The Magnitude Scale• A measure of the apparent

brightness

• Logarithmic scale • Notation: 1m.4 (smaller brighter)• Originally six groupings

– 1st magnitude the brightest

– 6th magnitude is 100x dimmer

• So a difference of 5mag is a difference of brightness of 100

• Factor 2.512=1001/5 for each mag.

Absolute Magnitude

• The absolute magnitude is the apparent magnitude a star would have at a distance of 10 pc.

• Notation example: 2M.8

• It is a measure of a star’s actual or intrinsic brightness called luminosity

• Example: Sirius: 1M.4, Sun 4M.8– Sirius is intrinsically brighter than the Sun

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

The Interstellar Medium

• Dust dims and reddens the light from distant stars

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”)