Star Classification
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4Star Classification
Stars are classified by their spectra (the elements that they absorb) and their temperature.
There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and
M. O and B stars are uncommon but very bright; M stars are common but dim.. An easy
mnemonic for remembering these is: "Oh be a fine girl, kiss me."
The
Sun
is a as a
G2V type
star, a
yellow
dwarf and
a main
sequence
star.
Hertzsprung - Russell Diagram
The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface
temperature) vs. its
luminosity
(intrinsic brightness or absolute magnitude). On it,
astronomers
plot stars'
color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3
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; this stage generally 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) eventually become faint
white dwarfs
(hot, white, dim stars) that are
below the main sequence. These hot, shrinking stars have depleted their nuclear fuels and will
eventually become cold, dark, black dwarfs.
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