Brown Dwarfs: Crash Course Astronomy #28

Summary

This script explores the intriguing concept of brown dwarfs, celestial objects that lie between planets and stars in terms of mass. Unable to initiate hydrogen fusion, they cool and radiate heat, eventually appearing cold and dark. Discovered relatively recently, brown dwarfs have expanded our understanding of the cosmos, with thousands now identified. They exhibit unique characteristics, such as not increasing in size with added mass, and their atmospheres can even rain molten iron. The script also ponders the possibility of an even closer star system to Earth than Proxima Centauri, challenging our current astronomical knowledge.

Takeaways

• 🌌 Stars are massive celestial bodies that undergo nuclear fusion in their cores, converting hydrogen into helium and emitting energy.
• 🌍 Planets, including gas giants like Jupiter, lack the mass necessary for core fusion and rely on gas pressure to balance their gravity.
• 🔥 Jupiter requires significantly more mass to initiate fusion, being only 1% of the mass needed to become a star.
• 🚀 The concept of a celestial body with a mass between that of a planet and a star, known as a brown dwarf, was introduced in the 1960s.
• 🔮 Brown dwarfs form similarly to stars but do not sustain heat due to the lack of fusion, eventually cooling and appearing black.
• 🏷️ The term 'brown dwarf' was coined by Jill Tarter, despite the fact that these celestial bodies are not actually brown.
• 🌡️ Astronomers classify stars by their temperature, with the coolest being M class stars, followed by L, T, and Y class stars, which include brown dwarfs.
• 🔬 The discovery of lithium in a celestial body's spectrum can indicate the presence of a brown dwarf, as it is not fused in their cores like in regular stars.
• 🌟 The first confirmed brown dwarf, Teide 1, was found in the Pleiades star cluster, with about 50 times the mass of Jupiter.
• 🌌 The Wide-field Infrared Survey Explorer (WISE) mission has discovered hundreds of brown dwarfs, expanding our understanding of these objects.
• 🌈 Brown dwarfs can exhibit unusual colors, including magenta, due to the absorption of specific light wavelengths by molecules in their atmospheres.

Q & A

What is the primary difference between stars and planets in terms of their ability to generate fusion?

Stars are massive enough to fuse hydrogen into helium in their cores, generating energy, whereas planets, even gas giants like Jupiter, are too small to generate fusion.

At what mass does an object become capable of initiating hydrogen fusion?

An object needs to have at least 0.075 times the Sun’s mass, or roughly 75 times the mass of Jupiter, to initiate hydrogen fusion.

What is a brown dwarf, and how does it differ from both stars and planets?

A brown dwarf is an object with more mass than a planet but not enough to initiate hydrogen fusion like a true star. It forms like a star but cools and doesn't sustain significant heat.

Who coined the term 'brown dwarf' and why?

Jill Tarter coined the term 'brown dwarf' to describe these objects that emit light in the infrared and almost nothing in the visible spectrum, even though stars can’t literally be brown.

What significant discovery did astronomers make in the Pleiades cluster in 1995?

Astronomers discovered Teide 1, a sub-stellar object with lithium in its spectrum, confirming it as the first true brown dwarf.

How do astronomers distinguish brown dwarfs from normal stars using lithium?

Brown dwarfs lighter than about 65 times the mass of Jupiter don't fuse lithium, so the presence of lithium in their spectrum helps distinguish them from normal stars.

What was unique about the brown dwarf companion of Gliese 229 discovered in the 1990s?

Gliese 229b had lithium and methane in its atmosphere, making it cooler than Teide 1 and leading to the creation of the T dwarf classification.

How did the Wide-field Infrared Survey Explorer (WISE) contribute to the study of brown dwarfs?

WISE, launched in 2009, scanned the entire sky in infrared light, discovering hundreds of brown dwarfs and identifying a new classification, Y dwarfs.

Why do some brown dwarfs appear magenta instead of red?

Some brown dwarfs have molecules in their atmospheres, like methane and water, which absorb specific colors of light, blocking more red light than blue, making them appear magenta.

What unusual characteristic do brown dwarfs have concerning their size and mass?

As brown dwarfs gain more mass, they get denser instead of larger, so even a brown dwarf twice as massive as Jupiter isn't much bigger in size.

