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.