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Name: Date: Oct.6.21
Student 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.
1. If the headlights are equally bright, which motorcycle is closer?
Explain: The one on the left is closer
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?
We cannot conclude an absolute answer
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 too 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?
The yellow star
B. Which star takes longer to pulse?
Also, the yellow one
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 star takes longer to pulse
Get the Gizmo ready:
Activity A:
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 light becomes its brightest in 12 days, increases quickly decreases slowly
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:
2018
, Left probe time: 1.1 Right probe time:13.5
C. What is the time difference between the two brightness peaks?
This is the period of the Cepheid.
12.4
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.
record on doc
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 star A-091?
7004
B. Turn on Show mean brightness. What is the mean brightness of A-091?
Record on doc
Record this value in the Gizmo and on your Data worksheet.
3. Observe: Click Record spectrum. Stars emit light at a variety of wavelengths. Just as white
light is separated into a rainbow when it passes through a prism, starlight can be separated
into its constituent colors when it passes through an instrument called a spectrograph. The
result is a pattern of colors and black bars called an absorption spectrum. The bars
represent wavelengths of light that are absorbed in the star’s atmosphere. Each star’s
unique spectrum depends on the temperature and composition of its atmosphere.
Look at the spectrum of A-091. Sketch the lines of its stellar spectrum in the diagram below.
4. Record: Click Done. Select the other Cepheid variable star in the Andromeda Galaxy (A-
171). Determine and record the period, mean brightness, and spectrum of this star. Then,
click Return to map and select Region B.
There is a total of ten Cepheids hidden in the nine regions of the Gizmo. Record the period,
mean brightness, and spectrum of all ten stars both in the Gizmo data table and on your
own data table.
5. Compare: By comparing Cepheid variables that were all about the same distance away,
Henrietta Leavitt discovered that larger, more luminous Cepheids had longer periods. Look
at the periods of the ten stars in your data table.
A. Based on the periods of the stars, which three stars are the most luminous?
The ones with the longest pulse, D819, A091, C197
B. Find two stars with similar periods, and therefore similar size. List these stars, their
periods, and their mean brightness below.
Star: A091 Period: 12.4 Mean brightness: 7004
Star: G958 Period: 10.7 Mean brightness: 12.94
C. Compare the apparent brightness of these two stars. Which star do you think is
farther away? They would be the dimmer light, G958
Activity B: Get the Gizmo ready:
Luminosity and Select the DISTANCE tab.
distance Check that Luminosity vs. Period is selected.
2018
Name: Date: Oct.6.21
Student 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.
1. If the headlights are equally bright, which motorcycle is closer?
Explain: The one on the left is closer
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?
We cannot conclude an absolute answer
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 too 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?
The yellow star
B. Which star takes longer to pulse?
Also, the yellow one
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 star takes longer to pulse
Get the Gizmo ready:
Activity A:
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 light becomes its brightest in 12 days, increases quickly decreases slowly
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:
2018
, Left probe time: 1.1 Right probe time:13.5
C. What is the time difference between the two brightness peaks?
This is the period of the Cepheid.
12.4
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.
record on doc
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 star A-091?
7004
B. Turn on Show mean brightness. What is the mean brightness of A-091?
Record on doc
Record this value in the Gizmo and on your Data worksheet.
3. Observe: Click Record spectrum. Stars emit light at a variety of wavelengths. Just as white
light is separated into a rainbow when it passes through a prism, starlight can be separated
into its constituent colors when it passes through an instrument called a spectrograph. The
result is a pattern of colors and black bars called an absorption spectrum. The bars
represent wavelengths of light that are absorbed in the star’s atmosphere. Each star’s
unique spectrum depends on the temperature and composition of its atmosphere.
Look at the spectrum of A-091. Sketch the lines of its stellar spectrum in the diagram below.
4. Record: Click Done. Select the other Cepheid variable star in the Andromeda Galaxy (A-
171). Determine and record the period, mean brightness, and spectrum of this star. Then,
click Return to map and select Region B.
There is a total of ten Cepheids hidden in the nine regions of the Gizmo. Record the period,
mean brightness, and spectrum of all ten stars both in the Gizmo data table and on your
own data table.
5. Compare: By comparing Cepheid variables that were all about the same distance away,
Henrietta Leavitt discovered that larger, more luminous Cepheids had longer periods. Look
at the periods of the ten stars in your data table.
A. Based on the periods of the stars, which three stars are the most luminous?
The ones with the longest pulse, D819, A091, C197
B. Find two stars with similar periods, and therefore similar size. List these stars, their
periods, and their mean brightness below.
Star: A091 Period: 12.4 Mean brightness: 7004
Star: G958 Period: 10.7 Mean brightness: 12.94
C. Compare the apparent brightness of these two stars. Which star do you think is
farther away? They would be the dimmer light, G958
Activity B: Get the Gizmo ready:
Luminosity and Select the DISTANCE tab.
distance Check that Luminosity vs. Period is selected.
2018
