Binary Systems : 4 Spectacular types of binaries and their applications

– “Binary Systems” written by Yash Bhat, reviewed by Muskan Agarwal.

What are binary star systems?

Have you ever wondered how planets started revolving around the sun? Well, technically they were moving in a different direction, but it was the sun’s gravity that started pulling them towards itself. Just imagine a situation where you throw a stone, and a situation where you tie it with a thread and start revolving it with your hand. The former was the situation of the planets before they started revolving around the sun and the latter represents the impact of the sun’s gravity.

But, these planets that we are talking about, are way smaller and lighter as compared to our sun. So, what do you think would happen, if an object, as heavy as the sun, comes in the influence of the sun’s gravitational pull? You might say they will bump into each other, well that is a possibility, but again what if the momentum of that object is not directed exactly towards the sun?

In this case, the sun and that heavy body would start revolving around each other, and it is this system that we call a binary system. If both bodies are stars, then we call it a binary star system or simply binary stars.

binary systems

Now, I would like to define binary stars in a more technical aspect. Binary stars are two stars that orbit a common centre of mass. The primary star is the brighter of the two, while the secondary star is the dimmer one (classified as A and B respectively). In circumstances where the stars are of comparable brightness, the discoverer’s designation is followed.

What are the different types of binary star system?

There are 4 main types of binary systems namely:

  • Visual binaries
  • Spectroscopic binaries
  • Eclipsing binaries
  • Astrometric binaries

There is another type of binary system called the “exotic type binaries” which does not fall under either of the four star categories.

  1. Visual binaries

A visual binary is a binary system in which the system’s component stars may be resolved separately using a telescope. The relative positions of the system’s members can then be plotted using long-term measurements. This information is gathered over time and utilised to compute the orbits of the stars.

Visual binary systems are systems that are relatively close to us in order to resolve the individual stars. They’re systems with component stars that are physically separated by tens to hundreds of AUs. The stars in such systems are gravitationally bound to one another, but they do not “interact” in the same way that other close binaries do, where one star may pull material from the surface of the other.

Many of the brightest stars in the night sky are visual binary systems. Centauri, which is 1.338 pc away, is a visible binary, with the two stars Cen A and Cen B separated by around 23 Astronomical Units, which is somewhat more than the distance between Uranus and the Sun. They have an 80-year orbital period between them.

Spectroscopic binaries

Most binaries are too distant for modern telescopes to resolve as visuals. Others are simply too close to be resolved in their own right. How can they be identified as binary, then? Doppler shifts in spectral lines have been used to detect the majority of binaries. Such systems are referred to as spectroscopic binaries.

If a binary system cannot be resolved into its component stars, the spectrum produced will be a combination of the spectra from each of the stars. One of these stars, A, may be travelling towards us while the other, B, is going away from us as they orbit each other.

As a result, A’s spectra will be blue-shifted to higher frequencies (shorter wavelengths), whereas B’s will be redshifted. When the stars move across our line of sight, there is no Doppler shift, and the lines remain in their normal places. As the stars continue to orbit each other, A’s spectral lines will shift towards the red end of the spectrum, while B’s will shift toward the blue.

Now I would like to give an example of this system. Mizar, or Ursae Majoris, was the first spectroscopic system found in 1889. Mizar was formerly thought to be a visual binary, but spectroscopic examination of the brighter of the two stars, Mizar A was revealed to be a spectroscopic binary.

Following studies, it was discovered that Mizar B was also a spectroscopic double, bringing the total number of stars in the system to four. Modern astronomers can now “separate” or resolve Mizar A into its component stars thanks to recent advances in optical interferometry and imaging techniques.

Eclipsing Binaries

Photometric measurement is used in the third way of detecting a binary system. The apparent magnitude of many stars changes on a regular basis. This could be due to one of two factors. It could be a single star that has its inherent luminosity change. Such stars are called pulsating variables . The second option is that it is a binary system with an orbital plane that is edge-on to us, causing the component stars to regularly eclipse one another. Such systems are known as eclipsing binary systems.



Periodic brightness dips characterize eclipsing binary light curves, which occur whenever one of the components is eclipsed. The primary eclipse has a larger loss in brightness than the secondary eclipse, which has a lesser drop. Unless the two stars are identical, a primary eclipse is more likely to cause a brightness decline than a secondary eclipse. As a result, one period of a binary system contains two minima. Why is it that one eclipse will result in a greater decrease in light than the other?

Assume you have two stars in mind: star 1 and star 2. Let star 1 be hotter than star 2. This means that it radiates more energy per unit surface area than the colder star 2 (remember, L T4), according to Stefan’s Law. As a result, when star 1 passes in front of (or is eclipsed by) star 2, more flux is blocked than when star 2 eclipses star 1. As a result, the major eclipse occurs whenever the hotter of the two stars is obscured. When a hotter star passes in front of a cooler star, a secondary eclipse occurs.


Astrometric Binaries

When monitored over time, some stars exhibit a disturbance or “wobble” in their proper motion. We can deduce that the disruption is caused by the gravitational attraction of an unseen partner if it occurs on a regular basis. A bright star and a fainter companion circle the same center of mass in our system. Binary systems detected by such astrometric means are called astrometric binary systems.

One of the best examples of this system would be Sirius. The presence of Sirius B, a considerably fainter white dwarf companion, causes minor disturbances or wobbles in the brilliant star Sirius A. Sirius was discovered as an astrometric binary system, but it is currently classified as a visual system.

Due to the necessity for long-term observations and the uncertainty in position and proper motion determinations, only a few binaries have been found astrometrically. With the next generation of space-based astrometric missions, this will undoubtedly alter. Sirius is the most well-known example of an astrometric binary system. It had a wobble in its proper motion, according to Friedrich Bessell in 1844. He deduced from this that the visible star, now known as Sirius A, must have an unseen (and so dim) partner, Sirius B.

Alvan Clark only saw this telescopically in 1862, and it is now recognized to be a dim white dwarf. Procyon, also known as A Canis Minoris, was discovered as an astrometric binary system. It, too, has a white dwarf companion, which may now be seen through telescopical observation.

“Exotic” type of binaries

The first-known binary pulsar, PSR J0737-3039, was one of the most fascinating celestial objects found in late 2003 on the Parkes radio telescope. It has a 23-millisecond pulsar, PSR J0737-3039A, and another pulsar, PSR J0737-3039B, which rotates every 2.8 seconds and orbits each other every 2.4 hours.

This unusual binary system has piqued the interest of astronomers all around the world because it serves as a fantastic test bed for General Relativity and the search for gravitational waves. It’s not only the first of its kind to be discovered, but it’s also an eclipsing binary system. However, because of relativistic influences on the orbit, it is only expected to remain in an eclipse orbit for another ten years or so.

How does studying about binary systems help astronomers in their field work?

Over four-fifths of the single spots of light we see in the night sky are actually two or more stars revolving around each other. Binary systems, which consist of only two stars, are the most common multiple star systems. These couples can be found in a variety of combinations that aid scientists in classifying stars and may have implications for the evolution of life.

Binary systems are the most accurate way for scientists to determine a star’s mass. Astronomers can compute the size of the pair as they tug on each other, and then use that information to determine temperature and radius. These characteristics aid in the identification of solitary main sequence stars throughout the universe.

Binaries are particularly significant to our understanding of the processes by which stars develop since a large fraction of stars exist in binary systems. The period and masses of the binary, in particular, reveal the quantity of angular momentum in the system. Binaries provide crucial information regarding the environment under which stars formed because this is a conserved quantity in physics.

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