Download Stellar Spectra

Lecture 9
Stellar Spectra
Announcements

Homework 5 is due on Monday.
Quantum states
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Unlike a planet going around the sun, electrons can
only orbit the nucleus in certain orbits that are fixed
distances from the nucleus.
These are called permitted orbits.
Electron Orbits
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Here’s an example:
The solid circles
represent
permitted orbits.
An electron can
circle the nucleus
in these orbits.
This is the
nucleus
Electron Orbits
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Here’s an example:
An electron could
not orbit in one
of these orbits,
because it is not
a permitted orbit.
This is the
nucleus
Electron Orbits
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Here’s How Electrons Move:
It takes energy
to push an
electron from a
low orbit …
Electron Orbits
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Here’s How Electrons Move:
…into a higher
orbit. The
farther the
electron must
travel, the
more energy is
needed.
Electron Orbits
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Here’s How Electrons Move:
The electron
can also
“jump”
multiple orbits,
if you give it
enough of a
push. For
example, it can
go from here…
Electron Orbits

Here’s How Electrons Move:
…all the way to
here (with
enough energy
for the push).
Electron Orbits
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Here’s How Electrons Move:
Once in a high
orbit, the
electron will
only stay there
for a little
while before it
automatically
jumps into a
lower orbit.
Electron Orbits

Here’s How Electrons Move:
The electron
can jump into
any lower
orbit, but it
must give off
energy to do
so. A bigger
jump gives off
more energy.
Summary

Electrons may only orbit the nucleus in
certain, specific permitted orbits.
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Electrons can jump from one orbit to the next.
An electron has to absorb energy to jump to a
higher orbit.
An electron has to give off energy to jump to a
lower orbit.
Atomic Spectra
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A very common way to
get an electron to
jump to a higher orbit
is to absorb a photon
of light.
But an electron can
only absorb photons
with just the right
energies to put them
in the higher orbits.
Atomic Spectra
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When an electron jumps
from a higher orbit to a
lower one, it gives off
light.
The light’s wavelength
(color) depends on the
exact amount of energy
the electron needs to
give off to make it down
to the lower orbit.
E = hf
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f = frequency (Hz)
h = 6.626x10-34 (Js)
(Plank’s constant)
Atomic Spectra
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Each type of atom
(hydrogen, helium,
oxygen, etc.) has its own,
unique combination of
permitted orbits.

