October 25, 2019

Ten things you didn’t know about binary stars

This story is also available in Spanish.

Over the past four years, while doing my PhD in astrophysics -or “science fiction” as one professor I know used to call it- I dedicated most of my time to studying astronomical objects called “binary stars”.

Not too long ago, I was giving a talk about my PhD research to an audience of non-experts. I was quite amused to discover that several people in the room thought that the term “binary stars” was related to computer coding. So I decided to share with you here the top 10 secrets I learned about binary stars, starting with what they actually are.

The dictionary defines “binary” as “composed of, or involving two things”. So a binary star is a system composed of two stars which orbit each other thanks to their mutual gravity.

You may wonder, why are these systems interesting? Well, most of the stars in the Universe come in pairs which makes our Sun, which is a single star, pretty unusual. Because these systems are quite common in the sky, it is important to understand how they live their lives, what they do meanwhile and how they die.

Top 10 secrets

1. They come in different sizes

Binary systems can be formed of stars with different masses and ages. Different combinations exist, for example, two Sun-like stars, or a giant star and a Sun-like star, or two white dwarfs or neutron stars, or even systems where one of them is a black hole. The most massive binary system known is called R144. The lightweight star in this system is about 95 times the mass of our Sun, and its heavyweight partner weighs as much as 205 Suns!

2. They trace different geometries in the sky

As they revolve around each other, binary stars can have either circular or elliptical orbits, thus they trace a circle or an ellipse in the sky.

3. Some like long-distance relationships

Some binary stars orbit each other at large distances. Mira, for example, is a system composed of a giant star and a white dwarf. Their separation is about 70 times the distance between Earth and the Sun, and so it takes them about 500 years to complete one orbit.

4. Some don’t like to be apart

In contrast, some double stars are very close to each other. HM Cancri, a system formed of two white dwarfs, is so close that they can make one full orbit around each other every 324 seconds!

5. Some like to cuddle

Some binary stars are so close that they can touch each other. Astronomers call these objects “contact binaries”. These systems may explain the hypothesised peculiar type of star called Thorne–Żytkow object, which is a giant that has at some point gobbled up its neighbouring neutron star.

6. Some are obsessed about looking young

As one of the stars grows older, it starts losing its material through what astronomers call stellar winds. Its nearby companion can then siphon it in by gravity. This means fresh fuel for the companion, so it gets rejuvenated! 

7. Some are stellar da Vinci’s

If one of the stars grows old enough to start shedding its outer layers, the companion star will paint, as it moves around it, beautiful spirals in the wind, which we can observe with telescopes such as the Hubble Telescope in space or with ALMA in Chile.

8. Some like to go bang

If both stars are elderly, they may lose almost an Earth’s worth of material once every month through stellar winds. These two winds may collide head-on, producing titanic collisions which heat the surrounding gas to multi-million degrees. Such collision radiates brilliantly in X-rays and thus can be detected by telescopes. 

9. Some can have babies

Wouldn’t it be cool to have two sunrises and two sunsets every day? Well, on planets that are “parented” by a binary system rather than a single star, this is indeed possible! There are two types of planet configurations around a binary system: the planet orbits both stars or orbits only one of the two. Unfortunately, the planets we have found orbiting two stars so far have no possibilities for life as we know it. 

10. Dead binary stars can communicate from the afterlife 

The mass of the star during its life determines the type of corpse it leaves behind when it dies. Light stars become white dwarfs while heavy stars die as neutron stars or black holes. All of these objects, but especially black holes, are mind-bogglingly dense. When two such objects revolve around each other they cause a small distortion in space-time like ripples in a pond. The denser the object, the stronger the distortion, and this can be measured on Earth as gravitational waves. Spooky!

Illustration is by Martha Rosas Vilchis, the author’s mother! Background image is the Great Orion Nebula by Joe Morris.  


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