Life Cycles Of Stars

The Stars

The Sun is a Star

Surface of the Sun

Solar Prominence

Solar Corona

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

Constellations• 89 in number• Based on Near Eastern myths via the Greeks• Some 17th-18th century Invented

Constellations• Now Have Formal Boundaries• Every star is in one and only one

constellation• Constellations are entirely human

inventions. They are not “Really” in the sky.

The Southern Cross

A Myth in the Autumn Sky

A Star Map

Western Constellations

Chinese Constellations

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

Parallax and the Distances of

Stars• Stars appear

identical all over Earth

• They do show slight parallax shift from opposite sides of Earth’s orbit

Parallax: pre-1997• Parallax is tiny - was once used as argument

against motion of the Earth• One second of arc = size of a quarter at 5

km (3 mi.)• Parallax angle of nearest star (4.3 l.y.) is

0.75”• Accuracy limited by Earth’s atmosphere• Fairly accurate to 30-40 l.y., rough to 100

Hipparcos

• Named for ancient Greek astronomer who catalogued the stars

• High Precision Parallax Collecting System• Launched by European Space Agency,

1989• Data Collection 1989-1993• Data Analysis 1993-1997

The Hipparcos Data• 118,218 stars measured: parallax and

motion• 22,396 accurate to 10% - a 20-fold

improvement• Stars out to 200-300 l.y. are known to

within 10%• 30,000 more accurate to 20%• All pre-Hipparcos distance data is obsolete

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, the Moon Does Not

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

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

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

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

The Hertzsprung-Russell Diagram

How Stars Form

• Collapsing gas and dust cloud

• Protostar - mostly infrared

Main Sequence Stars

• Brown Dwarf• Red Dwarf• Normal Star

All Objects Exist Because of a Balance Between Gravity and

Some Other Force• People, Planets-Interatomic Forces• Normal Stars-Radiation• White Dwarfs-Electron Repulsion• Neutron Stars-Nuclear Forces• Black Holes-No Known Force

How Stars Die• Main Sequence Stars Brighten With Age• The More Massive a Star, the Faster it Uses

Fuel• Giant Phase• White Dwarf• Supernova

– Neutron Star - Pulsar– Black Hole

Historical Supernovae

• 1006 - Chinese• 1054 - Chinese, European, Anasazi?• 1572 - Tycho’s Star• 1604 - Kepler’s Star• 1987 - Small Magellanic Cloud

(170,000 l.y.)

Life (Briefly!) Near a Supernova

• Sun’s Energy Output = 77 billion megatons/second

• Let’s relate that to human scales. What would that be at one kilometer distance?

• 77 x 1015 tons/(150 x 106km)2 = 3 tons• Picture a truckload of explosives a km away

giving off a one-second burst of heat and light to rival the Sun

Now Assume the Sun Goes Supernova

• Brightens by 100 billion times• Our 3 tons of explosive becomes 300,000

megatons• Equivalent to entire Earth’s nuclear arsenal

going off one km away - every second• This energy output would last for days

Planetary Systems

• Protoplanetary Disks• Accretion of Planets• Expulsion and Migration of Planets• About 120 extrasolar planets known• Our Solar System may be unusual?

Protoplanetary Disks in Orion