for life.

💡Neutron Star

A neutron star is the collapsed core remnant of a massive star that went supernova. The script mentions that if a star's core collapse stops when it's about 20 kilometers wide, it forms a neutron star. These objects are incredibly dense, composed almost entirely of neutrons, and are related to the video's theme as they represent one possible endpoint in the life cycle of a massive star.

💡Black Hole

A black hole is a region of spacetime with a gravitational pull so strong that nothing, not even light, can escape from it. The script explains that if a star is massive enough, its core collapse will continue until it forms a black hole. Black holes are relevant to the video's theme as they represent the ultimate fate of the most massive stars.

💡Red Supergiant

A red supergiant is a large, luminous star in a late phase of its life cycle, having exhausted its core hydrogen fuel. The script uses Betelgeuse as an example of a red supergiant, highlighting their immense size and luminosity. Red supergiants are important to the video's narrative as they illustrate the stages a massive star goes through before potentially becoming a supernova.

💡Blue Supergiant

A blue supergiant is a very luminous, hot, and massive star that is in a later stage of its life cycle. The script mentions Rigel as an example, noting that it emits over 100,000 times as much energy as the Sun. Blue supergiants are relevant to the video's theme as they represent another stage in the life of a massive star that could eventually explode as a supernova.

💡Neutrinos

Neutrinos are subatomic particles with very little mass and no electric charge, which rarely interact with other matter. The script explains that during a core collapse, a vast number of neutrinos are produced, carrying away a tremendous amount of energy. Neutrinos are crucial to the video's theme as they play a key role in the explosion of a supernova.

💡Nucleosynthesis

Nucleosynthesis is the process by which atomic nuclei are created from pre-existing nucleons. The script refers to 'explosive nucleosynthesis' during a supernova, where heavy elements are created in large quantities. This process is vital to the video's message, as it explains how the heavy elements necessary for the formation of planets and life are produced.

💡Iron

Iron is a chemical element and plays a critical role in the script as the element that, when fused in a star's core, consumes energy rather than producing it. This consumption of energy leads to the core's collapse, which in turn triggers a supernova. Iron is a key concept in the video as it marks the end of the fusion process in massive stars and the beginning of their end.

💡Crab Nebula

The Crab Nebula is a supernova remnant and pulsar wind nebula, the result of a supernova explosion that was observed on Earth in 1054 AD. The script uses the Crab Nebula as an example of the fantastic shapes that supernova remnants can form. It is relevant to the video's theme as it provides a concrete example of the aftermath of a supernova explosion.

Highlights

Stars are in a constant struggle between gravity and internal heat.

For stars like the Sun, the balance between forces tips in their twilight years, leading to a brief expansion before shedding outer layers.

More massive stars end their life with a supernova, a very big bang.

Fusion in a star's core creates heavier elements, with each step requiring higher temperatures and pressures.

Stars with more than 8 times the Sun’s mass can fuse carbon into neon, magnesium, and sodium.

As stars age, they transition from red giants to red supergiants, which are incredibly huge and luminous.

Betelgeuse and VY Canis Majoris are examples of red supergiants, with Betelgeuse being one of the brightest stars in the sky.

Blue supergiants, like Rigel, are extremely luminous and put out over 100,000 times as much energy as the Sun.

The core of a massive star resembles an onion with multiple layers of fusing elements.

Stars more massive than the Sun deplete their fuel much faster, with a 20 times solar mass star exhausting its fuel in a day.

Fusion of iron in a star's core is problematic as it consumes energy instead of producing it, leading to core collapse.

Core collapse results in an extremely rapid compression, potentially leading to the formation of a neutron star or a black hole.

The core's collapse triggers a shock wave and the creation of neutrinos, which carry away a vast amount of energy.

A supernova explosion is one of the most violent events in the Universe, outshining all the stars in a galaxy combined.

Supernova remnants form fantastic shapes, such as the Crab nebula, from a star explosion in 1054.

Supernovae are not a threat to us as they need to be at least 100 light years away to have real effects.

Supernovae are critical for our existence as they create and scatter the heavy elements necessary for life.

Elements like calcium, phosphorus, and iron in our bodies were created in supernovae.

Transcripts

Stars are in a constant struggle between gravity trying to collapse them and their internal

heat trying to inflate them. For most of a star’s life, these two forces are at an uneasy truce.

For a star like the Sun, the balance tips in its twilight years. For a brief glorious

moment it expands… but then blows away its outer layers, leaving behind the gravitationally

compressed core. It goes out with a whimper. Well, a whimper from a two octillion ton barely

constrained nuclear powered fireball.

But more massive stars aren’t quite as resigned to their fate. When they go out, they go out

with a bang -- a very, very big bang.

