You exist here, today.
Concept of “look-back time”
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Learning goal: be able to ….
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Explain what is meant by look-back time and apply
the concept to observations and evolution of
galaxies.
Name ____________________________________________________________ Date _________________
Looking at Distant Objects
(Adapted from Lecture-Tutorials for Introductory Astronomy, ©CAPER Team, Preliminary Edition, 2002)
01. Describe what a light year is (is it time? distance? speed?), giving some measurement comparisons
that will help someone who has never taken an astronomy class understand.
Imagine that you have received six pictures of six different children who live near six of the closest stars
to the Sun. Each picture shows a child on his or her 12th birthday. The pictures were each broadcast
directly to you via radio waves on the exact day of the child’s 12th birthday. From the closest to the most
distant the children are:
✴Savannah lives on a planet orbiting Alpha Centauri, which is 4.3 sly from our Sun.
✴Peter lives on a planet orbiting Barnard’s Star, which is 6.0 ly from our Sun.
Looking at distant objects: The Concept of Look-Back Time
Each child had exactly the same birth date in the Milky Way.
02. What were the shortest and longest lengths of time that it took for any one of the pictures to travel
from the child to you? Relate your answer to the distances to those children.
03. How old did each child look in his or her picture? ____________
04. If each child was 12 years old when he or she sent his or her picture to you, how old was each child
in his or her time frame when you received the picture?
Savannah ________ Peter ________ Celeste________ John________ Inga ________ Ron________
05. Is there a relationship between the current age of each child and his or her distance away from Earth?
If so, describe this relationship.
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cp11_looking_distant_objects.pages
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Looking at distant objects: The Concept of Look-Back Time
Each child had exactly the same birth date in the Milky Way.
!
02. What were the shortest and longest lengths of time that it took for any one of the pictures to travel
from the child to you? Relate your answer to the distances to those children.
03. How old did each child look in his or her picture? ____________
04. If each child was 12 years old when he or she sent his or her picture to you, how old was each child
in his or her time frame when you received the picture?
12+4.3
12+6.0
12+8.6
12+9.5
12+10.8
12+11.4
Savannah ________ Peter ________ Celeste________ John________ Inga ________ Ron________
05. Is there a relationship between the current age of each child and his or her distance away from Earth?
If so, describe this relationship.
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cp11_looking_distant_objects.pages
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03. How old did each child look in his or her picture? ____________
04. If each child was 12 years old when he or she sent his or her picture to you, how old was each child
in his or her time frame when you received the picture?
16.3 Peter ________
18.0
20.6
21.5
22.8 Ron________
23.4
Savannah ________
Celeste________
John________
Inga ________
05. Is there a relationship between the current age of each child and his or her distance away from Earth?
If so, describe this relationship.
Nearest
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Farthest
cp11_looking_distant_objects.pages
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HOWEVER, QUESTIONS 2 - 5 DO NOT REPRESENT HOW THE UNIVERSE WORKS! SO,
FORGET YOUR ANSWER TO #5!
All of the children were born at the same time; when one child turned 12, they all turned 12. Imagine
that the six pictures were broadcast by satellite to you from those planets, and that the pictures all
arrived at exactly the same time. To make this happen the child that is the farthest away sends his picture
first, taken when he was 12 years old. The next farthest child waits until she receives that picture, and
then sends her picture right away. This continues as signals travel to Earth. Check the figure for the
amount of time that passes between each child before he or she receives the pictures from farther out.
06. How old will each child look when we receive his/her picture?
Savannah ____ yrs Peter ____ yrs Celeste ____ yrs John ____ yrs Inga ____ yrs Ron ____ yrs ——-Nearest—————————————————————————————Farthest———-
Looking at distant objects: The Concept of Look-Back Time
Each child had exactly the same birth date in the Milky Way.
HOWEVER, QUESTIONS 2 - 5 DO NOT REPRESENT HOW THE UNIVERSE WORKS! SO,
FORGET YOUR ANSWER TO #5!
All of the children were born at the same time; when one child turned 12, they all turned 12. Imagine
that the six pictures were broadcast by satellite to you from those planets, and that the pictures all
arrived at exactly the same time. To make this happen the child that is the farthest away sends his picture
first, taken when he was 12 years old. The next farthest child waits until she receives that picture, and
then sends her picture right away. This continues as signals travel to Earth. Check the figure for the
amount of time that passes between each child before he or she receives the pictures from farther out.
