Stellar Spectroscopy

Goals of the Lab

To observe the spectra of several bright stars, in particular the absorption lines.
To attempt to match spectra you see with the template spectra that define spectral types, in order to identify the spectral types of the observed stars.

Requirements: The telescope, your logbook, a set of template spectra, and the spectroscope. There are pages of template spectra in the lab that the TA's can lend you during lab; you can also find some in Figure 11-5 of Pasachoff & Filippenko. We only have a limted number of spectroscopes, so only a few lab teams may do this lab at once.

Note: This is an advanced lab activity which requires familiarity with the telescope and careful observing. You will want to complete at least the Field of View and Stellar Observation labs to build your telescope experience before attempting this lab.

Contents

A Note about the Spectroscopes

The visual spectroscopes are checked out separately from the telescopes. They are expensive pieces of equipment and we cannot afford to buy more or to replace damaged units. Therefore:

Do not touch the glass surfaces with anything (fingers, cloth, etc.)
Do not drop the spectroscope!
Use the thumb screws sensibly (do not over tighten, loosen/tighten as necessary when moving/securing the spectroscope over the eyepiece)
When not in use, put it back inside its own box, and inside the box of parts.
Check out the spectroscope from a TA, and return it to a TA (who --> will note that you have returned it.

How to Use the Visual Spectroscope

First acquire, center and focus the target star in the field of the 25 mm eyepiece.
Put the visual spectroscope over the eyepiece and secure it in place with the 3 thumb screws. Do not over tighten the screws.
The field of view will now appear slightly off center and will look like in the diagram below. You will need to refocus the telescope so that the star appears as a narrow straight line perpendicular to the length of the spectrum (see diagram). The visual spectroscope contains a special lens which effectively distorts the point like appearance of the star. When out of focus, it will be oval, like a football. It is important to focus correctly as you will not be able to see the spectral features otherwise. Remember, you are trying to focus the brightest off-center orangish feature into a narrow, straight line.
For ease of viewing, you can rotate the visual spectroscope to suit your preference. Just loosen one thumb screw, rotate the spectroscope and tighten the screw. I find that having the spectrum displayed horizontally as shown in the diagram is more comfortable.

The spectroscope generates several new images. The star is now much dimmer and appears as an orange streak. To either side of the star, you will see the colorful "first order" spectrum. One side is closer to the center of the field of view and it is also much brighter, by design. Most of the light of the star has been deflected into one side of the "first order" spectrum, which is why the star looks much dimmer. Further away from the star, you will see a faint, nearly colorless, ghostly streak (at the right edge of the field of view in the diagram). This is the "second order" spectrum of the star. It is the same spectrum, but with twice as much stretching in wavelength. Its colors are washed out or invisible because it is much fainter (an effect of your eye). For the majority of stars, the second order spectrum will be too faint to be of interest. For the brightest stars, however, you may be able to see additional spectral lines which are not discernible in the bright first order spectrum. It is a good idea to also inspect the second order spectrum.

Part I: Observations

Below are 4 tables of bright stars from which you will select your targets for spectroscopic observations. Note that not all stars are visible in a given season/semester. It is recommended that you start with the brightest stars (at the top of each table) which are currently visible in the sky. All of these stars are plotted on the long constellation chart (SC001) which you should use to identify and locate them. You need to observe at least 2 stars from Table 1, 2 stars from Table 2, and 1 star from Table 3. During the Fall semester, you also need to observe the star in Table 4. This is the bare minimum required for this lab and you are encouraged to observe more stars from each list. You can also try your luck on other stars which are not listed (after you have met the above minimum requirement), keeping in mind that fainter stars will be much harder to observe with the spectroscope.

TABLE 1

Star Name	Right Ascension	Declination	Magnitude
Alpha Andromeda (Alpheratz)	0h 08min	+29^o 05'	2.1
Beta Orion (Rigel)	5h 14min	-8^o12'	0.1
Alpha Canis Major (Sirius)	6h 45min	-16^o 43'	-1.5
Alpha Gemini (Castor)	7h 35min	+31^o 53'	1.6
Alpha Leo (Regulus)	10h 08min	+11^o 58'	1.4
Alpha Virgo (Spica)	13h 25min	-11^o 09'	1.0
Alpha Lyra (Vega)	18h 37min	+38^o 47'	0.0
Alpha Aquila (Altair)	19h 51min	+8^o 52'	0.8
Alpha Cygnus (Deneb)	20h 41min	+45^o 16'	1.3
Alpha Pisces Austrinus (Fomalhaut)	22h 58min	-29^o 37'	1.2
Alpha Pegasus (Markab)	23h 05min	+15^o 12'	2.5

