If you look up to the sky you see stars with different brightness. Some stars are very bright, while others are just faint. This is partially because some stars are more luminous than others, but also because some stars are much further away than others. For an example have a look at the two pointers of the Southern Cross for instance. Alpha Centauri (the one the furthest from the Cross) seems brighter than Beta Centauri, also called Hadar (the one closest to the Cross). The reason for this is its proximity; it is in fact our closest stellar neighbour. The double stars in Beta Centauri are much more luminous than the triple stars in Alpha Centauri, but they are about 120 times further away. This causes the star to look less bright because the light is spread out over a much larger area as it travels away from the star.
Absolute Magnitude
To compare how stars would look if they were all at the same
distance, astronomers use a measure of intrinsic brightness called the absolute magnitude scale. This scale uses high positive numbers for the dimmest stars and negative numbers for the brightest ones. A star’s intrinsic brightness is related to its luminosity, which is the amount of energy it radiates into space per second. Once a star’s distance is known, its absolute magnitude can be determined and from this its luminosity. Stellar luminosities are generally expressed as factors of the Sun’s luminosity. There is a very large range of stellar luminosities, from less than one ten-thousandth to a million times that of the Sun.
To compare how stars would look if they were all at the same
distance, astronomers use a measure of intrinsic brightness called the absolute magnitude scale. This scale uses high positive numbers for the dimmest stars and negative numbers for the brightest ones. A star’s intrinsic brightness is related to its luminosity, which is the amount of energy it radiates into space per second. Once a star’s distance is known, its absolute magnitude can be determined and from this its luminosity. Stellar luminosities are generally expressed as factors of the Sun’s luminosity. There is a very large range of stellar luminosities, from less than one ten-thousandth to a million times that of the Sun.
Apparent magnitude
A stars brightness as seen from the Earth on the other hand, is described by its apparent magnitude. Again the smaller the number of a star’s apparent magnitude, the brighter the star. The faintest stars detectable with the naked eye have an apparent magnitude of +6, whereas the apparent magnitude of most bright stars lay between +2 and 0. The four brightest stars have negative apparent magnitudes. The brightest star in the sky is Sirius. It is, with 8.6 light years, the ninth closest star to Earth and has an apparent magnitude of -1.46. Sirius is a binary pair consisting of Sirius A (a main-sequence star that has twice the mass of the Sun and that is 23 times more luminous) and its companion, a white dwarf.
A stars brightness as seen from the Earth on the other hand, is described by its apparent magnitude. Again the smaller the number of a star’s apparent magnitude, the brighter the star. The faintest stars detectable with the naked eye have an apparent magnitude of +6, whereas the apparent magnitude of most bright stars lay between +2 and 0. The four brightest stars have negative apparent magnitudes. The brightest star in the sky is Sirius. It is, with 8.6 light years, the ninth closest star to Earth and has an apparent magnitude of -1.46. Sirius is a binary pair consisting of Sirius A (a main-sequence star that has twice the mass of the Sun and that is 23 times more luminous) and its companion, a white dwarf.
I hope you found this interesting again.
Happy stargazing
Miriam
Miriam
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