The size of black holes
November, 2014
Miguel Murguía-Romero
At the center of the Milky Way lies a black hole, a concentration of mass so dense that its gravity is so strong that no particle can escape, not even light. Black holes are more common than previously thought, as one apparently lives at the center of every galaxy. Although they share physical properties, black holes can differ in the amount of matter they contain by up to millions of times.
What is a black hole?
Black holes are very important objects in astronomy, as their study offers a better understanding of the universe. They may hold the keys to understanding the structure of matter and the history of galaxies, as well as the origin of the universe itself. A black hole is said to be a concentration of mass so dense that objects close to it are attracted and absorbed, and gravity is so strong that not even light can escape. This is why they are "observed" in space as dark regions, and the objects around them have orbits congruent with a center of great mass.
The Navel of the Milky Way
A few decades ago, it was suspected that black holes existed at the centers of spiral and elliptical galaxies; our galaxy, the Milky Way, is a spiral galaxy. Does it have a black hole at its center? A group of researchers, including American astronomer Andrea Ghez, has been studying the Milky Way for more than ten years to answer this question. This group of astronomers studied the orbits of stars near the center of our galaxy. This task was arduous, as interstellar space is rife with dust and gas, making observation difficult. In 2000, they were able to determine the orbit of one of the stars at the center of the galaxy; it has an orbit period of about 16 years. With this data, they were able to estimate that the invisible object orbiting the star would have a mass of about 4.1 million solar masses, which supported the hypothesis of the existence of a black hole. As of last year, they had been able to determine the orbits of just over 20 stars. With this information, they were able to adjust their estimates and strengthen the hypothesis of a black hole at the center of the Milky Way.
As is often the case in science, when you try to answer one question, many more arise: the surprising thing is that most of the stars near the center of the galaxy that the group of astronomers studied are very young stars, about 10 million years old, barely one-thousandth the age of the Milky Way, which is 13,200 years old. That is, if we imagine that the Milky Way existed for 24 hours, these young stars would have been born just a minute and a half ago. It is very difficult to explain how such young stars are found so close to the black hole, since interstellar dust and gas are drawn in, giving no time for stars to form.
A new method to study black holes
One of the methods currently used to measure the amount of matter in black holes is through the kinematics of the stars surrounding the black hole, or through the kinetics of interstellar ionized gas, but these methods only work for certain types of galaxies, such as young ones. A group of astronomers from Germany, the United Kingdom, and the United States proposed a novel method for measuring the mass of black holes based on the gravitational effect of the hole on the surrounding interstellar molecular gas.
Applying the new method, the research group studied the galaxy NCG4526 and estimated that the black hole at its center is 4x108 solar masses, or 400,000,000 solar masses, or 400 million times the mass of the Sun, almost 100 times the mass of the black hole at the center of the Milky Way.
Types of Black Holes
Black holes are generally classified into two types: stellar and supermassive. Stellar black holes form from the mass of stars with a mass of four to 16 times that of the Sun, so their mass is much smaller than supermassive ones, which are found at the centers of galaxies. The smallest black hole known to date is a stellar black hole found in our galaxy, which is almost four times the mass of the Sun and 24 km in diameter, which is close to the theoretical minimum size for black holes.
Thus, the difference in size between the two types of black holes is abysmal. While stellar black holes measure "only" four solar masses, supermassive ones can reach up to four hundred million times the mass of the Sun, that is, the difference in mass can be up to one hundred million times. Recently, intermediate-sized black holes have been discovered, about one hundred thousand times the mass of the Sun. Black holes of this type are believed to be found primarily in the centers of star clusters, that is, in the centers of groups of hundreds of thousands or millions of stars, but which do not constitute a galaxy, but rather are part of one. For example, about 150 star clusters have been discovered in the Milky Way.
There are many unanswered questions about black holes. For example, it has been discovered that two stellar-sized black holes coexist in a globular cluster in the Milky Way, a feat previously thought impossible. As if that weren't enough, it is suspected that up to five black holes coexist in that cluster.
References
Asociación de Biólogos Amigos de la Computación, A.C. (ABACoA.C.)
SERES Sistemas Especializados
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