1. Dr Paul A Daniels Guildford Astronomical Society 1
Astronomy Basics
H-R is not just
‘Human Resources’
Dr Paul A Daniels Guildford Astronomical Society 1

2. Stars have brightness and colour!
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3. But what about colour? I
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4. Colour Index Star Name Bayer B Magnitude V Magnitude B-V Colour Index
QSO B
14.24
16.5
-2.26
RMC135
12.63
13.48
-0.85
Bellatrix
γ Ori
1.38
1.60
-0.22
Rigel
β Ori
0.12
0.15
-0.03
Sirius
α CMa
-1.45
0.00
Vega
α Lyr
Altair
α Aql
0.97
0.75
0.22
Sun
-26.11
-26.77
0.66
Capella
α Aur
0.84
0.05
0.79
Arcturus
α Boo
0.82
Betelgeuse
α Ori
1.97
0.45
1.52
Aldebaran
α Tau
2.40
0.85
1.55
Antares
α Sco
2.89
1.05
1.84
Herschel’s Garnet Star
μ Cep
6.44
4.20
2.24
UU Aur
7.89
5.25
2.64
Hind’s Crimson Star
R Lep
13.51
7.76
5.75
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5. Dr Paul A Daniels Guildford Astronomical Society 5
Review of Apparent v. Absolute Magnitude
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6. Aug 1989: Hipparcos - accuracy of ±0.001”
Apparent Magnitude
The brightness of a star as seen from just above the Earth’s atmosphere
Absolute Magnitude
The brightness of a star as seen from a standard distance of 10 parsecs.
Used for comparing the intrinsic brightness of stars.
Nearest star’s parallax = ” ± ” ≡ 1/ = pc ≡ 3.26 x = ly
Accuracy of ±0.025”
By 1900 only 60 stars measured
Aug 1989: Hipparcos - accuracy of ±0.001”
Catalogue of 120,000 stars up to 1,600 ly
Dec 2013: Gaia – accuracy of ± ”
Will produce distances to 20 million stars with ~1% accuracy and 200 million stars with ~10%.
First catalogue out last year (2017).
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7. Dr Paul A Daniels Guildford Astronomical Society 7
The Magnitude-Distance Relationship
𝐼 𝑟 𝐼 10 = 1 𝑟 = 𝑟 2
𝐼 ∝ 1 𝑟 2
I = Intensity
r = Distance
𝑚 𝑟 − 𝑚 10 =−2.5 log 𝐼 𝑟 𝐼 10
mr = Apparent magnitude
m10 = Absolute magnitude
𝑚 10 = 𝑚 𝑟 −5 log 10 𝑟 −1
𝑚 𝑟 − 𝑚 10 =−5+5 log 10 𝑟
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8. Dr Paul A Daniels Guildford Astronomical Society 8
The Hertzsprung-Russell Diagram
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9. Dr Paul A Daniels Guildford Astronomical Society 9
Getting distance from the H-R diagram
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