P190 2016 LabIII Spectra Classification

8/18/2019 P190 2016 LabIII Spectra Classification

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Name___________________________________ Student #________________________

Lab Partners _____________________________________________________________

P H Y S I C S 1 9 0 : L A B I I I

S P E C T R A L C L A S S I F I C A T I O N :

L A B O R A T O R Y A N D O B S E R V A T O R Y

Goals

!Use a simple spectrometer to observe sources that illustrate the three categories ofspectra: i) the continuous spectrum produced by an ordinary incandescent filament; ii)the emission spectra produced by three different “mystery” gases, and iii) the absorp-tion spectra in the Sun’s light (assuming reasonably clear skies!).

!Try to identify the mystery gases by comparing your observations of their emissionspectra with a standard laboratory reference of the spectra of many different elements.

!Test drive a simulated spectrometer at the CLEA Virtual Observatory.

Equipment

1) Diffraction-grating and eyeglass spectrometers. 2) “Mystery gas” discharge tubes.

3) Wall chart of the spectral lines of various elements. 4) Computers with CLEA installed.

Background You will observe spectra using a simple diffraction-grating spectrometer, which uses a

transparent plastic sheet (or grating) with finely-spaced parallel grooves. The grating rein-forces the tendency of light to “diffract” (or bend) when going through an opening, causinglight of different wavelengths to emerge at different angles: this enables the identification ofthe wavelength of a particular colour of light from the diffraction angle. The blue-plasticspectrometer allows you to determine spectral line wave lengths. You will also tryout someeyeglasses that are made with diffraction gratings instead of lenses.

The lab has two parts. In Part I, you do “real” laboratory observations, in three sections.In Part II, which has four sections, you will use a simulated spectrometer at the CLEA Virtu-al Observatory to determine the “spectral type” of two stars (one of which is Alcyone, thebrightest star in the Pleiades), and to identify some elements in their atmospheres.

I): Spectra in the Laboratory

Ia) Incandescent Filament 1) Use the spectrometer to view white light from an incandescent filament (as your TA forhelp with these instructions!).

!First, take a look at the source through the eyeglass spectrometer. You will see rain-bows of continuous colour on both sides of the filament.

!Next, take the blue plastic spectrometer and point the entrance slit of the spectrome-ter towards the filament, and look through the viewing hole: when properly lined up,

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the entrance slit will be a bright vertical line on one side, and on the other side you willsee a continuous spectrum of colours.

! Above the spectrum is a scale that gives a physical position along the inside of thespectrometer housing in centimetres, and below the spectrum is a scale that gives thewavelength of the corresponding colour of light in nanometers (nm).

2) Record the approximate spread in wavelength of each colour in the following table.

Note: The spectrometer you are using has been “calibrated” so that the physical loca-tion of a spectral line on the scale can be read off as the wavelength of that colour oflight. Different colours get bent by different amounts when passing through the dif-fraction grating (red bends more than blue, for instance), so the spread across thescale is proportional to the wavelength. The calibration can be done by measuring the

positions of some spectral lines of known wavelength produced by standard sources.

Ib) Emission spectra of three “mystery” gases 1) Use the eyeglass spectrometer to view the discrete spectral lines produced by thethree “mystery gases”.

!The gases are contained inside discharge tubes. An electric voltage applied to thetube induces electric discharges in the gas; these cause electrons in the gas atoms to

jump to higher-energy atomic levels, after which they make transitions to lower levels,with the release of radiation at specific wavelengths characteristic of each gas.

2) Now use the blue plastic spectrometer to record the wavelengths of each source!Make a rough sketch showing the placement of some of the brighter lines in eachspectrum, in the table below, and write down the approximate wavelength next to theline. Represent the brighter spectral lines with thicker lines in your drawing.

Note: The wall chart may show the colours in an orientation opposite to that in thespectrometer (i.e. Violet may be on the left on the wall chart, instead of Red).

(units: nm) R E D O R A N G E Y E L L O W G R E E N B L U E V I O L E T

From:

To:

(units: nm) R E D O R A N G E Y E L L O W G R E E N B L U E V I O L E T

Gas A

Gas B

Gas C

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2) Compare your observations of the three spectra with the standard chart of spectra onthe wall of the laboratory, and try to identify the three mystery gases.! Write your spectral ID of each gas in the table on the next page.! Compare the identifications made by your group with other groups in the lab.! Your TA will tell you what the gases actually are.! Remember, this is a friendly competition ;)!

