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The Sun and the Stars
The Sun and the Stars
Dr Matt Burleigh
The Sun and the Stars
Dr Matt Burleigh
The Sun and the Stars
Classification of stellar spectra
Potted History :
1802 William Wallaston – showed that the spectrum of the
sun is not simply a continuous spectrum, but is broken
up by a series of dark lines (absorption lines).
1814 Joseph Fraunhofer identified ~600 lines in the solar
spectrum and measured wavelengths for approx half.
that number. Lines now known as Fraunhofer lines.
1863 Angelo Secchi – crude spectral typing
1864 William Huygens matched some of the Fraunhofer
lines to absorption lines seen in the spectra of other
stars.
1890s Edward Pickering (Harvard College Observatory)
starts project to obtain spectra of all stars observed
down to 8th magnitude
Subsequently shown that : small number of distinct patterns
of absorption lines seen in stellar spectra
 Spectral classification
Dr Matt Burleigh
The Sun and the Stars
Spectral Classification – Harvard Classification scheme
Work originally started by Henry Draper, continued by Annie Jump Cannon in 1910s, results published in the
Henry Draper Catalogue. Extended version of HD catalogue contains spectra of 225,000 stars down to 9th magnitude.
Annie Jump Cannon – spectra are dominated by lines of Hydrogen and Helium and small amounts of metals.
Original scheme had classes OBAFGKM (now extended to L, T and Y dwarfs)
Most important lines are Balmer series of H, neutral and singly ionised He, Fe, Ca H and K doublet,
the G band (CH molecule), neutral Ca, metal lines and Ti O.
excitation-ionisation
sequence
Scheme is based on (i) absence of lines
(ii) line strengths (W())
(iii) line ratios
Dr Matt Burleigh
The Sun and the Stars
The Hertzsprung-Russell Diagram (E. Hertzsprung and H.N. Russell)
Plot of surface temperature versus luminosity, or colour (e.g. B-V) versus absolute magnitude M,
and various other combinations
bright
Can distinguish
between early type
and late-type stars
dim
Hot
Cool
Dr Matt Burleigh
The Sun and the Stars
HCS is a temperature classification scheme – why?
Once a star has formed, there is little mixing of the core and surface material,
and few chemical reactions
 spectral differences reflect primarily differences in the surface temperature
O 50,000 K strong He+ lines, no H lines
B 20,000 K strong neutral He lines, v. weak H lines
A 10,000 K Strongest H lines, weak Ca+ emerge
F 7,500 K H grows weaker,Ca+ becomes stronger, metals
G 6,000 K Strong Ca+,Fe+ and other metals , H weaker
K 4,000 K Strong metal lines, weak CH,CN bands emerge
M 3,500 K strong TiO and VO emerge and strengthen
Cannon - further subdivided classes from 0 to 9 (0 hotter, 9 cooler), so that F5, lies halfway
between F0 and G0. In this scheme the Sun is classed as G2
Dr Matt Burleigh
The Sun and the Stars
Harvard classification scheme
Type
Colour
Surface Temp.
Characteristics
O
Blue
>25,000 K
B
Blue
11,000-25,000
neutral He lines in absorption
Rigel, Spica
A
Blue
7,500-11,000
H lines at max strength for AO,
decreasing from AO-A9
Sirius, Vega
F
Blue to White
6000-7500
metallic lines emerge and strengthen
G
White to Yellow
5000-6000
lines of neutral metals, Ca+ Fe+
K
Orange to Red
3500-5000
metallic lines dominate, weak blue
continuum
Arcturus,
Aldebaran
M
Red
<3,500
molecular bands of TiO noticeable
Betelgeuse
singly ionised He in emission or
absorption. Strong UV continuum
Examples
10 Lac
Canopus,
Procyon
Sun, Capella
Dr Matt Burleigh
Dr Matt Burleigh
The Sun and the Stars
Examples spectral classification sequences
Dr Matt Burleigh
The Sun and the Stars
1943 Yerkes SCS (or MKK – named after Morgan, Keenan and Kellman)
Extends Harvard classification by using line shapes to estimate surface gravities
gs 
GM *
R*2
MKK luminosity classification
Ia luminous Supergiants
Ib less luminous Supergiants
II luminous Giants
III normal Giants
IV sub Giants
V main sequence stars (dwarfs)
VI sub Dwarfs
In this extended scheme the sun is
classified as G2V
Dr Matt Burleigh
dwarf stars : smaller radii higher surface gravity c.f. giants
therefore higher surface density and higher surface pressure
– under higher pressure atoms collide more frequently,
collisions shorten lifetime of excited state.
smaller Δt, larger ΔE
(ΔE Δt ≈ h/2π)
=> lines are broader!
also at a given temperature, T, atoms are thermalised
more easily at lower pressure
Dr Matt Burleigh
The Sun and the Stars
MKK luminosity classification
Dr Matt Burleigh
Dr Matt Burleigh
The Sun and the Stars
Additional classes
Wolf Rayet, W or WR – mostly He atmos. - dying supergiants with fast stellar winds
subtypes WC, WN or WO (for Carbon, Nitrogen and Oxygen)
(Possible GRB/Hypernovae progenitors)
Carbon stars, C (originally R and N classification), red-giants/supergiants near end of lives
with excess, Carbon in atmosphere
WDs, D – stars are no longer undergoing thermonuclear reactions, simply cooling
subclasses designated A,B,O,Q,Z,C,X depending on optical lines present (ie atmospheric
composition)
L stars,T,Y dwarfs – Cool stars, T and Y dwarfs are not massive enough to undergo nuclear fusion
(Brown dwarfs)
Dr Matt Burleigh
The Sun and the Stars
Since spectral type is essentially a temperature sequence, we can construct a
colour-magnitude diagram, a diagram of Absolute magnitude Mv versus colour,
e.g. Hyades open cluster
Colour magnitude diagrams can be used to estimate distances – spectroscopic parallax
Dr Matt Burleigh
The Sun and the Stars
Spectroscopic parallax
From a stars spectrum, we can identify
i)
ii)
Spectral type (or colour)
Luminosity class
Since,
m  M  5 log d  5
These fix a position on the HR diagram from which
we can measure Mv
, the observed apparent magnitude m gives us the
distance modulus, and hence the distance to the star
Dr Matt Burleigh
The Sun and the Stars
Colour-colour diagrams
NB a colour is just a flux ratio
If stars radiated like blackbodies
then a colour-colour plot would
be unique and follow a straight
line
The kink in the “lazy S”
Is due to the Balmer
discontinuity
BB
U-B
(BD at 3640 falls into U-band)
B-V
Dr Matt Burleigh
The Sun and the Stars
When we looks at young clusters and globular clusters their colour-magnitude diagrams appear very different
Globular clusters consist of ~half a million stars
gravitationally bound, and located above the plane
of the galaxy. They are old metal poor stars.
Show extensive red-giant,AGB and horizontal branch
As well as main sequence
Young open clusters are metal rich pop 1 stars.
Dominated by their main sequence stars, they are
young so very few stars have evolved off of the main
sequence.
Age can be determined from the location of the
turn-off point, younger clusters will turn off at higher
luminosities
Dr Matt Burleigh
The Sun and the Stars
Globular cluster schematic colour-magnitude diagram
Dr Matt Burleigh