How Far Away Are the Stars?
ESO
To try to get a grip of the distances to the stars is not easy. Only two celestial bodies displays a surface to the naked eye observer, the Sun and the Moon. So they ought to be closer to us than all the other points of light. Actually, by pure chance the Moon and the Sun have almost the same apparent size on the sky. In reality, the Moon is 400 times smaller than the Sun, but is on the hand 400 times closer. In this way, nature has given us the possibility to, in rare instances, see a total eclipse of the Sun, allowing us to see the Sun's glimmering corona by naked eye.
Certain points move among the stars, although you may need a few days or weeks to notice it. They are the planets, the ones visible to the naked eye are Venus, Mars, Jupiter and Saturn. Occasionally, you can also glimpse Mercury, although always quite close to the Sun.
Actually, also the stars are moving. But it takes thousands of years before such changes can be noticed. Their distances are measured in light years. But how far away is in fact the most distant star you can see with unaided eye? There are various answers, depending on how dark your sky is, but also due to the fact that such distances are not always very well determined. One candidate is the star Rho Cassiopeiae. It has an apparent magnitude of 4.5 and is thus located in the constellation of Cassiopeia. It is known as a hypergiant and is considered to be 500 000 times more luminous than the Sun. Therefore it can be seen from a distance of about 10 000 light years.
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