White Dwarfs & Planetary Nebulae: Crash Course Astronomy #30

Summary

This script from Crash Course Astronomy explores the fascinating final stages of low mass stars like our Sun. After helium fusion ceases, these stars become white dwarfs—hot, dense remnants about Earth-sized. They may also form planetary nebulae, intricate glowing structures from the gas expelled during the star's death throes. The nebulae's shape and color reveal the star's life story, offering insights into stellar evolution. Despite their beauty, these nebulae are fleeting, lasting only a few thousand years, a brief moment in the cosmos.

Takeaways

• 🌞 The Sun, like other low mass stars, will eventually become a white dwarf after a series of expansions and contractions, and then fade away over billions of years.
• 🤔 White dwarfs are dense and small, with a size comparable to Earth but with a mass close to the Sun's current mass, making them incredibly dense.
• 🔥 Newborn white dwarfs are extremely hot, with temperatures exceeding 100,000 degrees Celsius, causing them to glow white and emit radiation in the ultraviolet and X-ray spectrums.
• 🌌 Planetary nebulae are formed when the outer layers of a dying star are expelled and then illuminated by the intense radiation from the central white dwarf.
• 🌌 The name 'planetary nebula' is a misnomer, originating from their appearance through early telescopes, which resembled small green disks.
• 🌌 Planetary nebulae display a variety of shapes, from round to elongated, spiral, and with jets or tendrils, indicating complex processes during their formation.
• 🌀 The shapes of planetary nebulae can be influenced by binary star systems or the presence of orbiting planets that may have been swallowed by the expanding star.
• 💫 The glow of a planetary nebula is primarily due to hydrogen and oxygen in the expelled gas, with oxygen emitting a characteristic green color.
• 📚 The study of planetary nebulae provides insights into the life and death of stars, contributing to our understanding of stellar evolution.
• 🌠 The visibility of a planetary nebula is fleeting, lasting only a few thousand years before the gas disperses and the glow fades, making them rare sights in the cosmos.
• ☀️ Unlike more massive stars, the Sun is unlikely to form a visible planetary nebula when it dies, as it will not be energetic enough to excite the surrounding gas.

Q & A

What is the ultimate fate of low mass stars like our Sun?

Low mass stars like the Sun will eventually expand and contract, blow off their outer layers, become white dwarfs, and then fade away over billions of years.

What is a white dwarf and why is it considered an awesome object?

A white dwarf is the dense, hot remnant of a star that has exhausted its nuclear fuel. It's considered awesome due to its incredible density and the fact that it's supported by electron degeneracy pressure, a quantum mechanical phenomenon.

How does the Sun's core change as it ages?

As the Sun ages, it fuses hydrogen into helium and eventually helium into carbon, with traces of oxygen and neon. When it runs out of helium, fusion stops, and the core becomes nearly pure carbon, leading to the cessation of fusion.

What is electron degeneracy pressure and why is it significant in white dwarfs?

Electron degeneracy pressure is a quantum mechanical effect where electrons resist being squeezed together more strongly than simple electric repulsion. It becomes the dominant force supporting the core of a star after helium fusion stops, leading to the formation of a white dwarf.

What is the size of a white dwarf in comparison to the original star?

A white dwarf is about the size of Earth, which is only 1% of the original width of a star like the Sun.

How dense is a white dwarf and what does a cubic centimeter of its material weigh?

A white dwarf is incredibly dense, with a single cubic centimeter of its material weighing about a million grams, or one metric ton.

What is the gravity on the surface of a white dwarf compared to Earth's gravity?

The gravity at the surface of a white dwarf is extremely high, easily topping 100,000 times the Earth's gravity.

What causes the gas around a newly formed white dwarf to glow?

The intense radiation emitted by the hot white dwarf causes the nearby gas, which was expelled during the star's final stages, to glow in response, forming what is known as a planetary nebula.

What is a planetary nebula and why is it called so?

A planetary nebula is a glowing cloud of ionized gas expelled from a star during its final stages, illuminated by the ultraviolet radiation from the central white dwarf. It was named so because early astronomers, observing them through telescopes, mistook them for small green disks similar to planets.

Why do planetary nebulae come in various shapes and not just spherical?

Planetary nebulae can have various shapes due to factors such as the star's rotation, the presence of a binary companion, and possibly interactions with orbiting planets, which can cause the expelled gas to form complex structures.

