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 star A-171 is the white star.
A. Which star reaches a greater apparent brightness?
ANSWER: Star A-171 is the brighter star.
B. Which star takes longer to pulse?
ANSWER: Star A-091 takes 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?
ANSWER: The brightness of the star is related to how quickly it pulses by
, Get the Gizmo ready:
Activity A:
● On the STARS tab, check that Region A: NGC
Period and
224 (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?
ANSWER: The star’s brightness kept on going up and down when it changes.
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 1.2
Right probe time 13.5
C. What is the time difference between the two brightness peaks?
ANSWER: The difference is 12.3 between the two brightness peaks.
This difference 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.
(Activity A continued on next page)
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