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STELLAR SPECTRA: Amateur Observations
Text - Peter Schlatter
I enjoy the way direct spectroscopy opens one more aspect
of our fascinating universe to the amateur astronomer. I hope
that this article will pique your curiosity about this interesting
field upon which most modern astronomy is based.
CONTENTS
Background
Amateur Equipment
Spectral Classification
How Spectral Lines are Created
Resources and Recommended Books
Recording Observations
Tables
BACKGROUND
Light from a radiating body - such as a star - may be dispersed
into its constituent wavelengths or colours (called a continuous
spectrum or continuum) with a prism or diffraction grating.
The atmospheric temperatures, densities and compositions of
the surface envelope of a star gives rise to various dark and
bright lines and bands within this spectrum. The dark lines are
called absorption spectra. The bright lines are called emission
spectra. These lines allow us to determine basic stellar characteristics,
estimate luminosities, distances,
magnetic fields, rotation speeds, and mass. We classify stars
according to their spectral lines.
It is possible for the amateur to explore this facet of astronomy
directly.
AMATEUR EQUIPMENT
I recently purchased a diffraction grating and a one-way lens
(from Jim Badura of Rainbow Optics of California) to observe absorption
spectra with an amateur telescope. (This product was reviewed
in the October 1995 issue of Sky and Telescope magazine.)
The diffraction grating screws into an eyepiece thread, then
the one-way lens is attached to the top of the eyepiece. On my
10 inch telescope, the diffraction grating produces a continuous
spectrum and, in the case of stars of about 5th magnitude or brighter,
a low dispersion pattern of absorption lines.
SPECTRAL CLASSIFICATION
Due to this past year's weather, I have not had as much opportunity
to become trained in the use of my spectroscope as I would have
liked. I have had time, however, for some initial observations.
The spectra I viewed may be broken down into three categories
- I am using part of a classification system that was originally
developed by Father Angelo Secchi in the
1860s.
TYPE I
The first category of spectra is easy to observe on hot, white
class A main sequence stars. It consists of a series of three
dark, narrow lines in the red (656 nm) - it is interesting to
note that you will be better able to see this line if your eye
is not dark-adapted - green blue (486 nm), and blue violet (434
nm) areas of the visible continuum. These are ,respectively, the
Balmer alpha, beta, and gamma lines that are produced by the hydrogen
atom. They, like all of the absorption features I will be discussing,
arise when electrons in the atoms and molecules in the surface
layers of a star absorb photons from the stellar continuum, which
arises from the interior of the star. Sirius A I V, Vega A 0 V,
Castor AI V and Altair A 7 V are good examples of stars that produce
these lines.
TYPE II
The second category of lines that can be observed is created
by the absorption spectra of more complex neutral atoms. These
lines appear in the cooler yellow and orange stars, the visible
majority of which are class III giants. They constitute a series
of lines running from orange to blue and indicate neutral atoms
such as sodium (589 nm yellow), iron (572 green, 438 blue), and
magnesium (517 green). Called metal lines, they are visible in
late G and early K stars, of which Aldeberan K 5 III, Pollux K
0 III, Capella G 8 III and Arcturus K 2 III are good examples.
TYPE III
The third category constitutes the spectrum of the cool red
giant star of spectral class M. Visible as a series of wide molecular
bands that run the full length of the spectrum, they arise from
the presence of titanium oxide molecules in the distended outer
surfaces of these stars. The bands are very distinct in stars
such as Betelgeuse M Ia, Arcturus M Ib, and Mu Cephi M 2 Ia. You
can also see strong wide bands in the Mira stars. I observed R
Leonis (M 8 IIIe) and O Ceti (M 7 IIIe), and the bands of these
latter two stars wipe out large parts of the continuum.
HOW SPECTRAL LINES ARE CREATED
The various types of spectra arise in the surface layers of
a star's atmosphere and are related to the temperature and density
at the star's surface.
On the surfaces of cool M stars, molecules that have formed
are not broken down as readily as they are in warmer stars. These
molecules will therefore be visible in the spectrum.
As we proceed to warmer stars, the molecules will not so easily
survive the higher temperatures, but heavier atoms will-and we
will see the metal absorption lines of the G and K class star.
This progression carries on into the still hotter A class star
where the lines of the simpler neutral hydrogen atom predominate.
