Ursa Minor is the name of a group of stars in the Northern Hemisphere. The word used in astronomy for a group of stars is “constellation”.
The stars and constellations have Latin names. Ursa Minor means The Little Bear. Its brightest star is called Polaris, and is centered over the North Pole. It is of great importance in helping sailors to find their bearing when navigating at night.
Star maps of the sky will help you locate the constellations.
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A star is a body of luminous gas, like the sun. But as stars are much farther away from the earth than the sun, they appear to be only small points of twinkling light. With the naked eye it is possible to see about 2,000 stars at any one time or place but with the most powerful telescope over 1,000 million stars are visible. Although light travels at 186,000 miles a second, the light from the stars takes many years to reach the earth.
Stars are not fixed in space, but are travelling in different directions at different speeds. Seen from the earth, these movements appear to be so small that groups of stars, or constellations, seem to have a permanent relationship. The star patterns we see in the sky are almost the same as those seen by our ancestors hundreds, or even thousands of years ago.
The sizes of stars vary tremendously, from less than the diameter of the sun to thousands of times its size. Most stars appear white when looked at with the naked eye, but some are bluish-white, yellow, orange and red. The varied colours are due to differences in surface temperature. The brilliant, white stars are the hottest with surface temperatures of several hundred thousand degrees. The less brilliant, orange and res stars have surface temperatures of about 2,000 degrees.
There are exceptions, however. Te red giant, betelgeux, in the constellation (or group) of Orion, appears to be brilliant because of its size. Its diameter is 250 million miles, which is greater than the diameter of the earth’s orbit round the sun.
Shooting stars which are sometimes seen moving across the night sky for a few seconds are really meteors. These small particles flare up as they strike the earth’s atmosphere and usually burn out.
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The Milky Way, or Galaxy, is the whole concourse of stars and other bodies which can be seen stretched across the heavens. It includes our own sun and its planets, as well as all stars visible to the naked eye. But the name is commonly restricted to the luminous band or belt where most classes of stars are concentrated.
The spiral arms of the Milky Way are rich in hot, bright stars, interstellar clouds of gas (mainly hydrogen) and dust. The first evidence of spiral arms was obtained in 1951 by the American astronomer W.W. Morgan, who identified three.
Our own system of sun and planets appears to be situated towards the inner edge of one of the arms, which is about 1,300 light-years away. The Andromeda nebula, a vast mixture of gaseous and solid matter, is visible as a small luminous patch in our sky. But it is comparable in size to the Milky Way and seems remarkably similar to our own galaxy.
The Palomar telescope, 200 inches in diameter, situated on Mount Palomar in California, has perhaps 1,000 million galaxies within the scope of its vision.
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In the beginning our universe was a mass of white-hot vapours and molten materials whirling about in space. Our world was formed from this. Astronomers believe it took millions of years for the cloud to cool, contract and begin to turn into molten rock.
Modern astronomers think that many millions of years ago there was a huge explosion in space. They do not know exactly what happened. But it is possible that our sun exploded or that a much bigger companion star of the sun became a supernova-that is, it broke up violently. The debris and blazing gases from this explosion were, it is thought, flung far into space.
For more millions of years our solar system boiled and bubbled. But very slowly, the fiery redness began to cool and condense into the nine planets and many smaller bodies. All these planets now revolve around the sun.
After further vast periods of time the lava of the earth began to solidify, developing over many millions of years, into the world as we know it today.
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