Star Not So Bright

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The stars in the sky are doing a lot more than twinkle. They're constantly evolving, burning nuclear fuel, and sometimes even interacting with other celestial objects. Francesca Valsecchi, Northwestern University graduate student in physics and astronomy, studies the lives of massive stars and their final fate – becoming a black hole. Valsecchi will talk about her work at a Junior Science Cafe event on Friday February 18 at the Evanston Public Library. We spoke with her for a preview.

What is a black hole?
The black holes I’m interested in are the evolutionary endpoint of massive stars. While evolving, a star burns what’s inside – hydrogen, helium, other elements – until it runs out of nuclear fuel. When this happens, it just collapses. If the star is massive enough, it collapses into a black hole.

Francesca ValsecchiFrancesca ValsecchiWhat are some of the characteristics of a black hole?
Black holes are among the densest objects we know of in the universe, but we cannot see them. We can see stars because they shine, and we have a fairly good understanding of their nature – how stars evolve, how they interact with their host environment, etc. But the physics behind the black hole formation process, how they interact with what is around them– these are things that are not completely clear. There are still fundamental questions about the nature of black holes that we do not understand.

How do you study them?
While isolated black holes are very hard to see, we can see black holes when they are coupled with a star that is feeding mass to them. These kinds of systems are called X-ray binary systems. When the black hole is gaining matter from a star, it becomes “X-ray bright,” meaning the black hole becomes a source of radiation that we can detect.

So one way to understand how black holes form and how they interact with their host environment is by studying these binary systems. By observing a binary system, we know how far apart the star and the black hole are, the masses of the two components, how many X-rays are emitted, the shape of their orbit – a circle, an ellipse – etc.

But we don’t know, from observing the system today, the mass of the star that became the black hole, or what kind of interactions it had with the other star in the system in the past. To try and understand this, we need to model the evolution of observed binary systems. This modeling has helped us understand what kind of star you need to form a black hole, and what might happen during the collapse process. X-ray binary systems are a unique laboratory for understanding the nature of black holes.

What is unique about M33 X-7, the X-ray binary system you’re studying?
There are many things. First of all, when the properties of this system were determined precisely, the system’s black hole was the most massive black hole to have formed from a star that we knew of. The star orbiting the black hole is the most massive ever seen in this class of system. Also, their orbital separation is small compared to the size of the star, and the orbit is not really a circle, which is unusual in these systems.

Finally, the star is unusually dim for its mass. It is only as bright as a normal star that’s, say, 50 times as massive as the Sun, while it has a mass 70 times that of the Sun.

We looked for an easy model to explain these peculiarities without introducing too complicated of physical effects, and we found a solution. The beauty of this is that we have a confirmation that our basic understanding of the evolution of stars and binaries is correct, because we can explain even this unusual system without having to invoke effects outside the standard physics of binary stars. This is encouraging, because it gives us confidence that we can make predictions for systems that haven’t been discovered yet, using the same tools.

Why isn’t the star as bright as it should be?
There are two reasons. The biggest contribution comes from the way we observe the system. We are looking at the system edge-on, meaning that we’re observing more or less the equator of the star (rather than the poles). Because the star and the black hole are so close (making the star feel its gravitational pull), and the star is rotating around itself, the star is not perfectly spherical. These gravitational and centrifugal forces cause it to deform into a teardrop shape.

Because the star is deformed, its luminosity is not uniform across its surface. If it was a sphere, it wouldn’t matter which way we looked at it – it would be equally as bright at any point. Since it is deformed, it is less bright in the equatorial region, and that’s the region we’re observing. So, we had to correct the standard models of an isolated, spherical star to account for the fact that we are looking at the dimmer, equatorial region of a deformed star.

The second reason for the low luminosity – and the cooler reason – is because the star didn’t evolve in isolation in its past, and instead interacted with its companion (the star that will become the black hole). Because the companion star exhausted its fuel first, we know that it was more massive than the star we see today.

While evolving, stars expand. Near the end of its life, the companion expanded enough to begin donating matter to the star we see today.  Because the companion was so massive and expanded so fast, the donation of matter was very rapid.  The star we see today could not adjust rapidly enough to take full advantage of the new fuel.   Since the star cannot burn the accumulated fuel as efficiently as a star that is “naturally” that massive, it appears under-luminous for its mass.

Want to learn more about the warped world of black holes and stars? Join Valsecchi at her Junior Science Café event on Friday, February 18 at 4:30 PM in the Loft at the Evanston Public Library (Main Library). Junior Science Cafes are free and open to middle and high school students, as well as their families and teachers. For more information, visit http://www.sciencecafe.northwestern.edu/.

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