In the core of a star, pressure and temperature are high enough that atomic nuclei can get

squeezed together and fuse. This releases energy, and creates heavier elements. Hydrogen

fusion makes helium, helium fusion makes carbon, and each heavier element, in general, takes

higher temperatures and pressures to fuse.

Lower-mass stars like the Sun stop at carbon. Once that builds up in the core, the star’s fate is sealed.

But if the star has more than about 8 times the Sun’s mass, it can create temperatures

in its core in excess of 500 million degrees Celsius, and then carbon will fuse. There are actually a lot

of steps in this process, but in the end you get carbon fusing into neon, magnesium, and some sodium.

What happens next hearkens back to what we found goes on in the Sun’s core as it ages:

fuse an element, create a heavier one, then that heavier one builds up until the core

contracts and heats up enough to start fusing it. So carbon fusion makes neon,

magnesium, and sodium, and those accumulate. The core heats up, and when it reaches

about a billion degrees, neon will fuse. Neon fusion creates more magnesium, as well as some oxygen.

These build up in the core, it shrinks, heats up to about 1.5 billion degrees, and then

oxygen fuses, creating silicon. Then THAT builds up until the temperature hits about

2-3 billion degrees, whereupon silicon can fuse.

Among a pile of other elements, silicon fusion creates iron. And that’s trouble. Big, big

trouble. Once silicon fusion stars, the star is a ticking time bomb.

But before we light that fuse, let’s take a step back. What’s happening to the outer

layers of the star? What do we see if we’re outside, looking back at it?

Because the star was born massive, it spent its hydrogen fusing days as a blue main sequence

star. Stars like this are extremely luminous, and can be seen for tremendous distances.

Like the Sun, though, a massive star changes when hydrogen fusion stops, its core contracts,

and then helium fusion begins. It swells up just as the Sun will, but instead of becoming

a red giant, it generates so much energy it becomes a red supergiant.

These are incredibly huge stars, some over a billion kilometers across! And they are

luminous. For example, Betelgeuse in Orion is a red supergiant, and one of the brightest

stars in the sky despite being over 600 light years away. From that distance, you’d need

a decent telescope to see the Sun at all. And that’s nothing compared to VY Canis

Majoris, the largest known star, which is a staggering two billion kilometers across.

We even have a special term for it: a hypergiant.

As the core switches back and forth from one fusion reaction to the next the outer layers

respond by contracting and expanding, so a red supergiant can shrink and become a BLUE supergiant.

Rigel, another star in Orion, is a blue supergiant, putting out over 100,000 times as much energy as the Sun!

OK, let’s go back to the core. It now looks like an onion, with multiple layers: iron

is building up in the center, surrounded by fusing silicon. Outside that is a layer of

fusing oxygen, then neon, then carbon, then helium, and finally hydrogen.

You might think massive stars would last longer because they have more fuel than lower mass

stars. But the cores of these monsters are far hotter, and fuse elements are far higher

rates, running out of fuel more quickly.

A star like the Sun can happily fuse hydrogen into helium for over 10 billion years. But

a star twice as massive as the Sun runs out of hydrogen in just 2 billion years. A star

with 8 times the Sun’s mass runs out in only 100 million years or so.

And each step in the fusion process happens faster than the one before it. In an extreme

case, like for a star 20 times the mass of the Sun, it’ll fuse helium for about a million

years, carbon for about a thousand, and neon fusion will use up all its fuel in a single year!

Oxygen lasts for only a few months.

Silicon fuses at a ridiculously high rate; the star will go through its entire supply

in — get this — a day. Yes, one day. The vast majority of a star’s life is spent

fusing hydrogen; the rest happens in a metaphorical blink of the eye.

Silicon fuses into a bunch of different elements, including iron. That inert iron builds up

in the core, just like all those elements did before, and just like before the iron

core shrinks and heats up.

But there’s a huge difference here.

In all the previous fusion stages, energy is created. That energy transforms into heat,

and that helps support the soul-crushing amount of stellar mass above the core.

But iron is different. When it fuses it actually sucks up energy instead of creating it. Instead

of providing energy for the star, it removes it. This accelerates the shrinking, compressing

the core, heating it up even more.

Even worse, at these temperatures and pressures the iron nuclei suck up electrons that are

whizzing around, which are also helping support the core. It’s a double whammy; both major

means of support for the star are removed in an instant — silicon fusing into iron is

happening so fast this literally takes a fraction of a second once it gets started.

The core gets its legs kicked out from under it. It doesn’t shrink, it collapses.

The gravity of the core is so mind-bogglingly strong that the outer parts crash down on

the inner parts at a significant fraction of the speed of light. This slams down on

the central core, collapsing from several hundred kilometers across down to a couple

of dozen kilometers across in just a few thousandths of a second!

