[History of Astronomy]
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The Brightest Stars
(Sun = 1)
(km / sec)
|6||Capella||Auriga||0.08||G8 F0||90 70||43||+30|
|10||Betelgeux||Orion||0.0 - 0.9||M2||105,000 v||640||+21|
Some examples of the names: Deneb is Latin for tail (because it marks the tail of The Swan - Cygnus); Antares is Greek for rival of Mars (because of its red colour); Aldebaran is Arabic for eye of the bull (because it marks the eye of The Bull - Taurus).
In the West, there are 88 recognised constellations; 48 of these date from Roman times and are known as the Classical Constellations. These include the 12 Zodiac constellations through which the Sun, Moon and planets always pass through. Constellations are always known by their Latin names.
Some examples: Canis Major means The Great Dog; Orion is The Hunter; Crux Australis means The Southern Cross.
Constellations are used by astronomers for convenience. We say that Sirius is in Canis Major rather than give its celestial coordinates.
In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.5 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. On this scale, Jupiter has a magnitude (at its brightest) of -2.6, Venus is at -4.4 and the Sun -27. The faintest stars visible to the naked eye are sixth magnitude. Pluto has a magnitude of +14, far too faint to be visible without a powerful telescope.
In the table it can be seen that Betelgeux varies its magnitude - some stars are variable in brightness.
The brightness of a star as seen from Earth depends on its intrinsic luminosity and its distance from Earth. A dim star may appear bright because it is close while a luminous star may appear faint because it is far away. This is why we say Apparent Magnitude.
Stellar spectra are classified into types. These types are given letters. The spectral type series is a temperature series. Moving from the hottest stars to the coolest, the series of letters runs O, B, A, F, G, K, M.
Each spectral type is subdivided into ten numbers. For example, A0, A1, A2, up to A9. A0 is hotter than A1. The table below gives more information.
Our Sun is a star of Spectral Type G2 with a surface temperature of around 6,000°C.
Luminosity can be measured indirectly by combining the apparent brightness of a star with its distance. It can also sometimes be measured directly from the spectrum.
Many stellar distances can be measured directly by trigonometry. As the Earth moves around the Sun, the star appears to shift its position against more distant stars. This effect is called parallax. It is a tiny effect but can be measured. The amount of the parallax depends on the diameter of the Earth's orbit around the Sun (just under 300 million km or 186 million miles) and the distance to the star. A star with a paralax of 1 second of arc (written 1") is said to be at a distace of 1 Parsec. 1 Parsec is equal to 3.26 Light Years.
Other stars can have their luminosity measured by their spectra or by other properties. When this is compared to their apparent brightness, a distance can be calculated.
For more on astronomical distances look at The Scale Of The Universe.
Radial velocity is easily measured by looking at the star's spectrum. The lines on the spectrum are shifted to the blue end if the star is moving towards us (the so-called blue shift) and to the red end if the star is moving away from us (red shift). The amount of this shift depends on the relative velocity between us and the star.