Stellar classification facts for kids

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Morgan-Keenan spectral classification
M stars are the coldest and O stars are the hottest in stellar classification. These stars are from the main sequence.

In astronomy, stellar classification is a way of grouping stars by temperature. Star temperature can be measured by looking at its spectrum, the type of light that the star shines.

Stars are also grouped into spectral types or classes by color. In general, a star's temperature determines its color, from red to blue-white. Spectral types are named with a letter: M, K, G, F, A, B and O. M stars are the coldest stars and O stars are the hottest.

The closest star to the Earth, the sun, is a class G star.

Harvard spectral classification

The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2,000 to 40,000 kelvin. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:

Note: The conventional color description describe only the peak of the stellar spectrum. However, the actual apparent colors the eye sees are lighter than the conventional color descriptions.

Class Surface temperature
color description
Actual apparent color Mass(solar masses) Radius(solar radii) Luminosity
Fraction of all
main-sequence stars
O ≥ 33,000 K blue blue ≥ 16 M ≥ 6.6 R ≥ 30,000 L Weak ~0.00003%
B 10,000–33,000 K blue white deep blue white 2.1–16 M 1.8–6.6 R 25–30,000 L Medium 0.13%
A 7,500–10,000 K white blue white 1.4–2.1 M 1.4–1.8 R 5–25 L Strong 0.6%
F 6,000–7,500 K yellow white white 1.04–1.4 M 1.15–1.4 R 1.5–5 L Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 M 0.96–1.15 R 0.6–1.5 L Weak 7.6%
K 3,700–5,200 K orange pale yellow orange 0.45–0.8 M 0.7–0.96 R 0.08–0.6 L Very weak 12.1%
M 2,000–3,700 K red light orange red ≤ 0.45 M ≤ 0.7 R ≤ 0.08 L Very weak 76.45%
L 1,300–2,000 K red brown scarlet Unknown Unknown Unknown Extremely weak
T 700–1,300 K brown magenta Unknown Unknown Unknown Extremely weak
Y ≤ 700 K dark brown dark purple Unknown Unknown Unknown Extremely weak

The mass, radius, and luminosity listed for each class are appropriate only for stars on the main-sequence portion of their lives and so are not appropriate for red giants. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2.

H-R diagram
The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

The Hertzsprung-Russell diagram is more often used in astronomy, because it relates three important variables: the absolute magnitude, the luminosity and the surface temperature. It is as important to astronomy as the periodic table is to chemistry.

Conventional and apparent colors

The conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars.

The Sun itself is white, though it is sometimes called a yellow star. This is a natural consequence of the evolution of human optical senses: the response curve that maximizes the overall efficiency against solar illumination will by definition perceive the Sun as white, though there is some subjective variation between observers.

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Stellar classification Facts for Kids. Kiddle Encyclopedia.