Stars surround Earth whichever way we look out. These stars all belong to one galaxy, our galaxy, the Milky Way.
This vast collection of stars which includes the Sun, our own star, is the one in which we live. At the start of the twentieth century we thought that the Universe consisted of just this one galaxy. In the intervening years we have discovered billions more, which exist beyond our own. Some of the lights we see in the night sky aren't individual stars but vast collections of stars; they are distant galaxies. We estimate there are between 100-125 billion galaxies in the visible Universe. Today we know that our Galaxy does not occupy a special place in the Universe, yet it remains exceptional to us. It is our home, the galaxy we know best, and the one whose stellar objects we can see and study in detail.
The Milky Way is an enormous collection of stars, and gas and dust, which together make up a roughly disc-shaped body that measure about 100,000 light years across and about 4,000 light years thick. A light year is the distance light travels in a year and is equal to 9.46 million, million kilometres.
We know there are hundreds of billions of stars in the Galaxy but we don't know just how many. The number of stars is calculated using the ﬁgure for the Galaxy's mass; the amount of material the Galaxy is made of. This is thought to be about 1,000 billion solar masses (1,000 billion Suns' Worth of material). This ﬁgure not only covers the visible stars, gas and dust, but also additional material that hasn't been found yet. This "dark matter" is thought to be as much as 90 percent of the Milky Way's total mass, and we don't know what it is made from, we only know it is there.
The total mass and our knowledge of the part of the Galaxy we live in are then used to estimate the number of stars in the Milky Way. Most astronomers agree a minimum of about 200 billion with a total of probably about 500 billion.
Most of the Galaxy's visible material consists of stars at various stages in the stellar life cycle. There are hot, bright, newly formed stars; middle-aged stars such as the Sun; older red giants; planetary nebulae and neutron stars, as well as black holes. The remaining ten percent is interstellar medium, that's predominantly hydrogen and dust that includes material shed by older stars, and material that will make new stars. Both old and young stars are densely packed in the centre of the Galaxy where they form a bulge; beyond this is the disc. Within the disc are pronounced spiral arms of stars. There are plenty of stars in the spaces between the spiral arms, but because the stars in the arms are young and bright these shine out. The brilliant light of these hot, newly formed stars give the disc a blue—white tinge. The bulge stars, which are much older, give the centre a yellow tinge.
Galaxies are classiﬁed into one of four main types (spirals, barred spirals, elliptical, and irregulars) according to their shape. It has been known for over half a century that the Milky Way is a spiral galaxy, and within the past decade astronomers have become increasingly convinced it is a barred spiral. This is because its spiral arms appear to radiate out from a bar-shaped bulge in the centre. Unfortunately, we cannot get a clear view of the centre, the spiral arms, or the Galaxy’s overall shape as we are looking from within the system; we can’t get that bird's eye view that we can of other galaxies.
The Sun and the Earth are about halfway from the Galaxy’s centre on the inner edge of one of the spiral arms, the Orion arm. This and the other arms are named after the constellation in which they are most noticeable in Earth’s sky. Working very roughly from the inside out, they are the Norma, Scutum—Crux, Sagittarius—Carina, Orion, and Perseus arms.
A sparsely populated spherical halo of old stars surrounds the disc. As Well as individual old stars the halo contains about 500 globular clusters, which are tightly packed spherical collections of hundreds of thousands of old stars. The halo is also believed to be the home of some of the Galaxy’s missing ‘dark matter’. All the stellar objects rotate around the Galaxy’s centre. The disc objects orbit within the plane of the disc; they travel individually at about the same speed and not as a solid disc such as a record on a turntable. The Sun orbits once every 220 million years or so at about 220 kilometres per second. Stars in the halo have more elongated orbits and these take them above, below and through the disc.
The centre of the Milky Way is hidden from us in optical wavelengths by dense dust and gas, but other wavelengths such as radio allow us to see through such obstacles. The Galaxy's exact centre is occupied by an intense radio source that seems to have no orbital motion. Less than a decade ago this object, called Sagittarius A* (pronounced A-star) was identified as a massive black hole. It is smaller than the size of Saturn's orbit and has a mass of about three million solar masses.
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