Finding the absolute Magnitude To figure out absolute

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 10 pc, its absolute magnitude will be a bigger number, i. e. it is intrinsically dimmer than it appears – If a star is farther than 10 pc, 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 d 2 B (1) • The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature: L R 2 T 4 (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 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. K 5 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 K 0 is very close to G, K 9 is very close to M.

Constructing a HR-Diagram • Example: Aldebaran, spectral type K 5 III, luminosity = 160 times that of the Sun L 1000 160 100 Aldebaran 10 1 Sun (G 2 V) O B A F G K M Type … 0123456789 012345…

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 ~ 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/cm 3 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 M 20 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 18 th 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 ~ 8000 K • 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 ~ 8000 K • 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 ”)