What element is responsible for the green glow in planetary nebulae and why was it initially a mystery?

Oxygen is responsible for the green glow in planetary nebulae. Initially, astronomers couldn't identify the element and called it nebulium, but later it was discovered to be extremely diffuse oxygen.

Why is the phase of a planetary nebula brief and rare to observe?

The phase of a planetary nebula is brief because the gas expands and thins out over a few thousand years, eventually stopping to glow. Although billions of stars die this way in the galaxy, this phase is short, making it rare to observe.

Will our Sun eventually form a planetary nebula when it dies?

It is unlikely that our Sun will form a planetary nebula when it dies. Most planetary nebulae start off as stars more massive and hotter than the Sun, and when our Sun dies, it will do so quietly without much visible fanfare.

Outlines

00:00

🌟 The Life and Afterlife of Low Mass Stars

This paragraph introduces the fate of low mass stars like our Sun, detailing their transformation into white dwarfs and the creation of planetary nebulae. Phil Plait explains the Sun's eventual exhaustion of helium and the cessation of fusion within its core, leading to the ejection of outer layers. The remaining core, composed mainly of carbon and electrons, is subject to electron degeneracy pressure, which, counteracting gravity, results in a dense, Earth-sized object. White dwarfs are characterized by their immense density, high gravity, and intense heat, which can ionize nearby gas, causing it to glow and form the beautiful planetary nebulae. The paragraph also touches on the historical misidentification of these objects and their true nature as revealed by modern astronomy.

05:02

🌌 The Complex Beauty of Planetary Nebulae

This section delves into the intricacies of planetary nebulae, challenging the earlier simplistic view of them as spherical shells of gas. With advancements in digital imaging, the diverse and complex structures of these nebulae have been revealed, including elongated shapes, spiral patterns, and jets. The explanation for these varied forms involves the dynamics of stellar winds and the potential influence of binary star systems or swallowed planets. The paragraph explores the hypothesis that the interaction of fast and slow stellar winds, along with the rotational effects of internal planets, could shape the nebulae's final appearance. It also discusses the spectral analysis that led to the understanding of the elements responsible for the nebulae's characteristic colors, such as the green glow of oxygen and the red hues of nitrogen and sulfur.

10:07

🌍 The Future of Our Sun and Stellar Spectacles

The final paragraph summarizes the key points about the end stages of low mass stars, emphasizing the formation of white dwarfs and the brief, luminous period of planetary nebulae. It clarifies that our Sun is unlikely to produce a planetary nebula due to its lower mass and energy output. The paragraph also teases the topic of next week's episode, which will cover the explosive deaths of high mass stars, suggesting a contrast to the quieter demise of stars like our Sun. The production credits for Crash Course Astronomy are also acknowledged, highlighting the collaborative effort behind the educational series.

Keywords

💡White Dwarf

A white dwarf is the remnant of a low mass star after it has exhausted its nuclear fuel and shed its outer layers. It is incredibly dense and hot, with a size comparable to Earth but a mass similar to that of the Sun. In the video, the white dwarf is described as the end state of stars like our Sun, where the core's gravity is balanced by electron degeneracy pressure, resulting in a small, Earth-sized object with immense density.

💡Electron Degeneracy Pressure

Electron degeneracy pressure is a quantum mechanical effect that arises when electrons are compressed to a point where they resist further compression. This pressure becomes significant in white dwarfs, where it counteracts the force of gravity and prevents further collapse. The video explains that this resistance is what allows a white dwarf to maintain its size despite the immense gravity pulling inward.

💡Planetary Nebula

A planetary nebula is a glowing cloud of ionized gas ejected from a star during its late stages of life. It is often referred to as one of the most beautiful objects in space due to its intricate and colorful structures. The video describes how the gas from a dying star, illuminated by the intense radiation from the central white dwarf, forms these nebulae, which can exhibit a variety of shapes, including spirals, jets, and tendrils.

💡Helium Fusion

Helium fusion is a nuclear reaction that occurs in the core of a star, where helium nuclei combine to form heavier elements such as carbon and oxygen. The video mentions that when a star like the Sun runs out of helium to fuse, it faces a crisis, as it cannot ignite the carbon nuclei due to insufficient mass, leading to the cessation of fusion reactions and the eventual formation of a white dwarf.

