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The Classification of Stars

The primary classification of stars is by spectra and temperature. There are seven main groups. In order of decreasing temperature, O, B, A, F, G, K, and M. This is known as the Spectral Type.

O and B stars are uncommon but very bright, while M stars are common but dim.
(A mnemonic for remembering this order is: "Oh Be A Fine Girl, Kiss Me.")

W: W is a very rare type of intensely hot star, with surface temperatures up to 50,000 K. There is only one example in the sky that is visible to the naked eye, in the Suhail al Muhlif system in the constellation Vela.
O: O-type stars are also relatively uncommon, but far more numerous than those of type W. These are bright blue stars which also have very high surface temperatures, in the range 25,000 K to 50,000 K. Examples are Alnitak (O9.5), Naos (O5), Hatysa (O9) and Heka (O8).
B: The B type is the first of the really populous classes. Stars of this type are blue in colour and burn hotly, with surface temperatures lying between 11,000 K and 25,000 K. Prominent examples of blue B-type stars are Rigel (B8), Achernar (B3), Agena (B1) and Spica (also B1).
A: A-type stars are those whose surface temperatures lie in the approximate range 7,500 K to 11,000 K. They are white in colour, and some of the brightest and most famous stars in the sky belong to this classification, including Sirius (A0), Vega (A0), Altair (A7) and Deneb (A2).
F: F-type stars lie between the A-type white stars and G-type 'true' yellow stars, and have a distinctly yellowish light. Their surfaces have a temperature between 6,000 K and 7,500 K. Sometimes called Calcium Stars, examples of this type include Canopus (F0), Procyon (F5), Algenib in Perseus (F5) and Wezen (F8).
G: The cooler a star, the more complex its chemistry tends to be. G-type stars, with temperatures ranging between 5,000 K and 6,000 K, have spectra that betray the existence of 'metals' (in this context, 'metal' refers to any element heavier than helium). Examples of yellow G-type stars are Alpha Centauri (G2), Capella (G5), Kraz (G5) and Mufrid (G0). The Earth's Sun is a G2 star, and also belongs to this type.
K: K-type stars are occasionally referred to as Arcturian Stars, after the brightest of their number. Their surface temperatures are between 3,500 K and 5,000 K, low enough for simple molecules to form. K-type stars are orange in colour, and among the brightest in the sky are Arcturus (K2), Aldebaran (K5), Pollux (K0) and Atria (K2).
M: The coolest of the common star types, red stars are classified as M-type. They have very cool surface temperatures below 3,500 K, allowing more complex molecules to form. Among the brightest red stars in the sky are Betelgeuse (M2), Antares (M1), Gacrux (M4) and Mirach (M0). The Sun's nearest neighbour in space, Proxima Centauri, is also a red star, classified as M5.

A number (in the range 0-9) is added after the primary letter which indicates the positionof a particular star within its lettered class.

The Hertzsprung - Russell Diagram

The Hertzsprung-Russell (H-R) Diagram is a graph that plots stars' color against luminosity. Information shown is color, temperature, luminosity, spectral type, and evolutionary stage. The diagram shows that there are three very different types of stars:

Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting hydrogen into helium. For these stars, the hotter they are, the brighter. These stars are in the most stable part of their existence; current calculations show that this stage lasts for about 5 billion years.
As stars begin to die, they become giants and supergiants (above the main sequence). These stars have depleted their hydrogen supply and are very old. The core contracts as the outer layers expand. These stars will eventually explode (becoming a planetary nebula or supernova, depending on their mass) and then become white dwarfs, neutron stars, or black holes (again depending on their mass).
Smaller stars (like our Sun) pass through a red giant (or sub-giant) stage and will eventually become faint white dwarfs that are below the main sequence. These hot, shrinking stars have depleted their nuclear fuels and will finally pass through the brown dwarf stage and ultimately become cold, dark, black dwarfs.

Spectral Classes

Star Type	Color	Approximate Surface Temperature in degrees Kelvin	Average Mass (Sun=1)	Average Radius (Sun=1)	Average Luminosity (Sun=1)	Lifetime in Millions of Years	Examples
W	Blue	50,000-70,000	40+	?	100,000	?	Gamma-2 Velorum (Regor)
O	Blue	> 25,000	60	15	1,400,000	10	10 Lacertra
B	Blue	11,000-25,000	18	7	20,000	100	Rigel Spica
A	Blue	7,500-11,000	3.2	2.5	80	1,000	Sirius Vega
F	Blue to White	6,000-7,500	1.7	1.3	6	3,000	Canopus Procyon
G	White to Yellow	5,000-6,000	1.1	1.1	1.2	10,000	Sol Capella
K	Orange to Red	3,500-5,000	0.8	0.9	0.4	50,000	Arcturus Aldebaran
M	Red	< 3,500	0.3	0.4	0.04 (very faint)	200,000	Bettelgeuse Antares

Subtypes

Within each stellar spectral class, stars are placed into subclasses (from 0 to 9) based on its position within the scale.

The Yerkes Luminosity Classification

Luminosity is the total brightness of a star. Luminosity is the total amount of energy that a star radiates each second, inclusive of all wavelengths in the electromagnetic spectrum.

In the Yerkes classification scheme, stars are assigned to groups according to the width of their spectral lines. For a group of stars with the same temperature, the luminosity class differentiates between their sizes (supergiants, giants, main-sequence stars, and subdwarfs).

