Black Holes

Black Holes:

When a star with a mass more than 20 times that of the Sun explodes as a supernova, it may leave behind a core so massive that even a neutron star is not its final state. The object's gravity may be so great that it goes on collapsing for ever, crushing the matter within it to an unimaginable state. As the object's density gets greater and greater, the force of gravity rises even more until nothing can escape — not even light. The object has become a 'black hole'. At the centre of the black — hole, the remains of the former star would be crushed to infinite density. in zero volume.

Alifye - Space and Astronomy - Black Holes

Black holes are invisible because they emit no light. However, objects can fall into a black hole, when they cross its‘ gravitational boundary. Once inside they can never escape. A black hole twice the Sun's mass makes a gravitational well 13 km. in diameter in space.

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Stars

Stars:

Stars form from gigantic clouds of gas and dust which are scattered throughout the spiral arms of our galaxy and in other galaxies. Many of these clouds appear to be dark. One such dark cloud is the Coal. Sack nebula in the constellation_ of the Southern cross. A cloud starts to form into stars when it becomes dense enough to collapse under the inward pull of its own gravity. As the cloud falls together, it separates into knots and clumps, each of which will eventually form an individual star. These stars — in — the - making are visible as small, dark blocks against the background of brightly glowing nebulae. A typical block known as a globule, is about the diameter of the solar system. At the dense central core of the still shrinking globule is formed the embryo star which is known as a protostar: At first the protostarshines a dul red, releasing heat which is produced by friction as material from the globule cascades down upon its surface. Eventually, the pressure' and temperature at the protostar's centre rise sufficiently for nuclear reaction to begin. These nuclear reactions mark the true birth of, new star. It is estimated that a star of the mass of the Sun takes about 10 million years to pass from the globule stage to the point when nuclear reactions begin.

Alifye - The Solar System - Stars

Since most of the original cloud consists of hydrogen and are made up Largely of hydrogen and helium. At atoms are crushed together to form helium. The conversion of hydrogen to helium releases energy which keeps the star life. Such stars are in the stable prime of their lives. This is the stage that the Sun and most other visible stars have reached today.

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