Surface densities cause some line strengths to weaken and others
to increase, depending on how different molecules or atoms react
to pressure and density. These are known as negative and positive
luminosity functions, respectively, and this is a prime method
of distinguishing between luminous giant stars and less luminous
main sequence stars. In denser Class A, main sequence stars, the
lines formed are much stronger than the lines of the supergiants
of the same spectral class. The supergiant stars have distended
surfaces and lower surface density. In a higher density medium
there are more collisions among atoms. This creates an increase
in line strength, a negative luminosity affect (because the more
luminous the star, the weaker the line strength). Recently, I
was able to make a brief observation of the fainter lines of the
A 2 Ia supergiant Deneb. The hydrogen beta line was only faintly
visible, and not nearly as distinct as that of a main sequence
class A star.
The observations I made are preliminary, and are not complete.
I expect to be able to refine and extend them considerably in
the future. (For example, I want to observe carbon stars.)
RESOURCES AND RECOMMENDED BOOKS
To compliment the use of a spectroscope, it is a good idea
to have some basic resources.
The Phillips Colour Star Atlas, and Starlist 2000
by Richard Dibon-Smith, contain information on stellar colours
and luminosities.
For the computer buff, the Guide program, which contains
the Bright Star Catalogue among its numerous databases,
is very good.
I highly recommend Stars and their Spectra, by James Kaler,
as a basic textbook. It is simply written, and contains a wealth
of information-including an excellent overview of all the various
stellar types, their characteristics, and why these arise.
With these resources, you can check observations against the
spectral and luminosity information they contain. This will greatly
facilitate the learning process.
RECORDING OBSERVATIONS
For the present, I am sketching the spectra I observe. I plan
to obtain a CCD camera and hope, by this method, to obtain still
fainter and more detailed spectra. (The use of a CCD to obtain
spectra was discussed in the fall 1995 issue of CCD Astronomy.
This article is also available, at the time of this writing, at
the Sky and Telescope home page.)
Clear skies.
TABLES
(from "Stars and their Spectra" by James B.
Kaler, Cambridge University Press, 1989)
Father Secchi's Classification System
Spectral Classes
Luminosity Classes
Wave Lengths of Colours
FATHER SECCHI'S CLASSIFICATION SYSTEM |
|
CLASS |
CHARACTERISTICS |
TYPE I |
Strong hydrogen lines: blue white stars
like Sirius and Vega |
TYPE II |
Numerous metallic lines (sodium, calcium,
iron), weakened hydrogen: Yellow or orange stars such as the
Sun, Capella, Arcturus |
TYPE III |
Prominent bands of lines, each of which
gets darker towards the blue (titanium oxide). Also metallic
lines of Type II: orange to red stars like Betelgeuse and Antares |
TYPE IV |
Bands that shade in the other direction,
deep red stars of at least magnitude 5: few visible to naked
eye-carbon stars |
TYPE V |
Bright spectrum lines (emission), either
in conjunction with, or instead of, absorption lines: rare (stars
loosing matter into space) |
SPECTRAL CLASSES |
|
|
|
CLASS |
COLOUR |
SPECTRAL FEATURES |
MAIN SEQUENCE SURFACE TEMPERATURE (K) |
O |
BLUE |
strong lines of ionized helium, ionized
metals, weak hydrogen lines |
40 000 |
B |
BLUE |
neutral helium lines, hydrogen lines
stronger |
25 000 |
A |
WHITE |
strong hydrogen lines, ionized calcium |
9 500 |
F |
WHITE |
strong ionized calcium lines, neutral
metals |
7 200 |
G |
YELLOW |
numerous strong ionized calcium lines,
strong neutral metal lines |
5 800 |
K |
ORANGE |
numerous strong lines of neutral metals |
4 900 |
M |
RED |
numerous strong lines of neutral metals,
strong molecular bands |
3 600 |
LUMINOSITY CLASSES |
|
CLASS |
LUMINOSITY CLASS |
0 |
EXTREME LUMINOUS SUPERGIANTS |
Ia |
LUMINOUS SUPERGIANTS |
Ib |
LESS LUMINOUS SUPERGIANTS |
II |
BRIGHT GIANTS |
III |
NORMAL GIANTS |
IV |
SUB GIANTS |
V |
MAIN SEQUENCE (dwarfs) |
VI |
SUB DWARFS (pop II - older main sequence
stars with low metal content: consequently warmer for spectral
class) |
VII |
WHITE DWARFS |
THE WAVE LENGTHS OF COLOURS |
|
WAVE LENGTH |
COLOUR |
360-440 |
VIOLET |
440-495 |
BLUE |
495-580 |
GREEN |
580-600 |
YELLOW |
600-630 |
ORANGE |
630-780 |
RED |
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