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GX 339-4, illustrated here, is a binary system of a black hole and a star. Astronomers were able to measure how the disk around the black hole shrinks for the first time. Image Credit: Credit: ESO/L. Calcada
I think a short discussion on the center of mass of a body will help.
Take out a long stick and try to balance it on a finger. If you’re successful, that means, you’re holding the stick right below its center of mass. If you balance the stick horizontally, and assuming it is uniform throughout like say a perfect meterstick, then you’ll find your finger right below the stick’s geometrical center. That is, for a meter stick, right below the mark that says 50 cm.
Hence, one can say that the meterstick’s center of mass is at the 50 cm mark. If you throw the meterstick into a spin, it will spin right about that center of mass. Now, not all bodies are uniform like the meterstick. Take out a hammer and throw it into a spin just like the meterstick.
In all likelihood, you’ll see that it wouldn’t spin about a point halfway along its longest side (i.e., along its handle). Instead, it would be wobbling. That’s because the hammer’s center of mass would be in or near its head. Try to balance a hammer horizontally. Surely, if you’re successful, your finger won’t be lying underneath a point halfway along its longest side.
Rather, you’ll be balancing it with your finger closer to its head.
In the universe, wherein we usually find pairs of celestial bodies wherein one may appear to be revolving about another – like the Moon about the Earth – the center of mass of each pair is called the barycenter.
Try to recall the spinning objects mentioned earlier. Yes, you may imagine the Earth-Moon pair as one body since they’re already joined by gravity. You might want to recall especially the spinning hammer.
For the Earth-Moon pair, the Earth can be likened to the hammer’s head. If this were so, then certainly, the revolution of the smaller body about the larger one would not be about the geometrical center of the latter.
In other words, the Moon is not really revolving around the Earth’s center. Hence, we can say that the barycenter of this pair is not in the geometrical center of the Earth. In some instances wherein the two bodies are comparable in size (such as Jupiter and the Sun), the barycenter can even be outside of either body.
In this case, both the Sun and Jupiter would be revolving about a point between them which is outside of either body. Such a barycenter is also common among pairs of stars known as binary star systems.
Want to read more about binary stars? Here are some articles from Universe Today:
What is a binary star?
Binary Star
NASA has an article entitled “Planets around binary stars” that provides a more detailed discussion on this matter. Other interesting questions regarding binary stars can be found in this link: Binary Star Systems.
Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:
Moons and the Drake Equation, Stars in the Void, and Rings Around Stars
Stellar Roche Limits, Seeing Black Holes, and Water on Mars
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