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
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
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
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!
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
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.
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
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
Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars
Star Formation(Compare: Solar System Formation)
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
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
Reminder: Kirchhoff’s Laws
Cool gas absorbs light at specific frequencies
Dark Lines: “fingerprints of the elements”
Looking Through Dust Clouds
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
Scattering in Earth’s Atmosphere
Dust Clouds• What happens to the blue light scattered by the
dust clouds?• It’s still there, and sometimes can be seen
M20 Pleiades
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
Dark Nebulae
• Classic Example: Horsehead Nebula in Orion
Can’t see what’s behind a dark nebula, that’s why we see it!
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
Dark Nebulae• Now you see it Now you don’t
• (infrared frequencies) (visible frequencies)
Rho Ophiuchi (visible light) Rho Ophiuchi (infrared)
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”)
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”)