Updated June 01, 2013

Categories of stars

Characteristics of stars

The man thought that the brightest stars could provide the figures. These groupings differ from one era to another and from one civilization to another.
Figures become traditional, often in connection with Greek mythology, are called constellations.
The stars of a constellation have nothing in common, if not to occupy, as seen from Earth, a neighboring position in the sky. It can be very distant from each other. However, the International Astronomical Union has defined a standard list of constellations, assigning to each region of the sky to help locate celestial objects. The stars have a mass of between about 0.08 and 150 times the mass of the Sun. This quantity determines the life of the star. In 2010, a team of astronomers led by Paul Crowther, professor of astrophysics at the University of Sheffield, discovered the most massive star with a mass greater than 300 times the mass of our Sun, is twice the 150 solar masses considered the maximum mass for a star. The star R136a1, found in the R136 cluster, is the most massive star observed with a mass of about 265 solar masses and a mass calculated at birth of 320 times the mass of the Sun. A very massive star is very bright but his life will be reduced.
The most massive stars generate powerful winds. "Being aged a little over a million years, the star most extreme R136a1 is already half of his life and has already undergone an intense dieting, losing a fifth of its initial mass during this period, which corresponds to more than fifty solar masses. " Paul Crowther said. Below the minimum mass, heat generated by the contraction is insufficient to start the cycle of nuclear reactions. Beyond the maximum, the force of gravity is insufficient to retain all the matter of the star when nuclear reactions begin.
Compared to our planet (about 12 756 km in diameter), the stars are enormous: the Sun has a diameter of about 1.5 million miles and some stars such as Antares or Betelgeuse has a diameter 800 times greater than our Sun.
The search for its stellar use across the range rather than the diameter of which remains a concept in two dimensions. The magnitude is a logarithmic scale of the radiative flux of the star. We distinguish the apparent magnitude which depends on the distance between the star and the observer, and the absolute magnitude, which is the magnitude of the star if it was arbitrarily placed at 10 parsec of the observer.

The absolute magnitude is of course directly related to the brightness of the star. The latter quantity is used by the models of stellar evolution, while the apparent magnitude is rather used for observations, since the eye has a logarithmic sensitivity also. Most stars appear white to the naked eye. But if we look closely at the stars, we can notice a color: blue, white, red and even gold. The fact that the stars show different colors remained a mystery.
Color used to classify stars according to their spectral type (which is related to the temperature of the star). The spectral types range from the more purple than red, that is to say, the warmer to the colder and are classified by the letters O B A F G K M. The Sun, for example, is spectral type G. But it is not sufficient to characterize a star by its color (its spectral type), we must also measure its brightness.
For a given spectral type, plus the star, the greater its light is strong. O, W and B stars are blue in the eye, the stars are white, stars F and G are yellow, the stars are orange K, M stars are red.

class	Description	temperature
O	super massive star	≥ 30000 K
W	Wolf-Rayet star	≥ 25000 K
B	massive star	10000 - 30000 K
A	large star	7300 - 10000 K
F	solar type	6000 - 7300 K
G	solar type	5300 - 6000 K
K	solar type	3800 - 5300 K
M	sub solar	2500 - 3800 K
C	carbon star	2400 - 3200 K
S	sub carbon star	2400 - 3500 K
L	hot brown dwarf	1300 - 2400 K
T	cool brown dwarf	600 - 1300 K
Y	sub brown dwarf	< 600 K

Table: Spectral types stars.

star clusters seen by the Hubble Space Telescope

Image: Image of the globular cluster Omega Centauri, taken by the Hubble Space Telescope with the Wide Field Camera 3 (WFC3), in 2009.
Credit: NASA, ESA, Hubble SM4 & the ERO Team. The color used to classify stars according to their spectral type (which is related to the temperature of the star). The spectral types range from the more purple than red, that is to say, the warmer to the colder and are classified by the letters OBAFGKM.
O and B stars are blue in the eye, the A stars are white, stars F and G are yellow, the K stars are orange, M stars are red.

Categories of stars: dwarf

nota : Brown dwarfs are not stars or rather they are failed stars. Their mass is between those of small stars and that of large planets. Indeed, over 8% of the solar mass, a protostar begins thermonuclear reactions and shines. Brown dwarfs are not massive enough but they radiate some heat, residue of its formation. It is possible that at the beginning of their formation they have started a thermonuclear fusion but they extinguished. Brown dwarfs have never reached the critical mass (13 times the mass of Jupiter, or 0.08 times the mass of the Sun) to catch fire and maintain a sustainable state. Brown dwarfs are difficult to observe, since they emit a weak radiation in the infrared.
Brown dwarfs: Teide 1, WISE 0855–0714