The star is doomed. Because all hell is about to break loose.

Now, at this point, one of two things can happen. If the star has less than about 20

times the Sun’s mass, the core collapse stops when it’s still 20 or so kilometers

wide. It forms what’s called a neutron star, which I’ll cover in the next episode.

If the star is more massive than this, then the collapse cannot be stopped by any force

in the Universe. The core collapses all the way down. Down to a point. The gravity becomes

so intense that not even light can escape.

A black hole is born.

We’ll cover black holes in a future episode as well. But for now, what happens when the

core collapses and suddenly stops?

The core of the star, whether it’s a neutron star or a black hole, is now extremely small

with terrifyingly strong gravity. It pulls on the star’s matter above it, HARD. This stuff

comes crashing down at a fantastic speed and gets hugely compressed, ferociously heating up.

At the same time, two things happen in the core. While this stuff is falling in, a monster

shock wave created by the collapse of the core moves outward, and slams into the incoming

material. The explosive energy is so insane it slows that material substantially.

The second event is that the complicated quantum physics brewing in the core generates vast

numbers of subatomic particles called neutrinos. The total energy carried by these little neutrinos

is almost beyond reason: In a fraction of a second, they carry away 100 times as much

energy as the Sun will produce over its entire lifetime

That’s an incredible amount of energy. Now, these little beasties are seriously elusive

and hate to interact with normal matter; one single neutrino could pass through trillions

of kilometers of lead without even noticing. But so many are created in the core collapse,

and the material barreling down on the core so dense, that a huge number of them are absorbed.

This vast wave of neutrinos slams into the oncoming material like a bullet train hitting

a slice of warm butter. The material stops its infall, reverses course, and blasts outward.

The star explodes. It explodes.

This is called a supernova, and it is one of the most violent and terrifying events

the Universe can offer. An entire star tears itself to shreds, and the expanding gas blasts

outward at 10% the speed of light. The energy released is so huge they can be seen literally

halfway across the Universe; they outshine all the stars in the rest of the galaxy combined.

The expanding material, called the supernova remnant, forms fantastic shapes. The most

famous is the Crab nebula, from a star we saw blow up in the year 1054. The tendrils

form as the material expands into the gas and dust that surrounded the progenitor star.

As remnants expand and age they become more tenuous. Some have bright rims as they push

into material between the stars; others form complex webs of filaments.

I’m often asked if there are any stars near enough to hurt us when they explode. The quick

answer is no. Even though supernovae are incredibly violent, space is big. A supernova would have

to be at least as close as 100 light years from us before we start feeling any real effects.

The nearest star that might explode in this way is Spica, in Virgo, and it’s well over

100 light years away. I say “might” explode, because it’s at the lower mass limit for

going supernova. It might not explode at all.

Betelgeuse will certainly explode some day, but it's too far away to hurt us.

We're pretty safe from this particular threat.

I’ll note that after all this, there IS another kind of supernova involving white

dwarfs, which we’ll cover in a future episode about binary stars. Happily, we’re probably

safe from them too. Breathe easy.

As terrifying and dangerous as supernovae are, there’s a very important aspect of

them you need to know. Supernovae are capable of great destruction,

but they’re also critical for our own existence.

When the star explodes, the gas gets so hot and is compressed so violently by the blast

that it undergoes fusion, what astronomers call explosive nucleosynthesis: Literally,

creating heavy elements explosively.

New elements are produced in quantities that dwarf the Earth’s mass. Calcium, phosphorus,

nickel, more iron… all made in the hellish forge of the supernova heat, and flung outward into the Universe.

It takes millennia or longer, but this material mixes with the other gas and dust clouds floating

in space. Sometimes, these clouds will be actively forming stars — sometimes the collapse

of the cloud to form stars may even be triggered by the supernova slamming into it! Either way, the

heavy elements created in the supernova will become part of the next generation of stars and planets.

Supernovae are how the majority of heavy elements in the Universe are created and scattered.

The calcium in your bones? The iron in your blood? The phosphorus in your DNA? All created

in the heart of the titanic death of a star. That star blew up more than 5 billion years

ago, but parts of it go on in you.

Today you learned that massive stars fuse heavier elements in their cores than lower

mass stars. This leads to the creation of heavier elements up to iron. Iron robs critical

energy from the core, causing it to collapse. The shock wave, together with a huge swarm

of neutrinos, blast through the star’s outer layers, causing it to explode. The resulting

supernova creates even more heavy elements, scattering them through space. Also, happily,

we’re in no danger from a nearby supernova.

Crash Course Astronomy is produced in association with PBS Digital Studios. They have a YouTube

channel with great videos -- go, just go over there, check their videos out. They’re fantastic.

This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,

and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited by

Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team, as always, is Thought Café.