Sagittarius A*

Our galaxy, the Milky Way, has a massive black hole at its center. It is known as Sagittarius A*, and scientists want to take a picture of its event horizon. In fact, they are planning to do just that.

If you are unaware, an "event horizon" is the point surrounding a black hole at which the gravitational pull becomes so great that it makes escape impossible. If this boundary is crossed, not even light can move fast enough to get out. The project, called the Event Horizon Telescope, has completed most of its technical preparations, as well as extensive theoretical calculations that are needed in order to complete the mission.

Notably, the size of the event horizon of Sagittarius A* is 24 million km across (about 15 million miles), 17 times bigger than our Sun. Surrounding it, however, are roiling clouds of gas and dust, which blaze with energy as they are sucked and squeezed furiously towards the proverbial mouth of the black hole.

Ultimately, theses features pose more challenges to the EHT scientists.

The historical attempt was discussed at the 227th annual meeting of the American Astronomical Society.

Looking into the Event Horizon

One of the most significant decisions for the scientists was choosing which wavelength of light they would use in viewing the event horizon.

Radio waves were chosen, as they are scattered much less by this material than visible or infra-red light. After several theoretical calculations, it was settled that 1.3 mm was to be the specific wavelength used. However, it was an "incredibly lucky coincidence", as Prof. Feryal Ozel, a member of the EHT team put it, that any wavelength at all was feasible.

Besides penetrating the black hole's dust cloud, Prof Ozel and her colleagues need the hot gas right at the event horizon to shine brightly in this wavelength—which they believe it does. The light also has to travel easily through the Earth's own atmosphere into the telescopes, which will be in Antarctica, Chile, Hawaii, Spain, Mexico, and Arizona.

The scientists are expecting to see a crescent. As the glowing gas is spinning around the black hole, and a dramatic Doppler effect should make it appear very bright on Earth.

Furthermore, Einstein's theory on General Relativity, which states that a mass, especially one as big as a black hole, bends space-time is on the line. That curvature can be calculated mathematically. The size of the shadow cast by Sgr A* should either match what is predicted by general relativity..or it won't.

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