A little star collision can really ruin your day. In fact, if two stars in the Milky Way galaxy were to collide – and if they were close enough to our Solar System – they could release sufficient gamma radiation to eliminate all life on Earth!

 

But not to worry. Solar collisions are relatively rare. In the Milky Way, astronomers estimate that there is a collision approximately every 10,000 years, so there’s no need to change your plans anytime soon.

This artist’s illustration shows two neutron stars in a binary formation spiraling in toward each other. Once they collide, the end result will be a massive explosion and the creation of a black hole. Image credit: Dana Berry, NASA

Why Do Stars Collide?

There are many kinds of stars in the heavens – red giants; white, red, and black dwarfs; blue “stragglers”; neutrons and pulsars; “main sequence” stars such as the Sun – and all types can experience a solar collision, or “star merge.” Two main factors tend to predispose stars to a cosmic merger:

  1. They are in globular clusters, which are breeding grounds for new stars, and the density of stars in the regions is relatively high.

 

  1. The stars are binary pairs in which two closely proximate stars orbit around their common center of mass. In cases where they are so close they start sharing each other’s atmospheres, the pairs are called contact binary star systems.

In the globular clusters of the Milky Way, there is likely a collision every 10,000 years or so.

This image from the Hubble Space Telescope shows globular cluster Omega Centauri, a region where stars are packed in 10,000 times more densely than in our own region of the Milky Way. Stars are much more likely to collide here than anywhere near us, and when they do, they are likely to form one more massive star, called a “blue straggler,” or create another binary system. Image credit: NASA, ESA, and the Hubble SM4 ERO Team

What Happens When Different Types of Stars Collide?

If two stars are close enough, they begin spiraling toward one another. Then many things start to happen. The mass of the smaller sun gives in to the ceaseless pull of its larger partner. The binary pair starts sharing atmosphere, and gravity begins to deform the smaller orb. But different stars, and different kinds of star pairs, react differently after this point. So let’s look at them individually.

Across the Universe, neutron stars collide and form black holes between one and ten times in a billion years.

  1. When fast-moving main sequence stars collide and create a nova, often the combined mass of both stars is flung out into the Universe and no trace of either star remains.

 

  1. Slower moving mid- to large-sized stars have a much more productive outcome. In these collisions, after a nova or a Type Ia supernova, the hydrogen present in both stars remains and begins to burn even brighter and hotter, creating a new, larger star called a “blue straggler.”

 

  1. When a small star, e.g., a white dwarf or a neutron star, collides with a large star such as a red giant, the outer layer of the larger star blows off entirely in a great explosion, and what remains in place of the ravaged red giant is a new white dwarf in a binary relationship with its progenitor.

 

  1. When a white dwarf collides with a star of its own magnitude (such as another white dwarf), both stars have their elements pulled apart in an explosive event of such force that only neutrons are left, and what remains is a small neutron star.

 

  1. And when two tiny neutron stars are drawn together, an amazing sequence of events unfolds. The larger one deforms and crushes the smaller, which erupts in a huge explosion of matter and energy. The remnant of the smaller neutron star then forms a spiral arm from the matter it has expelled, and this arm begins to fold around its larger partner. Finally, the larger neutron star accumulates so much mass that it collapses in on itself, resulting in the creation of a black hole.
This supercomputer simulation demonstrates what happens when two neutron stars collide and form a black hole. Image credit NASA/Goddard - Albert Einstein Institute

When Can We See a Star Collision?

Sooner than you might think. A binary system discovered in 2003, comprising two main sequence stars, seems to be on the verge of a collision. The system, called KIC 9832227, lies in the Cygnus constellation. It has been observed consistently since its discovery and is now known to be a contact binary star system. Astronomers predict that we will likely witness a large red nova explode in the sky sometime around the year 2022.

In this artist’s conception, two stars in a binary pair have begun to merge, even sharing the same atmosphere. As they continue their “dance of death,” they will eventually explode as a red nova. Image credit: ESO/L. Calçada

Because of its distance from the Earth, it’s been hypothesized that the stars in KIC 9832227 actually collided approximately 1,800 years ago. When we finally get to see the nova, it will appear as a magnificent red glow in distant Cygnus. The expected merger of the two stars will increase the brightness of the resulting star by a factor of 10,000.

 

For a tantalizingly short while the star will be one of the brightest lights in the heavens. The event will be must-see astronomy, for it is unlikely that we will have such an opportunity again in our lifetimes.