Issue link: https://wardsworld.wardsci.com/i/1440591
Yet another way to indirectly detect supermassive black holes and infer their mass is by measuring the velocities of stars in their vicinity. This approach has informed calculations of the mass of the supermassive black hole at the center of our Milky Way Galaxy, associated with the radio source Sagittarius A* (pronounced "A star"). The measurements are made in the near infrared with adaptive optics on large telescopes. The orbits of the stars are Keplerian ellipses, which allow calculation of the mass of the central body with high precision. The calculations reveal a black hole mass of about 4 million solar masses. Black holes and gravitational waves A recently developed way of studying black holes and infer- ring their existence is through gravitational waves, also known as gravitational radiation. These are ripples in the spacetime fabric of the universe, initially predicted—as well as black holes themselves—by Einstein's theory of general relativity in 1915. Massive objects, such as black holes, should create gravitational waves when they accelerate. The first instrument with the required sensitivity to detect the infinitesimal signatures of gravitational waves was LIGO. The instrument can detect displacements between mirrors, using interference properties generated by splitting laser beams into perpendicular directions, on the order of a ten-thousandth the diameter of a proton caused by gravitational waves passing through Earth. In 2015, LIGO detected the signature of two col- liding black holes with 36 and 29 solar masses apiece—a mass range not previously observed for stellar black holes (Fig. 3). The merger did not generate electromagnetic radiation or the emission of any other particles, thus showing how gravitational wave astronomy will allow the study of previously inaccessible natural phenomena. Multiple gravitational wave detections since are advancing our understanding of black hole proper- ties and the environments where they form. Though no specific environments are now known, and the current LIGO detections are of collisions billions of light-years away, eventually astrono- mers may find black holes in our galaxy in abundance in dense star clusters known as globular clusters. Since the 2015 detection of a black-hole merger, a neutron- star merger was detected not only in gravitational waves but also in light across the electromagnetic spectrum, ushering in a field known as multi-messenger astronomy. A highly antici- pated gravitational wave-spawning event is the collision of a black hole with a neutron star, which should yield observable light and therefore offer further insights into compact objects, astrophysics, and fundamental physics. Future gravitational wave observatories are planned to seek the signals theoreti- cally generated by sources including supermassive black hole mergers and from the earliest moments of the big bang. Fate of black holes According to general relativity, a finite mass can theoretically be compressed into a point of zero volume at the center of a black hole, creating an infinitely dense state of matter known Black Hole (continued) + ward ' s science Fig. 3 An artist's impression of two colliding black holes before their merger. (Credit: SXS)