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:
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?
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.
Anderson
A. Which star reaches a greater apparent brightness?
B. Which star takes longer to pulse?
close.
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?
, Get the Gizmo ready:
Activity A:
On the STARS tab, check that Region A:
Period and
NGC 224 (Andromeda Galaxy) is selected. If
brightness
not, click 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?
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 probe time:
C. What is the time difference between the two brightness peaks?
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
decrease your estimate
slowly and then of spike
the mean apparent
up on brightness
the 12th day. of star A-091?
B. Turn on Show mean brightness. What is the mean brightness of A-091?
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