Outlines

00:00

🌌 The Birth and Nature of Brown Dwarfs

This paragraph delves into the fundamental differences between stars and planets, particularly focusing on the mass and fusion capabilities. Stars, with enough mass, can fuse hydrogen into helium, creating an equilibrium between outward expansion and inward gravitational forces. Planets, on the other hand, lack the mass for fusion and rely on gas pressure for balance. The concept of an object with mass between a planet and a star, termed a brown dwarf, is introduced. Brown dwarfs, unable to sustain fusion, cool over time. The historical development of understanding these objects and the naming process by Jill Tarter is also discussed, highlighting the challenges in identifying and distinguishing brown dwarfs from stars based on their spectral characteristics.

05:02

🔍 The Discovery and Classification of Brown Dwarfs

This section narrates the journey of astronomers in identifying and classifying brown dwarfs. The discovery of L class stars, which were initially mistaken for brown dwarfs, and the subsequent realization that true brown dwarfs would not initiate core fusion are covered. The paragraph explains the use of the lithium test to differentiate brown dwarfs from regular stars, as brown dwarfs retain lithium due to insufficient core temperatures. The first confirmed brown dwarf, Teide 1, is mentioned along with its properties, marking a significant milestone in astrophysics. The introduction of T dwarfs with the discovery of Gliese 229b, which exhibited methane in its atmosphere, further expanded the classification system of celestial bodies.

10:04

🌑 The Spectrum of Brown Dwarfs and Their Physical Properties

The final paragraph discusses the expansion of our knowledge about brown dwarfs, from the discovery of the first brown dwarf to the identification of thousands more through the Wide-field Infrared Survey Explorer (WISE). The classification of brown dwarfs into different types, such as L, T, and Y, is explained based on their cooling and spectral characteristics. The peculiar physical properties of brown dwarfs are highlighted, including their color, which can appear magenta due to atmospheric molecules that absorb specific light wavelengths. The paragraph also touches on the unusual density characteristics of brown dwarfs, where increased mass leads to increased density rather than size. It concludes with a contemplation on the fine line between brown dwarfs and planets, and the possibility of discovering even closer brown dwarfs to our solar system.

Keywords

💡Stars

Stars are celestial bodies that are massive enough to undergo nuclear fusion in their cores, converting hydrogen into helium and releasing energy. This process generates heat that would expand the star, but the force of gravity balances it, maintaining an equilibrium. In the video, stars are contrasted with planets and brown dwarfs, highlighting the importance of mass in determining celestial body characteristics and behaviors.

💡Planets

Planets are celestial bodies that orbit stars but are not massive enough to initiate nuclear fusion in their cores. They are characterized by their gas pressure resisting compression, which balances their gravity, unlike stars. The script mentions Jupiter as an example of a gas giant planet that is far too small to generate fusion, emphasizing the distinction between planets and stars.

💡Brown Dwarfs

Brown dwarfs are celestial objects that fall in the mass range between the largest planets and the smallest stars. They are not massive enough to sustain hydrogen fusion but are warmer and more massive than planets. The term 'brown dwarf' is used in the script to describe these sub-stellar objects, which emit light primarily in the infrared spectrum and were named by astronomer Jill Tarter.

💡Hydrogen Fusion

Hydrogen fusion is the process by which hydrogen nuclei combine to form helium, releasing a tremendous amount of energy. This process is the primary source of energy for stars. The video explains that brown dwarfs lack the mass needed to initiate hydrogen fusion, which is a key factor in distinguishing them from stars.

💡Gas Pressure

Gas pressure is the force exerted by gas molecules on the walls of a container. In the context of the video, gas pressure is what balances the gravity of planets, preventing them from collapsing under their own weight. This is in contrast to the balance of forces in stars, which is due to the outward pressure from nuclear fusion.

💡Lithium Test

The lithium test is a method used by astronomers to distinguish brown dwarfs from regular stars. Brown dwarfs lighter than about 65 times the mass of Jupiter do not fuse lithium, whereas normal stars would deplete their lithium when they are still young. The script mentions the lithium test as a way to identify the presence of lithium in an object's spectrum, helping to confirm it as a brown dwarf.

💡Spectral Classification

Spectral classification is a system used by astronomers to categorize stars based on their temperature and spectral characteristics. The video script discusses the classification system, which includes classes O, B, A, F, G, K, M, L, T, and Y, with brown dwarfs being part of this classification, particularly in the L, T, and Y categories.

