Researchers recently discovered a black hole with a mass estimated at 3.3 times the mass of the Sun, which is significantly less than the lowest-mass black holes previously detected with approximately 5 times the Sun's mass. The new finding opens the door to the possibility that a sizable population of similarly low-mass versions of black holes has thus far escaped detection. Finding more members of this hypothetical population will help scientists better understand the origin and evolution of black holes, which are among the most extreme and enigmatic objects in the universe. See also: Black hole; Mass; Sun; Universe
Black holes famously exert gravity so strong that light cannot escape, rendering them invisible. However, these ultra-dense objects can indirectly reveal themselves as matter drawn into them heats up and emits radiation, or through their gravitational interactions with other objects such as stars. The latter method is how researchers turned up the new, record-setting low-mass black hole. The researchers combed through a dataset compiled by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), run from a telescope in New Mexico, of approximately 100,000 stars for those displaying movements characteristic of being paired with a black hole in a binary system, or a star system containing two objects with significant mass. One star in particular—an aged, voluminous type called a red giant—stood out for swinging in circles as it traveled through space, apparently moving about a common center of mass with a massive but unseen object. The orbital motion of the star indicated the strength of gravity exerted by its invisible companion, which equates with mass and puts the companion's mass at 3.3 times that of the Sun. The researchers have ample reason to conclude that the unseen companion in the study is not a different type of dense object, known as a neutron star, which can be as heavy as 2.17 solar masses. The scientists are confident in this conclusion due to the fact that beyond this mass level, a neutron star would collapse further in upon itself and transition into a black hole. See also: Binary star; Center of mass; Gravity; Neutron star; Orbital motion; Radiation; Star; Telescope
By more clearly delineating the mass boundary between neutron stars and black holes, scientists hope to improve theories describing how each object forms (in both cases, formation ultimately stems from stars with initial masses at least several times that of the Sun). Furthermore, by building a more accurate census of black holes in our galaxy, researchers can better account for mass distributions in the Milky Way Galaxy, as well as hone models of individual and group star formation. See also: Massive neutron star on the brink of collapse into a black hole; Milky Way Galaxy; Stellar evolution
