A105 Stars and Galaxies: Classifying Galaxies

Hubble’s Law Dec., 2015

Step 1: How do we estimate distances to galaxies?

Step 2 – The relationship between distance and recession velocity.

Below is a table of clusters of galaxies, which are known by the constellation in which direction they are found (e.g. the Virgo Cluster is seen in the direction of the constellation Virgo, although the galaxy cluster is vastly further away than the stars we see in the sky). For each galaxy cluster, the table lists the speed at which the galaxy is moving away from us and its distance in Megaparsecs (each Megaparsec is equal to 3.25 million LY).

Velocity Distance Cluster (km/sec) (Mpc) Virgo 1200 15 Perseus 5400 71 Coma 6600 83 Hercules 10500 150 Ursa Major I 15600 313 Leo 19500 337 Corona 21600 Borealis 347 Gemini 23400 402 Bootes 39300 650 Ursa Major II 40200 653 Hydra 60600 831 1000

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p 600 M (

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t 400 s i D

200 Distance - Velocity Relation

0 0 10000 20000 30000 40000 50000 60000 70000 Velocity (km/sec)

Plot the galaxies in the chart. Make a dot for each cluster, where the horizontal position is determined by the velocity and the vertical position is determined by the distance. Label each dot with the name of the cluster. Draw a single straight line which follows the general direction of the points you have plotted. The line should start at the origin (the (0,0) point).

1 Use the graph to estimate the recession velocity for a cluster at a distance of 1000 Megaparsecs:

Step 3 – Using Hubble’s Constant. A new galaxy cluster has been discovered that is moving away from us at 45,000 km/sec. Using the plot to estimate the distance to this new cluster.

Hubble’s constant has been determined most accurately by the ESA Planck mission. The currently accepted value is 67.8 ± 0.8.