💡Infrared

Infrared light is a type of electromagnetic radiation with wavelengths longer than visible light. The script mentions that brown dwarfs emit light primarily in the infrared spectrum, which is why they are difficult to detect and why the Wide-field Infrared Survey Explorer (WISE) was used to find them.

💡Methane

Methane is a molecule that can be found in the atmospheres of some celestial bodies, including brown dwarfs. The script describes how the presence of methane in the atmosphere of Gliese 229b indicated that it was cooler than Teide 1 and helped classify it as a brown dwarf, leading to the introduction of the T class of stars.

💡Deuterium

Deuterium is an isotope of hydrogen with one proton and one neutron in its nucleus. The video explains that brown dwarfs more massive than about 13 times Jupiter's mass can fuse deuterium, which is a distinguishing factor from true stars that fuse hydrogen.

💡Mass

Mass is a measure of the amount of matter in an object and plays a crucial role in determining the properties and behaviors of celestial bodies. The script discusses how the mass of an object dictates whether it is a planet, a brown dwarf, or a star, and how adding mass to brown dwarfs makes them denser rather than larger.

Highlights

Stars generate energy by fusing hydrogen into helium in their cores, creating an equilibrium between expansion and gravity.

Planets lack the mass to initiate fusion, relying on gas pressure to balance gravity.

Jupiter requires 100 times its current mass to initiate fusion in its core.

Astronomers in the 1960s theorized a minimum mass for stars to initiate fusion, below which they would be unable to sustain it.

Objects with less than 0.075 times the Sun’s mass are termed 'brown dwarfs', unable to sustain fusion.

Brown dwarfs were named for their cool temperatures and potential infrared emissions, though they are not literally brown.

Lithium presence in a celestial body can distinguish brown dwarfs from regular stars, as brown dwarfs retain lithium.

The first confirmed brown dwarf, Teide 1, was found in the Pleiades star cluster with about 50 times the mass of Jupiter.

Gliese 229b, another brown dwarf, was identified by its methane presence and cooler temperature compared to Teide 1.

The discovery of Gliese 229b led to the classification of T dwarfs, a new category of stars cooler than L class stars.

Brown dwarfs emit little to no visible light, with some appearing magenta due to atmospheric molecules absorbing specific colors.

WISE, the Wide-field Infrared Survey Explorer, has discovered hundreds of brown dwarfs by observing the infrared spectrum.

Brown dwarfs defy the typical relationship between mass and size, becoming denser rather than larger as mass increases.

The distinction between a brown dwarf and a planet is not always clear, with some having similar appearances but different formation processes.

Brown dwarfs can have unique atmospheric phenomena, such as rain of molten iron due to temperature-dependent condensation.

The closest known star system to Earth, after the Sun, is a pair of brown dwarfs called Luhman 16, located 6.5 light years away.

The existence of brown dwarfs challenges traditional astronomical classifications and may lead to further revisions in the field.

Transcripts

The sky, we now know, is full of stars AND planets. Stars are massive enough to fuse

hydrogen into helium in their cores, generating energy. The heat created by that process tries

to expand them, but their gravity balances that outward force, creating an equilibrium.

Planets, even gas giants like Jupiter, are far too small to generate fusion. The stuff

inside them resists being squeezed, so their gravity is balanced by simple gas pressure.

Jupiter is only about 1% the mass needed to have fusion going on in its core. That’s

a pretty big gap between a big planet and a small star. It’s natural to ask what would

happen if we dumped more mass onto Jupiter. Eventually it would become a star -- the pressure

in its core would get high enough to initiate hydrogen fusion.

But what if we stopped just short of that? What if we have an object far more massive

than a planet, but not quite massive enough to become a true star?

What sort of thing would we have then?

What indeed.

By the late 1950s, astronomers were starting to get a pretty good handle on how stars worked.

The mathematical equations that governed the physical processes of fusing hydrogen into

helium were being worked out, and applied to what we knew from observing the stars themselves.

In the 1960s the idea that you could have a star with a minimum mass was becoming clear;

if it had less than about 0.075 times the Sun’s mass, roughly 75 times the mass of

Jupiter, it simply lacked the oomph needed to squeeze hydrogen together hard enough to generate fusion.

What would such an object look like?

