Astronomy 2022 October 17-19

spectral property of stars, the most basic one, determined by the light emitted at a specific peak wavelength. Explained by Wein’s law and related to temperature

spectral property of stars, Photosphere of a star is surrounded by a thin, hot atmosphere, dark line spectrum, Kirchoff’s third law

hot, dense balls of incandescent gas, follow Kirchoff’s first law: they emit a continuous spectrum

the part of the star you see with your naked eye. The color you see is explained by Wein’s law. Below this level, the stars are opaque, and above, the star’s atmospheres are transparent.

depends on either temperature of photosphere, core, or envelope of a star

based on temperature and surface area; Stefan Boltzmann law

classifying by spectral types/spectra, can be used to break star’s light into a continuous and absorption spectrum

can be used to determine temperatures and/chemical compositions of stars, lines dark due to absorption of light at specific wavelengths, can be used to identify elements of a star’s outer atmosphere

1866, classified stars into four main classes based on spectral lines

Harvard computer, noticed stellar temperature was the distinguishing spectral feature. Reordered types by temperature and most old classes were thrown out, analyzed and classified 220k stars

proposed stars were 80% hydrogen, 19% helium, 1% metal, and that most spectral lines came from different ionization states of same elements/created by different temperatures

10,000k-50,000k, most hydrogen is ionized, only very weak hydrogen lines since there aren’t many electrons to make absorption lines

7,500k-10,000k, ideal excision conditions, strongest hydrogen lines

5,000k-6,000k, cool temperatures, limited ionization, weak hydrogen lines because electrons are in a ground state

classifications of stars by temperature, OBAFGKM hot-cool

OBAFGKM sequence later updated by subdividing each spectral type with numbers 0-9

blue, temp 30k-50k K, luminosity 100k+ L, mass 3-20m, life expectancy 1-500Myr

blue/white, mass 3-20m, temperatures 10k-30k K, luminosity 100-100k L, life expectancy 1-500 Myr

white, mass 1.5-3m, temperatures 7.5k-10k k, luminosity 10-100 L, life expectancy 500-3Gyr

yellow/white, mass 1.2-1.5m, temperatures 6k-7.5k K, luminosity 3-10 L, life expectancy 3-6 Gyr

yellow, mass .85-.1.2 M, temperatures 5k-6k K, luminosity .5-3 L, life expectancy 6-15 Gyr

yellow/orange, mass .5-.85M, temperature 3.8k-5k K, luminosity .01-.5 L, life expectancy 15-60 Gyr

red, mass .075-.5m, temperature less than 3.8k K, luminosity less than .01 L, life expectancy 60Gyr-6Ty

L, T, Y, failed stars

For every 1 O star there are 4.4k b stars, 20k A stars, 100k F star, 250k G stars, 400k K stars, 2.5m M stars

classified 10k stars by strength of absorption lines A-O, strongest to weakest hydrogen lines, Harvard observatory used objective prism to collect spectra of 220k stars

assigned by strengths or absence of specific spectral lines, sensitive to pressure and density in atmosphere

classification, color is determined by surface temperature, range 18k K- 60k K

classifications of stars determined by width/mass, range 1/10k L to 6M L, .07 m-315 m

.07 m-315 m, .8 R-2,150 R

luminosity, the total energy output of a star, depends on the temperature of photosphere T, and the surface area of photosphere determined by radius

luminosity of a star proportiona to the fourth power and it’s radius squared, explains luminosity radius temperature relationship, L=r^2 T^4

1a-0: extremely luminous supergiants
1a: brightest supergiants
1b: less luminous supergiants
11: bright giants
111: giants
1V: subgiants (intermediate)
V: main sequence dwarfs (m-s

density of a star is mass/volume (4/3piR^3). Main sequence stars: range of average densities, giants low densities (10^-7 g/cc), supergiants very low density (10^-9 g/cc), white dwarfs high density (10^-5 g/cc)

sensitive to pressure and density, determines luminosity classes, large stars have narrow lines and low pressure atmospheres, small stars have broader lines and high pressure atmospheres

OBAFGKM, 85% of stars lie along diagonal the band in HR diagram, luminosity .05-700k, temperature 3k to 40k, radius .6 to 18, mass .1-40

mostly K and M, same temperature as main sequence but brighter and larger, 6k-2k K, luminosity 1k-100k, low density stars 10^-7 g/cc

mostly K and M, largest stars with wide range of temperatures and luminosity, radius 20-2.15k R, luminosity 100k-6M, very low density 10^-9 g/cc

white hot and small, much fainter than main sequence but same temperature, radius .01r (earth size), temperature 60k kelvin, luminosity .005 L, high density 10^-5 g/cc