Is there a Black Hole at the Centre of our Galaxy?
Scientists believe that at the centre of the Milky Way there 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 Doppler studies of the orbits of stars around the Galactic Centre. They are rotating with a period of 5.6 days, so fast that the only explanation is the existence of a single object.
Another star 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.
Using the period plus spectral measurements of the visible companion's orbital speed leads to a calculated system mass of about 35 solar masses. The calculated mass of the dark object is 8-10 solar masses; much too massive to be a neutron star which has a limit of about 3 solar masses - hence a black hole theory.
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. Further evidence that strengthens the case for the unseen object being a black hole is the emission of X-rays from its location, an indication of temperatures in the millions of Kelvins. This X-ray source at Sagittarius A* exhibits rapid variations, with time scales on the order of a millisecond. This suggests a source not larger than a light-millisecond or 300 km, so it is very compact. The only possibilities that we know that would place that much matter in such a small volume are black holes and neutron stars, and the consensus is that neutron stars can't be more massive than about 3 solar masses.
The formation of supermassive black holes is a subject that is still under investigation. It is still not completely clear whether they were the condensing seeds for galaxies or whether they are a result of galaxy formation. Others have speculated that Sagittarius A* may be a so-called boson star - a theoretical entity composed of exotic elementary particles that may also be candidates for dark matter. These strange stars have no surface and interact with normal matter only through gravity.
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 diffraction 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.
Last updated on: Tuesday 20th June 2017
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