What are Binary Star Systems? — Astronomy Explained

Stars come in twos — most of the time

This article was originally published at: https://astronomyexplained.com/what-are-binary-star-systems/

Did you know over 50% of all stars in our Galaxy are in pairs or groups? Over 50% is a significant number! We call these binar star systems — if they are in pairs. You see how common binary star systems are? But what are binary star systems? Why are they so frequent and are they important?

Check this article out, too: History of Dark Matter: Journey Through Time

Binary star systems have two stars orbiting around a shared center of mass. These pairs move through space, held together by gravity. The Washington Double Star Catalog lists over 100,000 pairs of double stars as of June 2017. This huge number shows the variety of multiple-star systems in our Galaxy. These systems are diverse, from pairs we can see with telescopes to those we can only detect with special tools.

Stars like our Sun, for example, are not binary star systems; they are one-star systems. That’s why binary star systems are interesting to us. It may help to take a look at how planets and stars evolve in different environments than those we know. These pairs can go through special events like mass transfer and gravitational effects. These events shape their paths in ways single stars can’t.

Understanding Binary Star Systems: A Fundamental Overview

Binary star systems are a common feature in our universe — even in our Milky Way Galaxy. They consist of two stars orbiting around their common center of mass. Sir William Herschel first used the term “binary stars” in 1802. Today, we know that about 85% of stars in the Milky Way are part of binary or multiple-star systems.

Definition and Basic Concepts

Binary stars are gravitationally bound stars that orbit each other. Unlike optical doubles, which only appear close to Earth, binary stars are physically connected. These systems are more common than once thought (as I mentioned above, over 85% of stars are part of either binary or multiple-star systems). That’s why studying binary star evolution is a key area for us. We have to understand something that common.

These stars are gravitationally bound, meaning the force of gravity keeps them locked in their shared motion. Binary systems can vary in size, brightness, and distance between the stars. In some cases, these stars are so close that they share material, while in others, they are separated by vast distances but still connected through gravity. Binary stars are part of a larger category called multiple-star systems. These systems have three or more stars, like triple-star systems.

Gravitational Binding in Binary Systems

The key to understanding binary stars is about understanding their gravitational bond. These stars are constantly pulled towards each other by gravity. This center of mass is like a balance point, and its position depends on the masses of the two stars. If both stars have similar masses, the center of mass is located near the midpoint between them. If one star is much heavier, the center of mass will be closer to the heavier star.

As the stars orbit, they can exchange mass, affecting their individual development. We call this gravitational binding. Gravitational binding plays a significant role in binary star evolution. As stars age and change and they transfer mass between one another, they influence their growth, temperature, and lifespan.

What are Binary Star Systems and Their Classification Types

We classify binary star systems into three main types based on how we observe them. This classification type is somewhat similar to how we classify stars in general. These classifications are visual binaries, spectroscopic binaries, and eclipsing binaries.

1. Visual binaries: These are binary star systems where we can observe both stars separately through a telescope. This classification is possible when the stars are far enough apart to distinguish them visually. They are the simplest to spot. There are over 100,000 that we listed. Most are 10 to 20 Astronomical Units apart. Big telescopes can even spot them up to 1,300 light years away.
2. Spectroscopic binaries: In these systems, the stars are too close to be resolved individually through telescopes. So they are harder to find. Instead, we detect them by looking at their light spectra changes, the Doppler shifts. This shift occurs as the stars move toward or away from the observer during their orbital motion. We have thousands of these listed already.
3. Eclipsing binaries: These are systems where the orbital plane is aligned with our line of sight. One star blocks the light of its partners. This causes noticeable dips in brightness, which are carefully monitored to understand the stars’ sizes, distances, and orbital characteristics. Kepler satellite has found over 2,000 of these so far.

The Historical Discovery of Binary Stars

We first started to discover binary star systems after we invented the telescope. Mostly because we can’t see or distinguish them with the naked eye. That’s why their history doesn’t go as back as some other topics in astronomy. We’ve been studying them for about 200 years.

William Herschel’s Contributions

William Herschel was a pioneer in astronomy. He started studying double stars in 1779. By 1803, he found that many of these pairs were orbiting each other, held together by gravity. His amazing observations were the foundation for understanding these paired stars and their gravitational dynamics. He observed and cataloged numerous double stars using telescopes he built himself. This was the first systematic effort to analyze binary star systems. Additionally, he was the first to recognize that some double stars were not merely chance alignments but were gravitationally bound systems.

Development of Modern Binary Star Science

Herschel’s work set the stage for today’s binary star science. As technology improved, so did our ability to study these systems. We know that up to 50% of all stars are in binary systems. The most recent studies showed a deeper complexity of binary star systems. For example, within Perseus cloud, we found twenty-four multiple systems. About half of these are in elongated core structures, showing specific formation processes. This research suggests that each dusty core might give birth to two stars ( there is a Berkeley University study about this topic). This could mean that more stars are formed per core than we thought before.

Visual Binary Stars: Observing Double Star Systems

Visual binary stars are among the most fascinating and accessible types of binary systems for observational astronomy. These systems have seen two distinct stars that we can see individually through telescopes. The interesting bit about visual binary stars is that we can study the relative motion of the two stars around their common center of mass in much more detail over time.

Primary and Secondary Stars

In visual binary systems, stars can differ in brightness. The brighter one is called the primary, and the dimmer one is the secondary. The luminosity of the brighter one dominates the entire star system and it allows us to study its properties. The difference in brightness can also tell us about their sizes, masses, and stages of life. For example, Albireo in Cygnus has a golden primary star with a magnitude of 3.1 and a blue secondary at a magnitude of 5.1.

