How Are Binary and multiple stars possible?
What do stars have in common with humans? In this video, we are going to talk about
it. Humans like to socialize, staying together
and gossip a lot. We look for a company, we have friends and
some of us (the bravest ones!) have a partner! Since we are babies, we need someone to keep
us warm and safe. We strongly depend on others. This is a wonderful trait of the human race.
We cannot stand being truly alone. What if I told you that some stars follow
exactly this trait? Does it sound weird to you?
Let's dive into this video to get to know more about pairs of stars, known as binary
stars.
Binary stars are, very simply, stars orbiting around each other.
They are linked by their gravitational attraction and orbit around their common centre of mass.
The term binary was first used in this context by Sir William Herschel in 1802 when he
wrote:
If, on the contrary, two stars should really be situated very near each other, and at the
same time so far insulated as not to be materially affected by the attractions of neighbouring
stars, they will then compose a separate system, and remain united by the bond of their own
mutual gravitation towards each other. This should be called a real double star; and any
two stars that are thus mutually connected, form the binary sidereal system which we are
now to consider.
Here's how a binary system appears. Notice how the stars rotate around each other,
in some sort of dance.
Binary stars are a very important object in astrophysics because they can help us understand
some important things about planet formation and star masses.
Let's first discuss how useful they are in terms of hosting planets.
While a number of binary systems have been found to harbour extrasolar planets, such
systems are comparatively rare compared to single star systems. Observations by the Kepler
space telescope have shown that most single stars of the same type as the Sun have plenty
of planets, but only one-third of binary stars do.
In fact, we know that for a planet to be born we need a rocky grains disk from which the
so-called protoplanet forms. And in the other hand, we know that tidal
forces of massive objects in the neighbourhood of the rocky disk can disrupt it, leading
to a missed planet formation. That's exactly what happens in a binary system.
Even widely separated binary stars often disrupt the discs of rocky grains from which protoplanets form!
Only one third f binary stars do have planets.
But we should also keep in mind that simulations suggest that the presence of a binary companion
can actually improve the rate of planet formation within stable orbital zones by "stirring up"
the protoplanetary disk, increasing the accretion rate of the protoplanets within, where the
accretion rate is the growth in the mass of the protoplanetary disk in the time period.
Now try to think about it for a while: what would we see if Earth was not the third planet
of the solar system? What if Earth was created in a binary environment?
What would we see at that point? Well, the answer seems pretty clear: we would
have two stars. But could it be possible for us to exist?
The answer is: it depends. It is estimated that 50-60% of binary systems are capable
of supporting potentially habitable planets. Withing stable orbit ranges.
For stable orbit ranges we mean that, unluckily, a planet could actually be easily expelled
from its orbit, due to chaos arising from the dynamical systems of 3 bodies.
Science fiction has often featured planets of binary or ternary stars as a setting, for
example, George Lucas' Tatooine from Star Wars, and one notable story, "Nightfall",
even takes this to a six-star system. In reality, some orbital ranges are impossible for dynamic
reasons. Also, even in the range of stable orbits,
we found ourselves in trouble, and binary systems present serious challenges for eventual biospheres because
of likely extreme variations in surface temperature during different parts of the orbit.
However, if a planet does exist, does it orbit around one of the two stars? Or does it orbit
around both? The answer is: it can be the first case, or
the second as well. Planets that orbit just one star in a binary
system are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type"
or "circumbinary" orbits.
Very well. Now that we know what a binary star is, let's
understand how we discovered them. Do we need particular instruments to catch them? What
are our methods of observations?
Binary stars can be studied visually; spectroscopically; photometrically, by changes in brightness
caused by an eclipse; or astrometrically, by measuring a deviation in a star's position
caused by an unseen companion. We are going to spend some words about visually
and spectroscopically determined binary systems. Any binary star can belong to several of these
classes; for example, several spectroscopic binaries are also eclipsing binaries.
The ones that can be studied visually are called visual binaries.
