The spectral classification system for stars includes two main components: a letter denoting the general spectral class and a number from 0 to 9 indicating a more precise temperature or color within that class. Here’s an overview of how it works:
Spectral Classes (Letters)
Stars are classified into spectral types based on temperature, from hottest to coolest:
- O - Blue, extremely hot, very luminous (30,000–50,000 K).
- B - Blue-white, very hot (10,000–30,000 K).
- A - White, hot, often showing strong hydrogen lines (7,500–10,000 K).
- F - Yellow-white, moderately hot, with weaker hydrogen lines (6,000–7,500 K).
- G - Yellow, similar to our Sun, moderately warm (5,200–6,000 K).
- K - Orange, cooler than the Sun (3,700–5,200 K).
- M - Red, coolest of the main sequence stars (2,400–3,700 K).
Additional classes for cooler objects (not typically main-sequence stars):
- L - Brown dwarfs, cool, red to brownish, (~1,300–2,400 K).
- T - Very cool brown dwarfs, methane-rich atmospheres (~600–1,300 K).
- Y - Coldest known stars and brown dwarfs, (~less than 600 K).
Number Subclasses (0–9)
The 0–9 numbering scheme further refines each spectral class. Here’s how it works:
- 0: Hottest within that spectral class (e.g., G0 is at the warmer end of the G class).
- 5: Middle of the range for that class (e.g., K5 is an average K star, orange in color).
- 9: Coolest within that spectral class, almost ready to shift into the next cooler spectral class (e.g., A9 is very close to F0).
Examples:
- O2: An extremely hot and bright blue star, very high in temperature and energy output.
- B7: A relatively cooler B-type star, blue-white in color.
- G2: Similar to the Sun, a typical yellow main-sequence star.
- M6: A cool red dwarf, cooler and dimmer than M0 but warmer than an M9.
Additional Symbols
Sometimes, classifications include symbols to give more detail:
- V: Main sequence (dwarf star).
- III: Giant star.
- I: Supergiant star.
- e: Emission lines in the star’s spectrum.
- p: Peculiar features in the spectrum.
So, K5 V would indicate a mid-range, orange dwarf star in the main sequence, while B1 I would describe a supergiant blue star near the hotter end of the B class.
Stellar remnants
Unlike main-sequence stars, stellar remnants represent the end stages of stellar evolution and don’t conform to the main spectral classification. These objects are classified based on their properties as remnants rather than temperature and color alone.
- White Dwarfs: Extremely dense, Earth-sized remnants of low- and medium-mass stars (up to ~8 solar masses). Typically around 10,000 K but slowly cool over time. Often designated by the letter D, with subclasses like
- DA
- DB
- DO, etc. based on specific features in their spectra.
- Neutron Stars: The remnants of massive stars that have exploded as supernovae. They are Incredibly dense, with masses about 1.4 times that of the Sun, packed into a sphere about 20 km in diameter. Neutron stars can be put into types like:
- pulsars (rapidly rotating neutron stars emitting regular pulses of radiation)
- magnetars (neutron stars with extremely strong magnetic fields).
- Black Holes: Formed when extremely massive stars (over ~20 solar masses) collapse under their gravity after a supernova. Defined by their event horizons and can be classified by mass:
- Stellar-mass black holes (up to about 20 solar masses).
- Intermediate-mass black holes (100–1,000 solar masses).
- Supermassive black holes (millions to billions of solar masses).