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X9 in 47 Tucanae Animations
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Tour of X9 in 47 Tucanae
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In astronomy, a binary system is one where two objects are close enough that they orbit each other because they are gravitationally bound to one another. There are many possible combinations of binary systems: stars like the Sun orbiting each other, two neutron stars, a massive star and a smaller one, and so on. Binary systems are useful to astronomers for many reasons, including being useful laboratories to measure the masses of stars.

Recently, astronomers have found a particularly interesting binary. This pair has a white dwarf star in orbit around a black hole. While scientists have found this configuration many times before, this binary, known as X9, is special. That's because it has the closest orbit ever seen between a black hole and a companion star.

The X9 system is located in 47 Tucanae, a dense star cluster in the outskirts of the Milky Way. Using NASA's Chandra X-ray Observatory along with data from other telescopes, researchers determined that the white dwarf makes one complete orbit around the black hole in less than a half an hour. This means that the separation between the white dwarf and the black hole is about two and half times the distance between the Earth and the Moon. In cosmic terms, these two objects are extremely close together.

What does this close pairing mean for the objects in X9? The extreme proximity likely spells a bleak future for the white dwarf. Although scientists don't expect the white dwarf to fall into the black hole, they do think so much material will be pulled from the star that it will one day become some sort of exotic planet or even evaporate all together. In the short term, astronomers are going to watch this system very closely, because they don't know exactly how such an extreme system will behave.
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(Credit: NASA/CXC/A. Hobart)

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