The distance between stars in binary systems varies a lot. Mizar and Alcor in Ursa Major are about 12 arcminutes apart. Closer pairs like Epsilon Lyrae need telescopes to see, with components just 3 arcminutes apart. Some visual binaries aren’t true pairs but optical doubles. Alpha Capricorni’s components seem close but are 526 light-years apart!

Visual binary stars are like cosmic labs for studying stars. They are great sources to study star formation, evolution, and the number of multiple star systems in our Galaxy. In fact, we only observed about 5% of the total binary and multiple-star systems in our Galaxy.

Eclipsing Binary Systems and Light Variations

In eclipsing binary systems, one star passes in front of the other one. It’s interesting because when it passes in front of the other one, it creates a change in the other star’s brightness. These brightness variations are periodic, and we can schedule these to understand when they form light curves, which provide a wealth of information about the stars’ properties, including their sizes, shapes, and orbital characteristics.

Eclipse Types and Phases

We categorize eclipsing binary systems based on eclipse types. In a total eclipse, one star completely blocks the other star’s light. This creates a significant dip in the overall system’s brightness. Partial eclipses are when the stars do not align perfectly, and only part of the light is blocked. Annular eclipses are rare and happen when one star appears smaller than the other, leaving a visible ring of light.

The change in brightness in eclipse is what makes them special. For example, the Algol system has a 2.9-day cycle of eclipses. During the primary eclipse, the brighter star shines with about 92% of the system’s light. The dimmer star adds less than 8% to the total light. The duration and depth of these phases are critical for determining the relative sizes and luminosities of the stars.

A total eclipse has a flat-bottomed light curve. Partial eclipses, on the other hand, show gradual changes in light. Systems with stars of different sizes are easier to spot than those with similar stars.

Famous Binary Star Systems in Our Galaxy

As I said, our Milky Way galaxy is filled with binary star systems. We have thousands of these binary star systems. However, some of these are more famous than others. Sometimes because of their proximity, sometimes because of their size, or because of something very unique. Here are three very famous binary star systems in the Milky Way Galaxy.

Sirius A and B System

Sirius, also known as “Dog Star,” is the brightest star in our Galaxy (also visible from Earth!), and it’s part of a binary system. That’s why it gets more attention from astronomers. We named the two stars in this binary star systems Sirius A and Sirius B.

Sirius A is much brighter than Sirius B, shining 10,000 times more. Sirius A is a luminous main-sequence star and Sirius B is a white dwarf. It’s incredibly dense, with the mass of our Sun but only the size of Earth. It was once a massive star that exhausted its nuclear fuel and collapsed into a dense, compact remnant.

This system is a prime example of the evolution of stars. It show us how stars can end their life cycles. We studied the orbit of Sirius A and B around their common center of mass extensively. We learned a ton about white dwarf physics and binary dynamics.

Algol Binary System

Algol is known as the “Demon Star” in the constellation Perseus. It’s an eclipsing binary system. This means its brightness changes as its stars pass in front of each other. The system consists of a larger, cooler star and a smaller, hotter companion. Algol is a textbook example of mass transfer in binary systems, where material flows from the larger star to the smaller, more compact star due to its close proximity.

The periodic dimming of Algol has been observed since ancient times, making it one of the most historically significant binary systems.

Cygnus X-1 System

Cygnus X-1 is one of the most famous and intriguing binary star systems in the Galaxy. It has a massive blue supergiant and a stellar-mass black hole. That black hole was one of the first black holes that astronomers confirmed. This system emits powerful X-rays as matter from the supergiant is pulled toward the black hole, forming an accretion disk. The discovery of Cygnus X-1 really changed the study of black holes and their interactions with companion stars. It also sheds light on the extreme environments around black holes, the processes of X-ray generation, and the life cycles of massive stars.

Conclusion

In the end, it’s obvious that binary star systems are really important to astronomical research. So, when you wonder what binary star systems are, there is quite a bit of depth to that question. They are important to reveal complex stuff between stars. From their discovery to modern classifications, these systems have been providing critical insights into the universe

. Maybe there is much more about them that we haven’t discovered yet? Who knows! The Karl G. Jansky Very Large Array is one of our biggest helpers to study binary star systems. I hope there will be much more in the future so that we can observe much more than the current 5% in our Galaxy.

FAQ

What are binary star systems?

Binary star systems are pairs of stars that orbit around a common center. They are held together by their gravitational pull. These systems are like a cosmic dance, with two stars moving together in space.

How common are binary star systems?

Binary star systems are quite common in our Galaxy. It’s believed that about half of all stars in the Milky Way are part of a binary or multiple-star system. So, binary stars are more common than you might think.

Who discovered binary stars?

William Herschel was the first to show that many double stars are actually pairs orbiting each other. His work in the late 18th century started the study of binary stars as we know it today.

Can binary stars have planets?

Yes, binary star systems can have planets. These planets can orbit both stars or just one. Finding these planets is hard, but several have been discovered. They help us understand how planets form and where life might exist.

What’s a triple-star system?

A triple star system, or trinary system, has three stars bound together. They can be arranged in different ways, like a close binary pair with a distant third star. Triple star systems are less common but can be found in our Galaxy.

Originally published at https://astronomyexplained.com on January 31, 2025.