Ic) Absorption spectrum of the Sun (if clear skies!) 1) Before you go outside to look at the Sun’s reflected light, take another look at the con-tinuous spectrum produced by the filament, so you have a fresh mental image of a fea-tureless spectrum.

2) Look at light reflected off a sunlit concrete wall or a white-cardboard surface to viewthe absorption spectrum of the Sun, using the blue plastic spectrometer,

DO NOT USE EITHER SPECTROMETER TO LOOK DIRECTLY

AT THE SUN!! LOOK ONLY AT REFLECTED SUNLIGHT!! 3) Sketch the absorption pattern that you see in the table below.

!Indicate the positions of some prominent dark lines, and write down their approxi-mate wavelengths in the table.

!You may have to block daylight from your eye in order to see the faint spectrum in-side the spectrometer.

4) Use the list of solar absorption lines printed on the spectrometer to identify some ele-ments in the Sun’s atmosphere from your observations.

!List the elements and their wavelengths below, and describe the appearance of theabsorption lines (i.e., compare the thickness and darkness of the different lines).

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Y O U R I D O F T H E G A S A C T U A L I DGas A

Gas B

Gas C

R E D O R A N G E Y E L L O W G R E E N B L U E V I O L E T

Sun



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Part II) Spectra at the Observatory

Note: CLEA runs only on Windows. The Mac computers in the lab have been installedwith VirtualBox , a Windows emulator. All of the activities are to be carried out withinthe Windows emulation. If the emulator is not already running, launch VirtualBox fromthe Mac desktop, then click the green arrow in the “Virtual Manager” dialog.

IIA) Activate the telescope and slew to Pleiades

1) Double click on the “VIREO” icon on the Windows desktop to launch the observatorysimulator (VIREO=VIRtual Educational Observatory).

2) Login by clicking File>Login: enter the names of the students in your group (leave the

table number blank), click OK, then click Yes to confirm.3) Connect to the remote observatory by clicking on File > Run Exercise > “Classificationof Stellar Spectrum” (3rd from the top)

! A splash screen will appear with the above graphic: it will take a moment for that toclose and for a “VIREO Exercise” control panel to open.

4) In the VIREO Exercise panel, click Telescopes > Optical > Access 0.4 Meter.

! It will take a few moments for a dialog to appear with “You now have control of the0.4m (16”) Telescope”. Click OK.

5) Click the switch to open the dome.

6) With the dome now open, click the button below “Telescope Control Panel”.7) Click Tracking to turn on the telescope’s “clock drive”.

8) Click Slew > Set Coordinates.

! Enter the following Right Ascension (RA) and Declination (Dec):

"RA = 3h 46m 10s.

"Dec = 240 10’ 00”.

! Click OK. Another dialog pops up: click Yes to Confirm the Slew.

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IIB) Activate the Spectrometer and take data

1) Set the View slider to Telescope.

2) Verify that the Instrument slider (right side of dialog) is set to Spectrometer

3) You will see a pair of small red lines, probably somewhat off-centre from the star.

!These lines represent a slit that allows only a narrow beam of light to enter the spec-trometer: the spread of colours produced by different points in a beam of light overlap,

and if the beam is too wide the colours will be blended back to white light.!Centre the slit on the star.

4) Click the Access button below the Instrument slide to open the spectrometer controlpanel (the VIREO Reticon Spectrometer Reading window).

! The panel displays an object number for Alcyone - Object: N2230-02202.

5) Record the apparent magnitude of the star, shown at the bottom of the panel:

! App. Mag (V) = ____________.

6) Click “Go” to start the spectrometer.

! You can think of the spectrometer as a camera that takes an image of the spectrum

produced by a diffraction grating similar to the one you used in the lab.! For fainter stars, a longer exposure is needed to build up a good spectral image.

! Alcyone is relatively bright, and a few seconds of exposure are enough. The spec-trometer will stop automatically after about 10 seconds.

7) Click File > Data > Save Spectrum.

! CLEA will automatically set the filename to the object number: click Save.

! Close the spectrometer window: File > Exit Spectrometer

8) Alycone is a blue star, so let’s slew to a nearby red one for comparison.