💡Red Giant

A red giant is a stage in the life cycle of a star where it has exhausted the hydrogen fuel in its core and begins to burn helium, causing it to expand dramatically. The video explains that during this phase, the star's outer layers are expelled, and the core is exposed, which eventually leads to the formation of a white dwarf.

💡Stellar Evolution

Stellar evolution refers to the sequence of radical changes a star undergoes throughout its lifetime, from its formation to its eventual death. The video discusses how studying the death of stars, such as the formation of white dwarfs and planetary nebulae, provides insights into the broader process of stellar evolution.

💡Nebula

A nebula is a cloud of gas and dust in space, often the birthplace of stars. In the context of the video, the term specifically refers to a planetary nebula, which is the result of a star's outer layers being expelled and ionized by the radiation from the star's hot core, forming a glowing cloud of gas.

💡Binary Star System

A binary star system consists of two stars that orbit around their common center of mass. The video suggests that the interaction between a dying star and its binary companion can influence the shape of the resulting planetary nebula, with the companion's gravitational influence altering the distribution of the ejected gas.

💡Exoplanet

An exoplanet is a planet that orbits a star outside our solar system. The video mentions the hypothesis that planets within a star system might be engulfed by the star as it expands into a red giant, potentially influencing the star's rotation and the shape of the resulting planetary nebula.

💡Spectral Analysis

Spectral analysis is the study of the interaction of light with matter, often used to determine the composition of celestial objects by analyzing the light they emit or absorb. The video recounts the historical discovery of nebulium, which was later identified as oxygen, through spectral analysis of the glowing gases in planetary nebulae.

💡Star Lifecycle

The star lifecycle describes the stages a star goes through from its birth in a nebula to its eventual death, which can take various forms such as a white dwarf, neutron star, or supernova. The video emphasizes the importance of studying the end stages of stars, like the formation of white dwarfs and planetary nebulae, to understand the broader processes of stellar evolution.

Highlights

The Sun and other low mass stars will become white dwarfs after a series of expansions and contractions.

White dwarfs are dense, with a single cubic centimeter weighing about a metric ton.

Electron degeneracy pressure, a quantum mechanical force, supports the core of a star once helium fusion stops.

A white dwarf is about the size of Earth and has gravity over 100,000 times stronger than Earth's.

Newborn white dwarfs are extremely hot, with temperatures exceeding 100,000 degrees Celsius.

Planetary nebulae are formed when the gas expelled by a dying star is excited by the radiation from a white dwarf.

The name 'planetary nebula' is a historical misnomer, as these objects are not related to planets.

Planetary nebulae can exhibit a variety of complex shapes, including spirals and elongated forms.

Binary star systems may influence the shape of planetary nebulae due to the interaction of their orbital dynamics.

The idea that planets within a star could influence the formation of planetary nebulae was proposed by Noam Soker.

Oxygen within a planetary nebula glows green due to atomic physics, giving the nebula its characteristic color.

The glow of a planetary nebula is short-lived, lasting only a few thousand years before the gas disperses.

The Sun is unlikely to form a planetary nebula due to its lower mass and energy output compared to stars that do.

Higher mass stars, more than about 8 times the mass of the Sun, end their lives with a spectacular explosion.

Crash Course Astronomy is produced in association with PBS Digital Studios, offering a variety of educational content.

The script for this episode was edited by Blake de Pastino and consulted by Dr. Michelle Thaller, ensuring accuracy and quality.

Transcripts

Hey folks, Phil Plait here. In the last episode of Crash Course Astronomy, I talked about

the eventual fate of the Sun, and other low mass stars like it. After a series of expansions and contractions,

they blow off their outer layers, become white dwarfs, and fade away over billions of years. The end.

Except not so much. First, white dwarfs are pretty awesome objects, and worth investigating.

And second, when a star becomes a white dwarf it produces what is, quite simply, one of

the most gorgeous objects in space.

To recap, when the Sun ages, it undergoes a series of changes in its core. It’s fusing

hydrogen into helium now, today, and will eventually fuse helium into carbon, and it’ll

make a dash of oxygen and neon too. But when it runs out of helium to fuse, it’s in trouble.

It doesn’t have enough mass to squeeze the carbon nuclei together, so they can’t fuse.

Fusion is the Sun’s energy source. Once the core is nearly pure carbon, that power is switched off.

By this time, nearly 8 billion years from now, the Sun’s outer layers are gone, expelled

by all the shenanigans going on in the core. What’s left of our star is just its core,

exposed to the dark of space. Over the next few billion years it’ll cool and fade to black.

