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9.7 Option – Astrophysics: 3. Spectroscopy

Syllabus reference (October 2002 version)
3. Spectroscopy is a vital tool for astronomers and provides a wealth of information
Students learn to: Students:

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

Prior learning:
Preliminary module 8.2 The World Communicates
Preliminary module 8.5 The Cosmic Engine

Background:
Spectroscopy is the analysis of spectra. There are three types of spectra – continuous, emission (bright lines) or absorption (black lines.)

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perform a first-hand investigation to examine a variety of spectra produced by discharge tubes, reflected sunlight, or incandescent filaments

Sample Procedure

Safety note: Do not look directly at the sun with your eyes or through any optical device unless it has been designated as safe to use for this purpose. Keep a safe working distance from any high voltage or spark discharge apparatus.

Choose a variety of light sources for this experiment. Include light from an incandescent lamp (i.e., one with a filament), a fluorescent tube, a flame, light seen through various coloured solutions, and various discharge tubes as commonly found in school science laboratories. Since you should not point the spectroscope directly at the Sun, you could look at the sunlight reflected from a sheet of white paper or card. Comment on what effect this might have on the accuracy or validity of what you observe.

Examine the light from each source using a spectroscope. You should describe and draw each spectrum, indicating observable features of the spectrum. If it is possible to vary the voltage of the incandescent lamp, examine the light from this source at various power levels, commenting on both the change in brightness and range of colours observed. As an extension, you could use the scale in the spectrometer to observe and record the wavelength of features of each spectrum.

Group the spectra you have observed according to whether they are continuous spectra, line spectra, absorption spectra or a combination of these. Describe the common features of each type of spectrum.

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account for the production of emission and absorption spectra and compare these with a continuous blackbody spectrum

An alternative way of comparing the three types of spectrum is by using a table:

Continuous black body spectrum Emission spectrum Absorption spectrum

consists of a continuous range of frequencies without either bright or dark lines, appearing as a continuous range of colours

consists only of radiation at a number of discrete wavelengths, appearing as bright lines against a dark background

consists of a continuous range of wavelengths with discrete gaps at particular wavelengths, appearing as dark lines against a continuous background of colours

given off by hot solids, liquids and high pressure gases

produced by hot diffuse gases

produced when a continuous spectrum of light passes through a cloud of cool gas

all wavelengths are produced at some intensity

wavelengths produced depend on possible energy transitions within atoms of the gas

wavelengths absorbed depend on possible energy transitions within atoms of the gas

intensity varies smoothly with wavelength, with the maximum depending on the temperature of the hot

intensity of lines varies discretely with each wavelength, depending on the composition of the gas

darkness of lines varies discretely with each wavelength, depending on the composition and density of the gas

on blackbody emission, that allows you to specify different values of surface temperature, can be found at: Continuous Emission and Absorption Spectra (external website) Center for Astrophysics, Harvard Smithsonian, USA.

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describe the technology needed to measure astronomical spectra

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identify the general types of spectra produced by stars, emission nebulae, galaxies and quasars

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describe the key features of stellar spectra and describe how these are used to classify stars

Table of spectral classes of visible stars

Spectral Class Colour Surface temperature (K) Elements evident in absorption lines
O blue over 30 000 ionised He, weak H
B blue-white 30 000 – 15 000 neutral He, weak H
A white 15 000 – 10 000 strong H
F white-yellow 10 000 – 7 000 weak H, metals (Ca, Fe)
G yellow 7000 - 5000 strong metals, esp. Ca
K orange 5000 - 4000 strong metals; CH and CN
M red 4000 - 3000 strong molecules (incl. TiO)
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describe how spectra can provide information on surface temperature, rotational and translational velocity, density and chemical composition of stars

Depiction of Translational Velocity

The component of the translational velocity of a star perpendicular to the observer’s line of sight cannot be determined from the star’s spectrum, but only photographically over a lengthy period of time.

Depiction of chemical composition
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analyse information to predict the surface temperature of a star from its intensity/wavelength graph

Equation for Wein's Law

where (W = 2.89 x 10-3 m.K). Rearrange the equation so that T is the subject of the equation, then substitute in the value of wavelength from the graph.

Example:

The intensity/wavelength graph of a star is given below. Use this curve to predict the star’s surface temperature.

Graph showing typical Black Body curve

Solution:

From the curve the wavelength at which the intensity is a maximum is approximately 5.8 x 10-7 m. The star’s surface temperature is then 5000 degrees from substitution into the equation:

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