Lab Report for Lab #12: Our Expanding Universe Get the spectrum of a galaxy

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Sum 10 Name: ________________________________

Lab Report for Lab #12: Our Expanding Universe

  1. Get the spectrum of a galaxy. Go to and select “Spectra” under the heading “Data”. Once there, enter “NGC1050” for the “object name” and click on the “submit query” button. The second spectrum that appears is one for the galaxy NGC 1050 and shows data for the wavelengths from 5445 to 7899 Angstroms. (This information appears in the far right column of the table.)

  1. Determine the wavelength of the tallest peak in the spectrum. Launch the Specview Applet by clicking on the word “Specview” below the spectrum for NGC 1050. This applet will allow you to display the data using any units, to display reference lines and to measure the wavelength of the emission lines shown. You may have to change the units.

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  1. Calculate the redshift of the galaxy. Usually the laboratory wavelength of the tallest peak in the spectrum is 6562.8 Angstroms. The redshift of the galaxy is equal to the observed wavelength (what you measure) minus the laboratory wavelength, divided by the laboratory wavelength. The number you get should be a number between zero and 1, closer to zero. Round off your answer to the nearest 1000th.

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  1. Calculate the recessional speed of the galaxy. The recessional speed of the galaxy is the redshift, times the speed of light. Use the speed of light in km/s, so that your answer will be in km/s. Check the answer you get against the value on NED. To check the value on NED, go to the website above and click on “Redshift” under the heading of “Data”. Once there, enter “NGC 1050” for the “object name” and click on the “redshift” button. Scroll down to find the recessional speed, which will be labeled “Velocity” or “Helio. Radial Velocity”. Ask your instructor which value you should use if the values are very different from one another.

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  1. Look up the luminosity distance for the galaxy. Go back to the website and look up the galaxy using the “by name” option under the “objects” heading. At the very bottom of the page, there is a section called “Cosmology-Corrected Quantities”. There should be at least three different distances listed. Use the “Luminosity Distance”. This distance is calculated using the luminosity and apparent brightness of the galaxy. This distance does not depend on the motion of the galaxy, so it is what we will use as an independent measure of distance.

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  1. Determine the value of the Hubble constant. Repeat steps 1 through 5 to get the heliocentric radial velocities and luminosity distances for at least nine other galaxies from the list found at the end of this lab. Make sure the velocities are all greater than 1000 km/s. (This is because the motions of galaxies that are moving at slower speeds is not dominated by the expansion of the universe.) To determine the value of the Hubble constant, plot the data you collected on a graph using Excel. Put the speed on the y-axis and the distance on the x-axis. Make sure speed is in units of km/s and distance is in Mpc. The slope of the line that fits these data points is the Hubble constant.

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  1. Compare the value of the Hubble constant you determined to the current accepted value (72 km/s/Mpc) and to Hubble’s original value (500 km/s/Mpc). Determine the percent difference (accuracy) between your value and the other two values above, and the percent error (precision) between your value and the currently accepted value. (The formulas can be found at the end of this lab, below the listing of galaxies.) The Hubble constant can also be used to calculate the age of the universe, assuming a constant rate of expansion. To perform this calculation, all you have to do is divide the number of km in a Mpc by the Hubble constant value, then divide it by the number of seconds in a year. That will give you the age of the universe in years. Use your textbook or the internet to find these conversion values.

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OPTIONAL STEP 8. For the final product of this exercise, write a report in the style of a scientific publication. You should have an abstract explaining the goal of the experiment and a brief description of the experiment and the results. The introduction should describe the history behind the experiment and methodology. Use your textbook and reliable internet resources (like NASA websites) as resources for this part. Be sure to address why one should be skeptical of the determination of Hubble’s constant through a correlation of recessional speeds with distance. The next section would be the Data and Observations section where you describe how you acquired the data for this experiment and what methods you used to acquire it. Here, you should explain what types of spectra you used to get your recessional speeds; also, you should describe the process you used to extract the data for NGC 1050. The next section is the Analysis section. Here you present the graph you made of the data you collected and describe the analysis you did to extract the Hubble constant. Your comparison to the currently accepted and original values should be included in this section, as well as your calculations of the age of the universe. Finally, the Conclusions section will summarize the experiment and list the conclusions you drew from your data and your analysis. Don’t forget to include a References section and list all resources you used for this experiment.

List of Galaxies:

    • M58 NGC 4579

    • M61 NGC 4303

    • M88 NGC 4501

    • M77 NGC 1068

    • M99 NGC 4254

    • M100 NGC 4321

    • M104: The Sombrero Galaxy NGC 4594

    • M109 NGC 3992

    • M84

    • M60 NGC 4649

    • M87: Virgo A, central galaxy in the Virgo cluster NGC 4486

% Difference = Difference in values × 100%

Average in values 1

% Error = Difference in values × 100%

Accepted value 1

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