Those numbers are given labels such as 'U−V' or 'B−V', which represent the colors passed by two standard filters (e.g. Other modern stellar classification systems, such as the UBV system, are based on color indices-the measured differences in three or more color magnitudes. Each star is assigned a spectral class (from the older Harvard spectral classification, which did not include luminosity ) and a luminosity class using Roman numerals as explained below, forming the star's spectral type.

The modern classification system is known as the Morgan–Keenan (MK) classification. Main-sequence stars arranged (from right to left) O to M Harvard classes The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main-sequence stars, class sd (or VI) for subdwarfs, and class D (or VII) for white dwarfs. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. In the MK system, a luminosity class is added to the spectral class using Roman numerals. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest ( O type) to the coolest ( M type). The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. In astronomy, stellar classification is the classification of stars based on their spectral characteristics.

System sequence diagram online