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Spectral classification

Stars. Spectral classification.

The stars are classified according to their color and size. The first classification criterion is its color, which sometimes can be seen from the Earth. This color is given by the star surface's temperature. Depending on their color are classified in a "Spectral Classification" table designated by the letters O, B, A, F, G, K, M and L. There is a nemotecnic rule to remember the spectral classification that is: Oh Be A Fine Girl Kiss Me Linda.

The blue stars has the higest surface temperature, and are classified as type O stars. The lowest temperature are the reds, which are M and L kind. Let's see it with more detail:

O spectral type O: stars very hot and bright and notables for its brilliant blue color. These stars have lines of neutral and ionized helium very prominent and have weak hydrogen lines. Most of its radiation is emitted in the ultraviolet. They can shine over a million times our Sun.

B spectral type B: extremely bright, like Rigel in the Orion constellation, a blue supergiant. The specters of these stars have lines of neutral helium and moderate hydrogen lines. These stars, like the O-type, are so massive that consume their energy much faster than other smaller stars releasing vast amounts of energy and lived for a short period of time (just a few million years). They tend to be in the regions of stellar formation in which were born, and by this they're usually observed in stars groups formed in the interior of dust giant clouds. At Orion constellation we find the closest example.

A spectral type: this group belong to the most common stars that we can see with the naked eye. Deneb, in the Swan constellation, is a high brightness star like Sirius (also a type A), the brightest star we can see from Earth in the Canis Majoris constellation, close to us but not quite as big as Deneb. Las estrellas de clase A tienen pronunciadas líneas de hidrógeno y poseen también líneas de metales ionizados. The A class stars have uttered hydrogen lines and have also ionized metals lines.

F spectral type: located in the main stream of Herptzsrung-Russell are high mass stars and very brigh. Their specters are characterized by weak hydrogen and ionized metals lines. They are white with a light yellow tone.

G spectral type: are the best khown because our Sun belongs to this category being a G2 star-type. They have hydrogen lines even weaker than those F type and also have ionized and neutral metals lines.

K spectral type K: orange color stars something colder than the Sun. Some of them are giants and even supergiants as Antares, while other stars of this group, as beta Alpha Centauri belongs to the main sequence. Tienen líneas de hidrógeno muy débiles y en ocasiones algunas líneas correspondientes a metales neutros. They have very weak hydrogen lines and sometimes a few neutral metals lines.

M spectral type: The most common group of all due for the large number of stars that belong to this type. The red dwarfs belong to this class and more than 90% of all stars, such as Proxima Centauri. The M class also counts with most of the giants and some supergiants as Arcturus and Betelgeuse. The specter of a M star has neutral metals lines but not hydrogen lines are normally shown. Titanium oxide can form intense lines in the M stars type. They have a wide range of sizes, there are enormous called "supergiants" and smaller called "dwarfs". The Sun is a yellow star medium G type. Betelgeuse, a supergiant, has about 610 times the diameter of our Sun.

See, for a better understanding, the Hertzsprung-Russell's diagram.

Celestial magnitudes.

The magnitude of a star is a number that indicate his brilliance. There is the apparent magnitude (the brightness with which the star is seeing from Earth) and the absolute (indicates the real brightness of the star).

The brightest a star is the smaller magnitude he has. Sirius, the brightest star in the firmament, has a magnitude of -1.46. Polar Star, which shines much less has a 2.10 magnitude.

Although the oldest catalog was prepared by the Greek Hiparcus (towards 127 BC), was in 1856 when the astronomer Norman Pogson created the scale used now and is known as the "Pogson's scale." It stipulates that a star of 1 st magnitude has a luminous intensity apparent 2.512 higher than a 2nd magnitude star, and this, in turn, a luminous intensity 2.512 times greater than a 3rd magnitude star, and so on. This scale of values extends on one hand toward zero and negative numbers, to cover brighter space objects like the Sun, the Moon, Venus, Mars, or the star Sirius, and ,on the other hand, extends a range from zero to positive numbers, in wich, with each increment, we have weaker visible shinnig stars.

Apparent magnitudes scale
Celestial Objetc Apparent Magnitud
The Sun -26,8
Full Moon -12,6
Maximun brigtness of Venus -4,4
Maximun brigtness of Mars -2,8
Brightest star: Sirius -1,46
Faintest stars visible in an urban neighborhood with naked eye +3,0
Faintest stars observable with naked eye under perfect conditions +6,0
Faintest objects observable in visible light with Hubble Space Telescope +30

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