ii-2b. magnitude 2015 (main ref.: lecture notes; fk sec.17-3) b 1 / d 2 lec 2
TRANSCRIPT
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= 1001/5
Therefore, six magnitudes must have ratios = 1001/5 = 2.512
1 2.512 2.5122 2.5123 2.5124 2.5125
1 2.512 6.310 15.851 39.818 100.023
Note” the smaller the magnitude, the brighter the star! Table II-1
•Sun : 26.7•Full Moon: 12.6•Venus: 4.4•Serius (brightest star): 1.4
•Pluto: +15.1•Largest telescope: +21•Hubble Space Telescope: +30
•EX 7 Modern Magnitude
(See Fig. II-5 for more details.)
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Astronomers often use the magnitude scaleto denote brightness
• The apparent magnitude scale is an alternative way to measure a star’s apparent brightness
• The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs
Fig. II-5: The Apparent Magnitude Scale
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Math Expression
m = m2 – m1 = 2.5 log ( b1 / b2 ) Eqn(6)
See examples in FK Box 17-3.
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EX 8: Venus m1 = 4;
dimmest star we can see m2 = + 6.
How many times brighter is Venus than the faintest star we can see?
Ans: 10,000 times brighter
(See class notes, also FK Box 17-3, Example 1)
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EX 9: RR Lyrae, variable: bpeak = 2 bmin.
What is the magnitude change?
Ans: 0.75
(See class notes, also FK Box 17-3, Example 2)
EX 10
(#)
(#) Note: If use m = 1.12, we get 2.8 times as bright.
EX 10
2.8
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2b-(ii) Absolute Magnitude M
• Absolute Magnitude M = m a star would have if it were located at 10 pc
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Math Expression
m – M = 2.5 log ( bM / bm ) Eqn(7)
m – M = 5 log ( dm / dM ) Eqn(8a)
dM = 10 pc; dm = true distance
m – M = 5 log d (pc) – 5 Eqn(8b)
(See lecture notes for derivation.)
Distance Modulus DM = m – M Eqn(9)See FK Box 17-3 for DM(=m – M) vs d(pc) .
e.g., DM = 4 d = 1.6 +20 105
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EX 13: A Star with m = +6 (faintest we can see by unadied eyes) at d = 20pc.
What is the absolute magnitude? Ans: M = + 4.5 (See class notes.)**************************************************************
EX 14: Suppose we are at 100 pc away from Sun. Can we still see Sun with naked eyes? What is m of the sun then? Note: Msun = 4.8 (see Ex 12).
Ans: No, too faint to be seen.Reason: m = 9.8 > 6 (See class notes and FK Box 17-3, Example 4.)
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Study more examples in FK Box 17-3.
Luminosity Function: The Population of Stars
(See FK pp 472-473)