Binary and Multiple Stars: Crash Course Astronomy #34

Summary

This script explores the fascinating world of binary and multiple star systems, debunking the myth that stars like our Sun are solitary. It explains how many stars are gravitationally bound in pairs or groups, with some even merging into contact binaries. The episode delves into visual, spectroscopic, and eclipsing binaries, revealing how these systems contribute to our understanding of stellar masses, sizes, and life cycles. It also touches on the dramatic phenomena of stellar novae and supernovae, highlighting the importance of binary stars in astrophysics.

Takeaways

• 🌞 The Sun is the only star in our solar system, with planets, moons, asteroids, and comets orbiting it, but no other star is part of our cosmic family.
• 🔭 Telescopes reveal that many stars in the sky are not single, but travel with companions, contrary to what the naked eye might suggest.
• 👥 Some stars appear close together by chance and are called 'optical double stars', while others are physically orbiting each other and are known as 'binary stars'.
• 🪐 It's estimated that about a third to half of all stars are part of binary or multiple star systems, indicating that stars often form in groups.
• 🌌 The naked eye can see a binary star system in the Big Dipper's handle, where Mizar and Alcor are two stars that have been historically used as an eye test.
• 🌟 Binary stars are thought to form together from dense lumps in a gas cloud, collecting material until they become true stars.
• 🔬 Visual binaries can be observed with telescopes and, over time, their orbital motion can provide crucial information about their masses and other properties.
• 🌈 Spectroscopic binaries are close enough that they cannot be separated with telescopes, but their motion is revealed through the Doppler shift in their light spectra.
• 💫 Mizar and Alcor, part of the Big Dipper, are part of a sextuple star system, with Mizar being a binary of spectroscopic binaries and Alcor being a spectroscopic binary itself.
• 🌠 Stars can form in larger groups beyond binaries, such as triple, quadruple, and even sextuple systems, with Polaris being an example of a pentuple system.
• 💥 Some binary stars, like 4U 1820-30, have such strong gravity that they orbit each other in minutes, while others can have orbits that last centuries.
• 🌑 Eclipsing binaries are systems where one star passes in front of the other, causing a dip in the total light observed, providing insights into their sizes and distances.
• 🤝 Contact binaries are stars that have become so close they physically touch, sometimes leading to mass transfer and dramatic changes in their evolution.
• 💥 Mass transfer in close binaries can lead to periodic explosions called 'stellar novae', and if a white dwarf reaches a critical mass, it can result in a supernova explosion.

Q & A

What is the Sun's status in the solar system regarding other stars?

The Sun is the only star in our solar system, with planets, moons, asteroids, and comets orbiting it, but no other stars are part of our solar system family.

What is the common misconception about stars based on visual observation?

The common misconception is that stars, like the Sun, are single entities, but in reality, many stars have companions and travel the universe together.

What are 'optical double stars'?

Optical double stars are stars that appear close together in the sky by coincidence, even though they are actually very far apart in space.

What distinguishes binary stars from optical double stars?

Binary stars are stars that are physically orbiting each other, as opposed to optical double stars, which only appear close together due to line-of-sight coincidence.

What fraction of stars are estimated to be part of a binary or multiple star system?

Approximately a third to a half of all stars in the sky are part of a binary or multiple star system.

Which well-known binary star system is visible to the naked eye and has historical significance?

The binary star system consisting of Mizar and Alcor, visible in the handle of the Big Dipper, is known for being used as an eye test in ancient times.

How do binary stars typically form?

Binary stars almost certainly form together from two dense lumps in the same gas cloud, each collecting material until they become true stars.

What is the significance of visual binaries in the field of astrophysics?

Visual binaries are important because observing their orbital motion allows for the determination of the actual size and shape of their orbits, and subsequently, the masses of the stars, which is crucial for understanding stellar properties.

How do spectroscopic binaries differ from visual binaries?

Spectroscopic binaries are stars that orbit so closely together they cannot be split even with the largest telescopes; their binary nature is revealed through the Doppler shift observed in their spectra as they move towards and away from us.

What is unique about the binary star system Mizar and Alcor in terms of their classification?

Mizar and Alcor form a sextuple star system, with Mizar being a visual binary of spectroscopic binaries and Alcor being a spectroscopic binary.

What is the phenomenon where a binary star system's orbit appears edge-on from Earth, causing periodic dimming?

This phenomenon is known as an eclipsing binary, where one star passes in front of the other, blocking its light and causing a dip in the total light observed from the system.

