Redshift

In this lab, you will investigate the redshifts of stars and galaxies. Your book describes redshift in several places, but for your reference it is defined as:

z = (λobserved -λrest) / λrest

This quantity is called redshift, and has already been found for all SDSS spectra. Although you don't have to calculate the redshift for your objects, I will ask you to calculate the velocity that would produce the observed redshift. You can do this with the equation for the Doppler effect:

cz = v

where c is the speed of light (lets all use c=3.00 x 105 km/s), z is the redshift, and v is the velocity. You'll investigate redshift by collecting some stars and galaxies that have spectra.

• 1. Navigate to https://dr18.sdss.org/optical/plate/search and click "Search" in the search box in order to load a list of over 6000 "Plates". Each plate is unique and was made to collect spectra for a specific part of the sky. I wasn't able to make the search box work, so pick any plate and click the blue "eye" symbol in the last column. This loads the "Spectrum Search" page for the plate you chose. 

  2.  The "Spectrum Search" page lists the hundreds of objects that this plate targeted for spectra. The columns we'll use in this lab are the redshift ("z"), class (galaxy, qso, or star), and the last column, which links to an information page for each specific object.

  3. Load a collection of stars in redshift order by first--in the search box--choosing the class "STAR" and clicking "Search". When the table loads, click the up-down arrows on the redshift column to sort it whichever way you like.

  4. For both the lowest and highest redshift stars in the table, record their redshift, and click the blue symbol in the last ("CAS") column to open the star's information page. Save the thumbnail image to your document

  5. In your document, label the thumbnail images of the stars with their redshift and velocity (use the redshift equation, v = cz, as explained above).

  6. Do the same for galaxies by selecting only GALAXY in the search box (you will have to click on STAR to unselect it) and searching again. Sort by redshift and copy thumbnail images and redshifts for the lowest and highest redshift galaxies into your document.

  7. Now create a page that includes:

     1. The thumbnail images of the lowest and highest redshift stars from your sample, labelled with their redshift and velocity (use the redshift equation, v = cz, as explained above).

     2. The thumbnail images of the lowest and highest redshift galaxies from your sample, labelled with their redshift.

     3. A short paragraph answering the following questions:

        • What is the average redshift for the two stars you found, and for the two galaxies?

        • You likely found a significant difference between the average redshift of the stars and the galaxies. Does it make sense? Explain.

        • For the galaxy with the greatest redshift, calculate its velocity (i.e. it's speed and direction). Considering the discussion in 17.1, is this galaxy actually moving through space this quickly? Why or why not?

You're welcome to use any tool that can arrange text on a page, but please submit something Canvas can open (Microsoft Word, Powerpoint, or a pdf).

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