The Archmage's Ruminations

Attack of the Giant Space Diamonds!


Blogmaster Helen Cothrel, hc261409@ohio.edu

Calm down, everybody. Giant space diamonds aren’t actually attacking–but they are out there, and eventually, our own Sun will join their ranks. One, for example, is BPM 37093, which was named “Lucy” in 2004 in tribute to the Beatles. Back in 2004, BBC News posted a short article about the star, but their explanation of how this 10 billion trillion trillion-carat rock wasn’t stellar. So, I’d like to take this opportunity to look a little closer at the mega-diamond, and talk about some more of the physics behind it. (I promise: no math!)

Lucy diamond white dwarf star

Artist’s conception of Lucy. Image source: http://www.spacetoday.org

Lucy is a thrilling find, but her discovery didn’t come out of the blue. Astronomers had known for a while that white dwarf stars–like Lucy–could eventually form diamond cores. We have a pretty good understanding of what is called “stellar evolution,” which is simply the way a star “evolves” over its lifetime, and it was this knowledge that led astronomers to look for evidence of a star like Lucy. So, how does a giant space diamond come to be?

Well, to look back in time at Lucy’s evolution, we can look to something conveniently close by: our own Sun. At this point it is important to note that the following path leading to becoming a diamond white dwarf can only be followed by stars whose masses are relatively close to the mass of the Sun.

By many standards, our 4.5 billion-year-old Sun is actually quite young. That is, it has yet to live out most of its life. Right now, the Sun is what is called a “main-sequence star,” which means it is currently on the main sequence of the Hertzsprung-Russell diagram.

h-r diagram

The Hertzsprung-Russell diagram. The vertical axis is luminosity, or how much energy the star is emitting per second. The horizontal axis is the surface temperature of the star. image source: http://deskarati.com

The Sun is currently burning through huge amounts of hydrogen via nuclear interactions in its core. In stars such as the Sun, which are considered to be low-mass stars, hydrogen in the core is being converted to helium through nuclear fusion. Eventually, all of the hydrogen in the core will be used up, and a core of helium will be left over. There is still hydrogen in the star, but the core–the star’s nuclear furnace–will be helium. With a star like our Sun, the temperature and density left over in the new helium core won’t be high enough for nuclear reactions to continue.

With nuclear reactions in the core halted, the helium core will contract. Essentially, it has to shrink because it’s no longer producing enough energy to prevent its slow collapse. Energy from the core will leak into a surrounding layer of remaining hydrogen, which will fuse into more helium. This helium will be added to the current core. As the core shrinks, fusion will continue in the surrounding hydrogen.

At this point, the structure of the helium core will be squeezed on the quantum level. The core will keep shrinking and the mass will keep increasing (thanks to continuing hydrogen fusion), and the star will get hotter and hotter. As more energy is released, the increased temperature of the outer layers of the star will cause them to expand.  The star will grow to 100 times its previous size, and it will turn off the main sequence of the Hertzsprung-Russell diagram onto the giant branch.

“The sun compared to the Red Giant star it will eventually become.
Image Credit: Department of Physics, NCKU” (www.astrobio.net)

The star will “move up” the red giant branch. Eventually, there will be enough energy for a “helium flash” to occur, in which the entire helium core almost instantly starts fusing into carbon. This leads to a runaway effect of increasing temperature and energy, resulting in a huge explosion of the helium core. Even during the explosion, the core is buried far enough within the red giant that the outer layers of the star will just be pushed out.

Helium fusion within the core will continue, where carbon will build up. Eventually, the star will be left with a carbon core surrounded by a helium-burning “shell.” Again, the core will shrink, but for a star with a mass close to our sun, there will never be enough energy for a “carbon flash” to occur. Instead, the remaining hydrogen and helium will burn up. The temperature of the star will drop, because the hydrogen- and helium-burning reactions won’t be as efficient as they were before. What will be left is the carbon core. This core is the white dwarf that emerges at the end of a red giant’s life.

The resulting white dwarf no longer has the energy for nuclear reactions to continue. The carbon star shrinks (to something close to the size of Earth) and cools. Under the force and pressure from its own mass, the carbon atoms will be squeezed into a crystal structure: diamond.

This is the end of the star’s journey. The carbon in the white dwarf continues to crystallize into diamond, and after long enough, all that will be left is a giant hunk of diamond floating in space. So it happened for Lucy, so it will happen for our Sun, brought to you by the wonders of physics.

Pretty much everything about this I learned from lectures on astrophysics given by Dr. Matthias Dietrich & Dr. Douglas Clowe at Ohio University.

Thoughts? Questions? Leave a comment!

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