Star Type Classifier

Classify stars by their spectral type and luminosity

Input a star's temperature, color, and luminosity to determine its spectral classification (O, B, A, F, G, K, M) and position on the Hertzsprung-Russell diagram.

How to Use

  1. 1
    Input the star's temperature and luminosity

    Enter the surface effective temperature in Kelvin (ranging from below 3,000 K for M dwarfs to above 30,000 K for O stars) and the luminosity in solar units or absolute magnitude. These two parameters place the star precisely on the Hertzsprung-Russell diagram.

  2. 2
    Identify the spectral type and luminosity class

    The tool applies the Morgan-Keenan (MKK) classification system to return the spectral type (O, B, A, F, G, K, or M) and luminosity class (Ia, Ib, II, III, IV, V, VI, or VII) based on your inputs. The Sun classifies as G2V: a G-type main sequence star at subtype 2.

  3. 3
    Review the HR diagram placement and stellar properties

    Examine the interactive HR diagram showing your star's position relative to the main sequence, giant branch, supergiant region, and white dwarf sequence. The tool also provides estimated mass, radius, age on the main sequence, and the most likely evolutionary fate for your star.

About

Stellar classification is the systematic categorization of stars by their spectral features, temperatures, and luminosities, providing the observational foundation for understanding stellar physics and populations. The Harvard spectral classification system, developed primarily by Annie Jump Cannon between 1901 and 1924, assigned the OBAFGKM sequence based on the strengths of hydrogen and metal absorption lines in tens of thousands of stellar spectra. Cannon classified over 350,000 stars in the Henry Draper Catalogue, a monumental achievement that remains a primary reference.

The two-dimensional Morgan-Keenan-Kellman (MKK) system added luminosity classes in 1943, enabling the distinction between dwarfs, giants, and supergiants that share similar surface temperatures but differ enormously in radius and intrinsic brightness. This distinction was critical for understanding the physical mechanisms of stellar evolution: giant and supergiant classification indicated that stars of similar spectral type could be in very different evolutionary phases. The combined spectral type plus luminosity class encodes the approximate mass, age, and destiny of any star in a compact notation.

Modern automated spectroscopic surveys including APOGEE, GALAH, and 4MOST classify millions of stars per year using data-driven methods that extract temperature, surface gravity, metallicity, and individual element abundances simultaneously. Machine learning classifiers trained on high-resolution spectral libraries can reproduce human expert classifications with high accuracy and extend them to chemical dimensionality beyond the classical system. These surveys are revealing the chemo-dynamical history of the Milky Way with unprecedented statistical power.

FAQ

What do the letters O, B, A, F, G, K, M mean in stellar classification?
The OBAFGKM sequence is a temperature sequence from hottest to coolest, originally derived from spectral line strengths observed in stellar spectra. O stars (above ~30,000 K) show ionized helium absorption lines. B stars (10,000-30,000 K) display neutral helium and hydrogen Balmer lines. A stars (7,500-10,000 K) have the strongest hydrogen Balmer lines. F and G stars (5,200-7,500 K) show increasing calcium and metal lines. K stars (3,700-5,200 K) display strong calcium and numerous metal lines. M stars (below 3,700 K) show molecular absorption bands from titanium oxide. The sequence is remembered with the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me."
What is luminosity class and what does Roman numeral V mean?
The luminosity class (I through VII) distinguishes stars of the same spectral type but different physical sizes and evolutionary states. Class V denotes main-sequence (dwarf) stars sustained by core hydrogen fusion. Class III indicates giant stars that have evolved off the main sequence, with larger radii and higher luminosities. Class Ia and Ib are luminous and less luminous supergiants, respectively. Class VI represents subdwarfs with lower metallicity and luminosity than main-sequence stars. Class VII is the white dwarf classification. The classification was introduced by W.W. Morgan, P.C. Keenan, and E. Kellman in their 1943 MKK atlas.
Why are some stars classified as Wolf-Rayet or carbon stars outside the OBAFGKM system?
The OBAFGKM system covers most normal stars but has been extended with additional types. Wolf-Rayet stars (type W) are massive stars shedding their outer layers at tremendous rates, showing broad emission lines from carbon, nitrogen, or oxygen. Carbon stars (type C) are cool giants with excess carbon in their atmospheres producing distinctive molecular bands. S stars show s-process element enhancements. L and T types classify brown dwarfs, and the Y type covers the coldest known brown dwarfs with ammonia absorption. Each extension preserves the principle of classification by spectral line features, which encode the surface temperature and chemical composition.
How does the Hertzsprung-Russell diagram reveal stellar ages?
In a stellar cluster where all stars formed at the same time, older clusters show a main-sequence turnoff at lower luminosities and temperatures as progressively less massive stars evolve off the main sequence. By fitting theoretical isochrones to the observed HR diagram of a cluster, astronomers determine the cluster's age to within 10 to 20%. Globular clusters with turnoffs near spectral type F to G have ages of 10 to 13 billion years. Open clusters with turnoffs at type B to A are young, typically less than 100 million years. The Sun's main-sequence lifetime of about 10 billion years means it is roughly halfway through its hydrogen-burning phase.
What spectral type is the Sun and what does that imply?
The Sun is classified as G2V: a G-type main-sequence star at numerical subtype 2 (on a scale where G0 is hotter and G9 is cooler within the G range). Its effective temperature is about 5,778 K, its luminosity is by definition 1 L☉, and its mass is 1 M☉. As a G2V star it will remain on the main sequence for another roughly 5 billion years. The G-type classification means it produces a broad solar spectrum peaking in the green-yellow range, though it appears white to observers outside Earth's atmosphere. G-type stars are among the most commonly studied hosts in exoplanet surveys given their similarity to the Sun.