For an explanation, see the main splats page
The principles of galaxies
- Stars occur in groups called galaxies, and unlike constellations, these stars really are grouped together, but they are all a very long way off.
- Our local galaxy is called the Milky Way, and it may be seen at night if you look up, so long as you are away from bright city lights which obscure it.
- In 1785 William Herschel suggested the Sun is part of a larger system of several million stars forming a thin disc, the start of our notion of the Milky Way.
- Over twenty years of observation, William Herschel was able to catalogue some 2500 stars of the Milky Way, using telescopes to measure their distances.
- William Herschel estimated that there might be as many as several million stars in the whole of the Milky Way, but this turned out to be a severe underestimate,
- The Milky Way galaxy is about 100,000 light years across, and contains around 300 billion stars. The centre of the galaxy is 27,000 light years away.
- The nearest star to the Sun is part of a three-star system, Proxima Centauri, 4.3 light years away, most stars are a great deal further away.
- In 1927, Jan Oort proved Bertil Lindblad's view of the Milky Way as a rotating spiral and found its rotational speed by analysing the motions of distant stars.
- When we look at any galaxy other than the Milky Way, all we can see is a single blob of light, because the individual stars are too far away to see.
- The nearest galaxy to ours is 2.2 million light years away. Forget galaxies far, far away, we can't even get to the nearest one in our lifetime.
- Galaxies are very large groupings of stars: there are about a hundred billion stars in our galaxy and there may be a hundred billion galaxies that we can see.
- There are probably 100 billion galaxies in the universe, containing an average of around 100 billion stars in each, and an unknown number of planets near them.
- The farthest known galaxy from Earth is known as 4C41.17. On red-shift data, it is around 15 billion light years away from our galaxy, and moving further away.
- The largest known galaxy is Abell 2029, which has an estimated 100 trillion stars and a diameter of 6 million light years, 60 times that of our galaxy.
- The groups of stars that we call constellations are generally all within a few hundred light years, though some are just a few light years away.
- As they are seen from earth, nearby stars seem to form constellations, but from another direction, these constellations would appear quite different.
- In 964, the Islamic scientist Al-Sufi noted in his 'Book of the Fixed stars' that he had seen a small fuzzy object, which we now call the Andromeda Nebula.
- All the other elements in the universe are formed from hydrogen, because stars carry out a form of fusion which produces energy and forms heavier atoms.
- Clouds of dust occur in interstellar space, and sometimes make it hard to see the stars lying beyond them. This dust in space may become stars in the future.
- Some stars emit gamma radiation or X-rays, rather than (or as well as) light and so may only be seen by using a directional antenna capable of detecting X-rays.
- The universe is expanding: distant stars show a redshift proportional to their distance. The Hubble constant can be used to relate distance to red-shift.
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