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# TAP 704- 5 Red shift

By Leroy Mills,2014-05-07 13:59
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TAP 704- 5 Red shift

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?).

-101A? = 1x10 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

? click ‘Set Coordinates’ and type in its RA and dec or

? 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.

8-1Speed of light, c = 3.00 x 10 ms.

-1-1Hubble constant, H = 75kms Mpc. 0

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??em

where ?????? and λ is the emitted wavelength and λ the wavelength received by emrecrecem

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.

Hd0To find the distance, d, use [Think carefully about the units of each quantity] ?zcRecord your values of z and d in your table of galaxies.

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://www.gettysburg.edu/academics/physics/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

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