We have one star in the solar system: the
Sun. Sure, it has lots of planets, moons, asteroids, and comets it shleps with it as
it moves through space, but no other STAR is part of our family. The Sun is alone. Based on that, you might naturally think that,
like the Sun, stars are single, too. They sure look that way by eye. But when you point a telescope at the sky,
you find that this is NOT the case. A lot of stars travel the Universe with companions…
and sometimes more than one. With so many stars in the sky, some appear
close together just by coincidence, even though in space they’re actually very far apart.
We call these “optical double stars”. By the 18th century astronomers were starting
to recognize that many stars that appeared close together really WERE physically orbiting
each other. We call these BINARY stars, to distinguish them from the coincidentally close
together DOUBLE stars. Although the numbers are a little bit uncertain, something like
a third to a half of all stars in the sky are part of a binary or multiple star system. One such binary system is visible to the naked
eye, and has been known for thousands of years. You may have seen it yourself! The star marking
the kink in the handle of the Big Dipper is actually two stars, one called Mizar, and
a fainter one called Alcor. They’re close enough together that you need decent eyesight
to separate them, and they were actually used as an eye test in ancient times. Binary stars almost certainly form together,
near each other in the gas cloud that was their original stellar nursery. Instead of
a single clump collapsing and forming a star, like our Sun, there are two such dense lumps, and
they both collect material until they become true stars. There are lots of different kinds of binary
stars. If the two stars can be seen separately using a telescope they’re called a VISUAL
BINARY. This is kind of a fluid classification; as telescopes get better stars that are closer
together can be resolved. These kinds of stars are fairly common; the
brightest star in the night sky, Sirius, is a visual binary. It’s a luminous blue star
about twice the mass of the Sun orbited by a smaller, much fainter white dwarf. It’s
funny, too: as I mentioned in an earlier episode, white dwarfs can be very hot and energetic,
and emit light at much higher energy than normal stars. When you observe Sirius with
an X-ray telescope, the white dwarf is by far the brighter of the two! Visual binaries are important, because, if
you observe them long enough you might be able to see their orbital motion. If we can
measure their distance from Earth then the actual size and shape of their orbits can
be determined, and in turn — using the math and physics of gravity —this can be used
to find the masses of the stars; the only way we know to get accurate measurements of
stellar masses is when they’re in binaries. And once we know the masses of the stars,
as we saw in Episode 26, we can learn everything else about them: How big they are, how brightly
they shine, and even how long they live. It’s no exaggeration to say that observing binary
stars opened up the new scientific field of astroPHYSICS, applying physics to astronomy…
and that led to us understanding everything we do about the Universe today. Not bad. Not all binaries are visual binaries, though.
Some stars orbit so closely together that we can’t split them even with our biggest
telescopes. So how do we know they’re binary? Spectroscopy! As the two stars orbit each
other, over time one will appear to be heading toward us while the other circles away, and
vice versa as they switch sides. While we may not see that motion directly, if we take
spectra of their light, breaking it up into individual narrow colors, we can see the Doppler
shift in their spectra. On their merry—go-round path, one undergoes a redshift as it moves
away, and the other has a blue shift as it moves toward us. These kinds of stars are
called SPECTROSCOPIC BINARIES. Remember Mizar and Alcor, the Big Dipper eye
test stars? I said they were a binary system, but I lied. Well, I understated. In even a
small telescope you can see that Mizar is actually a visual binary… but it turns out
that both of those two stars making up Mizar are actually SPECTROSCOPIC binaries, too.
Mizar is a binary binary star! Even better: Alcor is a spectroscopic binary, too! Since
Mizar and Alcor orbit each other, it turns out they make up a sextuple star system, SIX
stars all gravitationally bound to one another. Obviously, stars can be in bigger groups than
binaries. There are triple star systems, quadruple, and more. Polaris, the north star, is actually
a pentuple system, composed of five stars. It’s possible lots of stars are born in
multiple systems. However, it’s pretty hard to get a stable system like that; if the orbits
aren’t just right some of the stars will tend to get ejected from the system. What
we see today are the ones that, coincidentally, got things just right. Even then, they may
not be stable in the long run. Was the Sun born in such a multiple system?
