Name: Mack Dell Date: April 7th, 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 headlight on the left
Explain: Since the headlights are equally bright, the motorcycle that is seemingly brighter would
be closer since the light is less spread out, and is more concentrated.
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 between the motorcycle and bicycle?
If the light is actually small and on a bicycle, then the light must be more powerful to just as bright
with a smaller light.
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? Star A-091 reached a greater
apparent brightness.
B. Which star takes longer to pulse? Star A-171 took longer to 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?
, Based on these stars, the brighter a star is, the longer its pulse is.
Activity A: Get the Gizmo ready:
● On the STARS tab, check that Region A: NGC 224
Period and (Andromeda Galaxy) is selected. If not, click
brightness Return to map and select Region A.
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?
The first day it jumps up to around 10,000 brightness, then steadily lowers to around 4000
brightness over about 10 days, then over two days goes back up to 10,000
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: 1.0 d Right 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.
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 headlight on the left
Explain: Since the headlights are equally bright, the motorcycle that is seemingly brighter would
be closer since the light is less spread out, and is more concentrated.
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 between the motorcycle and bicycle?
If the light is actually small and on a bicycle, then the light must be more powerful to just as bright
with a smaller light.
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? Star A-091 reached a greater
apparent brightness.
B. Which star takes longer to pulse? Star A-171 took longer to 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?
, Based on these stars, the brighter a star is, the longer its pulse is.
Activity A: Get the Gizmo ready:
● On the STARS tab, check that Region A: NGC 224
Period and (Andromeda Galaxy) is selected. If not, click
brightness Return to map and select Region A.
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?
The first day it jumps up to around 10,000 brightness, then steadily lowers to around 4000
brightness over about 10 days, then over two days goes back up to 10,000
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: 1.0 d Right 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.
