Black holes and their secrets
28 March 2023
In 1972, the Uhuru spacecraft found a bright X-ray source in the Cygnus constellation, right next to a bright blue-white star with the well-known name HD226868. Because typical blue stars don’t make a lot of X-rays, astronomers quickly came to the conclusion that there must be a companion object that doesn’t make a lot of visible light. The X-ray source, which is now known as Cygnus X-1, has a physical size smaller than Earth but a mass at least six times that of our Sun, according to careful observations of the blue star’s dynamics and variations in X-ray luminosity.
In a similar vein, a massive X-ray flare was observed in 1989 close to another Cygnus star, V404 Cygni. The X-ray source was found to be between 10 and 14 times the mass of the Sun, according to Doppler effect measurements. The X-ray source and the star go around each other once every 6.5 days, indicating a very close binary system and a small companion. Cygnus X-1 and this one are both far too massive to be neutron stars—pulsars—because pulsars cannot grow beyond about three times the mass of the Sun.
Right: Several star orbits have been mapped around Sagittarius A*, the bright X-ray source in the middle of the Milky Way. Scientists have determined that Sagittarius A* is smaller than the Solar system and has a mass four million times greater than our Sun. Link.
A third example is Sagittarius A*, which means “A star” when spoken aloud, and it is an even brighter X-ray source in the center of our Milky Way galaxy. Astronomers estimated Sagittarius A*’s mass to be approximately 4 million times that of our Sun by observing the motion of the stars in its orbit. The stars can actually be observed directly and their motion can be plotted, unlike the previous two examples; Using Kepler’s laws of motion, even students in Astronomy 101 can use the collected data to determine the X-ray source’s mass. Another advantage of direct observation is that Sagittarius A* cannot be larger than Uranus’ orbit, which is approximately 20 times the distance between Earth and the Sun. To put it another way, whatever is at the center of our galaxy is physically too small to be a cluster of stars and has a mass that is far greater than that of any star (which can reach as high as 200 to 300 times that of the Sun, making them extremely luminous). Nearly every galaxy’s core has been observed to contain objects that are similar to Sagittarius A*.
Naturally, all of this is a tease: Everyone is aware that these three X-ray sources, in addition to numerous other objects, must all be black holes according to the scientific consensus. In this post, I will elaborate on the observational evidence for black holes and attempt to distinguish what we know about black holes from many of the more speculative and contentious issues that surround these enigmatic objects.
What is a black hole?
Right: Particle “trapping” by the event horizon is depicted by the trajectories near a black hole. The paths are not precisely calculated; this figure was created for clarity rather than scientific accuracy. Link. Credit: Francis Matthew
Because black holes are difficult to observe, indirect evidence of their existence is necessary. The following is a good functional definition: A black hole is a small object whose gravity is so strong that anything too close can’t escape. A boundary known as the event horizon will separate the black hole’s “interior” from the rest of space, according to Einstein’s general theory of relativity. Think about how a particle is affected by gravity: The trajectory of the particle won’t be affected if it is far from the black hole. The gravitational pull slightly deflects the particle as it gets closer; The particle may enter an orbit as it gets even closer. The event horizon is the point at which, if it gets very close, its path is so curved that it will never again be able to get away.
This misconception must be clarified: Nothing is sucked into black holes. A black hole’s gravity is stronger than that of other objects due to its large mass and small size, but gravity is the same for all objects. Earth’s orbit would not change if a black hole of equal mass replaced our Sun. Naturally, Earth would freeze over because the Sun’s light is so important to our planet’s life, but our orbit would still be the same size and shape.) In a similar vein, the fact that our galaxy contains a massive black hole at its center does not imply that our Solar System will eventually collide with it; Because Sagittarius A* is simply too far away, even if you completely eliminated that black hole from the galaxy, there would be no change to the path our Solar System takes through the Milky Way.
The mass of the black hole and the amount of spin it has are the two properties that control the size and shape of the event horizon as well as the strength of the gravitational attraction. Estimating the event horizon’s size is relatively simple: A black hole with a radius of three kilometers and mass equal to that of the Sun One with a radius of 6 kilometers and twice the mass of the Sun etc. The event horizon becomes spherical as a result of rotation, creating an odd region known as the ergosphere where nothing can remain still.