What are contact binaries and how do they affect the stars involved?

Contact binaries are binary stars that are so close together they physically touch, potentially stretching into teardrop shapes due to tidal effects and leading to mass transfer between the stars, which can alter their evolution.

What is the Algol Paradox and how does it occur?

The Algol Paradox refers to a contact binary system where the more evolved star is the white dwarf, which is the opposite of what is typically expected. This occurs due to mass transfer from the initially more massive star to the initially less massive star, reversing their roles over time.

What are recurrent novae and how do they form?

Recurrent novae are a type of binary star system where a white dwarf accretes matter from a companion star, and when enough hydrogen accumulates on the white dwarf's surface, it undergoes a thermonuclear explosion, creating a bright flare. This process can repeat periodically.

What catastrophic event can occur if a white dwarf in a binary system accumulates enough mass?

If a white dwarf in a binary system accumulates a mass around 1.4 times that of the Sun, it can undergo a runaway thermonuclear event, leading to a supernova explosion, which completely destroys the star.

Outlines

00:00

🌟 The Solitary Sun and the Social Stars

The script begins by emphasizing the uniqueness of our solar system's star, the Sun, which travels space accompanied by planets, moons, asteroids, and comets but without another star. This leads to a common misconception that stars, like the Sun, are solitary. However, upon closer inspection with a telescope, it's revealed that many stars have companions, contradicting the initial assumption. The text introduces the concepts of 'optical double stars', which are stars that merely appear close together but are actually distant, and 'binary stars', which are physically orbiting each other. It's estimated that a significant portion of stars are part of binary or multiple star systems. An example of a binary system visible to the naked eye is the two stars Mizar and Alcor in the Big Dipper. The script explains that binary stars likely form together from dense lumps in a gas cloud, and there are different types of binary stars, including 'visual binaries', which can be seen separately with a telescope, and 'spectroscopic binaries', which are identified through the Doppler shift in their spectra. The importance of studying binary stars is highlighted as it allows for the measurement of stellar masses, a key to understanding various properties of stars and contributing to the field of astrophysics.

05:03

🌌 Diverse Dynamics of Binary Stars

This paragraph delves into the various dynamics and phenomena associated with binary stars. It starts with the possibility of long-lost siblings of the Sun existing in the universe, which may be challenging to identify due to the vast distances and time scales involved. The discussion then moves to the diverse orbital periods of binary stars, ranging from very short to very long, exemplified by the peculiar binary star system 4U 1820-30, which completes an orbit in just over 11 minutes. The concept of 'eclipsing binaries' is introduced, where one star passes in front of the other, causing a dip in the total light observed from the system. This phenomenon can provide insights into the stars' sizes, masses, and other properties. The paragraph also explores 'contact binaries', where two stars are so close they physically touch, leading to unique interactions and transformations, such as the Algol Paradox, where mass transfer between stars can reverse their expected evolutionary stages. Additionally, the text describes how mass transfer in binary systems can lead to periodic explosions known as 'stellar novae' or even 'recurrent novae', and in extreme cases, the entire white dwarf can undergo a 'runaway thermonuclear event', leading to a supernova explosion.

10:06

💥 Supernovae and the Life of Binary Stars

The final paragraph wraps up the discussion on binary stars by focusing on the dramatic end stages of their life cycles. It describes how a white dwarf in a binary system, after accumulating enough mass from its companion, can undergo a catastrophic explosion known as a supernova. This process is distinct from the supernova of a high-mass star but releases a comparable amount of energy. The supernova not only destroys the white dwarf but also illuminates the surrounding universe. The paragraph concludes by summarizing the key points about binary stars, their formation, the various types, and the significant role they play in astrophysics, including the study of supernovae. The script credits the production team and consultants behind 'Crash Course Astronomy', highlighting the collaborative effort in creating the educational content.

Keywords

💡Solar System

The solar system consists of the Sun and all the celestial bodies that are gravitationally bound to it, including planets, moons, asteroids, and comets. It is the primary context of the video, as it discusses the Sun as the only star in our solar system and the various celestial bodies that accompany it through space.

💡Binary Stars

Binary stars are a pair of stars that are physically orbiting each other, as opposed to optical double stars which only appear close together by chance. The concept of binary stars is central to the video, illustrating how a significant portion of stars in the universe are not solitary but part of a system, with the video providing examples such as the stars Mizar and Alcor.