E.g. two hydrogen atoms
have the same permitted
orbits, but the permitted
orbits for a hydrogen atom
are different than for an
oxygen atom.
Atomic Spectra
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This means that each
type of atom absorbs
and emits radiation
only at certain, distinct
wavelengths (colors)!
Emission Spectra
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In a hot, thin gas, atomic collisions “pump”
electrons up into high orbits.
Emission Spectra
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As the electrons jump down into lower orbits, they
give off photons of light at very specific
wavelengths (colors) based on the size of the
jump.
Emission Spectra
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The resulting spectrum looks like narrow colored
lines on a dark background, and is called an
emission spectrum. The pattern of colored lines
is unique to the element.
Absorption Spectra
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A thin (relatively) cool gas has electrons mostly in
lower orbits.
Absorption Spectra
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If you shine a light with all colors (i.e. white light)
through the gas, only photons of the correct colors
will be used to “jump” the electrons to higher
orbits.
Absorption Spectra
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The result is a spectrum that looks like a
“rainbow” with narrow, dark lines located where
the bright lines would be in the atom’s emission
spectrum…
Putting it all Together
“Blackbodies” give off light at all
wavelengths, a continuous spectrum
Putting it all Together
A low-density gas excited to emit light will
do so at specific wavelengths and thus
produce an emission spectrum.
Putting it all Together
If light comprising a continuous spectrum
passes through a cool, low-density gas, the
result will be an absorption spectrum.
Stars Have Absorption Spectra
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The photosphere of a
star is very hot, but it
is much cooler than
the inside of the star.
So most stars (except
the very, very hot or
very, very dense ones)
show absorption
spectra.
Stellar Spectra
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The pattern of dark
lines (called spectral
lines) in the spectrum
tells you about which
gasses are in the
photosphere of the
star.
Spectral Classes
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Hydrogen is the most common element in
the universe – makes up most (75% by
mass) of the material in a star.
The dark lines in the visible light part of the
hydrogen spectrum are created by
electrons jumping from the 2nd orbit out
from the nucleus to higher orbits.
Spectral Classes
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In stars with very cool
photospheres (about
3,000 K), there are very,
very few hydrogen atoms
with electrons that have
been “bumped” by
collisions into the 2nd orbit
out.
So the spectra of these
stars have very weak
(hard to see) hydrogen
lines in visible light.
Spectral Classes
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In fact, red stars have
photospheres so cool,
that molecules can
actually form!
Molecules make the
spectrum look very
complicated by adding
LOTS of dark lines…
Spectral Classes
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The warmer the
photosphere, the more
collisions “bump”
electrons into the 2nd
orbit out, and the
stronger the hydrogen
lines become.
Spectral Classes
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BUT, once the
photosphere reaches
10,000 K, the collisions
become so violent that
they bump electrons totally
off the hydrogen atoms!
A hydrogen atom without
an electron can’t make
absorption lines!
Spectral Classes
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So the hydrogen lines in
the star’s absorption
spectrum become stronger
and stronger, until you get
to stars with photospheres
above 10,000 K.
Then, as the photosphere
gets hotter, the hydrogen
lines rapidly get weaker!
By the time the
temperature hits 40,000 K,
the hydrogen lines are
totally gone!
Spectral Classes
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In the 1900s, Annie
Cannon, a Harvard
Astronomer,
developed a
classification system
for stellar spectra
based on how strong
their hydrogen lines
were.
Spectral Classes
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In Cannon’s system,
spectra with the strongest
hydrogen lines were
classed as “Type A”
Weaker lines were “Type
B”
Weaker still were “Type C”
All the way to “Type O” –
which show no hydrogen
lines.
The Spectral Classification System
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In 1910, Cannon’s system was adopted by
the International Astronomical Union with
only a few changes:
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Several rarely-assigned classes were removed.
The remaining classes were each sub-divided
into 10 sub-classes.
They were rearranged in order of stars with
decreasing surface temperature.
The Spectral Classification System
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The major spectral classes, in order from hottest to coolest
were now:
O
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B
A
F
G
K
M
Each major spectral class is subdivided into ten numbered
sub-classes, going from the hottest (0) to the coolest (9).
For example, class A is sub-divided, from hottest to coolest,
into:
A0 A1 A2 A3 A4 A5 A6 A7 A8 A9
The Spectral Classification System
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Different classes of
stars show different
characteristic sets of
absorption lines.
The Sun is a G2 star.
Mnemonics to
remember the
spectral sequence:
Oh
Oh
Only
Be
Boy,
Bad
A
An
Astronomers
Fine
F
Forget
Girl/Guy,
Grade Generally
Kiss
Kills
Known
Me
Me
Mnemonics
The Spectral Classification System
Modern spectra are usually
recorded digitally and
represented as plots of intensity
vs. wavelength
Stellar Spectra
F
G
K
M
Surface temperature
O
B
A
Spectral Classes
M2 (3,000 K)
A1 (9,800 K)
G2 (5,700 K)
B8 (11,000 K)
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So the spectral class of a star tells you how strong
the hydrogen lines are in the star’s spectrum,
which relates to the surface temperature of the
star.
Other Things We Learn From Spectral
Lines
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If the star (or the star’s surface) is moving
toward or away from us, the star’s spectral
lines undergo a Doppler shift.
Motion measured in this manner is only that
part of the star's motion that is directly
toward or away from the observer (radial
velocity).
Red Shift
Spectral absorption (or emission) lines are seen
to be shifted toward the red end of the
spectrum (red shift) if the motion is away from
the observer.
Blue Shift
Spectral absorption (or emission) lines are seen
to be shifted toward the blue end of the
spectrum (blue shift) if the motion is toward the
observer.
Doppler Shift
Doppler Shift
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Here’s the equation for the Doppler shift:
v / c =  / 
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In this equation:
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v is the speed toward or away from us.
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c is the speed of light (3×108 m/s)
 is how far the spectral line is shifted from where it is
supposed to be.
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It’s positive if away from us.
It’s negative if toward us.
It’s positive for a redshift and negative for a blueshift.
 is the wavelength in the spectrum where the spectral line is
supposed to be.
For Next Time
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Read Units 49 and 51 for Monday.