Photo: Smartworld.it
Black holes are quite threatening objects.
Stars, planets, dust, and any other material that gets too close fall inside. There is an intense flash of radiation and nothing comes back out. At the centers of galaxies like ours, there are black holes with masses millions of times that of the sun. There are many less massive and much smaller black holes around, formed during the final collapse and explosion of dying massive stars.
Astronomers estimate that there are about 100 million of them scattered throughout our galaxy. This suggests that there may be one or two quite close to us. In fact, one has just been discovered. It is about 1,600 light years away. That’s close. Some of the stars we see in the night sky with the naked eye are further away than that.
This new discovery has not been seen directly; black holes are small and black, invisible against the black background of the sky. We are forced to use indirect methods in the search. Black holes are perhaps the strangest objects in nature. However, the recipe for making them is simple: a mass of material, gravity and some initial compression to start the contraction.
The force of gravity on the surface of a body depends on two things: its mass and its size. For a given mass, the smaller the size, the stronger the gravitational force that tries to make the body contract. If we compress something enough, its gravity becomes stronger than the body’s ability to resist the compression, so it shrinks. As the contraction continues, gravity strengthens and the contraction continues.
Our current understanding, gained from observing other things in the universe and what we can do in the lab, doesn’t tell us where this indiscriminate contraction will end. At some point in the contraction, the gravity on the surface of the body becomes so intense that even light cannot escape. If this is the case, then how can we find these objects?
One method is to look at the X-rays, light and other radiation emitted by the material as it spirals in and out of the hole. The radio images of black holes we have show a dark, roughly central region surrounded by a luminous ring. This method works when the black hole absorbs nearby stars, planets, and other material.
However, there are many black holes that do not “feed” and produce no visible radiation. They are called “dormant” black holes. In this case, we’re looking for stars that orbit objects that have the right mass to be black holes, but are otherwise invisible.
We see stars at the core of our galaxy in close orbits around the central black hole. By measuring these orbits, we can estimate the mass of the object they orbit. This also works on a smaller scale. Many stars are members of multiple star systems, where two or more stars are born together and stay together, orbiting each other.
If a star in one of these systems is invisible, perhaps because it has become a black hole, its presence and mass can be determined by analyzing the orbits of its visible siblings. That’s how this nearby black hole was discovered.
Astronomers have discovered a star orbiting something invisible. After careful observations using various astronomical instruments, the invisible object was identified as a black hole. However, there is still a problem. The star orbiting the black hole is a sun-like star.
It seems that two stars were born together; one was much more massive than the other. The massive one shone very brightly for several million years and then exploded, ending up as a black hole.
The other star had a mass similar to that of the Sun. Such stars survive for billions of years. It must have been around when the massive star exploded. Since the stars lie close together, about the distance between the Sun and the Earth, the sun-like star had to be destroyed.
How he survived the explosion is a big mystery.
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After sunset, Jupiter lies in the southeast and Saturn in the south. Mars rises later. The Moon will reach its first quarter on November 30.
This article was written by or on behalf of an outside columnist and does not necessarily reflect the views of Castanet.
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