That might seem like the end of the story. But I skipped a step, and it’s a beauty.

When helium fusion stops, the Sun’s core will have about half the mass of what the

Sun does today; the rest will have blown away into space around it. What remains is basically

composed of electrons and carbon nuclei, mixed with a small amount of a few other elements.

So what kind of an object are we left with here?

You may know that like charges repel; electrons have a negative charge and repel each other.

The tighter you squeeze them, the stronger that pressure.

There’s also a second force, called electron degeneracy pressure. It’s a result of some

of the weird rules of quantum mechanics (for you QM nerds, it’s The Pauli Exclusion Principle).

This describes how sub-atomic particles behave on teeny scales. One of these rules is that

electrons really hate to be squeezed together, above and beyond simple electric repulsion.

This resistance is phenomenally strong, and becomes the dominant force in supporting the

core of a star once helium fusion stops.

By the time this electron degeneracy pressure balances the core’s immense gravity, the

core is only about the size of Earth, 1% the original width of the Sun.

And it’s called a white dwarf.

Listing its characteristics is enough to melt your brain. Ironically, everything about it

gets amplified as its size shrinks. It becomes ridiculously dense; a single cubic centimeter

of it, the size of a six-sided die, has a mass of a million grams — one metric ton.

An ice cream scoop of white dwarf material outweighs 60 cars.

Because it’s so dense, the gravity at the surface of a white dwarf screams up, easily

topping 100,000 times the Earth’s gravity. If you have a mass of 75 kilos, you’d weigh

7,500 tons if you stood on the surface of a white dwarf.

Not that you can. Stand there, I mean. You’d be flattened into a greasy smear.

But not for long. Newborn white dwarfs are hot; they can glow at a temperature of upwards

of 100,000 degrees Celsius. If you were on the surface, you’d be a vaporized and ionized smear.

Their intense heat makes them white, and they’re small. Hence their name.

They’re so hot they also glow in the ultraviolet, even in X-rays. Weirdly, though, because they’re

so small, they’re actually quite faint. The closest one to us, Sirius B, can only

be seen with a telescope even though it’s nine light years away, one of the ten closest

known stars! Over 10,000 white dwarfs have now been found in our galaxy.

Still, any gas near a newly formed white dwarf is likely to be affected by the intense radiation pouring out of it.

And hey, wait a sec. When a star like the Sun is in its final death throes, it expels

its outer layers as a gaseous wind. You don’t suppose…?

Yup. By the time that white dwarf forms, the gas blown off hasn’t gotten very far from it,

at most a light year or two. That’s plenty close enough to get zapped by the white dwarf radiation,

causing that gas to glow in response. What does something like THAT look like?

Why, it looks like this.

This object is what we call a planetary nebula. It’s a funny name, and like so many other

names it’s left over from when these objects were first discovered. The astronomer William

Herschel — the same man who discovered infrared light and the planet Uranus — gave them

that name, because they appeared like small green disks through the eyepiece.

The first planetary nebula was discovered in 1764 by the French astronomer Charles Messier,

who spent years scanning the skies looking for comets. He kept seeing faint fuzzy objects that he mistook for

comets, so he decided to make a catalog of them, a sort of “avoid these objects” list. That list is now a staple

of amateur astronomers, because ironically it contains some of the best and brightest objects to observe.

Among them is M 27 — the 27th object on Messier’s list — one of the biggest planetary nebulae in the sky,

and one of my favorites; I love seeing it through my telescope when it’s up high in the summer.

Planetary nebulae can be a bit tough to observe; most are small and faint. On film, even with

long exposures, they can appear to be little more than disks. For a long time, they weren’t

thought to be terribly complicated; when a star becomes a red giant and blows off its

outer layers it’s rotating very slowly, so the wind should blow away in a sphere surrounding

the star. Many planetaries (as we call them for short), are round and look like soap bubbles,

pretty much what you expect when you look at an expanding shell of gas.

But with the advent of digital detectors, their fainter structures became clearer, and the

true beauty of these phenomenal objects was revealed. Some are elongated. Some have spiral patterns.

Some have jets shooting out on either side. Some have delicate tendrils streaming away from them.

In fact, only a handful of the hundreds known are actually circular!

Clearly, there’s more to planetaries than meets the eye.

If the wind from a star blows off in a sphere, how can planetaries come in all these fantastic shapes?

