Transcript 05spectralclasses

Classification of Stellar Spectra
Late 1800s: first high-quality spectral measurements of stars
What are the main features – and how to classify them?
Spectral Lines
• Balmer absorption lines occur
when an incoming photon
causes an electron in the n = 2
level in hydrogen to jump to a
higher level.
• The hotter a star, the more likely
that hydrogen electrons will be
in the n = 2 level
• But for very hot stars, hydrogen
will lose its electrons completely
• Balmer lines thus reach their
maximum “depth” in the spectra
of stars with T=9250 K, so let’s
call those stars class ‘A’
Balmer
emission
series
Dependence of Spectral Lines vs.
Temperature
Stellar Spectral Lines
• Why do spectral lines
depend upon temperature?
– Populations of various
atomic states depends
upon temperature
• Degeneracy of levels
– Stage of ionization
• Depends on Pressure and
density…
• Depends somewhat on
composition of star as well
The Spectral Sequence
• In 1890, Edward Pickering and
his assistant at Harvard
classified thousands of stellar
spectra at Harvard.
• Named them ‘A’ through ‘Q’
based on Balmer depth.
• Approx. 20 years later,
blackbody theory was
developed.
• A. Cannon ‘improved’ the
scheme and re-ordered it by
temperature: O,B,A,F,G,K,M
• Subdivided each into 0 through
9 (AO: hot – A9:cooler)
• Later on, L and T were added.
E. Pickering and his
housekeeper W. Fleming
A. J. Cannon classifying one of
200,000 spectra by eye for 25¢ an
hour ($6 today)
Spectral Type Classification System
O B A F G K M (L T)
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50,000 K
1,000 K
Temperature
The Spectral Sequence and Temperature
Cecilia
PayneGaposchkin
40000 K
20000 K
“Stars”
9000 K
7000 K
Our sun: G2 
5500 K
4500 K
3000 K
“Brown
dwarfs”
(no H fusion)
2000 K
< 1300 K
Hertzsprung-Russell diagram
• In early 1900s, H. Russell and E.
Hertzsprung independently
plotted absolute V magnitude
against spectral class
• Most stars fall along a band
(main sequence)
• Some are very luminous
compared to main sequence
stars of their spectral class
(implied large radius)  giants
• Some are very underluminous
for their class  white dwarfs
Star with Hipparcos
parallax distance
measurements
Note multiple axis
labels
HR Diagram from Gaia Parallax
Measurements
Stellar Luminosity Classes
• In 1930s, W. Morgan and P. Keenan noticed that stars with the
same temperature could have different Balmer absorption depths.
• Called the narrowest ones I and the deepest ones VI
A0 I
A0 II
A0 V
Spectra of three A0 stars of different luminosity class
Origin of Luminosity Classes
• At higher pressure, the gas particles in a stellar atmosphere are
closer together and can interact more frequently.
• The energy levels of the atoms are perturbed, so that a wider
range of photon frequencies can be absorbed.
Narrow line,
low density
pressure
Pressure broadening of a CO2
absorption line
Morgan-Keenan Luminosity classes
Betelgeuse
Arcturus, Capella
Most common type
(includes our Sun)
Sirius B
Morgan-Keenan Luminosity classes
Recall that luminosity
class varies with
surface gravity, which
varies as M / R2
Leads to luminosity
class regions on the
H-R diagram
Hertzsprung-Russell diagram
•
Stefan-Boltzmann law gives lines of constant radius:
Main Sequence Relations
Higher luminosity  higher mass  higher temp  shorter
lifetime
Note the
increasing
mass and
shorter
lifetimes as you
climb the main
sequence
Stars leave the
main sequence
toward the end
of their lives
Spectroscopic Parallax Method
• Can use H-R diagram to
estimate absolute magnitude
of star given its spectral type
and luminosity class
• Use apparent magnitude and
distance modulus formula:
• Scatter of +/- 1 magnitude
results in a factor of 1.6
uncertainty in distance