GIZMOS Student Exploration: Big Bang Theory – Hubble’s Law 2021 (answered)

tudent Exploration: Big Bang Theory — Hubble’s Law

Vocabulary: absolute brightness, absorption spectrum, apparent brightness, Big Bang theory, blueshift, Cepheid variable, Doppler shift, Hubble constant, Hubble’s law, luminosity, megaparsec, period, redshift, spectrograph

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

Standing by the side of a lonely highway at night, you see two motorcycle headlights, one in each direction. The headlight on your left appears brighter than the one on your right.

If the headlights are equally bright, which motorcycle is closer?


Suppose the dim-looking headlight on the right is actually a small light on the front of a bicycle. What can you conclude about the distance of the motorcycle and bicycle?

Gizmo Warm-up

In 1912, an astronomer named Henrietta Swan Leavitt studied a class of stars called Cepheid variables. These stars change from bright to dim to bright again. Her discoveries led to a method of measuring distances to other galaxies and eventually helped to support the Big Bang theory of the origin of the universe.

In the Big Bang Theory — Hubble’s Law Gizmo™, select Region A. Look at the image of the Andromeda Galaxy, a galaxy relatively close to our own Milky Way galaxy.

Locate the two Cepheid variables, the stars that change in brightness over time. Star A-091 is the yellow star, and A-171 is the white star.

Which star reaches a greater apparent brightness?

Which star takes longer to pulse?

Because both stars are in the same galaxy, they are about the same distance from Earth. Based on what you see, how is the brightness of the star related to how quickly it pulses?

Activity C:


Get the Gizmo ready:

Select the SPECTRAtab.

On the DATA pane, click the right arrow so the Redshift (z) column is visible.

Introduction: Have you ever listened to the siren of an ambulance? As the ambulance passes by, the pitch of its siren gets lower. This occurs because of the Doppler shift. Sound waves are compressed as the ambulance approaches, causing the pitch to be higher. As the ambulance drives away, the sound waves are spread apart, causing the pitch to be lower.

A similar effect occurs if a star or galaxy is moving relative to Earth. When a star approaches Earth, the light waves it emits are compressed, causing its light to be shifted toward the blue end of the spectrum. This is called blueshift. When a star moves away from Earth, its light is shifted toward the red end of the spectrum, a phenomenon called redshift.

Goal: Measure and interpret the redshift of Cepheid variable stars.

Check: In the bottom half of the Spectra tab, you should see 10 stellar spectra. If you are missing any, go back to the Stars tab, find the missing star, and record its spectrum.

Observe: You can measure redshift by comparing the wavelength of a black absorption line on the star’s spectrum with an equivalent line on the reference spectrum. Drag the spectrum for star C-197 into the top part of the Spectra tab.

Do you notice any difference in the locations of the absorption lines in the reference spectrum and the spectrum for star C-197? Describe.

Measure: Drag the probes to equivalent lines in each spectrum. Then, zoom in on the graph by clicking +. Adjust the probes so they are exactly on each absorption line.

What is the wavelength of the line on the reference spectrum

What is the wavelength of the line on star C-197’s spectrum?

Has the spectral line of star C-197 been shifted to the right (redshift) or to the left (blueshift)?

Is the galaxy that contains star C-197 moving toward our galaxy (the Milky Way) or away from our galaxy? Explain.

(Activity C continued on next page)

Activity C (continued from previous page)

Calculate: Redshift (z) is calculated by dividing the wavelength of the observed absorption line (λobs) by the wavelength of the equivalent reference line (λref), and subtracting 1. (If the redshift is negative, the light is blueshifted

What is the redshift of star C-197?

Check your answer by turning on the Redshift calculator.

Record: Repeat the procedure to measure the redshift of each star. Record your answers in the Gizmo and on your data sheet.

Which star has the greatest redshift? Greatest blueshift?

Infer: The Big Bang theory states that the universe began at a single point, and that the universe is still expanding today.

Are most stars redshifted or blueshifted?

What does this tell you about the galaxies that contain these stars?

How does this relate to the theory that the universe is expanding?

Analyze: Compare the redshifts you measured to the distances to the various stars.

In general, what do you observe about the redshifts of stars that are relatively close to Earth?

What do you notice about the redshifts of stars that are very far away?

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Category GIZMOS
Release date 2022-02-16
Latest update 2022-02-17
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Language English
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