One the great triumphs of 20th century astronomy was the large scale mapping of the Universe’s structure. A lot of credit in this endeavor goes to Edwin Hubble, after whom the Hubble Space Telescope was named.
Hubble observed the spectra of distant galaxies using the 2.5m reflecting telescope on Mount Wilson during the 1930s. He learned that the farther a galaxy is from our Milky Way galaxy, the more its spectral lines are shifted toward the red end of the spectrum. This is the so-called ”redshift”.
Redshift can be interpreted as a Doppler effect. The faster an object is receding from us, the redder (lower frequency) we see the light from that object. The fact that the light from farther galaxies is more redshifted means that farther galaxies are moving away from us faster, which is the Hubble Law in a nutshell.
Laboratory Tools
For this lab you will just need plotting software like Google Sheets or Microsoft Excel. The data you will need is in the tables below.
Assignment
1) Most of the galaxies used in calibrating the Hubble Law were relatively close normal spiral galaxies. Some sample normal spiral galaxies are listed as Normal Spirals (N1,… N5) in the table below. Using any plotting software, create a Hubble Diagram with Distance (in Mpc) on the x axis and Velocity (in km/s) on the y axis, and plot all the galaxies from the Normal Spirals table. Find and report the equation for the trend line.
| Galaxy Label | Recession Velocity [km/s] | Distance [Mpc] |
| N1 | 7,200 | 123 |
| N2 | 14,670 | 153 |
| N3 | 29,430 | 399 |
| N4 | 44,600 | 614 |
| N5 | 65,700 | 856 |
2) The Hubble Law is v = H x d. What is the value of H (Hubble constant) from the equation that you found? Calculate the percent difference with respect to the standard value of 70 km/sec/Mpc, %diff = (H – 70)/70 ×100.
3) Other types of galactic objects include Seyfert galaxies (S) and Quasars (Q). Seyfert galaxies are spirals with very luminous cores and quasars might be the powerful nuclei around the supermassive black holes that inhabit the center of spiral galaxies. Recession velocities are presented in for five Seyfert galaxies and four quasars. Using the relationship you found (Hubble Law), compute distances for all these objects and plot all of them in the same Hubble diagram as the others. Make sure you use different symbols. Note: Since distances are unknown, you must “flip” the equation to d = v/H.
| Galaxy Label | Recession Velocity [km/s] | Distance [Mpc] |
| S1 | 39,000 | |
| S2 | 60,000 | |
| S3 | 75,000 | |
| S4 | 90,000 | |
| S5 | 180,000 | |
| Q1 | 81,000 | |
| Q2 | 135,000 | |
| Q3 | 195,000 | |
| Q4 | 210,000 |
Questions
- Remember that when you are looking at galaxies farther away, you are looking farther back in time (for example, 1 pc = 3.26 light years; 500 Mpc ≃ 1.63 billion light years). As a result, by looking at your plot, can you infer whether spiral galaxies were more common in the far past than now? How about quasars?
- What is different in the plot between the normal galaxies N1-N5 and the spirals and quasars you added to the plot?
- The top velocity in your plot is 210,000 km/s. At what velocity do you think you can no longer interpret the redshift as a doppler shift?
