TAP 704-5: Red shift The CLEA software enables you to simulate controlling a telescope so that it points at a selected galaxy, and then using a spectrometer to record the light received over a range of wavelengths. From this spectrum you can measure the observed wavelengths of one or more absorption lines in the galaxy s spectrum. You are then able to calculate each galaxy s red shift, z, and use Hubble s law to calculate its distance. If you have collected data for several galaxies, a spreadsheet will speed up this calculation. Finally, you can plot a two-dimensional chart showing the distances of galaxies that lie in various directions from Earth. This chart will show you how the galaxies are distributed in space. In order to obtain and plot data on a large number of galaxies, collaborate with other students and arrange that you each select different galaxies. Then pool your results to make a combined plot. You will need a computer running the CLEA activity The Large Scale Structure of the Universe. a computer running a spreadsheet program Specialist terms The CLEA software uses some terms and units that may be unfamiliar. Wavelengths are expressed in units of angstroms (A ). 1A = 1x10-10 m, so 10A =1 nm. For example, a particular shade of blue light might have a wavelength λ = 4500A = 450nm. Astronomical positions are expressed in right ascension (RA) and declination (dec). Declination is equivalent to latitude, and runs from +90 (above the Earth s North Pole) to -90 (above the Earth s South Pole). Fractions of degrees are usually expressed in arcmin and arcsec. Right ascension is equivalent to longitude, but is expressed in units of time rather than angle it is related to the time taken for the sky to appear to move relative to a fixed reference direction. An apparent rotation of 360 takes 24 hours, so each hour of RA is equivalent to 15 of longitude. Fractions of hours are usually expressed in minutes and seconds of time. All the galaxies used in this activity lie within a fairly small slice of the sky, covering a range of 60 in RA and 5 in dec
Galaxies are generally referred to by their numbers in the NGC or IC catalogues (two large comprehensive catalogues giving the positions of thousands of galaxies plus various other items of data), or by two four-digit numbers that indicate their approximate RA and dec. For example, the galaxy 1245 + 2715 has an RA close to 12 h 45 min and a dec close to +27 15. The spectra of galaxies all have two very noticeable absorption lines, known as the calcium H and K lines. (They are produced by calcium in the cool outer layers of stars.) What to do Using the CLEA software Preparation You will need to use Table 2 from the CLEA student manual for the activity The Large Scale Structure of the Universe. This table lists over 200 galaxies with their RA and dec. There are also two blank columns headed mag and radial velocity. Change the headings of these columns and use them to record the red shift, z, and distance, d, for each galaxy that you study. (The units of d should be megaparsecs, Mpc.) Load the CLEA activity The Large Scale Structure of the Universe. From the File menu, select Run. From the Telescope menu select the 1.0 or 0.9 m telescope. Locating a galaxy The screen shows a control panel and an image of the inside of a telescope dome. You begin by simulating the use of a small telescope (a finder ) to point the main telescope towards a chosen galaxy. Click Dome to open the shutters and reveal a view of the sky. Click Tracking to ensure that the telescope keeps pointing at your chosen galaxy as the Earth rotates. Choose a galaxy from the CLEA table (Table 2 in the student manual for this activity). Either
or click Set Coordinates and type in its RA and dec click Tracking and use the N, S, E, W buttons to bring your chosen galaxy into view. You have now made the telescope point at your chosen galaxy and you should see its image in the red square in the centre of the sky image similar to that below: Obtaining data The next stage is to use the main telescope to make detailed observations of your galaxy. Click Change View to switch from the finder to the main telescope. You will see a larger image of you galaxy, with red lines indicating the position of a spectrometer slit. You may need to use the N, S, E, W buttons to align the slit with the centre of the image
You are now ready to collect data. Click on Take Reading. Click on Start Count. The screen will show you a graph of intensity against wavelength that builds up gradually as light enters the telescope. Allow the data to accumulate until the signal-to-noise ratio is at least 20 and you can clearly see dips in the collection of points on the graph. Click stop count. The software will then average the points and produce a line graph
