Dr Paul A Daniels
Guildford Astronomical Society
1
Stars have colour!
Dr Paul A Daniels
Guildford Astronomical Society
2
But what about colour?
•I
Dr Paul A Daniels
Guildford Astronomical Society
3
Colour Index
Star Name
Bayer
B Magnitude
V Magnitude
B-V Colour Index
QSO B0005-239
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
-1.45
0.00
Vega
α Lyr
0.00
0.00
0.00
Altair
α Aql
0.97
0.75
0.22
-26.11
-26.77
0.66
Sun
Capella
α Aur
0.84
0.05
0.79
Arcturus
α Boo
0.97
0.15
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
R Lep
13.51
7.76
5.75
Hind’s Crimson Star
Dr Paul A Daniels
Guildford Astronomical Society
4
Secchi Spectral Classification
•
Angelo Secchi developed the classification
scheme during the 1860s and 1870s.
Class
Colour
Description
Examples
I
III
Orion
III
Subtype
II
III
White/Blue
Heavy hydrogen lines.
Vega, Altair
Yellow
White/Blue
Orange/Red
Moderate hydrogen lines.
Narrow hydrogen lines
Complex band spectra.
Sun, Arcturus, Capella
Rigel, Bellatrix
Betelgeuse, Antares
Yellow
Moderate hydrogen lines.
Sun, Arcturus, Capella
Orange/Red
Complex band spectra.
Betelgeuse, Antares
Red
Strong carbon bands.
Carbon stars.
R Lep
Emission lines.
γ Cas, Sheliak
IV
V
Dr Paul A Daniels
Guildford Astronomical Society
5
Draper Spectral Classification
•
Developed by Edward C. Pickering in the 1880s.
•
The Draper Catalogue of Stellar Spectra published in 1890.
Secchi Class
I
II
III
IV
Draper Class
Description
A, B, C, D
Heavy hydrogen lines.
E, F, G, H, I, K, L
M
N
Not in the catalogue
O
Wolf-Rayet stars with bright emission lines
P
Planetary Nebulae
Q
Weird!
Spectrum of WR137 – a Wolf-Rayet star
Dr Paul A Daniels
Guildford Astronomical Society
6
Harvard Spectral Classification
Class
O
B
A
F
G
K
M
•
•
•
•
•
Effective
Temperature
≥ 30,000 K
10,000–30,000 K
7,500–10,000 K
6,000–7,500 K
5,200–6,000 K
3,700–5,200 K
2,400–3,700 K
Conventional Colour
Description
blue
blue white
white
yellow white
yellow
orange
red
Actual
Apparent Colour
blue
deep blue white
blue white
white
yellowish white
pale yellow orange
light orange red
Superceded the Secchi classification from about 1890.
Antonia Maury and Annie Jump Cannon at Harvard re-ordered
and dropped some of the letters (1897-1901).
The class is associated with effective surface temperature.
The classes may be subdivided (0-9), e.g., O9.7, A0, G2, K9.
The Yerkes classification extends the Harvard classification to
include Wolf-Rayet stars (W), White Dwarfs (D) & Carbon Stars (C)
and adds more letters…
Dr Paul A Daniels
Guildford Astronomical Society
Star
Class
Bellatrix
B2III
Vega
A0Va
Sun
G2V
Arcturus
K0III
Betelgeuse
M1-M2 1a-ab
Antares
M1.5Iab+B2.5V
Hind’s Crimson Star
C7,6e (N6e)
7
Harvard Spectral Classification
• OBAFGKM =
Oh! Be A Fine Girl, Kiss Me!
• WOBAFGKMRNS =
Wow! Oh Be A Fine Girl Kiss Me Right Now Sweetheart!
Wow! Oh Be A Fine Guy Kiss Me Right Now Sweetie!
Wow! Oh Be A Fine Girl Kiss Me Right Now…… Smack!
Obese Balding Astronomer Found Guilty Killing Many Reluctant Nonscience Students.
Overseas Bulletin: A Flash! Godzilla Kills Mothra! (Rodan Named Successor).
Oh Backward Astronomer, Forget Geocentricity; Kepler’s Motions Reveal Nature’s Simplicity!
Organs Blaring and Fugues Galore, Kepler's Music Reads Nature's Score.
Out Beyond Andromeda, Fiery Gases Kindle Many Radiant New Stars.
Oh, Bring A Fully Grown Kangaroo My Recipe Needs Some!
Oven-Baked Ants, when Fried Gently, and Kept Moist, Retain Natural Succulence.
Only Boring Astronomers Find Gratification Knowing Mnemonics!
Credit to http://www.star.ucl.ac.uk/~pac/obafgkmrns.html
Dr Paul A Daniels
Guildford Astronomical Society
8
Review of Apparent v. Absolute Magnitude
Dr Paul A Daniels
Guildford Astronomical Society
9
• 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
= 0.7687” ± 0.0003”
≡ 1/0.7687 = 1.3009 pc
≡ 3.26 x 1.3009 = 4.243 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 ±0.00002”
 Will produce distances to 20 million stars with
~1% accuracy and 200 million stars with ~10%.
 First catalogue due out this year (2016).
Dr Paul A Daniels
Guildford Astronomical Society
10
The Magnitude-Distance Relationship
1
2
𝐼𝑟
10
2
𝑟 =
=
𝐼10 1 2
𝑟2
10
1
𝐼 ∝ 2
𝑟
I = Intensity
r = Distance
mr = Apparent magnitude
m10 = Absolute magnitude
𝑚𝑟 − 𝑚10
𝐼𝑟
= −2.5 log10
𝐼10
𝑚10 = 𝑚𝑟 − 5 log10 𝑟 − 1
Dr Paul A Daniels
Guildford Astronomical Society
11
Next time…
• Review of Apparent v. Absolute Magnitude
http://astro.unl.edu/animationsLinks.html
https://www.youtube.com/watch?v=Kqe6F-Qf9Tk
Dr Paul A Daniels
Guildford Astronomical Society
12
Next time…
• Review of Apparent v. Absolute Magnitude
• Stellar Classification
• The Hertzsprung-Russell Diagram
• Luminosity/Distance determined from HR Diagram
• Stellar Evolution
Dr Paul A Daniels
Guildford Astronomical Society
13
Stellar Classification
• Review of Apparent v. Absolute Magnitude
Class
Effective
Conventional Colour
Actual
Main-sequence Mass
Main-sequence Radius
Fraction of All
Temperature
Description
Apparent Colour
(Solar Masses)
(Solar Radii)
Main-sequence Stars
O
≥ 30,000 K blue
B
10,000–30,000 K blue white
A
7,500–10,000 K white
≥ 16 M☉
≥ 6.6 R☉
~0.00003%
deep blue white
2.1–16 M☉
1.8–6.6 R☉
0.13%
blue white
1.4–2.1 M☉
1.4–1.8 R☉
0.6%
blue
F
6,000–7,500 K yellow white
white
1.04–1.4 M☉
1.15–1.4 R☉
3%
G
5,200–6,000 K yellow
yellowish white
0.8–1.04 M☉
0.96–1.15 R☉
7.6%
K
3,700–5,200 K orange
pale yellow orange
0.45–0.8 M☉
0.7–0.96 R☉
12.1%
M
2,400–3,700 K red
light orange red
0.08–0.45 M☉
≤ 0.7 R☉
76.45%
Dr Paul A Daniels
Guildford Astronomical Society
14