Stellar spectra reveal a star's secrets through its light. Temperature plays a crucial role, determining which absorption lines appear. Cooler stars show more lines, while hotter stars have fewer. This pattern helps astronomers classify stars into spectral types.

From the hottest O-type stars to the coolest Y-type brown dwarfs, each spectral class has unique characteristics. These classifications help us understand a star's composition, temperature, and evolutionary stage. Brown dwarfs blur the line between stars and planets, adding complexity to our cosmic understanding.

Stellar Spectra and Classification

Temperature effects on absorption lines

• Surface temperature of a star determines the appearance of absorption lines in its spectrum
• Atoms in the star's outer layers absorb specific wavelengths of light forming absorption lines
• Strength and presence of absorption lines vary with temperature
• Cooler stars like red dwarfs have more absorption lines in the visible spectrum
  • Lower temperatures allow atoms to remain in lower energy states resulting in more absorption
• Hotter stars like blue giants have fewer absorption lines in the visible spectrum
  • Higher temperatures excite atoms to higher energy states reducing the number of atoms available for absorption
• The strongest absorption lines correspond to the most abundant elements in the stellar atmosphere
  • Hydrogen lines prominent in stars with temperatures around 10,000 K (spectral type A)
  • Calcium lines strong in stars with temperatures around 6,000 K (spectral type G)

Characteristics of spectral classes

• Stars are assigned spectral classes based on surface temperatures and spectral features
• Classes arranged in decreasing temperature order: O, B, A, F, G, K, M, L, T, Y
• O-type stars are the hottest with surface temperatures exceeding 30,000 K
  • Appear blue with few visible spectrum absorption lines
  • Prominent lines include ionized helium (He II) and highly ionized metals
• B-type stars have surface temperatures between 10,000-30,000 K
  • Appear blue-white with strong hydrogen Balmer lines and neutral helium (He I) lines
• A-type stars have surface temperatures between 7,500-10,000 K
  • Appear white with the strongest hydrogen Balmer lines among all classes
• F-type stars have surface temperatures between 6,000-7,500 K
  • Appear yellow-white with weaker hydrogen lines and more prominent calcium (Ca II) lines
• G-type stars like our Sun have surface temperatures between 5,000-6,000 K
  • Appear yellow with strong calcium (Ca II) and many metal lines
• K-type stars have surface temperatures between 3,500-5,000 K
  • Appear orange with strong metal lines and weak hydrogen lines
• M-type stars are the coolest main-sequence stars with surface temperatures below 3,500 K
  • Appear red with strong molecular bands like titanium oxide in their spectra
• L-type objects are cool, low-mass stars and brown dwarfs with temperatures between 1,300-2,500 K
  • Spectra dominated by metal hydride bands and alkali metal lines
• T-type objects are cool brown dwarfs with temperatures between 700-1,300 K
  • Spectra dominated by methane absorption bands
• Y-type objects are the coolest known brown dwarfs with temperatures below 700 K
  • Spectra characterized by ammonia absorption features

Brown dwarfs vs planets

• Brown dwarfs and planets distinguished by mass and ability to sustain fusion reactions
• Brown dwarfs have masses between ~13-80 Jupiter masses
  • Massive enough to fuse deuterium (heavy hydrogen) in their cores
  • Not massive enough to sustain regular hydrogen fusion
  • Deuterium fusion provides temporary energy but once depleted, brown dwarf cools and contracts
• Planets have masses below the deuterium-burning limit (~13 Jupiter masses)
  • Insufficient mass to sustain any type of core fusion reaction
  • Form through accretion of material in a protoplanetary disk around a young star
• Boundary between the most massive planets and least massive brown dwarfs is unclear
  • Objects with masses close to 13 Jupiter masses may be called "sub-brown dwarfs" or "super-Jupiters"
• Brown dwarfs and planets have overlapping temperature ranges
  • Coolest brown dwarfs (Y-type) have temperatures comparable to some planetary atmospheres
  • Distinguishing them requires mass measurements or observations of their formation environments

Radiation and Stellar Spectra

• Stars emit blackbody radiation, a continuous spectrum of electromagnetic radiation
• Wien's displacement law relates a star's surface temperature to the peak wavelength of its emission
• The stellar atmosphere, a layer of gases surrounding the star, affects the observed spectrum
  • Opacity of the atmosphere determines which wavelengths of light can escape
  • Ionization of atoms in hot stellar atmospheres influences spectral features
• Stellar spectra consist of a continuous spectrum with superimposed absorption or emission lines
  • Absorption lines form when cooler outer layers absorb specific wavelengths
  • Emission lines can appear in certain conditions, such as in very hot or active stars