By Girvan McKay
The world of a new born child is very small. It consists only of the space occupied by the child and its mother, and its not even aware of that because its eyes cant focus.
A little later, its world extends by a few square feet when its put into its playpen. Still later, that world grows to take in the room, then the house, then the street, the town, and country that the child lives in.
Instead of a few feet, the childs environment ranges from hundreds to a few thousand kilometers, depending on the size of the country. When we go to school we learn about other countries and we're told that out Earth has an average diameter of 12,735km (or 7,010 miles).
Looking up into the sky at night we see our nearest neighbour, the Moon and we learn that its diameter is a quarter that of Earth - giving it an area about the same as the total surface of the North and South American continents. The distance between Earth and the Moon is 388,000km (238,900 miles), the equivalent of 27 Earths laid in a row. The Apollo journeys to the Moon took between 3 and 4 days.
As you know, when we're measuring distances beyond our Solar ystem we dont use kilometers or miles. We use the term Light Year (Ly). A light year is the distance that light travels in one year. Light travels at 299,793km (186,000 miles) per second, so we say that e.g. the Moon is 1.3 lights seconds from Earth. However, we can still measure the distance from here to the Sun and the other planets in kilometers or miles, if we want to. For example, the Sun is 150 million km (nearly 93 million miles) or 8.3 light minutes from Earth. If we could fly to the Sun in an ordinary aeroplane, it would take something like nearly 20 years to reach it. Of course rockets travel much faster than planes can, but maybe that gives some idea of how far the Sun is.
Unmanned spacecraft have managed to reach or approach all of the planets except pluto, which is 5,764 million km (3,582 million miles) away, or simply 5 light hours away. An Israeli astromomer calculated that if we could travel to Pluto in a jet plane, it would take more than 500 years. In other words, its 25 times farther away from us than the Sun is. Maybe in our lifetime an unmanned space craft will get to Pluto.
If the distances within our Solar System seem great, when we look beyond it they become mind boggling! The distance to the nearest star, Proxima Centauri, is 4.2 light years, or 280,000 times the distance from Earth to the Sun. Our fastest space probes would take25 to 30 thousand years to get there. When we look at the stars we know that we're not only looking out into space, but also back in time. With the naked eye we can see stars up to something more than 500 lightb years away, but we see them as they were when the first Europeans landed in America, not as they are now.
Another measure of distance used in Astronomy is tha parsec i.e. about 3.26 light years. When we speak about distant stars we can use the term gigayear as a unit of time, that is, one thousand million years. Now you can realise the kind of scale involved.
But if you think that these are huge, they're nothing compared to galactic distnaces. Although our Solar System is huge to us, its nothing compared to the size of the Milky Way, the galaxy that its a part of. The Milky Way has the shape of a colossak Catherine Wheel and our Solar System is away out on the margin of one of the arms on the wheel. Our galaxy contains more than one hundredd thousand (100,000,000,000) stars. We dont know how many of these other stars have planets round them though some have been detected. The diameter of the Milky Way is about 100,000 light years.
The galaxies aern't isolated. They're grouped together in clusters, which can contain anything from ten to several thousand galaxies. The cluster we belong to is called the Local Group. The nearest neighbouring galaxies to us are called the Magellanic Clouds, about 170,000 light years away. But the typical distance between members of galaxy clusters is measured in millions of light years. The diameter of the Local Group is about 4 million light years. Its possible to see with the naked eye the Andromeda Galaxy, our nearest galactis neighbour about 2.2 million light years away, on a clear night. But we see it as it was before the emergence of the first man on Earth. A rocket launched from earth would take 12 thousand million years (or 12 billion years) to get there - the very age of the Universe itself.
Every star orbits around the center of its galaxy. Our Sun takes 240 million years to orbit the center of the Milky Way. In every direction that we look in the sky there are galaxies. Its calculated that there are more than a hundred thousand million of them. The most distant objects observed so far are the quasars, the active nuclei of young galaxies. They're more than 10 thousand light years away. To put it simply, a quasar is a point-like source of light only visible in large telescopes, often releasing intense radio emissions, and showing a large red shift, which means that they are very remote and moving away at a great speed. Because they're so far away, the energy they are giving out must be immense. Quasars are among the many very mysterious objects in the univers and we know very little about them.
Perhaps we should say something about how we classify stars. We do this according to their brilliance. The apparent brightness of a star depends as much on its actual brightness as on its distance from us. A star that is very bright but very far away ma seem to be faint, whereas a less bright star may seem bright to us if its relatively near. Our sun is brighter to us than any object in the sky and yet its actual brightness is less than that of an average star.
When classifying stars astronomers apeak of stellar magnitude.
*Those stars which show the greatest apparent brightness are called first magnitude stars.
*The faintest stars we can see with the naked eye are magnitude 6, sometimes 7 depending on conditions.
*Between these magnitudes we have stars of the second, third etc magnitude.
*The scale is extended beyond the sixth to those stars that are so faint that they can only be senn through a telescope. With the biggest telescope its possible to observe stars down to magnitude 25. The scale also includes negative magnitudes for the most brilliant stars. Sirius, the star that has the greatest apparent brightness, has a magnitude of -1.58. The Sun's magnitude is -27, Venus is -4.3 (average) - although, of course, its isn't a star and it doesn't shine by its own light.
Its important to notice that the brighter an object seems to be, the less is its magnitude - not the other way around! When Mary Glennon lectured to T.A.S. she told of how to calculate a stars real brightness - what astronomers term its absolute magnitude. If we call the apparent magnitude of star m, and d is its distance in light years, and M represents its absolute magnitude, then:
M = m - 5log10d + 7.57
As we know, the most distant objects in the Universe are so far away that there's no way we can observe them using even the most powerful optical telescopes in the world. We have to use radio telescopes to detect them. However, now that we are able to put telescopes like Hubble into orbit, it should be possible to see farther and farther into the remote reaches of space and maybe even to find clues as to whether there is life elsewhere in the universe, and to solve some of the greatest mysteries of the Cosmos.
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