06. How old will each child look when we receive his/her picture?
19.1 yrs Peter ____
17.4 yrs Celeste ____
14.8 yrs John ____
13.9 yrs Inga ____
12.6 yrs Ron ____
12 yrs Savannah ____
——-Nearest—————————————————————————————Farthest———-
07. How old is each child in his or her time frame when you finally receive all of their pictures?
23.4 yrs Peter ____
23.4 yrs Celeste ____
23.4 yrs John 23.4
Savannah ____
____ yrs Inga 23.4
____ yrs Ron 23.4
____ yrs ——-Nearest—————————————————————————————Farthest———-
08. In 1987, astronomers discovered a new supernovae in the Large Magellanic Cloud (the “LMC”),
which they named SN1987A. However, the Large Magellanic Cloud is ~170,000 light years away.
Savannah ____
____ yrs
yrs Peter
Peter ____ yrs Celeste ____ yrs John ____ yrs Inga ____ yrs Ron ____ yrs Savannah
——-Nearest—————————————————————————————Farthest—————-Nearest—————————————————————————————Farthest———08. In
In 1987,
1987, astronomers
astronomers discovered a new supernovae in the Large Magellanic Cloud (the “LMC”),
08.
which they
they named
named SN1987A.
SN1987A. However, the Large Magellanic Cloud is ~170,000 light years away.
which
When did
did this
this supernova
supernova actually occur? ___________________________
When
09. Astronomers
Astronomers can
can routinely
routinely detect galaxies that are as far as 11 billion light-years away. Although we
09.
are just
just detecting
detecting the
the light
light from these galaxies now, how many years ago was the light from these
are
galaxies emitted?
emitted? _____________________
_____________________
galaxies
10. Assuming
Assuming that
that the
the Universe
Universe is 13.7 billion years old, the maximum age any galaxy can have today is
10.
about 13
13 billion
billion years
years (it
(it took a few hundred thousand years for galaxies to start forming). Are the
about
apparent ages
ages of
of these
these distant
distant galaxies very young or very old? How old are they in their time frame?
apparent
11. Shown
Shown below
below are
are 44 galaxies with redshifts of approximately 0.1 (nearest), 1.5, 2.5, and 3.5
11.
(farthest). Explain
Explain how
how we
we use
use the
the concept
concept of
of look-back
look-back time
time to
to understand
understand the
the evolution
evolution of
of galaxies.
galaxies.
(farthest).
!!
!!
!!
___________________________________________________________________________________
___________________________________________________________________________________
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Looking back:
Hubble Deep
Field
http://imgsrc.hubblesite.org/hu/db/videos/hs-1996-01-a-low_mpeg.mpg
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Learning goal: be able to ….
•
Explain what is meant by look-back time and apply
the concept to observations and evolution of
galaxies.
The most distant galaxies (those about 12 - 13
billion light years away) in the Hubble Ultra Deep
Field look about ....
A. 12 or 13 billion years old.
B. 4.5 billion years old.
C . 1 or 2 billion years old.
D. like nearby galaxies, only redder.
(Old light from young galaxies)
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37. Using the concept of look-back time, which one of the following answers is
correct? (2014)
a. The galaxies with the highest redshifts will look the oldest.
b. The most distant galaxies will look the youngest.
c. The closest galaxies to us will look the youngest
d. The galaxies with the highest redshifts will be the oldest.
(2010) 35. Which one of the following reasons is NOT a correct completion of
this sentence? Studying distant galaxies help us learn about galaxy evolution
because we observe
a. what the Milky Way may have looked like and theorize events leading to how it
looks today.
b. the birth of galaxies and the mergers that possibly led to elliptical galaxies.
c. and analyze data from centuries of telescope time to get the evolution of a single
galaxy.
d. galaxies at different distances, thus at different ages, and therefore at different
evolutionary stages.
HAND IN THE PARTICIPATION
ACTIVITY AS YOU LEAVE.
PLEASE WRITE IN BOTH SECTION
LETTERS OR TA’S NAME.
GO DAWGS!
https://youtu.be/oAVjF_7ensg
Looking
waaaayy back:
Hubble UltraDeep Field
Hubble ultra-deep field
Hubble Ultra-Deep Field in Perspective
What steps were necessary in order for
astronomers to create that 3-D perspective??
Answer on a FULL sheet of paper and turn in as you leave
lecture today. Include your name and section (or TA name).
Bring in knowledge of:
celestial sphere
telescopes
spectra
redshift
velocity
Hubble’s Law
distance
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