TABLE 2

Star Name	Right Ascension	Declination	Magnitude
Beta Andromeda	1h 10min	+35^o 37'	2.1
Alpha Taurus (Aldebaran)	4h 36min	+16^o 30'	0.8
Alpha Orion (Betelgeuse)	5h 55min	+7^o 24'	0.5
Alpha Scorpius (Antares)	16h 29min	-26^o 26'	0.9
Alpha Hercules	17h 15min	+14^o 23'	3.1
Beta Pegasus	23h 04min	+28^o 05'	2.4

TABLE 3

Star Name	Right Ascension	Declination	Magnitude
Beta Cetus (Diphda)	0h 43min	-17^o 59'	2.0
Alpha Aries	2h 07min	+23^o 28'	2.0
Gamma Andromeda	2h 04min	+42^o 20'	2.2
Alpha Auriga (Capella)	5h 17min	+46^o 00'	0.1
Alpha Canis Minor (Procyon)	7h 39min	+5^o 14'	0.4
Beta Gemini (Pollux)	7h 45min	+28^o 01'	1.1
Alpha Bootes (Arcturus)	14h 16min	+19^o 11'	0.0

TABLE 4

Star Name	Right Ascension	Declination	Magnitude
Beta Cygnus (Albireo)	19h 31min	+27^o 58'	3.2

Procedures

Record the date, time, viewing conditions (sky), the name of the star and its color (as viewed through the telescope before you attach the spectroscope).
Observe the star through the 25mm eyepiece without the spectroscope. Make a note in your observing log about any color you can see in the star (is it whitish, reddish, yellowish, etc.).
Put the spectroscope on the 25mm eyepiece. Focus the spectroscope as described above in "How to Use the Visual Spectroscope".
Sketch the spectrum in your observing log.
- The spectrum appears about 4 times longer than it is wide, so start by drawing two parallel lines representing the edges of the spectrum, with a length of about 4 their separation. Write down the color at each end of the spectrum for reference.
- Pencil in the spectral features you see, taking care to draw (or note) their relative darkness, sharpness, width, etc., as well as where they appear in the spectrum (color).
- Supplement your sketch with notes as appropriate. If you are not sure that you are seeing a particular line, you can draw it with a dotted line, or make a note of it.
- Observe the spectrum carefully, taking your time, and sketch the relative positions of the lines and bands as accurately as possible to help you identify them later.

Example

Below is the record (sketch and notes) of an actual observation made with this equipment (try to write more neatly though). You will find addditional examples in the binder kept in the strorage shed of the observing facility.

Note on Beta Cygnus (Albireo): You will notice that this is a binary star with a strong color contrast between the two components. Orient the spectroscope so that the spectra of the two stars are side by side (i.e. with minimal overlap) so you can study them separately. It is unlikely that you will see any spectral line in either star. What you need to pay attention to is the appearance of the continuum spectrum (the colors), and the differences between the two (if you see any).

Part II: Analysis

You can now make sense of your observations, either indoors at a later time (on a cloudy lab night, for example) or at the end of the lab. You will need the stellar spectra chart (ask your TA), which is a check out item and needs to be returned when you are done. The chart contains an article that reviews stellar spectroscopy at the introductory level and it will help to refresh your memory by reading it. You can also find a picture of the spectra of different types of stars in your textbook (see "Requirements" at the top of the lab).

Some neat color figures of stellar spectra and the emission lines of the principle elements can be found at http://www.erols.com/njastro/faas . Click on "Data/Simulation", then "Astronomical spectra on the Web" and then either of "Color plots of the spectra of stars along the main sequence" or "Color plots of the optical emission line spectra of the elements"..

The analysis of your observations is done by comparing them with the color spectra shown at the bottom of the chart.

Questions to Answer

For each star you observed:
- Identify each of the spectral features you have observed (TiO band, H beta line, Ca I line, etc.). If some features cannot be identified (or some appear to be missing), make a note of it and try to find a reason why.
- Determine the approximate spectral type of the star (indicated on the left-hand side of the chart).
Note: It is not hard to look up the "right answer" with information readily available to you. Getting the "right answer" is not what will get you a better grade, but rather the care you took in making the observations, how you analyze them and the conclusion you reach. Imperfect observations are expected and are the norm in real life. So brush away the temptation to "cheat" and do this lab honestly. It will be much more interesting for you and you will learn more.
Do you find a relation between the color of a star (as viewed through the eyepiece without the spectromoter attached) and its spectral type/features?
Based on your observations, can you order the stars you observed in a continuous sequence of spectral types/appearance?
Using the HR diagram on the back of the chart (p50), compare your own determination of the spectral types with the ones determined by astronomers. Do you see differences? Are there some stars where you saw only very weak lines (if any), while other stars of similar type showed spectral features more readily? Can you explain why?
If you observed Beta Cygnus (Albireo), can you explain the difference in color of the two stars from the appearance of their spectra?

Astronomy 103, Summer, 2006