Stars are classified into five main luminosity classes. These are the five classes:

I Supergiants: Very massive and luminous stars near the end of their lives. They are subclassified as Ia or Ib, with Ia representing the brightest of these stars. These stars are very rare - 1 in a million stars is a supergiant. The nearest supergiant star is Canopus (F0Ib) 310 light years away. Some other examples are Betelgeuse (M2Ib), Antares (M1Ib) and Rigel (B8Ia).
II Bright Giants: Stars which have a luminosity between the giant and supergiant stars. Some examples are Sargas (F1II) and Alphard (K3II).
III Normal Giants: These are mainly low-mass stars at the end of their lives that have swelled to become a giant star. This category also includes some high mass stars evolving on their way to supergiant status. Some examples are Arcturus (K2III), Hadar (B1III) and Aldebaran (K5III).
IV Subgiants: Stars which have begun evolving to giant or supergiant status. Some examples are Alnair (B7IV) and Muphrid (G0IV). Note also Procyon which is entering this category and therefore is: F5IV-V.
V Dwarfs: All normal hydrogen-burning stars. Stars spend most of their lives in this category before evolving up the scale. Class O and B stars in this category are actually very bright and luminous and generally brighter than most Giant stars. Some examples are the Sun (G2V), Sirius (A1V), and Vega (A0V).

Main Sequence Stars - Young Stars

Main sequence stars are the central band of stars on the Hertzsprung-Russell Diagram. These stars' energy comes from nuclear fusion, as they convert Hydrogen to Helium. Most stars (about 90%) are Main Sequence Stars. For these stars, the hotter they are, the brighter they are. The sun is a typical Main Sequence star.

DWARF STARS

Dwarf stars are relatively small stars, up to 20 times larger than our sun and up to 20,000 times brighter. Our sun is a dwarf star.

YELLOW DWARF

Yellow dwarfs are small, main sequence stars. The Sun is a yellow dwarf.

RED DWARF

A red dwarf is a small, cool, very faint, main sequence star whose surface temperature is under about 4,000 K. Red dwarfs are the most common type of star. Proxima Centauri is a red dwarf.

Giant and Supergiant Stars - Old, Large Stars

RED GIANT

A red giant is a relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K). They are frequently orange in color. Betelgeuse is a red giant. It is about 20 times as massive as the Sun about 14,000 times brighter than the Sun, and about 600 light-years from Earth.

BLUE GIANT

A blue giant is a huge, very hot, blue star. It is a post-main sequence star that burns helium.

SUPERGIANT

A supergiant is the largest known type of star; some are almost as large as our entire solar system. Betelgeuse and Rigel are supergiants. These stars are rare. When supergiants die they supernova and become black holes.

Faint, Virtually Dead Stars:

WHITE DWARF

A white dwarf is a small, very dense, hot star that is made mostly of carbon. These faint stars are what remains after a red giant star loses its outer layers. Their nuclear cores are depleted. They are about the size of the Earth (but tremendously heavier)! They will eventually lose their heat and become a cold, dark black dwarf. Our sun will someday turn into a white dwarf and then a black dwarf. The companion of Sirius is a white dwarf.

BROWN DWARF

A brown dwarf is a "star" whose mass is too small to have nuclear fusion occur at its core (the temperature and pressure at its core are insufficient for fusion). A brown dwarf is not very luminous. It is usually regarded as having a mass between 10²⁸ kg and 84 x 10²⁸.

NEUTRON STAR

A neutron star is a very small, super-dense star which is composed mostly of tightly-packed neutrons. It has a thin atmosphere of hydrogen. It has a diameter of about 5-10 miles (5-16 km) and a density of roughly 10 ¹⁵ gm/cm ³.

PULSAR

A pulsar is a rapidly spinning neutron star that emits energy in pulses.

Binary Stars:

DOUBLE STAR

A double star is two stars that appear close to one another in the sky. Some are true binaries (two stars that revolve around one another); others just appear together from the Earth because they are both in the same line-of-sight.

BINARY STAR

A binary star is a system of two stars that rotate around a common center of mass (the barycenter). About half of all stars are in a group of at least two stars.

Polaris (the pole star of the Northern Hemisphere of Earth) is part of a binary star system.

ECLIPSING BINARY

An eclipsing binary is two close stars that appear to be a single star varying in brightness. The variation in brightness is due to the stars periodically obscuring or enhancing one another. This binary star system is tilted (with respect ot us) so that its orbital plane is viewed from its edge.

X-RAY BINARY STAR

X-ray binary stars are a special type of binary star in which one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole. As matter is stripped from the normal star, it falls into the collapsed star, producing X-rays.

Variable Stars - Stars that Vary in Luminosity: CEPHEID VARIABLE STARS

Cepheid variables are stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. Cepheid Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through. Polaris and Delta Cephei are examples of Cepheids.

MIRA VARIABLE STAR

Some Mira Variable Stars	Magnitude Range	Period (days)
R Carinae	3.9-10.5	308.7
R Centauri	5.3-11.8	546.2
Mira (Omicron Ceti)	3.4-9.3	332.0

A Mira variable star is a variable star whose brightness and size cycle over a very long time period, in the order of many months. Miras are pulsating red giants that vary in magnitude as much as a factor of many hundred (by 6 or 8 magnitudes). Mira variables were named after the star Mira, whose variations were discovered in 1596.

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