Binary Stars

Even though more than half of the stars in our universe are binary stars, a majority of us tend to ignore the more profound understanding and meaning behind these stars. In fact, most of us get our knowledge about binary stars from the star system on the planet ‘Tatooine’ from Star Wars. Well, there’s a lot more to binary stars than that! 

Binary stars are two stars locked in each other’s gravitational pull. So, why are binary stars such a big deal? Binary stars are the only means of directly determining the stellar masses of stars other than our Sun. For astronomers, stellar mass is an eminent property that helps them determine the life cycle and fate of a star. To understand this application of binary stars, we must start from the beginning.

How do binary stars form? Often, binary stars are born from a parent nebula. The parent nebula consists of two component stars that are created separately. After their creation, solar winds (a stream of charged particles released from the upper atmosphere of a star) get rid of the remaining nebular material. This leaves behind the two stars in such close proximity that they cannot escape each other’s gravitational pull. On a few occasions, even a free star can fall into another star’s gravitational pull to become locked in an inescapable orbit and form a binary star system.

The classification of binary stars is based on either how we detect them or by an actual process. There are five major types of binary stars based on how we observe them. These are optical binaries, visual binaries, astrometric binaries, spectroscopic binaries, and contact binaries.

To begin with, we have the optical binary system, which is nothing but an illusion(Sorry, Mysterio! The universe is a better illusionist!). In these types of star systems, we have two stars that lie in the same line of sight for us on Earth, but in reality, the two stars are far apart. However, not all stars seen together from Earth have to be optical binaries. Visual binaries belong to the actual binary star system and can be easily spotted with the help of a telescope. 

Fig1: An optical binary star systems.

A binary star system can also be an astrometric binary system, where we only see one of the binary stars. The other ‘shy’ star cannot be seen unless the motion of the visible binary star is analyzed. If the visible star has an oscillatory motion, we can conclude that it is a binary star system. This oscillatory motion is a result of the gravitational pull the invisible star has on the visible one. Furthermore, we also have the spectroscopic binaries, where only one source of light is seen. If the spectrum of the star shows Doppler shifts from red-shift to blue-shift periodically, it is a binary star system.

Fig2: A spectroscopic binary star system.

The last type of binary star system we’ll cover in this article is my favourite, because of its exotic nature. This is the contact binary system. Their component stars are so close to each other that they literally touch each other or have merged into a single giant envelope.

Fig 3: Contact binary star system formation.

In the contact binary star system above, we can see the term ‘LaGrange point’ mentioned, which is basically the point where the gravitational forces of two bodies produce an enhanced region of attraction and repulsion. Also mentioned is the term ‘Roche lobe.’ This is a region around a star in a binary system, where the orbiting material is gravitationally bound to that star.

Before we finally learn about finding the masses of binary stars, we need to familiarize ourselves with the three Kepler Laws:

1) All planets move about their star in elliptical orbits with the star acting as one of the foci.

2) A radius vector joining any planet to its star sweeps out equal areas in equal lengths of time. 

Fig4: Kepler’s second law

3. The squares of the time periods of revolution of the planets are directly proportional to the cubes of their mean distances from their star. 

If we assume that the masses of the component stars in a binary star system are similar, we can adopt the Kepler’s law, and modify it to: 

  1. The stars orbit each other in elliptical orbits, with the center of mass (or barycenter) as one common focus.
  2.     The line between the stars (the radius vector) sweeps out equal areas in equal periods of time.
  3.  The square of a star’s period is directly proportional to the cube of its average distance from the center of system mass.

Using these three laws, scientists have arrived at the equation:

MA + MB = 4π2r3/GT2

where MA is the mass of star A and MA is the mass of star B. Thus, this equation can be used to find the mass of binary stars.

To conclude, most of the stars in our galaxy are binary stars, and they are the only other way for us to measure the masses of stars other than the Sun. However, calculating masses is just one aspect of the importance of binary stars. What other significance do these stars have? How are binary stars related to variable stars? All these questions will be answered soon! So, keep reading!!! And continue to speculate, innovate, till you constipate!

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