Man must rise above the Earth—to the top of the atmosphere and beyond—for only thus will he fully understand the world in which he lives.

                            --Socrates

We know that sun is the only star of our solar system and all the planets are orbiting around the sun. Now, is it possible that there is star system where there are two stars present or even more? Well, although it’s hard to think about this, cause it is counterintuitive but this kind of star system does exist. Now, the question that can arise is how do they orbit? How can they be stable?

The answer is, they orbit around their mutual center of mass because of their gravity. Center of mass is closer to the massive and far from the lower mass star. Even it was proposed that our solar system is a binary star system. Approximately, half of the star systems around the sun is binary systems or multiple star systems, where there can be more than two stars. Although some of the stars are far apart from each other and we think they are not binary. But, when we investigate closely we see they are orbiting around a common center. In the naked eye, we can see that there are two stars who are so close but when we put a telescope we see they are far apart. They are called optical binary stars. There are different kinds of binary stars. I will try to give a good description of them in my text below. So, let us begin.

History

Ancient Indian people used to test someone's eyesight showing them two stars. These stars are so close to each other that most of the people couldn't detect them with their naked eye. They are known as Mizar and Alcor. They are the first ever known binary system. In modern days, we have very highly sensitive telescopes. Through these telescopes, astronomers discovered that Mizar itself is not a single star, rather a few stars orbiting each other. There are mainly four stars orbiting each other. It was also thought that Alcor is the fifth star of the system. But later it was found that Alcor is not alone, he has a companion too. This made them sextuplet.

Alcor and Mizar |source

Galileo's student first noticed the binary system Mizar through a telescope. A week later he told it to Galileo. Galileo Included this in his paper as Mizar A and Mizar B. During the end of 19^th century it was found that Mizar A itself a binary and beginning of 20^th century it was found even Mizar B is also binary. Until now a lot of binary and multiple systems have been found.

Binary star formation

Binary star formation can happen in the same way as single star formation. A vast cold molecular cloud can form a star. These clouds are made of dust and gas. As this cloud is so cold, it is so dense. So these gas and molecules come close together to form molecules and it gets even denser.

Binary star formation |source But after a critical temperature, it starts to heat up because of pressure and get denser in the center. As the center is denser than others parts it will break into fragments. This fragments can form protostars. These protostars can be captured by each others gravity and orbit around a common center of gravity and form binary or multiple star system. There are other ways of forming binary star system. Two single stars can be captured by each others gravity and form a binary system. Even one star can be spilled into two. Until now forming binary star has never been observed.These methods above are suggested by astrophysicists and they think this is how a binary system can form.

Classification of Binary stars

There are actually four types of vinary stars. They are --

1. Visual binary star

2. Spectroscopic binary star

3. Eclipsing binary star

4. Contact binary star

Visual binary star

We can observe them through visual observation. They are far enough to be observed visually. To observe visual binary stars, the observation should be long. As their orbital period can be from ten to thousands of years. Visual binary stars are made of two stars. One star can be brighter than the other. Sometimes it can be really difficult to separate one from the other if their difference in brightness is so much. But we can look through the telescope and find the center of mass around which the bigger star is orbiting. This is how we can separate one star from the other. The brighter star is called Primary star and the fainter is called comapainion.

Orbits of visual binaries |source

Visual binary stars are orbiting around each others mutual center of gravity. When they are far from each other the move slowly. When they come close they move faster.

Usually for the visual binary star we can see the orbit of both stars from earth. The orbit we are seeing is not the actual orbit. It is just the projection of the actual orbit. The angle between the actual orbit and projection should be less than 90 degrees.

Angle between actual and apparent orbit |source

The orbits of visual binary stars are usually elliptical. There can be two reasons. First, their orbits are actually elliptical. Second, Their orbits are actually circular but because of the angle between actual orbit and projection, they look elliptical.

Visual binary stars are separated by angular distance. So, telescopes with better resolution will find out more and more visual binaries.

Spectroscopic binary star

When in a binary star system, two stars are so close or one is much brighter than the others, we can't separate them by telescopes. We use spectrometry to separate them. If the difference in brightness is so much, then we can get the spectra of only one star. We can see the blue or red shifted line. When the star is moving towards observer we see a blue shift and when moving away from the observer, we see a redshift.

Detection of spectroscopic binary |source

But if the stars are of similar brightness. Then we can see the spectroscopic blue and red shift oscillating depending on the stars approaching and going far. If the angle between the actual orbit and the projection is zero degree, we can't detect them.

Ecilipsing binary star

In a binary star system, the star's orbits can be so close that there happens eclipse. Like solar eclipses. They are called eclipsing binary stars. We use the light curve to identify them. Originally one of the stars is brighter than the other star. So, when the brighter star eclipses less bright we see dips in the spectra because the total light of the other star is eclipsed. When the less bright comes in front of brighter one, there are dips but is less than before. Studying the light cure we can determine the angle of the orbital plane, their period of revolution, the radius of each component, their temperature as well as their orbital orientation.

Eclipsing binary |source

Contact binary star

There are binary star system where the stars are so close to each other that they exchange mass. This kind of stars are called contact binary. Most of them are usually ecliptic binary star. If they get too close they can form a peanut shape. Let us think of two stars one is heavier blue star another is a red dwarf. Because of nuclear fusion the blue star will lose mass and as the red dwarf doesn't lose mass, it can swallow the mass of the blue star and get bigger and bigger and the blue star can transform into a white dwarf. Later again this process can go on.

Contact binary star |source

Is our solar system a star binary sistem

During the end of 20th century, some astronomer stared claiming that our sun has another companion. Its a red dwarf star which is still undetected. They named it Nemesis. Their theory was that it can explain a lot events like the extinction of dinosaurs. They even think that its orbit can hamper the Oort clouds. Today we have so much sensitive telescope and no stars like this even found still.

Conclusion

Learning about binary stars we can learn about a lot parameter of stars in the system like their mass, radius effective temperature etc. Especially contact binary stars are of special interest as there can happen so many astonishing events.

References

[1] https://en.wikipedia.org/wiki/Binary_star

[2] https://www.universetoday.com/24203/what-is-a-binary-star/

[3] https://www.space.com/22509-binary-stars.html

[4] http://www.astronomy.com/news/2014/01/new-studies-give-strong-boost-to-binary-star-formation-theory

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