Name: Shelby Fisher Date: 10/07/2021
Student Exploration: Big Bang Theory – Hubble’s Law
Directions: Follow the instructions to go through the simulation. Respond to the questions and
prompts in the orange boxes.
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.
1. If the headlights are equally bright, which motorcycle is closer?
The one on the left
Explain: If the light that you are seeing is brighter that means that they are closer to
you than the other motorcycle.
2. 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?
The motorcycle covers more distance than the 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.
1. 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.
A. Which star reaches a greater apparent brightness? A-091
B. Which star takes longer to A-091
pulse?
2. 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?
,The brighter the star, the more time it takes to pulse.
, Activity A: Get the Gizmo ready:
On the STARS tab, check that Region A: NGC 224 (Andromeda Galaxy) is sel
Period and brightness
Introduction: Two factors determine how bright a star appears to an observer: its luminosity, or
absolute brightness, and its distance from the observer. A star may appear bright because it is a
large, luminous star, or because it is very close. It is only possible to use a star’s apparent
brightness to determine its distance if you know the star’s luminosity. Henrietta Leavitt’s work
on Cepheids provided the key to solving this problem.
Question: How do Cepheids allow astronomers to measure intergalactic distances?
1. Collect data: Locate and select the yellow Cepheid variable star (A-091) in the lower left
section of the Andromeda Galaxy. Click the Collect data button. You will see a graph of the
apparent brightness of the star over time.
A. How does the star’s apparent brightness change over time?
It goes bright and then it dims, and then it gets bright again but then dims after
13 days.
B. Turn on Show time probes. Set the left probe at the first brightness peak, and the
right probe at the second brightness peak. List the time represented by each probe:
Left probe time:Right
1.0 d probe time: 13.3 d
C. What is the time difference between the two brightness peaks?
12.3 d
This is the period of the Cepheid.
D. In the DATA tab, record the name of this star and its period. Do the same on your
paper Data worksheet, located on the last page of this document.
2. Collect data: The apparent brightness of the star is shown on the y-axis of the graph. The
brightness is given as the ratio of the star’s brightness to the Sun’s brightness if viewed from
a standard distance of one megaparsec (1 Mpc), which is about 3.26 million light years. For
example, a brightness of “4,000” means that the star appears 4,000 times as bright as the
Sun would appear if observed from a distance of 1 Mpc.
A. What is your estimate of the mean apparent brightness of 7004
star A- 091?
B. Turn on Show mean brightness. What is the mean brightness of A- 7004
091?
Record this value in the Gizmo and on your Data worksheet.
Student Exploration: Big Bang Theory – Hubble’s Law
Directions: Follow the instructions to go through the simulation. Respond to the questions and
prompts in the orange boxes.
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.
1. If the headlights are equally bright, which motorcycle is closer?
The one on the left
Explain: If the light that you are seeing is brighter that means that they are closer to
you than the other motorcycle.
2. 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?
The motorcycle covers more distance than the 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.
1. 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.
A. Which star reaches a greater apparent brightness? A-091
B. Which star takes longer to A-091
pulse?
2. 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?
,The brighter the star, the more time it takes to pulse.
, Activity A: Get the Gizmo ready:
On the STARS tab, check that Region A: NGC 224 (Andromeda Galaxy) is sel
Period and brightness
Introduction: Two factors determine how bright a star appears to an observer: its luminosity, or
absolute brightness, and its distance from the observer. A star may appear bright because it is a
large, luminous star, or because it is very close. It is only possible to use a star’s apparent
brightness to determine its distance if you know the star’s luminosity. Henrietta Leavitt’s work
on Cepheids provided the key to solving this problem.
Question: How do Cepheids allow astronomers to measure intergalactic distances?
1. Collect data: Locate and select the yellow Cepheid variable star (A-091) in the lower left
section of the Andromeda Galaxy. Click the Collect data button. You will see a graph of the
apparent brightness of the star over time.
A. How does the star’s apparent brightness change over time?
It goes bright and then it dims, and then it gets bright again but then dims after
13 days.
B. Turn on Show time probes. Set the left probe at the first brightness peak, and the
right probe at the second brightness peak. List the time represented by each probe:
Left probe time:Right
1.0 d probe time: 13.3 d
C. What is the time difference between the two brightness peaks?
12.3 d
This is the period of the Cepheid.
D. In the DATA tab, record the name of this star and its period. Do the same on your
paper Data worksheet, located on the last page of this document.
2. Collect data: The apparent brightness of the star is shown on the y-axis of the graph. The
brightness is given as the ratio of the star’s brightness to the Sun’s brightness if viewed from
a standard distance of one megaparsec (1 Mpc), which is about 3.26 million light years. For
example, a brightness of “4,000” means that the star appears 4,000 times as bright as the
Sun would appear if observed from a distance of 1 Mpc.
A. What is your estimate of the mean apparent brightness of 7004
star A- 091?
B. Turn on Show mean brightness. What is the mean brightness of A- 7004
091?
Record this value in the Gizmo and on your Data worksheet.
