Download Wednesday, April 17 - Otterbein University

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
O
B
A
F
G
K
M
Temperature
30,000 K
20,000 K
10,000 K
8,000 K
6,000 K
4,000 K
3,000 K
Color
blue
bluish
white
white
yellow
orange
red
Examples
Rigel
Vega, Sirius
Canopus
Sun,  Centauri
Arcturus
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
L
1000
160
100
Aldebaran
10
1
Sun (G2V)
O B A
F
G
K
M
Type
… 0123456789 0123456789 012345…
The
HertzprungRussell 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 ~ 107 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

• (infrared frequencies)
Rho Ophiuchi (infrared)
Now you don’t
(visible frequencies)
Rho Ophiuchi (visible light)
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 gas
Temperatures ~ 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”)