Temperature affects strength of absorption lines

Temperature affects strength of absorption lines
• Example: Hydrogen lines are relatively weak in the hottest star because it is mostly
ionized. Conversely, hotter temperatures are needed to excite and ionize Helium so
these lines are strongest in the hottest star.
• Spectral Classification: Before astronomers knew much about stars, they classified them
based on the strength of observed absorption lines.
• Classification by line strength started as A, B, C, D, . . . , but became: O, B, A, F, G, K,
M, (L) A temperature sequence!
• Cannon’s system officially adopted in 1910
• “Oh be a fine girl/guy kiss me”
• “Oh brother, astronomers frequently give killer midterms”
• Stellar sizes: Almost all stars are so small they appear only as a point of light in the
largest telescopes
• A small number are big and close enough to determine their sizes directly through
geometry
• F = o~ T^4
• L = 4piR^2 o~ T^4
• Stellar size: indirect measurement
• Stars have a huge range of sizes

• Stefan’s Law: F = o~ T^4
• Luminosity is the flux multiplied by entire spherical surface
• Area of sphere A = 4piR^2
• Luminosity = 4piR^2 o~ T^4 or L infinity R^2 x T^4
• Hertzsprung-Russell (HR) Diagram
• HR diagrams plot stars as a function of their luminosity and temperature
• About 90% of all stars (including the Sun) lie on the Main Sequence . . . where stars
reside during their core Hydrogen-burning phase
• Parallax
• | | (eyes) The closer your finger is to you, the bigger the angular shift is
• d = 1pc or (3 LY)
• parcet – amount of shift
The HR Diagram: 100 Brightest Stars
• Most of these luminous stars are somewhat rare – they lie beyond 5pc
• We see almost no red dwarfs (even though they are very abundant in the universe)
because they are too faint
• Several non-Main Sequence stars are seen in the Red Giant region