nota : Red dwarfs are small stars (0.08 and 0.4 solar mass) red and discrete, the surface temperature is low (between 2500 and 5000 K), which is why they glow in the red or orange.
These stars among the most numerous of the Universe, consume very little nuclear fuel (hydrogen) and thus have a very long life, estimated between tens and 1000 billion years. They contract and heat up slowly until all their hydrogen is consumed. Proxima Centauri or Alpha Centauri C, the nearest star to us is a red dwarf, and some twenty of the thirty other nearby stars in the solar system.
Red dwarfs: Proxima Centauri, Regulus C

nota : White dwarfs are residues of extinguished stars. This is the penultimate stage of the evolution of stars whose mass is between 0.3 and 1.4 times that of the sun. The density of a white dwarf is very high.
A white dwarf of one solar mass has a radius of the order of that of the Earth. The diameter of the white dwarf does not depend on temperature, but its mass, the higher its mass, the more its diameter is small. However, there is a value above which a white dwarf can not exist, it is the Chandrasekhar limit. Beyond this mass, the pressure due to electrons is insufficient to compensate for the gravity and the star continues its contraction to become a neutron star.
White dwarfs: Sirius B, 40 Eridani B

Categories of stars: dwarf

nota : Yellow dwarfs are stars of medium size, but astronomers classify stars in dwarf or giant. They have a surface temperature of about 6000 K and shine a bright yellow, almost white. At the end of his life, a yellow dwarf becomes a red giant and a white dwarf. Star reaches this stage when its heart has exhausted its primary fuel, hydrogen.
Helium fusion reactions start then, and while the center of the star contracts, its outer layers swell, cool and redden. Converted into carbon and oxygen, helium is exhausted in its turn and the star dies. The star then expels its outer layers and center contracts into a white dwarf the size of a planet.
Yellow dwarf: Soleil, α Centauri A

nota : Orange dwarfs are stars of the main sequence, type KV, K (spectral type), V (luminosity class). They are located between the yellow dwarfs like the Sun and red dwarfs like Proxima Centauri.
They have masses of the order of 0.5 to 0.8 times that of the Sun (between 500 and 800 Jupiter masses) and surface temperatures between 3500 and 5000 K.
Orange dwarf: Alpha Centauri B, Epsilon Eridani, Eta Cassiopeiae, Sigma Draconis, 61 Cygni

Star category: Giant

Giant white, blue and yellow, red super giant are hot and shiny. These stars are at least ten times larger than the Sun.
The blue giants are extremely luminous, absolute magnitude of -5, -6 and more. Very massive, they quickly consume their hydrogen and their lifespan is very short of the order of 10 to 100 million years, so they are very rare in the observable universe.
When hydrogen in its heart was consumed, then the giant blue merges helium. Its outer layers swell and its surface temperature drops to become a red supergiant. The star then produces increasingly heavy elements iron, nickel, chromium, cobalt, titanium...
It's in the stars that the matter we are made is created. At this point, the fusion reactions stop and the star becomes unstable.
Then it explodes in a supernova and dies by seeding the complex atom interstellar space. The explosion left behind a strange heart of matter that will remain intact.

This corpse is, according to its mass, a neutron star or a black hole.

Blue giant: Rigel, Deneb, Hadar,
Red giant: Aldebaran,
White giant: Procyon
Yellow giant: Pollux
Yellow supergiant: Canopus,
Blue supergiant: Achernar,
Red supergiant: Betelgeuse, Antares,

Star category: neutron star and black hole

Neutron stars are very small but very dense.
They concentrate the mass of a star like the Sun in a radius of about 10 km. These are very massive stars remains of more than ten solar masses.
When a massive star reaches the end of existence, it collapses on itself, producing an impressive explosion called a supernova.
This explosion scatters enormous quantities of matter in space but spares the heart of the star. This heart contracts and becomes largely a neutron star.
The density of a neutron star is roughly that of the atomic nucleus. These objects, called magnetars, have very strong magnetic fields. Along the magnetic axis spreads charged particles, for example electrons, which produce synchrotron radiation.
This field is so powerful that it deforms to the atoms that make up matter. In the absence of magnetic field, the atoms have a spherical shape, while subjected to super strong magnetic field, they take a tapered shape and align themselves along magnetic field lines, such as small needles placed end to end.

Black holes are massive objects whose gravitational field is so intense that it prevents any form of matter or radiation to escape. Black holes are described by the theory of general relativity. When the heart of the dead star is too massive to become a neutron star, it shrinks inexorably to form this astronomical object that is the black hole. Consideration since the 18th century, the theory supporting the existence of black holes, states that are objects so dense that its escape velocity exceeds the speed of light - that is to say that even light can not overcome their gravitational surface and remains trapped. This disturbing feature from the labels "black" and "obscure," but the more accurate term would probably be "invisible" because it is indeed a total absence of light. The theory also defines precisely the intensity of the gravitational field of a black hole. It is such that no particles crossing the horizon, boundary theory, can not escape.
While most stars are easily placed in one or other of these categories, it is only temporary phases. During its lifetime a star changes shape and color, and can move from one Category to another.