!

There is red star near the cluster centre, though it is much fainter than Alcyone.!It has a catalogue number of HIP 18508, and is located at:

"RA = 3h 57m 26s and Dec=240 27’ 43”

!Instead of using the automatic slew, try using the manual slew to find it, for more fun!

!Switch to the Finder View (you won’t be able to do that if the Spectrometer window isstill open), and use the NSEW buttons to move the telescope, watching the RA andDec indicators in order to slew the telescope in the right direction.

!When you are roughly centred on the star (which is quite faint, with only a few starsof similar brightness seen in the Finder), switch to the Telescope View, and accurately

centre the spectrometer slit on the star.9) Open the spectrometer again, which should display Object: N2230-00176.

!Record the star’s apparent magnitude: _____________.

10) Start the spectrometer, and since this star is so faint, let it run for about 20 seconds.!Notice how the data points fluctuate noticeably at the beginning of the exposure,since the image starts off so faint. As the camera exposure increases, the image be-comes brighter, and the recorded spectrum becomes more stable.

11) Click File > Data > Save Spectrum to save the data, and File > Exit Spectrometer.



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IIC) Analyze the data

1) In the VIREO Exercise panel, click Tools > Spectral Classification.

2) The Classify Spectra window opens. Click File > Unknown Spectra > Saved Spectra,and select the file for Alcyone that you saved before (filename N2230-02202).

3) In the same window, Click File > Atlas of Standard Spectra, and in the pop-up windowdouble click on “Main Sequence”.

!Three graphs (called traces) are displayed: the spectral reading from your file isshown in the middle (labelled “Unknown”), and comparison spectra for two standardstars in the top and bottom panels.

4) Look at the teal-colored panel in the upper-right of the window: this allows you to

change the comparison spectra, according to a standard labeling of star types that em-ploys a letter followed by a number (e.g. O5).

!We’ll cover stellar classifications in detail in the lectures, but here’s a summary.

! Astronomers have developed an alphabetical labeling scheme that has ended upout of order for historical reasons.

!The sequence runs from hotter star to cooler stars using the following letters:OBAFGKM. A popular nemonic to remember the order is “Oh Be A Fine Girl/Guy KissMe” (yes, once upon a time, most astronomers were male, and immature).

!The letters are linked to colour, and therefore to temperature. Some examples: “O”

stars are blue, “B” stars are blue-white, “G” stars are yellow like our sun, and “M” starsare red.

5) Click on the different star types in the teal panel, and look at the traces to find the onethat most closely matches the trace of Alcyone.

!You may find it helpful to change the display to show the photograph of the spectrumthat was captured by the camera (in greyscale).

"To do this, click File > Display > Comb. (Photo plus Trace).

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! Another very useful setting is File > Display > Intensity Trace, followed by File > Dis-play > Show Difference (Std minus Unkn).

"Try to find the spectral type that makes the difference trace comes out as closeto the horizontal (zero) line as possible.

!Record your best estimate of the classification of Alcyone: ______________.

6) Try to identify some elements in the atmosphere of Alcyone.

!First, in the Classify Spectra window, click File > Spectral Line Table, which causes apop-up window to appear, with an extensive list of spectral lines for many elements.

"The tables gives wavelengths in “Angstroms” (Å): 1 Å = 10-10 m = 10 nm.

!Click on the graph for Alcyone (“Unknown”), near the bottom of a prominent dip inthe spectrum: the Spectral Lines window will select the closest matching line.

!See if you can find lines for three different elements (remember that each elementhas multiple spectral lines). Record the results in the table below.

7) Repeat the procedure in step 5) to make your best estimate of the classification of thered star HIP 18508 (filename N2230-00176).

!Record your answer for the spectral type of HIP 18508 here: ________________.8) Repeat the procedure in step 6) to identify some spectral lines in HIP 18508, andrecord the results in the table.

IID) Close the observatory

1) Close the Classify Spectra window.

2) In the VIREO Exercise panel, turn off the Telescope Control Panel, then click theswitch to close the dome.

3) Click File > Exit Observatory, and confirm in the popup dialog.

W A V E -L E N G T H ( Å )

E L E M E N T W A V E L E N G T H ( Å )

E L E M E N T

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P190 2016 LabIII Spectra Classification