Well, it might form like a star, collapsing from a gas cloud just like the Sun did 4.6

billion years ago, but instead of turning on and becoming a star, it would simply sit

there, cooling. It might start off pretty hot, due to the physical forces that made

it, but it couldn’t sustain that heat. Like a charcoal ember, it would radiate its heat

away. After a few billion years it would be cold, black, and for all intents and purposes dead.

As people started working out what such an object would be like, they tried to come up

with a name for them. These things were black, and small, but the name “black dwarf”

was already being used for another type of object. Some people called them sub-stellar

objects, but that wasn’t terribly catchy.

Really low mass stars are red, and these new objects would be so cool that they’d emit

light in the infrared, and almost nothing at all in the visible. So they’re somewhere

between red and black. Jill Tarter, then a young astronomer working in the field but

who later made a name for herself looking for aliens — and oh boy, we’ll get to

that later — dubbed them “brown dwarfs.” She didn't mean it literally; stars can’t

be brown. But the name stuck.

Work proceeded in figuring out what brown dwarfs were like, and a lot of progress was

made despite there not being any actual examples of them found.

But the hunt was on. Now as I talked about in Episode 26, astronomers classify stars

by their temperature. The hottest are O stars, then B stars which are slightly cooler, down

through A, F, G, K, and with the coolest stars being M.

But then, in 1988 astronomers found a star that was so cool it was distinct from even

the M class stars. It was the first of a new, cooler class of stars, so it was given the

letter L. Why L? Because there wasn’t any other astronomical object that used that letter, so why not?

Many more such L stars were found, but still these weren’t true brown dwarfs; these stars

were massive enough to initiate fusion in their cores.

Worse yet, when brown dwarfs are first born they’re very hot. They can mimic higher-mass

L stars for a while, looking just like them, making it hard to distinguish between the two.

But then a way out was found. A low-mass brown dwarf, it was determined, would have lithium

in it, whereas normal stars wouldn’t. Lithium is an element, the next one in the Periodic

Table after hydrogen and helium. It can be fused much like hydrogen can, and regular

stars would quickly use up their supply of lithium when they were still young. Brown

dwarfs lighter than about 65 times the mass of Jupiter wouldn’t fuse lithium at all.

Very careful observations of an object would be able to detect lithium if it were there.

That would provide a test to distinguish brown dwarfs from regular stars!

The lithium test isn’t perfect, but it does work under a lot of circumstances. Astronomers

began using it to look for actual, real brown dwarfs.

And so they found one.

In 1995, a group of astronomers was observing the Pleiades, a nearby cluster of stars that’s

visible to the naked eye. They were trying to find the faintest stars they could in the

cluster to get a complete sample of its membership. The advantage of this is that the distance

to the cluster was pretty well known, so a faint star in it must be very low mass.

They found an oddball object, which they named Teide 1. It was very red and cool, and best

of all, lithium was found in its spectrum. The best models of stellar mass showed that

it had about 50 times the mass of Jupiter, or 0.05 times the mass of the Sun. It was clearly sub-stellar.

Huzzah! The very first true brown dwarf had been found.

At just about the same time, astronomers found that another nearby star, called Gliese 229,

had an extremely faint companion. Spectra showed that it was even weirder than Teide 1. It

also had lithium, and so was clearly a brown dwarf. But its spectrum showed it had METHANE

in its atmosphere. Methane is a delicate molecule, and would break down even in the mild heat

of Teide 1’s atmosphere. This new object, called Gliese 229b, was even cooler than Teide 1.

It was looking like we needed yet another letter to classify stars. And so T dwarfs became a thing.

On a personal note, when I worked on Hubble Space Telescope, Gliese 229b was one of our

camera’s first targets after it was installed on Hubble in 1997. I was lucky enough to work

on the spectrum we took of it, and it was freaky. It emitted almost no light in the

visible part of the spectrum, and rocketed up in the infrared. I had seen a lot of stellar

spectra before, but nothing like this. Remember, Gliese 229b had only been discovered two years

before! I became so intrigued by it I wound up studying low mass stars and brown dwarfs

for several years after.

Well, it didn’t take long before more brown dwarfs were found.

In 2009, NASA launched the Wide-field Infrared Survey Explorer, or WISE, an orbiting observatory

designed to scan the entire sky looking at infrared light. It found hundreds of brown

dwarfs, and now at least 2000 are known, with more found all the time. Some are so cool

that they form yet another classification: Y Dwarfs.

