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Day 6) Measuring Distance 9.1-9.3
Name __________________
It is hard to measure astronomical distances. Today you will examine two of the most useful techniques.
1) Watch TEDEd How to measure extreme distances (0:00 - 1:08)
https://www.youtube.com/watch?t=315&v=Op3AYaJc0Xw
Stop at 1:08, where it says how do astrophysicists figure that out? After the table exercise below watch until
2:34 for a good review and introduction of parallax and standard candles and stop at 2:27. Watch the rest
after the light investigations for another 2.5 minutes of Cepheid variable, type 1A super novae. It ends with
time and space being connected.
Triangulation and Parallax
It is best to do this exercise on a table because it provides a right angle
and a steady base where you can put your eye at the same level.
1) One end of the table is the Earth and there is a star at a far corner.
a) Put a ruler as shown and measure the angle. _________o
b) Use these numbers to make a scale diagram at 1:10 on a separate piece of paper.
(Provide legal sized paper if you have long tables rather than desks)
c) Use the diagram to determine how long the table is. _______cm
d) Measure the length of the table using a meter stick. ________cm
e) How can you make your technique more accurate?
A larger base helps. Have them use the full width of the table. The biggest source of error is the angle.
f) Repeat the technique with your improvements. New value ______ cm
g) Use this technique to measure the distance to a far wall. ______ cm
h) Check your answer with a metre stick. ______cm.
i) What happens to the accuracy as the distance gets larger. Why?
It gets worse because the angle gets closer and closer to 90o so you need to measure with greater
precision.
j) What is the largest baseline that astronomers can use without leaving the Earth?
The diameter of the Earth’s orbit.
k) How do astronomers get bigger baselines?
They put satellites with recording equipment in large orbits. They do not send astronomers up for this.
Watch TEDEd: How to measure extreme distances (1:08 – 2:27)
Standard Candles
2) The brightness of stars will be explored with flashlights.
a) There are a variety of flashlights. Predict which will be brightest. Explain.
This question establishes that each flashlight has an intrinsic brightness or luminosity. In general the
bigger ones are brighter – but not if you compare LED to incandescent flashlights. The LED’s are brighter.
b) Point a flashlight at the floor, forming a circle of light. As the flashlight is moved away from the floor
the spot of light will become
A) dimmer, bigger
B) dimmer, smaller C) brighter , bigger D) brighter, smaller
Explain:
A) The circle of light will spread out more, and more area means less concentration of light. This can also
be demonstrated with a circle drawn on small balloon and then inflated further.
c) You have two flashlights, with different luminosities. How can you make them appear equally bright?
The brighter one needs to be farther away. Have students try shining flashlights at each other or a wall to
demonstrate this.
3) Astronomers measure brightness to determine how far away stars are. What else is needed?
They need to know the intrinsic brightness of the star. If this is known, they are called standard candles. To
determine the intrinsic brightness by measuring the brightness of stars close enough to use parallax.
4) The image above left shows the Big Dipper. Star A is twice as far away as Star B. This means that the
luminosity of Star A compared to Star B is
A) the same
B) double
C) quadruple
D) _________
D) If Star A appeared the same, then at twice the distance it would have to be four times brighter. However,
it appears even brighter so its luminosity must be even more.
5) The image above right shows a tiny section of the Andromeda galaxy. Star D is ______ than star C.
A) closer, more luminous
B) brighter, more luminous C) brighter, closer
D) all three
B) It is definitely brighter. All the stars in this small section of Andromeda must be basically the same
distance from Earth. Andromeda is 2.5 Ml-y away with a diameter of 0.220 l-y. A scale model of this
could be a picture of Andromeda https://www.flickr.com/photos/terryhancock/7862151292 held 2.5 m
away. The image shows a tiny part of this piece of paper.
6) The image above left shows a portion of the Hubble Deep Field image, which shows some of the most
distant objects ever found. All of the objects are galaxies of stars except for E, which is a single nearby
star. Which object is more luminous?
A) E
B) F
C) they are about the same
B) The two objects look equally bright, but are very different. The star is much closer and much less
luminous. The galaxies contain 100’s of millions of stars and are 100’s of millions times more luminous.
You can tell which objects are stars because of the lines that stick out from it. This is an effect of the optics.
7) The image above right shows galaxy NGC 4526 and near the bottom left you can see one of its stars - a
supernova - shining as brightly as the core of the galaxy which contains billions of stars. This type of
supernova is a standard candle. The rate at which it brightens and dims can be used to calculate its
luminosity. Astronomers can use this to determine the galaxy’s
A) distance
B) age
C) type
D) mass
Watch TEDEd: How to measure extreme distances https://www.youtube.com/watch?v=Op3AYaJc0Xw
A) distance. If you know luminosity and brightness you can calculate distance.
Textbook Consolidation: Read pages 365-369 and answer questions 1, 5. Compare parallax – which
astronomers use to measure distance - with the triangulation that you used in class.