At the centre of the Milky Way lies an unknown compact entity believed to be a supermassive black hole.
At the centre of the Milky Way lies an unknown compact entity. Located in the constellation Sagittarius and coinciding with an intense radio source called Sagittarius A* (pronounced A-star), it boasts a mass over 2.5 million times that of our Sun, squeezed into a region no greater than the distance between the Earth and the Sun. Unfortunately that's where our knowledge ends. Some believe it is a supermassive black hole while others have hypothesised more exotic objects. Scientists still know little about its nature or how it formed, and solving the mystery of this supermassive object is one of the greatest challenges in cosmology today. Our best evidence for a supermassive object comes from the orbits of stars around the Galactic Centre. They are rotating so fast the only explanation is the existence of a single object.
One in particular - S2 - appears to orbit in just 15 years, allowing scientists to deduce both the mass and volume of the central dark object. The astoundingly high density calculated has led researchers to reject simpler explanations such as dense clusters of dark objects - neutron stars, planets, star-sized black holes and so on - as these would become unstable within such a reduced region and collapse. Instead, the search is on for more exotic candidates.
Most in the astronomical community believe Sagittarius A* is a supermassive black hole, especially as some theories of galaxy formation indicate these reside at galactic centres, varying in size from millions to billions of solar masses. Some evidence comes from the intense radio emissions as Well as infrared observations of Sagittarius A*, which may be caused by the energy given off by gas and dust heated to millions of degrees before falling into the supermassive black hole. However, the question of how it formed is still open.
For now, there is no definitive evidence either way but researchers hope to have more answers soon. Upcoming projects such as ALMA and the European Southern Observatory’s VLT Interferometer will image the complex dynamics of our Galactic Centre in unprecedented detail, revealing its turbulent processes and perhaps even observe stars that orbit the supermassive black hole in as little as a year. Moreover, the next generation of infrared interferometers should allow astronomers to see the 'shadow' cast by the gravitational deﬂection of light rays near the black hole and the effects of the black hole horizon. As there is no horizon in boson stars, this would provide an extremely undeniable signature of what lies at the centre of the Milky Way.
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