lecture14_ch11

lecture14_ch11 - Things to Remember About the Sun When...

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Unformatted text preview: Things to Remember About the Sun When Thinking about the Stars Surveying the Stars • Solar core releases (mass-)energy by fusing H into He + E = mc2 + mass of 4 protons > mass of He nucleus • Two kinds of balance (solar thermostat) + pressure must balance gravity + energy from core must balance radiated energy • Hydrogen in Sunʼs core will eventually run out + Sun will not remain in balance forever Luminosity: Amount of power a star radiates (energy per second) How luminous are stars? Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter) Luminosity passing through each sphere is the same Relationship between apparent brightness and luminosity depends on distance: Brightness = Area of sphere: Luminosity 4π (distance)2 4π (radius)2 Divide luminosity by area to get brightness We can determine luminosity if we can measure distance and apparent brightness: Luminosity = 4π (distance)2 x (Brightness) 1 Parallax is the apparent shift in position of a nearby object against a background of more distant objects Parallax of Nearby Star Intro to Parallax Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun So how far are these stars? Parallax angle depends on distance Parallax angle vs. distance Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star Measuring parallax angle Most luminous stars: 106 LSun Least luminous stars: How hot are stars? 10-4 LSun (LSun is luminosity of Sun) 2 An object of ﬁxed size grows more luminous as its temperature rises T and L Every object emits thermal radiation with a spectrum that depends on its temperature Laws of Thermal Radiation Hottest stars: 50,000 K Coolest stars: 3,000 K (Sunʼs surface is 5,800 K) 1) Hotter objects emit more light at all wavelengths 2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies 106 K 105 K 104 K Ionized Gas (Plasma) Level of ionization also reveals a starʼs temperature 103 K Gas 102 K 10 K Liquid Solid Absorption lines in starʼs spectrum tell us ionization level 3 Lines in a starʼs spectrum correspond to a spectral type that reveals its temperature (Hottest) OBAFGKM (Coolest) Remembering Spectral Types (Hottest) OBAFGKM (Coolest) • Oh, Be A Fine Girl, Kiss Me • Only Boys Accepting Feminism Get Kissed Meaningfully. How massive are stars? • On Beautiful Afternoons, Friends Gather Keg Money. • Ofﬁcially, Bill Always Felt Guilty Kissing Monica Best way to remember the sequence: Make up your own mnemonic We measure mass using gravity Direct mass measurements are possible only for stars in binary star systems p2 = 1 a3 (M1 + M2) Need to know period (p) and average separation (a) of orbiting system. The orbit of a binary star system depends on strength of gravity Isaac Newton 4 Binary star systems are common (about half of all stars) Sirius is in a binary star system ( period ~ 50 years) Some binaries are eclipsing Some binaries are detected via their Doppler shifts. Most massive stars: 100 MSun Least massive stars: How do we classify stars? 0.08 M Sun (M Sun is the mass of the Sun) 5 Why are some red stars much more luminous than other red stars? Why are some red stars much more luminous than other red stars? Theyʼre bigger! Biggest red stars: 1000 Rsun Smallest red stars: 0.1 RSun A starʼs full classiﬁcation includes spectral type and luminosity class: I II III IV V - supergiant - bright giant - giant - subgiant - main sequence Examples: Sun - G2 V Sirius - A1 V Proxima Centauri - M5.5 V Betelgeuse - M2 I Remember 1) We measure brightness with a telescope. 2) How we measure distance through parallax. 3) We can figure out luminosity if we know distance and brightness. 4) We can figure out distance if we know luminosity and brightness. 5) How surface temperature is related to color or spectral type. 6) How we can estimate the masses of stars. 7) How we classify stars. 6 ...
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