PowerPoint Presentation Chapter 17 Opener

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Chapter 17a Properties of Stars
Outline
Chapter 17 Part A
Part a 1. The sky Constellations
Names Earths Motion and Sky 2. Distances
to the Stars Stellar Parallax
Nearest Stars Hipparcos satellite
Space Interferometer Mission (SIM) 3. Stellar
Motion proper motion Radial Motion
Outline
Part b 4. Brightness Apparent Magnitude
Absolute Magnitude 5. Temperature and Color
6. Classification of Stars Annie Jump
Cannon Spectrum
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Part A
Chapter 17 Part A
1. The sky Constellations Names
Earths Motion and Sky
Outline
2. Distances to the Stars Stellar
Parallax Nearest Stars Hipparcos
satellite Space Interferometer Mission
(SIM)
3. Stellar Motion proper motion
Radial Motion
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88 constellations are well-defined regions of
the sky
AST122 Robert Zimmerman University of Oregon
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Stars are also named with a Greek letter (a, b,
g) according to their relative brightness within
a given constellation the name of the
constellation Betelgeuse a
Orionis Rigel b Orionis
Proper names for the brightest stars
Orion Constellation
Orion
Betelgeuse
Rigel
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Daily Motion
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12 Zodiac
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Distance to the Stars About 6000 stars visible at
any one time
Stars that appear close in the sky may not
actually be close in space.
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Stellar Parallax

• p parallax angle in arc seconds
• Size of arc seconds
• Arcsecond1/3600 degree

d1/p distance in "Parsecs
1 Parsec 3.26 ly First
measured in 1838
Nearby stars appear to move with respect to more
distant background stars due to the motion of the
Earth around the Sun. This apparent motion (it is
not "true" motion) is called Stellar Parallax.
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Distance1/p (parsec)
Angle1 second of arc
p
Distance4 km
Parallax Angle of Alpha Centauri
0.75 arcsec Distance 1 pc/0.75 1.33 pc
1.33 x 3.28 ly 4.34 ly
Parallax of Altair 0.20 arcsec Distance 1pc /
0.20 5 pc 16.3 ly
Parallax can only be used for the nearest
stars, within 200 pc or 0.005 arcsec.
1 parsec 3.26 ly
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The Nearest Stars
Table of Nearest Stars Distance star
4.2 ly Proxima Centauri
4.3 ly Alpha Centauri A 4.3 ly Alpha
Centauri B
Proxima Centauri closest star 4.3
light-years. The red dwarf is bound to the double
star Alpha Centauri A and B, which it orbits over
millions of years. Alpha Centauri A yellow star
like our sun. Alpha Centauri B, orange-red. They
revolve around each other every 80 years.
Sirius A forms a double star with Sirius B.
Sirius B is a white dwarf whose mass to our
suns, but a diameter only three times the
Earths, making it 1/10000 the brightness of
Sirius A.
8.7 ly Alpha Sirius 8.7 ly Beta Sirius
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The Hipparcos satellite (launched by the European
Space Agency in 1989) measured precision
parallaxes to an accuracy of about
0.001-arcsec---1000 pc.
Hipparcos measured parallaxes for about 100,000
stars Got 10 accuracy distances out to about
100 pc Good distances for bright stars out to
1000 pc.
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launch date is about 2012. determine parallax
distances out to 25,000 parsecs,
encompassing the Galactic Center (8000pc away)
and the halo of the Galaxy.
The Future Space Interferometer Mission (SIM)
Why are Distances
Important? Distances are necessary for
estimating Total energy emitted by an object
(Luminosity) Masses of objects from their
orbital motions True motions through space of
stars Physical sizes of objects
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Proxima Centauri
Movement of the stars True Motion and Proper
Motion
1. True Motion ( Proper motion Radial Motion)
2. Radial Motion measured by Doppler Shift
3. Proper Motion is the projection onto the sky
of the star's true motion----observed Centauri
moves by 3.7"/year
Typical proper motion is
0.1" /year.
Proxima Centauri
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Proper Motion
Polaris, the North Star
Little Dipper
Big Dipper
Proper motions will cause the Big Dipper to
slowly change its apparent configuration over the
next 10,000 years.
How the Big Dipper will look after 10,000 years.
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Apparent magnitude Apparent magnitude is
the brightness of an object as it appears to
you Introduced by Hipparchus (160-127
B.C.) Brightest stars visible to unaided eye
Magnitude 1 Dimmest stars visible to unaided
eye Magnitude 6 a Magnitude difference of 1
corresponds to a factor of 2.5 in brightness 1st
magnitude stars are 100x as bright as 6th
magnitude stars.
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Magnitudes of Pleiades Stars
Pleiades
Apparent Magnitude (brightness) An increase of
1 in apparent magnitude corresponds to a decrease
by a factor of approximately 2.5 in brightness.

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Inverse-Square Law B L/d2
Luminosity is the amount of energy a body
radiates per second. (Absolute Magnitude)
Brightness is related to distance with an
inverse square relationship. ( apparent magnitude)
Luminosity
Double distance Brightness drops by 4 Triple
distance Brightness drops by 9 If you know
magnitude (Brightness) and distance Inverse
square law gives -------Luminosity
Brightness
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• Suppose that all stars had exactly the same
  luminosity.
• Then, the brightness of the stars would only
  depend on their distance.
• In fact, by measuring the apparent brightness, we
  could calculate the distance to the star using
  the inverse square law of light.
• Of course, we are not so lucky. Stars have a
  large range of luminosities, and their apparent
  brightness depends on distance, temperature, and
  size (surface area), etc.

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Apparent Magnitude
Absolute magnitude is the apparent magnitude of
a star if it were 10 parsecs away
Absolute Magnitude
The absolute magnitude is a distance-independent
quantity
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Absolute Magnitude(Luminosity)
The Absolute magnitude is the apparent magnitude
a star would have at 10 parsecs from the Sun.
Absolute Magnitude Luminosity, although in
different units
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Star Colors
Blackbody
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Classifying stellar spectra was done by women
at Harvard around the turn of the century.
1890
Stellar Classes
Annie Jump Cannon (1863-1941)

• Single-handedly classified more than 250,000
  stellar spectra.

Confirmed that classes made more sense if
arranged by temperature
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Temperature and Spectrum
Different atoms hold on to electrons with
different force
Too Cold--No Electrons
Below 6000 K most H electrons in lowest state
cant cause Balmer lines Above 15,000K most H
electrons completely lost (ionized)
Too Hot--No Electrons
From our text Horizons, by Seeds
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The End of Chapter 17a
Go to Chapter 17b