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About Stars: Measured Values

An explanation of the values given on the star pages:

Constellation: Tells where the star is in the sky. There are 88 different constellations.
Age: Is difficult to specify and often can only be approximated. Very big stars only live a few million years whereas small stars can have more than 100 billion years of life-span. Admittedly the universe itself is only 13.7 billion years old. Our Sun has an age of 4.6 billion years.
Distance: Our galaxy (Milky Way) has a diameter of circa 100 000 light-years. Our Sun is 8 light-minutes away from earth, the next other star, Alpha Centauri, 4.3 light-years. One light-year is about 9.5 trillion kilometers.
Spectral class: Tells the color (wavelength) and therefore the surface temperature. The designations span the stectrum in the order O B A F G K M R S C whereas O and B is blue, A and F is white, G is yellow, K orange and M - C red. R, S and C are stars with a special frequency of chemical elements. L and T is for brown dwarfs. Furthermore there are some special classes like W for Wolf-Rayet stars.
For an exacter definition the letters are followed by a number between 0 (shorter wavelength) and 9 (longer wavelenght). According to this the temperature is between 50 000 kelvin (O3 stars) and 2000 kelvin (M9 stars), beside extreme exceptions. O3 is the highest spectral class.
Visual magnitude: The brightness as seen from us. The smaller the value the brighter the star is. Sirius for example is with a negative value extremely bright. Up to 6.0 stars are just about to see with the naked eye at optimal conditions. 5 magnitudes make a difference of 100 times.
Luminosity: The absolute luminosity compared to our Sun (in units of solar luminosities). Generally it refers to the whole spectrum and not only to the interval of visual light.
Mass: In units of solar masses. The mass of the Sun is 1.9884 * 1030 kilogramm
Diameter: In units of the diameter of the Sun. This is 1 392 000 kilometers
Radial velocity: The movement of the star to us (positive value) or away from us (negative value). The horizontal speed is much more difficult to measure and isn't indicated.

The distance within a stellar system is given in AU, Astronomical Units. 1 AU accords 149 597 871 kilometers. This is the average distance of Earth and Sun.

From time to time a temperature is mentioned. This is, if not explicitly noted otherwise, always the surface temperature. It is measured in kelvin. To get degrees centigrade 273.15 must be substracted. To get it in fahrenheit please use the astronomical calculator as for other conversions as well.

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Spectres of the classes OBAFGKM
Graphic: AURA, NOAO, NSF
Pelican Nebula
The Pelican Nebula (IC 5070).
Photo: Nasa

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