Transcript Document

Stars
… how I wonder what you are.
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Goals
• Stars are Suns.
• Are they:
–
–
–
–
–
Near? Far?
Brighter? Dimmer?
Hotter? Cooler?
Heavier? Lighter?
Larger? Smaller?
• What categories can we place them in?
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Distance
• One proof of a heliocentric
Universe is stellar parallax.
– Tycho Brahe saw no parallax of
stars.
– Copernicus thought stars must be
too far away.
• Nearest star: Proxima Centauri
Parallax angle = 0.76 arcsec
– Tycho’s precision = 1 arcmin
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The Parsec
• What is the distance of an object
with a parallax angle of 1
arcsec?
Distance = 206,265 AU
• Call this distance 1 parsec (pc)
1 pc = 206,265 AU =3.3 lightyears
• 1 lightyear = distance light travels in
one year.
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Distances
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Distance(in parsecs)
parallax(in arcsec)
• Closest star: Proxima Centauri
parallax = 0.76 arcsec
Distance = 1.3 pc or 4.3 lightyears
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Brightness
• How bright are
they really?
• What is due to
distance?
• What is due to
luminosity?
• Luminosity:
– Total energy
radiated every
second.
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Magnitude
Scale
• The SMALLER the
number the BRIGHTER
the star!
– Every difference of 1
magnitude = 2.5x brightness.
– Every difference of 5
magnitudes is a 100x
difference in brightness.
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Star light, star
bright
• Sirius is magnitude –1.5
Polaris is magnitude 2.5
• Is Sirius really more
luminous than Polaris?
• No, Sirius is just closer.
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Apparent and Absolute
• Apparent Magnitude = the brightness (magnitude) of a
star as seen from the Earth.  m
– Depends on star’s total energy radiated (Luminosity) and its
distance
• Absolute Magnitude = the brightness (magnitude) of a
star at a distance of 10 pc.  M
– Only depends on a star’s luminosity
 distance

m  M  5log10 
 10pc 
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example
 distance

m  M  5log10 
 10pc 
• Our Sun:
– m = -26.8,
– distance = 4.8 x 10-6 pc
So: M = 4.8
• Polaris:
– m = 2.5,
– distance = 132 pc
So: M = -3.1
• Polaris is 1500 times more luminous than the Sun!
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Stellar Temperatures
How hot are stars?
• In Lecture 3 we
learned about
thermal radiation
and temperature.
• Since different
stars have
different colors,
different stars
must be different
temperatures.
Stellar Spectra
Hot
Cool
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Spectral
Classifications
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Orion Copyright – Tyler Nordgren
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Stellar Masses
How massive are stars?
• Kepler’s Laws – devised for the planets.
• Apply to any object that orbits another object.
• Kepler’s Third Law relates:
– Period: “how long it takes to orbit something”
– Semimajor axis: “how far you are away from that something”
– Mass: “how much gravity is pulling you around in orbit”
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a
P 
M
2
• Where M is the Total Mass.
• Can calculate the mass of stars this way.
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Binary Stars
• Most stars in the
sky are in
multiple systems.
• Binaries, triplets,
quadruplets,
etc….
– Sirius
– Alcor and Mizar
– Tatooine
• The Sun is in the
minority by being
single.
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Stellar Masses
How massive are stars?
3
a
P 
M
2
• Most stars have masses calculated this way.
• Result:
– The more massive the star, the more luminous it is.
– The more massive the star, the hotter it is.
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Stellar Radii
How big are stars?
• We see stars have different
luminosities and different
temperatures.
• Stars have different sizes.
• If you know:
50 mas
– Distance
– Angular size
• Learn real size.
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Atmospheric Seeing
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Stars are small
• Betelgeuse is the only star big enough to directly
see its surface with a normal telescope.
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Interferometry
• Combine the light from two or more telescopes to
simulate the RESOLUTION of one giant
telescope.
NPOI - optical
VLA - radio
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Optical
Interferometry
• NPOI simulates a single optical
telescope 65 meters in diameter.
• Resolve stars as small as 1.5 mas!
PTI - infrared
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Angular versus Linear
Supergiants, Giants and Dwarfs
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H-R Diagram
• Can order the stars we see by:
– Temperature (or spectral type)
– Luminosity (or absolute magnitude).
• And see where other qualities fall:
– Mass
– Radius
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The Main Sequence
• “Stars are characterized by
what holds them up.”
• 90% held up by heat of
Hydrogen fusion?
4H  He + Energy
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Main Sequence & Thermal Radiation
• The Main Sequence
makes sense!
• Hotter stars are bluer –
Wien’ Law
• Hotter stars are brighter –
Stefan’s Law
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Homework #6
• For 10/9:
• Read B15.6, 16.1 – 16.2, Ty10
• Do: Review Questions 1, 5, Problems 1, 2
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