We don’t really know. It’s certainly possible, and one way to find out would be to look for
stars that have a very similar elemental composition as the Sun. But the Sun was born billions
of years ago; plenty of time for any stars born with it to wander off. Even at relatively
slow speeds, 4.5 billion years is a long time, and for all we know they could be 50,000 light
years away and completely invisible to us. If there are long lost siblings to the Sun
out there, they may remain lost. Just like planets orbiting the Sun, binary
star orbits can be short, or very long. Some stars, separated by tens or hundreds of billions
of kilometers, can take centuries to orbit each other, while some are so close they may
only take days. One binary star, the most bizarre I know of, is called 4U 1820-30, and
it’s composed of a neutron star and a white dwarf. Their gravity is so strong, and they
are so close together, that they orbit each other in 685 seconds… 11.4 minutes… roughly,
the length of this episode. Like exoplanets, binary star orbits are tipped
every which-way to our line of sight from Earth. But for some of them, we see their
orbits edge-on, or very nearly so. For these binaries that means that every orbit we see
each of these stars pass in front of the other, blocking it from our view. We call these ECLIPSING
binaries. Eclipsing binaries are interesting, because
as one star blocks another, the total light we see from the system dips, just like in
a solar eclipse when the Moon blocks the Sun. Over the course of one orbit we see TWO such
dips, as the first star blocks the second and then half an orbit later when the second
passes in front of the first. If the two stars are similar, say both like
the Sun, then the two dips look very similar. But if one star is much brighter than the
other, then the two dips look very different. The brighter star dominates the total light
we see, so when the fainter star goes behind the brighter star, the light hardly drops
at all. But when that fainter star blocks the brighter one, we see a bigger dip in the
light. By carefully examining the sizes and shapes
of the dips this way, a lot of interesting information can be gleaned from the system,
including the sizes, masses, rotation rates, temperatures of the stars, the size and shape
of the orbit, and even the distance to the system. Some stars, like humans, enjoy cuddling. They
get so close together they become CONTACT binaries, literally two stars touching each
other. These are very strange objects. The stars can be stretched out into teardrop shapes
due to the mutual tidal effects. If they get very close together they merge into a double-lobed stellar peanut
shape, like two stars cocooned in shared material. This can make things really weird for them.
Imagine two stars born at the same time, perhaps a few millions kilometers apart, tightly orbiting
each other. One has, say, five times the mass of the Sun (so it’s a hot blue star), and
the other just one half (so it’s a red dwarf). The red dwarf doesn’t do much. It just slowly
fuses hydrogen into helium, glowing feebly. The bigger star, though, goes through its
nuclear fuel rapidly, and becomes a red giant. It blows off a wind of matter and loses mass.
Since the stars’ gravity depends on their masses, as the big star loses mass the orbits
get a little wonky, becoming more elliptical. But when the massive star swells, it gets
so big the two become a contact binary. A lot of the material leaving the higher mass
star gets dumped on the red dwarf, which starts to grow. Eventually, the big star loses most
of its mass and becomes a white dwarf, while what USED to be the lower mass star has grown,
and now might be more massive than the other star! It’s a bit like Robin Hood taking
from the rich and giving to the poor; if he gets too enthusiastic about it then the poor
become rich while the rich become poor. When we look at that binary system, we see
a white dwarf star that is clearly more evolved than a high mass one, the opposite of what
we expect! This is called the Algol Paradox, after the contact binary star Algol in Perseus
which shows this effect. Mass transfer between two stars can yield
even more dramatic results. Imagine this same system a couple of billion years later. The
high mass star has lost its outer layers, and is a dense white dwarf. The other star
eventually runs out of hydrogen fuel, and swells into a red giant. This material then
flows onto the white dwarf. White dwarfs have cruelly strong gravity.
If the hydrogen flowing onto its surface piles up enough, the gravity can squeeze it so hard
it fuses into helium. If the flow rate is just right, it piles up on the white dwarf
and then undergoes fusion in a single colossal flash, erupting in a huge explosive flare.
Some of these explosions can be incredibly violent, tens of thousands of times brighter
than the Sun! When this happens, a previously invisible
star can suddenly flare into visibility in the sky. These have been seen historically,
and called “Stellar novae”, for “new star”. I love the irony: These stars actually
have to be old, near the ends of their lives to go nova! But the name stuck. The explosion can blow out the stream of matter
falling from the other star, but when things settle down after a few weeks or month, the
matter stream can fall back on the white dwarf, and the whole cycle repeats. These are called
recurrent novae. If the matter stream is slower, the material
can fuse steadily, never piling up, so it never explodes. However, the mass of the white
dwarf still increases. If it reaches a mass of around 1.4 times that of the Sun, it gets
compressed by its own gravity so much that its temperature soars upward. It gets so hot
that carbon fusion initiates. And that a big problem. In a normal star,
it would just expand due to all the extra energy being generated. But a white dwarf
can’t; it’s ruled by electron degeneracy pressure. The extra energy just goes into
fusing more carbon, and what you get is a runaway thermonuclear event: All the carbon
EVERYWHERE INSIDE THE WHITE DWARF FUSES ALL AT ONCE. ALL of it. Basically a solar mass of carbon will instantly
fuse, releasing all that energy all at once. It’s like setting fire to a dynamite factory.
The star explodes. You get a SUPERnova. And it’s a completely
different process than what we saw when a high-mass star explodes, but coincidentally
it releases about the same amount of energy. The star tears itself to vapor, and gets so bright it can
be seen literally most of the way across the Universe. Ooh, this makes them very, very important
indeed… as you’ll see in a future episode. Today you learned double stars are stars that
appear to be near each other in the sky, but if they’re gravitationally bound together
we call them binary stars. Many stars are actually part of binary or multiple systems.
If they are close enough together they can actually touch other, merging into one peanut-shaped
star. In some close binaries matter can flow from one star to the other, changing the way
it ages. If one star is a white dwarf, this can cause periodic explosions, and possibly
even lead to blowing up the entire star. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch 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.
