Starlight and What it Tells Us

The Stars in the Sky

Vary in Brightness• Distance• SizeVary in Color• Color = Temperature

Star Names

• Proper star names mostly Arabic• Greek Letters, Numbers• Catalog Identifiers• Faint stars usually have no name

The Names of Sirius• Alpha Canis Majoris (Bayer, 1603)• 9 Canis Majoris (Flamsteed, 1725)• BD -16 1591 (Bonner Durchmusterung 1859-

1903)• HR 2491 (Harvard Revised Catalog, 1908)• HD 48915 (Henry Draper, 1918-1924)• ADS 5423 (Aitken Double Star Catalog, 1932)• HIP 32349 (HIPPARCOS, 1997)

The Heavens Are Not Changeless• The Stars Move– Most of our constellations would have been

unrecognizable to Neanderthal Man• The Solar System Moves– Very few of our nearby stars would have been

visible to the first humans• Stars are Born, Live and Die– Many of our brightest stars did not exist in the

days of the dinosaurs

Brightness of Stars

• Variations in distance and intrinsic brightness

• Scale based on one by Hipparcos 500 B.C.• Magnitude: Large Numbers = Fainter– One magnitude = 2.5 x– Five magnitudes = 100 x

Magnitudes• Planet around nearby star: 30• Pluto: 13• Faintest Naked-Eye Star: 6• Big Dipper Stars: 2• Sirius (Brightest Star) -1.6• Venus -4• Full Moon -12• Sun -27

Absolute Magnitude

• Altair and Deneb are about equally bright as seen from Earth

• Altair is 16 l.y. away, Deneb 1600

• Hence Deneb must be about 10,000 times brighter

Absolute Magnitude

• How bright a star would be at a distance of 32.6 l.y. (10 parsecs)

• Sun: 4.5 (inconspicuous naked-eye star)• Altair: 2.2• Deneb: -7.1 (bright as crescent moon)– Note: Deneb - Altair about 10 magnitudes = 100 x

100 = 10,000 times

Black-Body Radiation

• Objects Emit Radiation Because They Are Hot• Why “Black”? Because None of the Radiation

is Reflected from Some Other Source• The Sun Emits Black-Body Radiation, Mars

Does Not• Close Example of pure Black-Body radiation:

Peephole in a pottery kiln

Black Body Radiation

What’s The Source of the Light?

Color = Temperature

Why Black-Body Radiation is so Important

• Color is directly related to temperature• Temperature is the only determinant of color• Energy per unit area is the same if

temperature is the same– If two stars have the same color and distance,

difference in brightness is due to difference in size– Dwarf and giant stars are literally dwarfs or giants

Sirius and the Pup

Sirius and the Pup• Sirius M = -1.5; Pup M = 8.5• 10 magnitude difference • 100 x 100 = 10,000 times brightness distance• Sirius and the Pup are same color, therefore

same temperature (Pup is hotter)• Pup must have 1/10,000 the apparent area of

Sirius = 1/100 the diameter

Spectroscopy

• Different atoms absorb or emit specific wavelengths of light

• When light spread into a spectrum, the absorbed wavelengths show up as dark (missing) bands

• These spectral lines are indicators of:– Chemical composition– Physical conditions

Atoms and Radiation

The Solar Spectrum

Spectra and Spectral Lines

• Continuous Spectrum: Incandescent solids or liquids (steel mill) and dense hot gases (Sun’s photosphere)

• Emission Spectrum: Thin hot gases (fireworks, sodium or mercury vapor lights, Sun’s chromosphere

• Absorption Spectrum: Light shining through thin gases (Sun and star light)

How the Chromosphere Works

Spectral Lines are Affected By:

• Electrical and Magnetic Fields• Number of Electrons Atoms Have Lost

(Indicates Temperature and Pressure)• Motion (Doppler Effect)• Blue-shifted if Motion Toward Observer• Red-shifted if Motion Away From Observer

The Doppler Effect

What the Doppler Effect Tells Us• Radial Motion• Rotation of Stars– Approaching side of star blue-shifted, receding

side red-shifted• Unseen Companions (Stars or Planets)– Star oscillates around center of mass

• Surface and Interior Motions– Changes in Size– Interior Oscillations

Spectral Classification of Stars• W – very hot young stars expelling their outer

layers• Main Sequence: O, B, A, F, G, K, M (hottest to

coolest)– “Oh be a fine girl/guy, kiss me”

• Subdwarfs: L, T, Y (hottest to coolest)• Chemically Peculiar Stars: C, N, R, S• White Dwarfs: D

Spectral Signatures of Stars

• O: Ionized Helium• B: Neutral Helium• A: Strongest Hydrogen Lines• F: Ionized Calcium• G: Strongest Calcium Lines + Neutral Metals• K: Neutral Metals Dominate• M: Titanium Oxide

The Hertzsprung-Russell Diagram

The Main Sequence: O

• 30,000-60,000 K (Blue-white)• Absolute Magnitude -5• 1,000,000 times Sun’s Luminosity• 16 times Sun’s Diameter • 64 times Sun’s Mass• Lifetime: Less than a million years• Examples: Orion's Belt

The Main Sequence: B

• 10,000-30,000 K (Blue-white)• Absolute Magnitude -3• 20,000 times Sun’s Luminosity• 7 times Sun’s Diameter • 18 times Sun’s Mass• Lifetime: 10 million years• Examples: Spica

The Main Sequence: A

• Temperature: 7500-10,000 K (White)• Absolute Magnitude +0.5 • 40 times Sun’s Luminosity• 2 times Sun’s Diameter • 3 times Sun’s Mass• Lifetime: 600 million years• Examples: Vega, Sirius

The Main Sequence: F

• Temperature: 6000-7500 K (Yellow-White)• Absolute Magnitude +2.5 • 6 times Sun’s Luminosity• 1.5 times Sun’s Diameter • 1.7 times Sun’s Mass• Lifetime: 2.5 billion years• Examples: Procyon

The Main Sequence: G

• Temperature: 5000-6000 K (Yellow)• Absolute Magnitude +5 • 1 times Sun’s Luminosity• 1 times Sun’s Diameter • 1 times Sun’s Mass• Lifetime: 10 billion years• Examples: Sun, Alpha Centauri A

The Main Sequence: K

• Temperature: 3500-5000 K (Orange)• Absolute Magnitude +6 • 0.4 times Sun’s Luminosity• 0.9 times Sun’s Diameter • 0.8 times Sun’s Mass• Lifetime: 10 billion years• Examples: Alpha Centauri B

The Main Sequence: M• Temperature: 2000-3500 K (Red)• Absolute Magnitude +10 to +15 • 0.04 times Sun’s Luminosity• 0.5 times Sun’s Diameter • 0.4 times Sun’s Mass• Lifetime: 5 trillion years• 75% + of all stars• Examples: Barnard's Star, Proxima Centauri

Sub-Dwarfs

• L: 1300-2000 K, Borderline stars with alkali metals and metal hydrides

• T: 700-1300 K, Substellar, methane in spectra• Y <700 K, Substellar, ammonia in spectra

(predicted)