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# Distances to Galaxies

#### Click to view NGSS connections

Supports PE HS-ESS1-2 : Earth’s Place in the Universe
Supports DCI ESS1.A: The Universe and Its Stars, PS4.A Wave Properties
Engages in SEP 1:Asking questions, 3:Planning and carrying out investigations, 4:Analyzing and interpreting data,5:Using mathematics and computational thinking, 6:Constructing explanations, 7:Engaging in argument from evidence and CCC 1:Patterns, 6:Structure and function
The motion and make up of stars and galaxies provide evidence for the Big Bang theory.

## First, Observe and Appreciate

##### Next, examine a cluster of galaxies. In this Expedition we will be using a particularly rich set of galaxy clusters that were studied in the late 1950s and 60s by George Abell. The coordinates for each cluster are listed below. Use the links to explore one or more of the clusters. You should be inspired to poetry; if not, just appreciate these objects by recording some thoughts and observations before moving on. Note: the coordinates listed below may not be the center of the cluster. Pan around a little to be sure you observe the entire field.

RA              Dec

Abell 0085      10.4075      -9.3425

Abell 0779      139.955        33.7603

Abell 1132      164.58        56.7822

Abell 1377      176.7413    55.7389

Abell 1650      194.6926    -1.7530

Abell 2034      227.5547    33.5279

Abell 2199      247.1540    39.5243

Abell 2670      358.5423   -10.4050

## Record the Redshift to Each Cluster

##### One of the main objectives of the Sloan Digital Sky Survey is to measure redshifts accurately. Use information available through Navigate (or Explore if you are familiar with that tool) to determine the average redshift of each cluster:
• From the cluster image in Navigate, select the Objects with spectra box.
• In the Navigate window click on objects that have spectra (bounded by red boxes).
• Click the thumbnail spectrum graph that appears on the page.
• Locate and record the redshift (z= ) in the graph’s header.
• Think carefully about how you want to calculate the redshift to this cluster. Outline your methods to share with others.
• Calculate the redshift to each galaxy cluster using the methods you recorded.

## Assign a Relative Distance to Each Cluster

##### Hubble discovered that the best technique starts with identifying the brightest galaxy in the cluster. You can measure the size of the image in millimeters or pixels depending upon the tools you have available. The measurements you get are inversely proportional to the distances. More distant objects appear smaller. As a result, recording the inverse of the angular size assigns a smaller number to the clusters that are closer to us. (The smaller number represents the distance to the objects with large angular size.) This method more accurately represents the distribution of galaxy distances.
• Choose any technique or method available to you to measure the apparent size of the brightest galaxy in each cluster. Beware of foreground galaxies.
• Record the inverse of each measurement on your chart.