The gas around black holes can reach velocities so high that they heat up and shine. Like water down a bathtub drain, as the gas approach the black hole they move faster and faster. About 10% of super-massive black holes are surrounded by gas, which slowly fall onto them. There is another way to “see” black holes from greater distances. Beyond those, even our most sophisticated telescopes cannot distinguish the movements of individual stars in the centers of galaxies. The method used to discover the super-massive black hole in the center of our galaxy-measuring the movement of stars around it-can be used only for the closest galaxies. The challenge in trying to investigate black holes is that they are, obviously, black-they do not emit light-and so they are very hard to detect. Even more puzzling is that super-massive black holes can be seen already in the early Universe-when the Universe was younger than the time it should take such black holes to form. We do not understand how the universe forms such monstrous objects. In fact, super-massive black holes probably lie at the center of all large galaxies and can be as massive as a 100 billion suns. The Milky Way is not the only galaxy hiding such a monster. The black hole at the center of the Milky Way, with its mass of 4.5 million suns, could fit inside the orbit of Mercury. Ghez).īlack holes are extremely dense objects, equivalent to shrinking the entire Earth down to the size of a ping-pong ball. The stars can be seen orbiting an invisible object, located at the very center of the image, having a gravitational force of 4.5 million suns (Photo credit: A. The stars are marked by circles, denoting their positions each year between 19.Figure 1 - The motion of stars around the center of the Milky Way Galaxy.And not just any black hole, but a super-massive black hole. What object could produce the gravitational force of 4.5 million suns and not produce any light? A black hole. But, at that point in space, nothing was emitting any light. The stars were seen to orbit a point in space as if they were affected by the gravity of 4.5 million suns ( Figure 1). There, they found a group of stars and followed their movements for over a decade. 1 They developed instruments and methods to obtain a clear picture of what is going on at the heart of the Milky Way. Two scientists, Andrea Ghez and Reinhard Genzel, received the 2020 Nobel Prize in Physics for looking that monster right in the face. Not only because it requires piercing through interstellar “dust” that absorbs visible light, but also because there is a monster lurking there. It is not easy staring into the center of our galaxy, the Milky Way. In this article, we will discuss different ways we can “see” black holes, and particularly what we do and do not yet understand about stars getting “tidally disrupted” by them. These star-destroying events can help us to discover the locations of the most massive black holes in the Universe, but only if we know how to find and interpret them. When stars get close to super-massive black holes they can be torn apart, which produces a relatively brief but informative flash of light. Black holes are the most densely packed objects in the Universe. But, in recent years, astronomers have developed a new way to investigate a type of invisible and distant objects-super-massive black holes. Studying invisible objects in space that are hundreds of millions of light years away may sound impossible.
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