Download Spectroscopy – the study of the colors of light (the spectrum) given

Spectroscopy –
the study of the colors of light
(the spectrum) given off by
luminous objects. Stars have
absorption lines at different
wavelengths where the energy
is precisely correct to excite
the electrons to a new level.
Different elements show
different absorption lines,
so the composition can
be determined by the
spectrum of the light
produced.
However, differences in
the absorption spectra
of different stars is not
due to differences
in composition,
but due to differences
in temperature.
Stars over 25,000K show
intense lines of singly-ionized
helium and multiply-ionized
heavier elements (O, N, Si).
There are no Hydrogen lines
because the hydrogen is
mostly ionized, so no lines
due to excited electrons.
Only blue stars
are hot enough to
completely ionize
hydrogen.
Stars about 10,000K show
mostly H-lines. These
lines are produced by
electrons moving between
the 2nd and 3rd orbitals
eliminating those
wavelengths.
Stars about 10,000K show
no He, O, or N lines
because these electrons
are too tightly held in their
orbitals. Calcium and
Titanium lines are common
because they easily lose
their electrons.
Stars about 6000K,
like our Sun, have few
strong lines of ionized
elements; the elements
are too cool to ionize.
They have few H-lines.
Stars about 3000K,
red stars, have weak
H-lines, show weak
lines for neutral heavy
atoms. No lines are
seen from ionized
elements.
Spectral Classifications
Early researchers
designed a scheme of
classification based on
the spectra of stars.
At this time atomic theory
was lacking so all the lines
were not understood. (Most
importantly, the absence of
H-lines was not understood.)
It was believed that the
abundance of hydrogen
varied from lots to none.
Stars were classified by
Hydrogen-line intensity.
They used a system of
letters from A through P,
thinking A had more
hydrogen than P due to
the strength of the H-line.
The abundance of Hydrogen
is actually similar for all stars,
the different intensities of
H-lines from one star to
another is due to differences
in temperature causing
different levels of ionization.
Stars are more meaningfully
classified by surface
temperature. So, the A to P
classes were realigned by
temperature. The result is the
spectral classes: OBAFGKM
(the other letters have
been dropped from usage)
In this system, O is the
hottest type of star, M is
the coolest. A mnemonic
device for remembering
these classes is:
Oh, Be A Fine Guy, Kiss Me!
Each letter is
subdivided into
subclassifications,
0 through 9.
The lower the number,
the hotter the star.
Using this system
our Sun is a G2
(cooler than a G1,
hotter than a G3).
The star Vega is a A0.
Barnard’s Star is a M5.
Betelgeuse is a M2.
Spectral
Class
Surface
Temp
Prominent
Absorption
Lines
O
30000K
Strong Ionized He
Multiply-ionized Heavies
Faint Hydrogen lines
B
20000K
Neutral He Moderate
Singly-ionized Heavies
Hydrogen lines moderate
A
F
10000K
8000K
Faint Neutral He
Singly-ionized Heavies
Hydrogen lines strong
Example
Rigel (B8)
Vega (A0)
Sirius (A1)
Singly-ionized Heavies
Neutral metals
Canopus (F0)
Hydrogen lines moderate
Spectral
Class
G
Surface
Temp
6000K
Prominent
Absorption
Lines
Singly-ionized Heavies
Neutral metals
Hydrogen lines moderate
Example
Sun (G2)
Alpha
Centauri (G2)
K
4000K
Singly-ionized Heavies
Neutral Metals Strong
Arcturus (K2)
Hydrogen Faint
Aldeberan (K5)
M
3000K
Neutral Atoms strong
Molecules moderate
Hydrogen-very faint
Betelgeuse
(M2)
Barnard’s Star
(M5)
To construct a
Hertzsprung-Russell
diagram astronomers
must first find the star’s
surface temperature.
This can be done using a
Plank curve or the
spectrum.
Second , the luminosity
must be found. Finding the
luminosity is either: easy, if the
distance to the star and the
apparent brightness are known
they can be used to find the
luminosity from the inverse
square law; or it is impossible,
if the distance isn’t known.
The Main Sequence - The stars
are not evenly distributed on a
Hertzsprung-Russell diagram.
Most of the stars range from
high temperature and high
luminosity to low temperature
and low luminosity. (In other
words, cool stars are faint and
hot stars are bright, duh.)
Hot stars tend to be larger,
cooler stars tend to be smaller.
Large, hot, bright stars are
found to the upper left.
These are blue giants and
blue supergiants. Small, cool,
faint stars are to the lower
right. These are the red
dwarfs. Our Sun is in the
middle of the range.
HR diagrams are biased
in favor of blue giants
(so much easier to see)
and against red dwarfs
(hard to see). Actually red
dwarfs make up
approximately 80% of all
the stars in the galaxy.
Most stars lie on the
main sequence, but
there are some
notable exceptions:
the red giants and
the white dwarfs.
While red giants are rare,
they are very visible.
The distribution of types of
known stars:
90% of all stars are on the
main sequence, 9% of all
stars are white dwarfs, 1% of
all stars are red giants.
The main sequence can be
used to find the distance to
a star using the apparent
brightness and the temperature
(color). This use of a
Hertzsprung-Russell diagram to
find the distance of very distant
stars is called spectroscopic
parallax.
Luminosity is
comparable to absolute
brightness; this is
compared to the
apparent brightness to
find the distance by the
inverse square rule.
Spectroscopic parallax
depends on the
Principle of Mediocrity;
we assume that distant
stars are similar to
nearby stars.
Spectroscopic parallax is
simple to use if the star is
on the main sequence.
Fortunately, 90% of all
stars lie on the main
sequence. But, what if the
star is not on the main
sequence?
The width of the spectral line
seen in the spectra of stars is
determined by the density of
the gas producing the light.
The densities of these gases
is less for a red giant and
more for a white dwarf.
This lets astronomers using
spectroscopic parallax to
distinguish between red giants
and red dwarfs and between
white dwarfs and white giants.
Therefore the distance to the
other 10% of stars not on the
main sequence can be found.
Another way to distinguish types
of stars is by luminosity class:
Ia - bright supergiants
Ib - supergiants
II - bright giants
III - giants
IV- subgiants
V- main sequence dwarfs
VI - sub dwarfs