You should be able to identify the calcium H and K absorption lines quite close together at the short-wavelength end of the spectrum. Move the cursor on to the lowest point of one of the lines and click the left mouse button. The wavelength will appear on the screen. Record its value. Repeat for the other calcium line. Do NOT use the Record/Measure facility on the screen. Returning to the beginning To go back to the telescope control panel, click Return. A message will appear, warning that you have not used the on screen facility to record the spectrum. Click OK. You should now see the original telescope control panel. Repeat the procedure for other galaxies selected from the CLEA table. Processing the data In addition to the wavelength measurements you have obtained from CLEA, you will need to use the following information. In the laboratory, the wavelengths of the calcium H and K lines are 397.0nm and 393.3 nm. Speed of light, c = 3.00 x 10 8 ms -1. Hubble constant, H 0 = 75kms -1 Mpc -1. If you have measured several galaxies, set up a spreadsheet to calculate their red shifts and hence their distances. To calculate the red shift z use z = Δλ λ em where Δ λ = λ rec λem and λ em is the emitted wavelength and λ rec the wavelength received by the telescope. If you have measured the wavelengths of two lines, you will obtain two values that can be averaged to get a best value for red shift. H 0d To find the distance, d, use z = [Think carefully about the units of each quantity] c Record your values of z and d in your table of galaxies.
Practical advice Students use the CLEA software to simulate the collection of (real) spectral-line data and hence relate red shift to distance. This works well as a collaborative activity. Analysing all the galaxies listed would be a long and tedious task. The Activity Sheet guides students through their use of the simulations, in which they obtain measurements of wavelength and then use a spreadsheet to calculate red shifts and distances. Note that students should not use the in-built calculation facility provided by CLEA, as this takes a somewhat different approach to analysing the data. Hubble and the galaxies The purpose of this is to show students that it is only since the 1920s that people realised that there were other galaxies like ours beyond the Milky Way. This is partly because telescopes and instruments were getting better, and partly because no-one had previously made and interpreted the relevant observations. It was only with the discovery of Cepheid variable stars (and the 1912 work of Henrietta Leavitt that linked the Cepheid period of pulsation and luminosity.) that distances beyond the Milky Way galaxy could be estimated. Students may wonder if, when spectral lines are always red shifted, how astronomers know the lab wavelengths of the lines. They know this because all stars and galaxies contain the same elements, each with their own pattern of radiation, and always with the same ratios between the wavelengths even when they are shifted and the wavelengths have been accurately measured in the laboratory. The Doppler effect is invoked as an explanation for the red shift observed in the radiation from other galaxies though the explanation for red shifted galaxies should properly be called cosmological red shift it is not actually a Doppler effect, though the end result is the same. Alternative approaches For an alternative or extension to this activity, students will also need Internet access. They will also need a copy of Table 2 from the CLEA student manual for this activity. (The table is called Target galaxies in the PC red shift survey and is a long table containing over 200 entries.) In addition to using the CLEA software, or as an alternative, students could use an applet: http://jersey.uoregon.edu/hubble/hubble_plugin.html The following site has background information http://skolor.nacka.se/samskolan/eaae/summerschools/hubble.html and data enabling the Hubble constant to be deduced. Technician s note CLEA Software Contemporary Laboratory Experiences in Astronomy (CLEA) software is needed. You will also need PKUNZIP, or its equivalent, and an Acrobat reader. Your school/college computer services unit should have these available for your use should you not have them. The CLEA software needs to be downloaded from the Internet by going first to the CLEA homepage at http://www3.gettysburg.edu/~marschal/clea/cleahome.html. Click on the icon Software scroll down the page and follow the Hubble link. The manual is a pdf file.
External reference This activity is taken from Salters Horners Advanced Physics STA activity 29