STAR
Stars represent the most fundamental building blocks of a galaxy. They are luminous spheres of plasma held together by their own gravity, and serve as the anchor for (nearly) every star system in the universe. Stars are responsible for the creation of heavy elements, which are essential to the evolution of the universe, and their remains after death are frequently responsible for the birth of the next generation of stars.
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MAIN SEQUENCE SPECTRAL TYPES
A normal, hydrogen-fusing star spends the vast majority of its existence on the Main Sequence. This is a continuous and distinctive band of stars that can be classified based upon color and temperature.
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1 solar mass - Earth's sun
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luminosity - the absolute measure of light emitted by a star
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magnitude - the brightness of a star. The brighter the star, the lower the number assigned.
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SPECTRAL CLASS O
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
dark blue/violet
30,000° - 52,000° K
ionized atoms, helium
60-150 solar masses
1,400,000
-5
0.00000003125% of all stars
10 million years
The largest and brightest stars in the universe, Class O stars live fast and die young. Due to their immense size, they burn through their fuel much faster than other stars and end spectacularly in a blazing supernova, generally becoming a black hole. Planets are common around these stars, but due to their short life span, organic life generally doesn't have time to develop.
SPECTRAL CLASS B
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
blue
10,000° - 30,000° K
neutral helium, hydrogen
2-16 solar masses
20,000
-3
0.13% of all stars
100 million years
Massive, but not super giants, Class B stars are extremely luminous and relatively short-lived. When all fuel is exhausted at the end of their life cycle, they will almost certainly supernova and become a neutron star. Planets are common around Class B stars, but due to their generally short existences, complex life does not have time to develop.
SPECTRAL CLASS A
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
light blue
7,600° - 10,000° K
hydrogen and ionized metals
1.4 - 2.1 solar masses
80
+1
0.6% of all stars
1 billion years
Class A typically represents large, young stars that over the course of millions of years, cool into red giants. Class A stars do not have a convective zone and thusly do not have a magnetic field. As a consequence, they lack the means to generate strong stellar winds. Planets are common around Class A stars, however evolution tends to favor gas giants in these star systems.
SPECTRAL CLASS F
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
yellow-white
6,000° - 7,600° K
hydrogen, ionized metals, calcium, iron
1.0 - 1.7 solar masses
6
+3
3% of all stars
3 billion years
Many older stars will eventually retire into Class F after their stellar deaths, becoming a compact star known as a yellow-white dwarf (not to be confused with the white dwarf). These stars have no internal energy production, but may radiate heat for millions of years.
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Young stars may also form as Class F, though these are regular hydrogen-fusing celestial bodies. When they experience stellar death, Class F stars expand into a red giant, become a planetary nebula, and later a white dwarf.
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Planets are very common around Class F worlds, and complex life is possible, though generally limited to underwater or underground due to high levels of ultra-violet radiation.
SPECTRAL CLASS G
COLOR
SURFACE TEMPERATURE
COMPOSITION
..
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
yellow
5,300° - 6,000° K
ionized calcium, neutral and ionized metals.
0.84 - 1.15 solar masses
1
+5
7.6% of all stars
10 billion years
Class G stars are fairly common, long-lived and reasonably luminous, and while they emit a fair amount of ultra-violet radiation, this can be easily blocked by a planet's ozone. As such, Class G stars are well suited for planets to harbor a wide range complex life.
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As a Class G star ages, it will expand into a red giant, phase into a planetary nebula, and cool to a white dwarf.
SPECTRAL CLASS K
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
orange
3,900° - 5,200° K
neutral metals
0.5 - 0.8 solar masses
0.4
+10
12% of all stars
20-70 billion years
Given their immense longevity and low ultra-violet radiation emissions, Class K stars are ideally suited for planets to develop complex life. These stars eventually cool to become white dwarfs.
