The Family of Stars Please press “1” to test your transmitter.
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Transcript of The Family of Stars Please press “1” to test your transmitter.
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The Family of Stars
Please press “1” to test your transmitter.
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The Family of StarsWe already know how to determine a star’s
• surface temperature
• chemical composition
Now, see how we can determine its
• distance• luminosity• radius• mass
and how all the different types of stars make up the big family of stars.
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Distances of Stars
Trigonometric Parallax:
Star appears slightly shifted from different positions of the Earth on its orbit
The further away the star is (larger d), the smaller the parallax angle p.
d = __ p 1
d in parsec (pc) p in arc seconds
1 pc = 3.26 LY
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The Trigonometric ParallaxExample:
Nearest star, Centauri, has a parallax of p = 0.76 arc seconds
d = 1/p = 1.3 pc = 4.3 LY
With ground-based telescopes, we can measure parallaxes p ≥ 0.02 arc sec
=> d ≤ 50 pc
This method does not work for stars further away than 50 pc.
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Sirius, the brightest star in the sky, has a trigonometric parallax of p = 0.385 arc
seconds. What is its distance from Earth?
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1. 0.385 pc
2. 0.80 light years
3. 1.255 pc
4. 2.60 light years
5. 8.47 light years
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The method of trigonometric parallaxes (from ground based telescopes) allows us to
measure distances …
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1. only to objects in our solar system.
2. only to stars in our solar neighborhood within the Milky Way.
3. to stars throughout the entire Milky Way.
4. to stars and galaxies throughout the Local Group.
5. even to other clusters of galaxies.
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Intrinsic Brightness / Absolute Magnitude
The further away a light is, the fainter it appears.
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Intrinsic Brightness / Absolute Magnitude (II)
More quantitatively:
The flux received from the light is proportional to its intrinsic brightness or luminosity (L) and inversely
proportional to the square of the distance (d):
F ~ L__d2
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The stars A and B have the same intrinsic luminosity, but A is 5 times
further away from Earth than B. Then:
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1. Both stars will appear equally bright.
2. A will appear 5 times brighter than B.
3. B will appear 5 times brighter than A.
4. A will appear 25 times brighter than B.
5. B will appear 25 times brighter than A.
EarthStar BStar A
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Distance and Intrinsic Brightness
Betelgeuze
Rigel
Example:
App. Magn. mV = 0.41
Magn. Diff. Intensity Ratio
1 2.512
2 2.512*2.512 = (2.512)2 = 6.31
… …
5 (2.512)5 = 100
App. Magn. mV = 0.14
For a magnitude difference of 0.41 – 0.14 = 0.27, we found: Rigel appears 1.28 times brighter than Betelgeuze
But Rigel and Betelgeuze may be at quite different distances from us!
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Absolute Magnitude
To characterize a star’s intrinsic brightness, define Absolute Magnitude (MV):
Absolute Magnitude = Magnitude that a star would have if it were at
a distance of 10 pc.
If we know a star’s absolute magnitude, we can infer its distance by comparing
absolute and apparent magnitudes.
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Absolute Magnitude (II)
Betelgeuze
Rigel
Betelgeuze Rigel
mV 0.41 0.14
MV -5.5 -6.8
d 152 pc 244 pc
Difference in absolute magnitudes:
6.8 – 5.5 = 1.3
=> Luminosity ratio = (2.512)1.3 = 3.3
Rigel is actually 3.3 times brighter than Betelgeuze!
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The Size (Radius) of a StarWe already know: flux increases with surface temperature (~ T4);
hotter stars are brighter.
But brightness also increases with size:
A B
Star B will be brighter than star A.
Specifically: Absolute brightness is proportional to radius (R) squared, L ~ R2.
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Example:
Both Spica B and Sirius B are B-type stars, but Sirius B is a white dwarf star, with a
radius ~ 560 times smaller than Spica B.
Thus, since L ~ R2, Sirius B is intrinsically
5602 ≈ 320,000
times fainter than Spica B.
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Polaris has just about the same spectral type (and surface temperature) as our sun,
but it is 10,000 times brighter. Thus, Polaris’ radius is … the sun’s radius.
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1. the same as
2. 100 times larger than
3. 100 times smaller than
4. 10,000 times larger than
5. 10,000 times smaller than
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Organizing the Family of Stars: The Hertzsprung-Russell Diagram
We know:
Stars have different temperatures, different luminosities, and different sizes.
To bring some order into that zoo of different types of stars: organize them in a diagram of
Luminosity versus Temperature (or spectral type)
Lum
inos
ity
Temperature
Spectral type: O B A F G K M
Hertzsprung-Russell Diagram
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The Hertzsprung Russell Diagram
Most stars are found along the Main Sequence
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The Hertzsprung-Russell Diagram (II)
Stars spend most of their active life
time on the Main Sequence (MS).
Same temperature,
but much brighter than
MS stars
→ Must be much larger
→ Giant Stars
Same temp., but
fainter → Dwarfs
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Radii of Stars in the Hertzsprung-Russell Diagram
10,000 times the
sun’s radius
100 times the
sun’s radius
As large as the sun
100 times smaller than the sun
Rigel Betelgeuze
Sun
Polaris
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Luminosity Classes
Ia Bright Supergiants
Ib Supergiants
II Bright Giants
III Giants
IV Subgiants
V Main-Sequence Stars
IaIb
II
III
IV
V
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Examples:
• Our Sun: G2 star on the Main Sequence:
G2V
• Polaris: G2 star with Supergiant luminosity:
G2Ib