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You look up into the sky on a clear dark night, far from the intrusive glow of streetlights, you will see a faint band of light running across the heavens. This is the Milky Way, our own galaxy. It contains about two hundred billion stars and countless other objects besides. The Milky way viewed through binoculars is a magnificent sight, with thousands of stars in each field of view. Our galaxy is only one of
billions of other galaxies in the universe,
but it is special to us, because it is home to Earth and our solar system. The
Milky Way has been studied extensively by astronomers, and today our ideas about
it's nature are set on a more firm foundation. Long before the Sun and the solar system formed, before the galaxy existed, the universe was filled with gas - mainly hydrogen, with some helium. This gas was eventually to be turned into stars, planets and people. But before these things could happen, the galaxy had to form. Astronomers believe that
the galaxy formed out of a large, fairly spherical cloud of cold gas, rotating
slowly in space. At some point in time, the cloud began to collapse in on
itself, or condense, in the same way that the clouds which formed individual
stars also condensed. Initially, some stars may have formed as
the gas cloud began to fragment around the edges, with each fragment condensing
further to form a star or group of stars. The Milky Way galaxy in which
we now live is a very different place to the cold gas from which it formed over
16 billion years ago. No longer is it a spherical mass of hydrogen; today
astronomers with radio telescopes have charted the clouds of gas and have found
that the Milky Way is a Spiral Galaxy, one of countless others. Whilst we can't
"stand back" and see our galaxy as a whole, we can look out into space
and see other galaxies which we think may be similar to our own. The star: Sizes of stars: Stars form in a variety of sizes, and as they get older, different processes occur inside them causing many changes; the star evolves. The size of the star is one of the quantities which will change as it evolves. Astronomers divide stars into several different types depending on properties such as their size, age, temperature and luminosity, and so when we make a general comparison of stellar size, we must take care that we are comparing similar types. However, to gain some idea of the range of sizes, it is interesting to note that White Dwarf stars can be one thousand times smaller than our sun, whilst Red Giant stars can be over one hundred times larger than our sun. That means that stellar sizes cover a range of (approximately) 1,400 km to 1,400,000,000 km in diameter ! Colours, temperatures, twinkles: It may not be immediately obvious on a clear night, but stars have different colours. Although in the case of most stars our eyes are not sensitive enough to see the colours without aid, using binoculars or a telescope makes the effect more noticeable; photography shows the colours in their full glory. Stars can be seen in a variety of subtle shades, from the red star Aldebaran in the constellation of Taurus, to the brilliant white Vega in the constellation of Lyra, to the blue stars of the Pleiades shown in the image at the top of the page. These colours give us clues to the temperature of the star.
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Our Sun is a star, our sun is the closest
star we have. It is relatively very close to us so it looks very different to
the stars we see in the night time sky. However, this appearance is deceptive.
In reality, the night time stars are also vast globes of gas held together by
gravity, and creating large amounts of heat and light through nuclear fusion -
converting the hydrogen which was already present in the universe at early
times, into heavier elements like helium, oxygen, carbon, and so on. Stars are
huge factories producing the materials which make up the universe we see around
us.