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Supports PE HS-ESS1-2 : Earth’s Place in the Universe
Supports DCI ESS1.A: The Universe and Its Stars, PS4.A Wave Properties, PS4.B: Electromagnetic Radiation
Engages in SEP 4:Analyzing and interpreting data and CCC 1:Patterns, 6:Structure and function
The motion and make up of stars and galaxies provide evidence for the Big Bang theory.


Required: Pre-Flight Training – Spectra: SDSS Spectrum Graphs
Recommended: Pre-Flight Training – Redshift: The Basics

 

With some basic understanding about redshift and the tool of the SDSS spectrum graph in hand, you are prepared to explore how redshift is measured and how it can be used. We will need lots of spectra.

Accessing Data through the Science Archive Server (SAS)

If you want to look at a lot of spectra at one time, the easiest place to access them is through the Science Archive Server (SAS). SAS is the latest image and spectrum service for the SDSS. But before you can use SAS, you need a starting place.
beaker smallSpace is vast and infinite, and with 500 million stars in the SDSS database, there are enough stars to go around for all researchers. Everyone should be able to start in a unique location, or Special Place (as you proceed in your experimentation and data collection, you will be able to build upon your knowledge base for your Special Place since you will continue to revisit it in other activities). If you completed the activity, My Special Place in the Database, you already have a set of coordinates as your own Special Place. Use the RA and Dec that you recorded during that activity. If you do not yet have your own Special Place, you will use RA and Dec to define a starting place (which will then become your Special Place, your very own “little plot of space real estate” to explore evermore).
If you already have a Special Place in the Database, great. Start there. If you don’t or need additional help to locate a starting place, you can choose one using the Constellations Notebook.

I have a starting place Use constellations notebook

Working in SAS

When you arrive at an SAS Spectra List page, bookmark the location. Next, notice that each row represents a different object that was captured by the spectrograph. The Survey, Plate ID, and MJD columns are identical. This set of data was gathered under the same observing goals (Survey) using the same spectroscopic plate (Plate) on the same day (MJD). It isn’t until you get to the fourth column (Fiber #) that the information becomes unique. Scroll down and notice that there are either 640 or 1000 objects in the list depending upon the survey. You can reorder any column by clicking the up-down arrows on the column heading.

This example of an SAS Spectra List shows 13 column headers and 6 rows of example data. It can be seen that the data in each of the first three columns is identical as all data is from the same plate. As described in the instructions, the data has been limited by entering the greater than sign followed by the number 15 into the box below the Signal to Noise ratio column header and the word STAR into the box below the Class column header. The Redshift column header is circled in red and an enlarged inset box is used to emphasize using the up and down arrows to sort this column by clicking the header.

 

beaker smallLaunching into an exploration of redshift requires that we observe several different types of spectra. We can use the top row on the spectra list in SAS to sort the columns using a variety of commands. If you are familiar with SAS, sort your table to display stellar spectra with a signal-to-noise ratio greater than 15, ordered by increasing redshift. Follow the instructions below if you need help.

 

  • Begin at your starting place in SAS.
  • Select objects that have less jagged spectral graphs by typing “>15” in the signal-to-noise ratio column – r(s/N)2.
  • Narrow your table to display only stars by typing STAR in the Class column.
  • Click the Redshift column header to sort the column in ascending order.
Note the range of redshifts in the table.

 

What does zero redshift look like?

With your collection of stellar spectra at hand, locate a star with zero redshift. Clicking the plot link (or anywhere in the row, for that matter) opens the interactive spectrum tool. There can be a lot of different shapes for the continuum of a star. This activity is not concerned with the shape of the graphed line, so it doesn’t matter which stellar spectrum you pick. For now, focus on the bumps and dips (absorption and emission lines). Recall that the graph is the combination of the individual contributions of light from many different kinds of atoms.

 The colorful hydrogen absorption spectrum ranging from violet to red is shown directly above the color lines of the hydrogen emission spectrum. The dark lines on the absorption spectrum line up with the color lines on the emission spectrum. Dark lines correspond to dips on a spectrum plot while color lines correspond to peaks. An x axis is shown spanning 400 to 700 nanometers with an arrow calling attention to the H alpha line at 656 nanometers.

 This example of the spectral plot for a chosen star is shown alongside a table of Line Measurement Information. Both the plot and the table are found by clicking on the plot link in the SAS Spectra List. The Hydrogen Emission Spectrum is superimposed across the bottom of the plot so that corresponding H alpha, H beta and H gamma line up. An arrow is used to connect the location of these three lines on the graphic to the row that displays data for each in the table.

beaker smallWe begin by focusing on the pattern of light produced by hydrogen atoms (see image on left). Take note of any patterns you see or other features you are curious about. It’s likely there is an activity or resource in Voyages to help you explore further.
With your zero redshift spectrum in view, investigate the location of some of the absorption lines on your graph. The basic steps are listed below. Use the image below for a little help. For a lot more help, try the video, SAS Interactive Spectrum.
  • Locate the wavelength of the Hydrogen Alpha line on the Line Measurement Information table in the Rest Wavelength column. Record that number.
  • Locate the same line on the graph. If the redshift is zero, it should be in the same location reported. Later, this will not be as easy, so practice now. Do the numbers match exactly? Describe what you observe.

Find Redshifted Spectral Lines

beaker smallAstronomers determine redshift by locating patterns in the absorption or emission lines in a spectrum. They determine the amount the pattern is shifted from the standard that is produced in the laboratory as seen in the image on the right.
  • Open a spectrum in the interactive tool.
  • Locate the familiar hydrogen alpha line. Record its location on the x-axis.
  • Observe several different galaxies with different redshifts. What else do you notice about the list of spectral lines recorded for each galaxy?

 The idea of redshift is demonstrated by showing the absorption spectrum of a stationary object above an absorption spectrum of the same type of object that is moving away from the observer. The dark absorption lines of the redshifted object are shifted to the right, or redder, end of the spectrum.

Remember, the range of wavelengths your eye sees remains the same. Only the position of individual spectral lines changes.

 These two images show spectral plots of a galaxy on the left and a star on the right. A continuous emission spectrum is superimposed towards the bottom of both plots ranging from violet at 4000 angstroms to red at 7000 Angstroms. This emphasizes the resting location of the colors of the spectrum. An emission spectrum showing H alpha and H beta emission lines are superimposed on each plot, shifted to match the actual location of these lines on the graph. It can be seen that the spectral lines of the galaxy plot on the left have been shifted to the right, towards the red end of the spectrum. It can also be see that the spectral lines of the star plot on the right have been shifted to the left towards the blue end of the spectrum.

graph-22

Calculate Redshift

beaker smallWe use the observed position of a known absorption or emission line and the position where we would expect to find the feature with no redshift (rest wavelength in SDSS) to calculate a value for redshift that can be compared.
z = (?observed – ?rest) / ?rest
Using what you know about SAS spectrum plots, demonstrate this calculation for one galaxy.

The SDSS Filters and Redshift

Just as the portion of the electromagnetic spectrum that our eyes see does not change, so are the SDSS filters fixed with respect to the spectrum. However, the amount (flux) of light that is captured in each of the filters does change with increased redshift. The simulation below demonstrates this. Observe the changes in the continuum of the spectrum as well as the nature of the light transmitted by each filter. Record these observations.