SPECTRAL CLASS M
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
red
2,500° - 3,500° K
ionized atoms, helium
0.2 solar masses
0.04
+15
76% of all stars
1 trillion years or more
The smallest and coolest type of star on the main sequence, Class M stars are also the most abundant. While solar evolution tends to favor rocky terrestrial plants around these stars, they tend to orbit very close. As a result, these planets are tidally locked and bathed in radiation. Life is not impossible around these stars, but it is definitely not common. Due to their low mass, Class M stars burn through fuel very slowly, giving them an impressive lifespan in excess of a trillion years.
SPECTRAL CLASS L
COLOR
SURFACE TEMPERATURE
COMPOSITION
MASS
LUMINOSITY
MAGNITUDE
ABUNDANCE
LIFETIME
dim red
less than 2,500° K
metal hydrides, alkali metals
less than 0.2 solar masses
0.02
+20
possibly very numerous
1 trillion years or more
Cooler and dimmer than even Class M, the meager Class L still has enough mass to support hydrogen fusion and is therefore a star. Planets are rare and limited to small rocky worlds.
EXTENDED SPECTRAL TYPES
Not all stars fit comfortably into the Main Sequence. Older stars often age out of the sequence, and many small, dim stars never reach it.
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CLASS T: BROWN DWARF
A substellar object that has mass between the heaviest gas giant planets and the least massive stars. While they are not very luminous, planets have been known to orbit them.
CLASS R: RED GIANT
Stars of Classes A, F, G, and K age out of the Main Sequence when they exhaust their supply of hydrogen and begin thermonuclear fusion of hydrogen in an outer shell. The star cools to about 2,500 K, but the shell is tens of thousands of times bigger than the star that birthed it, making red giants extremely bright, despite their low mass. Many red giants will form a planetary nebula before cooling into a white dwarf.
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CLASS D: WHITE DWARF
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CLASS N: NEUTRON STAR
When a Class B star reaches the end of its life, it ends with a supernova. Because these stars do not have sufficient mass to form a black hole, the core of the star is instead crushed into a tiny neutron star. Measuring no more than 10 kilometers in diameter, neutron stars are extremely dense; a single spoonful of a neutron star would weigh 3 billion tons on Earth. With temperatures in excess of 600,000 K, it will theoretically take these stars billions of years to cool.
After a red giant billows into a planetary nebula, solar winds will eventually disperse the nebula, leaving behind a dim, low-mass star about the size of a planet, known as a white dwarf. Essentially a dead star, it will over the coming eons--a span of billions upon billions of years--dissipate all of its remaining heat and become a (theoretical) black dwarf.
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Oddly enough, a white dwarf might still have planets. Any worlds that were untouched by the star's red giant and planetary nebula phases would remain, forever orbiting a dead star.
CLASS P: PULSAR
A pulsar is a highly magnetized neutron star that emits beams of electromagnetic radiation from its poles (often with very, very short rotational periods).
CLASS Y: YELLOW HYPERGIANT
An absolutely massive star with an extended atmosphere, a spectral class from A to K, and a mass of 20-60 solar masses. They are among the most luminous stars in the universe, with an absolute magnitude of -9. They are also very rare, with only 15 known in the Milky Way. These stars have clearly evolved off the main sequence, and are thought to be a rare progression of Class B stars.
CLASS H: BLUE SUPERGIANT
Stars with an initial mass of over 25 solar masses tend to very quickly exit the Main Sequence and increase in luminosity to become the extremely rare blue supergiant. These short-lived stars will either blow up in a supernova or lose their mass to stellar winds and become a Class W star.
CLASS W: WOLF-RAYET
An extremely rare type of giant star that is believed to be a Class O in an advanced stage of stellar evolution. Greater than 25 solar masses and boasting temperatures in excess of 25,000 K, they are more than a million times brighter than Earth's sun. Internal pressure within these stars causes intense stellar winds that blow away much of the star's mass. Like any other Class O star, a Wolf-Rayet is doomed a short existence that ends with a supernova and a black hole.