💡Optical Double Stars

Optical double stars are pairs of stars that appear close together in the sky but are not physically associated with each other; they are merely a line-of-sight coincidence. The script mentions these to contrast with binary stars, emphasizing that while they look close, they are actually very far apart in space.

💡Visual Binary

A visual binary refers to a binary star system where the two stars can be separately observed using a telescope. The video uses Sirius as an example of a visual binary, highlighting how advancements in telescope technology have allowed us to resolve closer stars and observe their orbital motion.

💡Spectroscopy

Spectroscopy is the study of the interaction between light and matter, and in the context of the video, it is used to identify spectroscopic binary stars. These are systems where the two stars are too close to be resolved by a telescope, but their orbital motion can be inferred from the Doppler shift in their spectral lines.

💡Spectrophotometric Parallax

While not explicitly mentioned in the script, the concept of determining the distance to stars through spectrophotometry is implied. It is a method that involves measuring a star's brightness across different wavelengths and comparing it to its known intrinsic brightness to calculate its distance.

💡Stellar Masses

The masses of stars are a fundamental property that can be accurately determined through the study of binary stars. The video explains that by observing the orbital motion of binary stars, astronomers can apply the laws of physics and gravity to calculate the masses, which in turn helps in understanding other properties of the stars.

💡Eclipsing Binaries

Eclipsing binaries are a type of binary star system where the orbits are aligned in such a way that one star periodically passes in front of the other, causing a dip in the total light observed from the system. The video describes how this phenomenon can provide insights into the size, mass, and other properties of the stars.

💡Contact Binaries

Contact binaries are binary star systems where the stars are so close that they physically touch, often deforming into teardrop shapes due to tidal forces. The script uses this concept to illustrate the complex interactions and mass transfers that can occur between stars in such systems.

💡Algol Paradox

The Algol Paradox refers to a phenomenon observed in contact binary systems where the more massive star has evolved into a white dwarf, and the less massive star has grown larger, an inversion of what is typically expected. The video uses this term to highlight the unexpected outcomes of mass transfer in binary systems.

💡Novae and Supernovae

Novae and supernovae are types of stellar explosions. Novae occur in binary systems where a white dwarf accretes matter from a companion star and undergoes a thermonuclear explosion on its surface. Supernovae, on the other hand, are the explosive deaths of stars, marking the end of their lifecycles. The video discusses these phenomena to demonstrate the dramatic events that can occur in binary star systems.

Highlights

The Sun is the only star in our solar system, with no other stars as part of our celestial family.

Contrary to the apparent isolation of stars, many travel the universe with companions, challenging the notion of solitary stars.

Optical double stars are pairs that appear close together by chance, but are actually far apart in space.

Binary stars are physically orbiting each other, unlike optical doubles, and make up a significant portion of stars in the sky.

The naked-eye visible binary system of Mizar and Alcor in the Big Dipper has been known for thousands of years and used as an ancient eye test.

Binary stars likely form together from dense lumps in a gas cloud, leading to the birth of two stars instead of one.

Visual binaries are those where the two stars can be separately observed with a telescope, such as Sirius, the brightest star in the night sky.

Observing visual binaries over time allows for the measurement of their orbital motion and the determination of their masses, a key to understanding stellar properties.

Spectroscopy reveals the binary nature of stars too close to separate visually, through the Doppler shift in their spectra, identifying spectroscopic binaries.

Mizar and Alcor, initially thought to be a simple binary, are revealed to be part of a complex sextuple star system.

Stars can form in larger groups beyond binaries, such as triple, quadruple, and even sextuple star systems, like the one including Mizar and Alcor.

The possibility that the Sun was born in a multiple star system is considered, though its siblings may be lost to the vastness of space and time.

Binary star orbits can vary greatly, from those taking centuries to complete to the extremely close 4U 1820-30, which orbits in just 11.4 minutes.

Eclipsing binaries occur when one star passes in front of another, causing a dip in the total light observed, providing insights into their properties.

Contact binaries are a unique phenomenon where two stars are so close they physically touch, potentially merging into a peanut shape.

Mass transfer in close binaries can lead to the Algol Paradox, where a more evolved white dwarf is paired with a higher mass star.

Recurrent novas and supernovae are extreme outcomes of mass transfer in binary systems, where a white dwarf can accumulate matter and explode.

Binary stars are crucial for astrophysics, providing insights into stellar masses, sizes, brightness, and lifespans.

Transcripts

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.