Photo: Smartworld.it
Black holes are rather menacing objects.
Stars, planets, dust and any other material that gets too close fall into it. There is an intense flash of radiation and nothing comes out. There are black holes with masses millions of times that of the sun at the center of galaxies like ours. There are many less massive and much smaller black holes around, formed during the final collapse and explosion of dying massive stars.
Astronomers believe there are about 100 million scattered throughout our galaxy. This suggests that there might be one or two very close to us. In fact, one has just been discovered. It is about 1600 light years away. This is close. Some of the stars we see in the night sky with our naked eyes are further away than that.
This new discovery has not been seen directly; black holes are small and black, invisible against the black background sky. We are forced to use indirect methods in research. Black holes are probably the strangest objects in nature. However, the recipe for making them is simple: a mass of material, gravity and some initial compression to initiate 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 shrink the body. If we compress something enough, its gravity becomes stronger than the body’s ability to resist the compression, so it shrinks. As the contraction progresses, gravity becomes stronger and the contraction continues.
Our current understanding, derived from observing other things in the universe and from what we can do in the lab, doesn’t tell us where this runaway contraction will end up. At some point in the contraction, gravity on the surface of the body becomes so strong that not even light can escape. If this is the case, how can we find these objects?
One method is to look at X-rays, light and other radiation emitted by the material as it spirals and disappears into the hole. The radio images we have of black holes show a dark, roughly central area surrounded by a bright ring. This method works when the black hole consumes nearby stars, planets, and other materials.
However, there are many black holes that do not “feed” and do not produce observable radiation. These are called “dormant” black holes. In this case we look for stars orbiting 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 are orbiting. This also works on a smaller scale. Many stars are members of multiple star systems, with two or more stars born together and staying 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. This is how this nearby black hole was discovered.
Astronomers discovered a star orbiting something invisible. After careful observations using different astronomical instruments, the unseen 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 huge one shone brightly for a few million years and then exploded, ending up as a black hole.
The other star had a mass similar to that of the Sun. These stars survive for billions of years. It must have been when the massive star exploded. Since the stars are close together, about the distance between the Sun and Earth, the Sun-like star should have been destroyed.
How it survived the blast is a major puzzle.
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After sunset, Jupiter is in the southeast and Saturn is in the south. Mars rises later. The Moon will reach its first quarter on November 30.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.