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Stellar Spectra Classification Exercise



Introduction

This activity allows you to introduce the classification of stellar spectra
from a few different perspectives. It utilises actual intensity-plot
spectra of stars obtained by the Sloan Digital Sky Survey project accessed
through SkyServer: http://skyserver.sdss.org

Details on spectroscopy including how they are obtained and how stars may
be classified according to their spectra can be found as part of the
Australia Telescope Outreach and Education website section on the
Astrophysics option:
http://outreach.atnf.csiro.au/education/senior/astrophysics/spectroscopytop.
html
There are nine pages focusing on spectroscopy that directly address the
syllabus requirements. They cover historical background, how we obtain
spectra, classification schemes, examples, activities and questions.

Background details on spectroscopy on SkyServer site at:
http://cas.sdss.org/dr3/en/proj/advanced/spectraltypes/

The specific stellar spectra used in this activity are from:
Browse Plate: http://cas.sdss.org/dr3/en/tools/getimg/plate.asp
Plate: 266/51630

The SkyServer website has an online exercise on stellar spectral
classification suitable for HSC students at:
http://cas.sdss.org/dr3/en/proj/advanced/spectraltypes/studentclasses.asp

Notes:
The wavelength unit on the SDSS spectra is ?, the angstrom. 1 ? = 1x10-10m.


Types of spectra:

|Type of spectrum |Photographic example |
|Continuous (or continuum)|[pic] |
|Absorption (dark line) |[pic] |
|Emission (bright line) |[pic] |

The Balmer Series (visible H lines)
|Balmer line |Wavelength |
| |(nm) |
|H? |656.3 |
|H? |486.1 |
|H? |434.1 |
|H? |410.2 |
|H? |397.0 |


Wien's Law and Blackbody Curves

The behaviour of most stars approximates that of black body radiators. This
means that they emit some energy at each wavelength but also have a peak
wavelength and region in which they emit most of their energy.
[pic]
In the early 1890s Wilhelm Wien investigated thermodynamics and coined the
term black body for an ideal radiator. He discovered that the wavelength of
maximum energy emitted from a perfect black body was inversely proportional
to the temperature of the body, ?max is proportional to 1/T. This
relationship is known as Wien's Law in his honour.
[pic]
Equation 1: Wien's Law, where ?max is the peak wavelength, T is the
effective temperature of the black body and W is a constant called Wien's
Constant and has a value of 2.898 x 10-3 for ?max in meters and T in
kelvins.
(Note: This equation is not explicitly required for the NSW HSC course)
Wien's Law is a highly significant relationship. He used it to estimate the
effective temperature of the Sun at 6,000 K which was much closer than the
previous estimate of 10,000 K predicted using the Stefan-Boltzmann law of
radiation. The value of Wien's relationship is that if you can measure the
wavelength of maximum intensity from a spectrum, you can use it to
calculate a value for the effective temperature. let us see how this works
in the example below. Remember, this is an actual stellar spectrum so it
approximates a black body and has absorption lines in it rather than being
a perfect continuum Planck curve.

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A

B

C[pic]

D[pic]

E[pic]

F[pic]

G[pic]

H[pic]

I[pic]

J[pic]

K[pic]

L[pic]

M[pic]

N[pic]