finding the absolute magnitude to figure out absolute magnitude, we need to know the distance to the...
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Finding the absolute Magnitude• To figure out absolute magnitude, we need to
know the distance to the star
• Then do the following Gedankenexperiment:– In your mind, put the star from its actual position to a
position 10 pc away– If a star is actually closer than 10pc, its absolute
magnitude will be a bigger number, i.e. it is intrinsically dimmer than it appears
– If a star is farther than 10pc, its absolute magnitude will be a smaller number, i.e. it is intrinsically brighter than it appears
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Measuring the Sizes of Stars
• Direct measurement is possible for a few dozen relatively close, large stars– Angular size of the disk and known distance
can be used to deduce diameter
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Indirect Measurement of Sizes
• Distance and brightness can be used to find the luminosity:
L d2 B (1)
• The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature:
L R2 T4 (2)
• We can compare two values of absolute luminosity L to get the size
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Sizes of Stars• Dwarfs
– Comparable in size, or smaller than, the Sun
• Giants– Up to 100 times
the size of the Sun
• Supergiants– Up to 1000 times
the size of the Sun
• Note: Temperature changes!
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Classification of the Stars: Temperature
Class Temperature Color Examples
O 30,000 K blue
B 20,000 K bluish Rigel
A 10,000 K white Vega, Sirius
F 8,000 K white Canopus
G 6,000 K yellow Sun, Centauri
K 4,000 K orange Arcturus
M 3,000 K red Betelgeuse
Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me
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The Key Tool to understanding Stars: the Hertzsprung-Russell diagram
• Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature)
• To obtain a HR diagram: – get the luminosity. This is your y-coordinate. – Then take the spectral type as your x-coordinate, e.g.
K5 for Aldebaran. First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K0 is very close to G, K9 is very close to M.
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Constructing a HR-Diagram• Example: Aldebaran, spectral type K5III,
luminosity = 160 times that of the Sun
O B A F G K M Type… 0123456789 0123456789 012345…
1
10
100
1000
L
Aldebaran
Sun (G2V)
160
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The Hertzprung-
Russell Diagram• A plot of absolute
luminosity (vertical scale) against spectral type or temperature (horizontal scale)
• Most stars (90%) lie in a band known as the Main Sequence
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Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars
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Star Formation(Compare: Solar System Formation)
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Where Stars come from: the Interstellar Medium
• Gas– Single atoms and molecules– Mostly hydrogen (90%), 9% helium; deficient in heavier
elements
• Dust– Microscopic clumps of atoms/molecules– Size ~ 107 m, similar to the wavelength of visible light– Composition is not well known
• Temperature depends on the proximity of stars, typically ~100 K
• Density is very low!– Gas: about 1 atom/cm3 D; Dust: even less dense
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How do we know it’s there?
• Cold gas or dust doesn’t glow– they are dark
– We might “see” them blocking light of other objects (Dark Nebulae)
• Gas & Dust clouds are very dilute– they might not be blocking other object’s light totally
– Usually they will reduce (redden) the light of other objects
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Reminder: Kirchhoff’s Laws
Cool gas absorbs light at specific frequencies
Dark Lines: “fingerprints of the elements”
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Looking Through Dust Clouds
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Seeing Through Gas and Dust• EM radiation is appreciably
scattered or absorbed only by particles with size comparable to its wavelength (or larger)
• Gas– Emission and absorption
lines
– Doesn’t block EM radiation
• Dust– Grain size is comparable to the wavelength of visible light
– Dims visible light and high frequency EM radiation
– Transparent to longer wavelength radio and infrared radiation, though
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Scattering in Earth’s Atmosphere
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Dust Clouds• What happens to the blue light scattered by the
dust clouds?• It’s still there, and sometimes can be seen
M20 Pleiades
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Nebulae
• Any irregularly shaped cloud of gas and dust• May be bright or dark, depending on temperature• Types:
– Emission (bright) Nebulae
– Dark Nebulae
– Reflection Nebulae
• Historic Remark: Only some of the 109 “nebulae” catalogued by Charles Messier in 18th Century are actual nebulae; most are star clusters and galaxies
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Dark Nebulae
• Classic Example: Horsehead Nebula in Orion
Can’t see what’s behind a dark nebula, that’s why we see it!
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Dark Nebulae
• Dark Nebulae do emit light of their own, though
• Temperatures ~ 10 to 100 K; black body radiation peaks in the radio to infrared frequencies
fpeak in infrared frequencies
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Dark Nebulae• Now you see it Now you don’t
• (infrared frequencies) (visible frequencies)
Rho Ophiuchi (visible light) Rho Ophiuchi (infrared)
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Emission Nebulae
• Regions of hot glowing gas– Temperatures ~ 8000K
• Made to glow by ultraviolet radiation emitted by new O- or B-type (hot) stars located inside
• Emission lines from the nebula are easily distinguished from the continuous spectrum and absorption lines of stars within
• Color predominantly red, the color of a particular hydrogen emission line (the “H line”)
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Emission Nebulae Example: Orion Nebula (M 42)
• hot glowing gasTemperatures ~ 8000K
• Made to glow by ultraviolet radiation emitted by young O- or B-type (hot)
stars located inside
• Color predominantly red, the color of a particular hydrogen
emission line (“H”)