Transcript The Stars The Sun is a Star Surface of the Sun.

The Stars
The Sun
is a Star
Surface of the Sun
Solar Prominence
Solar Corona
The Stars in the Sky
Vary in Brightness
• Distance
• Size
Vary in Color
• Color = Temperature
Star Names
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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• All pre-Hipparcos distance data is obsolete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
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Planet around nearby star:
Pluto:
Faintest Naked-Eye Star:
Big Dipper Stars:
Sirius (Brightest Star)
Venus
Full Moon
Sun
30
13
6
2
-1.6
-4
-12
-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
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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
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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
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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