The term visual here means that we can observe it as a real double star in our telescope
(or even high-powered binoculars!). This happens not so often because we need
the angular separation between the two components to be great enough to permit them to be observed.
And this depends on how much they are separated physically, and on how distant they are.
Talking of visual binaries, the brightest star is called Primary, and the other is called
a secondary star. Fun fact: if the stars are equally bright,
the discoverer designation for the primary is customarily accepted.
However, visual stars are not so simple to find.
And here the Doppler effect comes into play. Sometimes, the only evidence of a binary system
is indeed the Doppler effect on its emitted light.
In very short and simple terms, the doppler effect shifts the wavelength of the light
spectra of a star towards the red part of the spectrum, if the star is moving away from
us; it shifts towards bluer wavelengths if the star is approaching us.
This means that, if we see some variations during a period of time in the light spectra
of a star, going from bluer to redder wavelengths or vice versa, we can say pretty safely that
is a binary star. In these systems, the separation between the
stars is usually very small, and the orbital velocity very high.
Ok, this is all good. But what are the main difficulties and challenges
when it comes to finding a binary system? For example, suppose you are screening the
sky with your own telescope, and suddenly you find two stars, very close to each other.
How can you distinguish a real binary system? Is it possible that you have a perspective
biased view, isn't it? That's exactly the case.
That's the reason why astronomers coined another term for stars that appear very close in the
sky, but when we account for the third dimension we find they are not bounded.
They call them Double stars, where double means they're optical double stars.
Some double stars are binary stars, but not all of the binary stars are double stars.
I bet you got the difference between them.
So, for now, we know that some stars are happy to be connected.
They are happy to be bounded, and one depends on the other.
But, as it happens for humans, sometimes a star is not linked just to another one, but
to three of four, or even more of them! These are called multiple systems.
A binary system is multiple systems consisting of 2 stars.
When it comes to multiple systems, the discussion about planet formation is pretty the same,
but the chaos theory here dominates and usually, a planet is going to be expelled very quickly.
Poor planet!
And now, let's try to answer the last question, maybe the most important one.
Is there any binary system in the Milky Way? How many stars are binary stars, in our galaxy?
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Of all the stars systems in the Milky way, about one-third of them are bounded in binary
or multiple systems. If we take a number of 100 billion stars in the Milky way, this means
there are about 30 billion binary systems waiting to be studied!
Okay, now we know a lot of things about binary systems!
But what are they? Can we give some famous examples of binary systems?
The first one is Antares. Antares is a star in the Scorpion constellation,
and it is a red supergiant in a binary system with a hotter blue main-sequence star Antares-B.
Another example is the Famous and beautiful star Eta Carinae.
If you take a picture of Eta carinae you will see it is going under strong loss mass processes,
because it is a blue variable star. The secondary star appears to have a higher
temperature than the primary. I want also to mention the example of R-Aquarii.
R Aquarii is in the constellation of Aquarius and shows a spectrum that simultaneously displays
both cool and hot signatures. This combination is the result of a cool red
supergiant accompanied by a smaller, hotter companion. Matter flows from the supergiant
to the smaller, denser companion.
Also, just to give an example of a not yet confirmed binary system, we can mention Mizar
and Alcor. They are stars in the Ursa Major constellation
and are separated by almost 12 arc minutes. Although they share the same origin and the
same motion right in space it is not certain that they are gravitationally linked to each
other.
The last thing I want to say about binary stars is that, as a main-sequence star increases
in size during its evolution, it may at some point exceed its Roche lobe, meaning that
some of its matter ventures into a region where the gravitational pull of its companion
star is larger than its own. The result is that matter will transfer from
one star to another through a process known as Roche lobe overflow, either being absorbed
by direct impact or through an accretion disc. The mathematical point through which
this transfer happens is called the first Lagrangian point.
It is not uncommon that the accretion disc is the brightest (and thus sometimes the only
visible) element of a binary star.
"This video ends here! Thanks for watching everyone! Do you like binary stars?
Is there any binary star you would hear about? Let us know in the comments below! And remind
me to keep it curious! See you next time on the channel!”