Lesson 5 – Stellar Classification

Ordering the Stars

By the late nineteenth century, astronomers had accumulated spectra for thousands of stars. The challenge was to make sense of the enormous variety — stars of wildly different colours, brightnesses, and spectral patterns. The solution was classification: grouping stars by their properties to find underlying patterns, just as Mendeleev had organised the elements into the periodic table.

Two properties turned out to be the most fundamental: temperature (revealed by colour and spectral features) and luminosity (intrinsic brightness — how much energy a star actually emits). Plotted against one another, they revealed a structure of breathtaking order.

The Spectral Classification System

The modern spectral classification system — established by Annie Jump Cannon at Harvard in the early twentieth century — arranges stars into types by surface temperature. The sequence, from hottest to coolest, is remembered with the mnemonic Oh Be A Fine Girl/Guy, Kiss Me:

Each type is further subdivided by a number 0–9 (hottest to coolest within the type), plus a luminosity class. The Sun is classified as G2V — type G, subdivision 2, luminosity class V (main sequence). This compact label summarises its temperature, colour, intrinsic brightness, and evolutionary stage.

Luminosity and Absolute Magnitude

The apparent magnitude of a star is how bright it looks from Earth — which conflates true brightness with distance. A star can appear brilliant simply because it is nearby. To compare stars fairly, we use absolute magnitude: how bright the star would appear if placed at a standard distance of 10 parsecs.

Luminosity is the total power output of a star, measured in watts or in multiples of the Sun’s luminosity (L☉). It depends on both temperature and size:

The Hertzsprung-Russell Diagram

In 1910, Ejnar Hertzsprung and Henry Norris Russell independently plotted stellar luminosity against surface temperature for large samples of stars. The resulting diagram — the HR diagram — is one of the most important tools in all of astrophysics. Stars do not scatter randomly across it. They cluster into distinct regions that correspond directly to stages of stellar evolution.

The key regions of the HR diagram are:

• The Main Sequence — A diagonal band running from hot-luminous upper left to cool-dim lower right. About 90% of all stars, including the Sun, reside here. A star’s position on the main sequence is determined by its mass.
• Red Giants and Supergiants — Upper right region. Large, cool, luminous stars that have exhausted core hydrogen. The Sun will become a red giant in about 5 billion years.
• White Dwarfs — Lower left region. Small, hot, but dim — stellar remnants that have shed their outer layers. The Sun’s eventual fate.
• Horizontal Branch and AGB — Intermediate evolutionary stages visited by stars after the main sequence, visible as secondary groupings.

Luminosity Classes

The Roman numeral appended to a spectral type indicates the luminosity class — the star’s evolutionary state and size category:

Beyond OBAFGKM — Extending the Sequence

The classical sequence has been extended for objects cooler than M-type stars. Type L (1,300–2,200 K) covers the boundary between the coolest stars and brown dwarfs. Type T covers warmer brown dwarfs, and type Y the coldest known substellar objects, with temperatures below 500 K — cool enough that water clouds may form in their atmospheres. These extensions were added in the late 1990s following discoveries from infrared surveys.