It turns out the real situation is more complicated. As usual.

When a star is a red giant, it spins slowly, and blows off a dense but slow solar wind.

If there’s nothing else happening to the star, then that wind will blow outward in

all directions, spherically. However, as those outer layers of the star peel away, they expose

the deeper, hotter part of the star. The star starts to blow a much faster, though far less

dense wind. That wind catches up with and slams into the slower wind.

When that happens, you get that idealized soap bubble nebula.

But some stars are binary; two stars that orbit very close together. We’ll go into

detail on them in a later episode, but, if the dying star has a companion, they may circle

each other rapidly. That will shape the wind, forcing more of it outward in the plane of

the stars’ orbits due to centrifugal force.

The overall shape of the expanding gas is flattened, like a beach ball someone sat on.

When the fast wind kicks in, it slams into that stuff in the orbital plane and slows down.

But there’s less stuff in the polar direction, up and down out of the plane. It’s easier

for the wind to expand in those directions, and it forms huge lobes of material stretching

for trillions of kilometers. That’s a very common shape for planetary nebulae.

But to explain the shapes we see, the two stars would have to orbit improbably close.

Most binaries aren’t that tight. So what can cause these shapes?

When I was in graduate school, my Master’s degree advisor, Noam Soker, came up with a

nutty idea; maybe the stars had planets, like in our solar system. If the star expanded into

a red giant and swallowed them, it would take millions of years or more for the planets

to vaporize. And for all that time they’d be orbiting INSIDE the star, moving faster

than the star itself. Like using a whisk to beat eggs, the planets inside the star would

spin it up, causing it to rotate faster… fast enough to explain the shapes of planetaries.

That was in the early 1990s. A few years later, the first exoplanets were found, and we saw

that massive planets orbiting very close to their stars were common. I suspect this is why we see

so many weird and fantastic shapes in planetaries; their progenitor stars swallowed their planets.

So planetary nebulae really may owe their existence to planets! And we come… full circle.

The glow in a planetary nebula is due to the hot central white dwarf exciting the gas.

Most of the gas is hydrogen, which glows in the red. However, a lot of the gas is oxygen.

There’s not nearly as much oxygen as hydrogen, but kilo for kilo oxygen glows more brightly than hydrogen

due to the atomic physics involved. This oxygen glows green, giving planetaries their characteristic hue.

Funny: When this green glow was first analyzed spectroscopically, astronomers couldn’t

identify the responsible element making it. They dubbed it nebulium, but eventually figured

out is was just extremely tenuous oxygen.

Other colors can be found, too. Nitrogen and sulfur glow red, and oxygen can emit blue

as well, all adding to the beauty of these celestial baubles. But these aren’t just

pretty pictures: The structure, color, and shape of a planetary nebula tells us about

the life of the star that formed it. We learn even more about stellar evolution by studying how stars die.

Mind you, the gas in a planetary nebula is still expanding, cruising outward from its

initial momentum of being thrown off the star. Eventually, the gas expands so much it thins

out, and it stops glowing. That takes a few thousand years, so when you see a planetary

nebula you’re seeing a very short snapshot of the life, the death, of a star. And that’s

why we don’t see many; though there are billions of stars in the galaxy that die this way,

this phase is very brief. Enjoy looking at them while you can. And what of the Sun?

Will it, one day in the distant future be at the center of a planetary nebula it expels as it dies?

Ehh probably not. When the Sun becomes a white dwarf, it most likely won’t be energetic

enough to make the surrounding gas glow; most planetaries start off as stars more massive

and hotter than the Sun. When our Sun dies, it’ll go quietly and without a lot of visible fanfare.

Alien astronomers, if they’re out there in 8 billion years, may not even notice.

But more massive stars do make quite the spectacle. And if they’re really massive, more than

about 8 times the Sun’s mass, they really and truly make a scene when they die. They explode.

But that’s for next week. Mwuhahahaha.

Today you learned that when low mass stars die, they form white dwarfs: incredibly hot

and dense objects roughly the size of Earth. They also can form planetary nebulae: huge,

intricately detailed objects created when the wind blown from the dying stars is lit

up by the central white dwarf. They only last a few millennia. The Sun probably won’t

form one, but higher mass stars do.

Crash Course Astronomy is produced in association with PBS Digital Studios. Why don’t you

head on over there and check out their YouTube channel -- they have lots of great videos

there. 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 is Thought Café.