Download Stellar Luminosity

Stellar Luminosity
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Apparent brightness (flux) is a measure of how bright a star appears on Earth
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Luminosity is a measure of how much energy per second (W) a star emits
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The apparent brightness of an object declines with distance (inverse square)
Luminosity
Apparent brightness (flux) =
4π × (distance)2
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If we measure apparent brightness (energy/sec/m2) and we know distance, we can get the luminosity of the star
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For Sun, apparent brightness = 1370 W/m2
and d = 150 million km = 1.5 × 1011 m
L = 4π (1370 W/m2 )(1.5 × 1011 m)2
= 3.9 × 10 26 W
Two identical stars, one 5 light years from Earth, and a second 50 light years from Earth are discovered. How much fainter does the farther star appear to be?
A
square root of 10 B
10
C
100
D
1,000
E
the farther star does not appear fainter, since it is identical
Distance and Parallax
• It is relatively easy to measure apparent brightness of a star
• Distance is much harder to measure
• For nearby stars (d ≤ 3000 ly) we can use the technique of parallax
• You can quickly understand parallax by putting your finger in front of your face, then alternate closing your two eyes -­ note how your finger appears to move relative to the more distant objects in the room (Image at right)
Distance and Parallax
• As the Earth orbits the Sun, relatively nearby stars appear to move relative to more distant stars
Interactive Figure 15.3
• Because even the nearest stars are so distant, there is a simple relationship between distance and apparent angle a star moves
1
d (in parsecs) =
p (in arcseconds)
• 1 parsec ≈ 3.26 light years
Which of the following stars is closest to us?
A
Procyon (parallax angle = 0.29")
B
Ross 780 (parallax angle = 0.21")
C
Regulus (parallax angle = 0.04")
D
Sirius (parallax angle = 0.38")
On Earth, the parallax angle for the star Procyon is 0.29 arcseconds. If you were to measure Procyon’s parallax angle from Venus, what would the parallax angle be? (Note: Earth’s orbital radius is larger than Venus’s orbital radius
A
more than 0.29 arcseconds
B
0.29 arcseconds
C
less than 0.29 arcseconds
D
zero arcseconds (no parallax)
The star Gamma Gemini has an apparent magnitude=1.9
and Epsilon Gemini has an apparent magnitude=2.9
How do the observed fluxes of the two stars compare? A
FGamma Gem = 0.1 FEpsilon Gem
B
FGamma Gem = 0.4 FEpsilon Gem
C
FGamma Gem = 2.5 FEpsilon Gem
D
FGamma Gem = 10 FEpsilon Gem
The brightest star in the constellation Taurus (which is named Aldebaran) has twice the flux of the 2nd-­
brightest star in that constellation (Elnath).
How do the magnitudes of the two stars compare? A
Aldebaran is 0.3 magnitudes brighter
B
Aldebaran is 0.75 magnitudes brighter
C
Aldebaran is 1.0 magnitude brighter
D
Aldebaran is 2.5 magnitudes brighter
In 1843, the massive star Eta Carinae flared remarkably, increasing in magnitude to m = −1 (becoming the 2nd-­brightest star in the entire sky) before fading away again to 8th mag. How much did its luminosity change then? A
• By a factor of 109 B
• By a factor of 104.5
C
D
• By a factor of 4×103
• By a factor of 2×104
Stellar Luminosities
• Stellar luminosities vary from 0.0001 L¤–1,000,000 L¤, ten orders of magnitude
• Note that most of the stars in this image are at the same distance, so their relative apparent brightness is the same as their relative luminosities
• Note that there are many more faint stars than bright stars, suggesting that less luminous stars are far more common