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.
