As the Earth orbits the Sun during this period, the observer (taking measurements at the opposite sides of the Earth's orbit) notices an apparent movement of the star compared to more distant stars. To determine the distance to a star, astronomers measure the apparent change in its position over one year. Next article: Stellar Parallax > Stellar Parallax Andromeda is the nearest major galaxy to the Milky Way.The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are small satellite galaxies of own galaxy visible from the southern hemisphere.The Pleiades Cluster is a nearby open cluster of stars also known as the Seven Sisters.Proxima Centauri is the closest star to the Sun.Light Time is the distance as measured if travelling at the speed of light:.It is clear from this table why different units are used for defining distances to different objects. Table 3.1: Distances to various astronomical objects in different units. "The distance at which an object has a parallax of one arc second."Īn arcsecond is equivalent to 1/3600 of a degree, that is an angle of one second of arc (") is equal to one sixtieth of one minute of arc ('), and one minute of arc equals one sixtieth of a degree.ġ parsec = 3.26 light years = 3.09 x 10 13 km = 206 265 AU Since stars are distant, the parallax angle is very small and is usually measured in arc seconds (fractions of a degree) rather than degrees. The Tycho Catalogue - detailed distribution and data map of a further 1.2 million stars.The Hipparcos Catalogue - 120 000 stars to a precision 200 times better than any previous observations.As a result of the mission two catalogues of observations were produced: Launched in 1989 the ESA Hipparcos mission used the parallax method to observe positions of stars within the galaxy. Astronomers use this effect to measure the distance to stars by determining the angle between the lines of sight of a star from two different positions of the observer. This apparent movement is known as parallax. If you hold one finger at arm's length in front of your face and close each eye in turn, you will see that the finger appears to move compared to distant objects behind it. Astronomers use many different ways to determine just how far away a star is. Furthermore, how bright a star appears is ultimately no indication as to how close it is to us. Although the stars we see with the naked eye look similar in size, they vary enormously in their distance from the Earth. On a clear, dark night we may be able to see a few thousand stars in the sky, a tiny proportion of the billions of stars that are thought to exist in the Milky Way alone.
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