Binary Mergers: Watching Stars Collide in the Heavens

Next time you bask in sunlight, remember our Sun may be an oddity. More than half the specks of light in the night sky are in fact multiple stars orbiting each other. Have you ever seen any binary mergers? Depending on the type of star, the chances of it being alone in a system range from anywhere between 43% and 85%. Astronomers have discovered some truly strange systems with multiple stars, including quintuplets and sextuplets.

Not just the number, but the type of star can vary. Red dwarf stars are most common, cooler and smaller than our Sun. Stars can also be huge: UY Scuti has 21 billion times the volume of our Sun. If it were at the center of our solar system it would engulf Jupiter. And then there are neutron stars, only a few miles across but with more mass than our Sun. Systems can also feature black holes – places where gravity is so intense that not even light escapes.

Systems with multiple stars or black holes sometimes experience mergers, instances when the celestial bodies collide. Scientists can witness these mergers with telescopes. The growing field of gravitational wave astronomy is improving observational abilities, allowing the detection of gravitational waves that ripple outward from mergers.

Change in the brightness of binary star system

Filling Space spoke with Dr. Eleonora Troja, an astrophysicist who studies compact binary star mergers at NASA, to learn about her experience observing this interesting phenomenon.

***

What are binary mergers and how does studying them advance scientific knowledge?

(Compact) binary mergers are pairs of exotic stars, such as two neutron stars or a neutron star and black hole, which spend millions (and sometimes billions) of years rotating around each other.

These objects are extremely dense. To get the same density of a neutron star, you should squeeze the Himalaya into a teaspoon, and black holes are even denser! Because of their incredible density and fast rotation, they are very efficient in producing gravitational waves. The more gravity waves they produce, the closer they get, and the faster they rotate. This process goes on and on until they finally get so close that they collide and merge forming a new black hole or a massive neutron star.

There are many reasons to study these systems. Whereas most of their long life is quite uneventful, the last few moments are incredibly rich of surprises. They become cosmic furnaces forging the heaviest metals in the universe, such as gold, platinum and uranium. They also produce powerful beams of energy, which dump into the universe as much energy as our sun will produce in its entire life. We can see these jets because they emit intense flashes of gamma rays. And let’s not forget about the strong gravitational signal that now we can catch with advanced observatories like LIGO and Virgo. We can use this signal to learn more about black holes, how they form, and about our universe and the rate of its expansion. They are truly treasure troves.

What is one of the most exciting things you have studied in your investigations of binary mergers?

No doubt, GW170817, the neutron star merger discovered by LIGO last year.

In just two short weeks we solved decades-long mysteries about these objects, and I saw it unraveling day by day.

One of the most exciting moments was when I saw for the first time the data from the Hubble Space Telescope. Those were real data, taken from a real object just a few hours earlier, yet they looked as if taken out of a textbook. I was looking at the radioactive glow of the heavy metals, the so-called kilonova. The infrared light captured by Hubble (and other telescopes on Earth) was so similar to the theoretical predictions that it seemed unreal.

In just two short weeks we solved decades long mysteries about these objects, and I saw it unraveling day by day. Click To Tweet

While I was still in awe for the kilonova and I thought this could not get any better, the data from the Chandra X-ray Telescope came in and made me doubt my own eyes. At the same spot where the kilonova was, I was now seeing a source of X-ray light.

That was a big surprise because dozens of scientists had looked at it before me, and found nothing. So, what was I seeing?

I had to call a colleague of mine in Japan and ask him to please look at the data himself and tell me whether he was seeing the X-ray light too.

It turns out, that was a real source of X-ray light from the powerful beam of energy launched right after the binary merger, and we are still looking at it with Chandra to learn more about this historical event.

How did you become interested in binary mergers?

Partly by chance, and partly by choice.

By chance, because I won a fellowship to pursue my graduate studies and the fellowship had a theme: “Gamma-ray bursts”. In order to receive financial support for my studies, my research had to relate to this topic. I have to say I wasn’t too thrilled at the idea as I was more interested in another branch of astrophysics (cosmology, that is the study of our universe as a whole and its evolution), but in the end I accepted.

Once I started to study the subject of gamma-ray bursts I was soon drawn to this enigmatic connection between gamma-ray flashes, compact binary mergers, and gravitational waves, which became the main focus on my PhD thesis. I guess I was intrigued by the mystery surrounding these objects as well as their immense discovery potential. And now that we can detect gravity waves from these mergers, we can actually use them to do cosmology!