The combination of large mass and tiny size means a black hole's gravity is more intense than other objects, but gravity obeys the same rules whatever the object. If it gets very close, its path is so curved it will never again straighten out enough to escape – that's the event horizon.Ī common misconception must now be addressed: black holes do not suck anything in. Closer in, the particle gets deflected slightly by the gravitational influence closer still, the particle may fall into some kind of orbit. Think about the effect of gravity on a particle: far from the black hole, the particle's trajectory won't be affected. According to Einstein's general theory of relativity, there will be a boundary called the event horizon which separates the “interior” of the black hole from the rest of space. A good functional definition is as follows: a black hole is a compact object whose gravitational influence is so strong that anything coming too close cannot escape. Credit: Matthew Francis.īlack holes by their nature are difficult to observe, so the evidence for their existence is by necessity indirect. This figure is created for clarity, not for scientific accuracy – the paths are not precisely calculated. Right: Trajectories near a black hole, showing how the event horizon “traps” particles. In this post, I will flesh out the observational evidence for black holes and attempt to separate what we understand about black holes from a lot of the more conjectural and controversial issues surrounding these mysterious objects. Objects similar to Sagittarius A* have been observed at the heart of nearly every galaxy.Īll this is a tease, of course: everyone knows the scientific consensus is that these three X-ray sources, along with with many other objects, must be black holes. In other words, whatever is at the center of our galaxy is far more massive than any star (which top out around 200-300 times the mass of the Sun, and stars that huge are incredibly luminous objects anyway), and is physically too small to be a cluster of stars. Another bonus from direct observation is that the size of Sagittarius A* can be no larger than the orbit of Uranus – about 20 times the distance from Earth to the Sun. Unlike the previous two examples, the stars can actually be observed directly and their motion plotted with the data collected, even Astronomy 101 students can calculate the mass of the X-ray source using Kepler's laws of motion. Using the motion of stars orbiting around Sagittarius A*, astronomers determined its mass to be approximately 4 million times the mass of our Sun. Link.Ī third example is an even brighter X-ray source at the center of our Milky Way galaxy, known euphoniously as Sagittarius A* ("A star" when spoken aloud). Using data of the stars' motion, scientists have determined Sagittarius A* is 4 million times the mass of our Sun and smaller than the Solar system. Right: Sagittarius A*, the bright X-ray source at the center of the Milky Way, with several star orbits mapped. Both this and Cygnus X-1 are far too massive to be neutron stars – pulsars – which cannot grow beyond about 3 times the Sun's mass. The star and the X-ray source orbit each other once every 6.5 days, which indicates a very close binary system – and a small size for the hidden companion. Doppler effect measurements showed the X-ray source to be between 10 and 14 times the mass of the Sun. Similarly, in 1989 a huge X-ray flare was detected in the vicinity of another star in Cygnus, known as V404 Cygni. Careful observations of the dynamics of the blue star and variations in the X-ray luminosity show that the X-ray source, known today as Cygnus X-1, has a mass at least 6 times that of our Sun, but a physical size smaller than Earth. Ordinary blue stars don't produce a lot of X-rays, so astronomers quickly concluded there must be a companion object that doesn't give off much visible light. In 1972, a bright X-ray source was discovered by the Uhuru satellite in the constellation Cygnus at the same location as a bright blue-white star with the highly memorable name HD226868. Left: Chandra X-ray Telescope image of Cygnus X-1, the first black hole candidate discovered.
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