So now we have O B A F G K M L T and Y. You’re on your own for an acronym here.

So if brown dwarfs aren’t brown, what color are they?

Some are so cool they don’t emit visible light at all, so they’d be black. You could

be right over one and you wouldn’t see it.

But some are still warm, and so give off some visible light, feeble as it might be. What color would they look?

Funny thing. They might be magenta.

You’d think they’d be really red, because of their temperature. But it’s a bit more

complicated than that. Remember, they have molecules in their atmospheres that absorb

specific colors of light. In some brown dwarfs, there are molecules like methane and even

water—well, steam at those temperatures—that are pretty picky about what colors they absorb.

Some of these molecules block more red light than blue, so that messes with their colors,

making them look magenta.

WISE takes pictures in the infrared, which our eyes can’t see. To make pictures, astronomers

map each infrared color to one our eyes can see. So an image using the shortest wavelength

infrared detector is displayed as blue, the medium wavelength one green, and the longest

one red. Brown dwarfs put out a lot of light in the intermediate wavelength WISE sees,

so weirdly, they appear green in WISE pictures. That does make them easy to spot in those

images, even when thousands of other stars are visible, too.

The physical nature of brown dwarfs is just as weird as you’d expect. For one thing,

they have a very unusual characteristic: As they get more massive, they don’t get any bigger.

Usually, if you dump mass onto an object it gets bigger; take two lumps of clay and smush

‘em together and you get one more massive, slightly larger lump. Same with planets and stars.

But brown dwarfs are different. At their cores the density is very high, and the physics

is a bit different than what you’d expect. The details are complex but the end result

is that when you add more mass to them they actually get DENSER, not bigger. This effect

becomes important right around the mass of Jupiter, which means that a brown dwarf twice

as massive as our biggest planet won’t actually be a whole lot bigger.

So what’s the difference between a small brown dwarf and a really big planet? Well,

not much. Nature isn’t as picky as we are about having narrowly-defined borders between

classes of objects. Some people say a planet forms from a disk of material around a star,

growing larger as it accretes stuff, while a brown dwarf collapses directly from a cloud

of gas and dust. But then you could have two objects the same mass, and which look exactly

the same, yet one would be a planet and one a brown dwarf, depending on how they formed.

That strikes me as… inconvenient.

Astronomers are still debating this. And it gets worse.

For example, as I said before, brown dwarfs over 65 times Jupiter’s mass fuse lithium.

It turns out that ones more massive than about 13 times Jupiter can also fuse deuterium,

an atom that’s very similar to hydrogen, except it has a proton and a neutron in its

nucleus. But neither of these fuses actual hydrogen, so they’re not considered true

stars. That’s still a little arbitrary, so again I don’t make too much of a fuss about it.

I think it’s best not to think of them as planets or stars, but something with characteristics

of both. For example, the way their atmospheres behave depends a lot on how hot they are.

In some, iron is vaporized, a gas. In others, they’re just cool enough that iron condenses

out of the atmosphere… which means it literally rains molten iron!

One more thing. The nearest star to the Sun is a red dwarf called Proxima Centauri, which

orbits the binary star Alpha Centauri. It’s about 4.2 light years away. In 2013, astronomers

announced the discovery of a binary pair of brown dwarfs, called Luhman 16. They’re

only 6.5 light years away, and became the 3rd closest known star system to Earth.

You gotta wonder: Could there be an even fainter, cooler brown dwarf closer to us? We know there’s

none in our solar system, even out in the Oort cloud; it would’ve been seen by now

by one of several different sky surveys. But a light year or two out? Maybe. Is Proxima

Centauri REALLY the closest star, or will we find one even closer? It seems unlikely,

but no more unlikely than the existence of brown dwarfs themselves. Maybe sometime soon

we’ll have to rewrite astronomy textbooks. Again.

Today you learned that brown dwarfs are objects intermediate in mass between giant planets

and small stars. They were only recently discovered, but thousands are now known. More massive ones can

fuse deuterium, and even lithium, but not hydrogen, distinguishing them from “normal” stars. Sort of.

Crash Course Astronomy is produced in association with PBS Digital Studios. Head on over to

their YouTube channel and see even more awesome videos. 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.