Hubble National Teach-In
Nationwide Galaxy Count
The Nationwide Galaxy Count is a project to get students, teachers, and
schools from across the country to collaborate on estimating the number
of galaxies in the observable universe.
The Hubble Ultra Deep Field
One of the Hubble Space Telescope's most important images is the
Hubble Ultra Deep Field (HUDF). The HUDF is the deepest visible light view
of the universe, where “deep” is an astronomy term for seeing very faint
light. This extremely long exposure, combining about 11 days of Hubble
observations, shows a vast number of galaxies stretching across the
universe.
There are so many galaxies in the HUDF that counting them by hand
would be a very long and very repetitive task. In fact, astronomers write
specialized computer programs to identify and count the galaxies (and
stars) in images like these.
Furthermore, the HUDF image shows only a very small patch of the night
sky. One cannot help but wonder, if there is such an abundance of
galaxies in this tiny region, how many galaxies are there across the entire
universe? The Nationwide Galaxy Count attempts to address that
question through a collaborative project.
Sampling and Estimation
The basic idea is to use representative sampling to estimate the number of
galaxies in the HUDF and the universe. If one counts the galaxies in a small
region (a sample), one can then estimate the number of galaxies in a
larger region.
This extrapolation is just a multiplication by the ratio of the region sizes. For
example, if the larger region is 10 times the size of the smaller region, then
it can be estimated to have 10 times the number of galaxies. For this
estimate to be accurate, the sample region must be an average (or
representative) region of the whole.
2 The Hubble Ultra Deep Field
For the HUDF, we have created 100 sample regions that we call “cutouts”.
These HUDF cutouts are numbered from “00” to “99”. Each HUDF cutout
contains an appreciable number of galaxies, but not so many that it is too
arduous to count by hand.
The HUDF cutouts are available in 10 PDF files with 10 HUDF cutouts in
each. These files can be downloaded from the Hubble National Teach-In
web page. Nationwide Galaxy Count is at the bottom of the page.
http://hubble25th.org/go/Teach-In
3 Materials
-
Pens or Pencils
1 Recording Sheet per group (located in this packet)
This activity can be completed with or without a computer
To complete the activity with a computer
o Download the HUDF cutouts
http://hubble25th.org/go/Teach-In
o Place the files in a directory students can access.
To complete this activity without a computer
o Download and printout the HUDF cutouts
http://hubble25th.org/go/Teach-In
o Files can be printed in color or black and white
NOTE: Due to the differing contrast of a computer screen and a
printout, one may get different counts. This is a good point to discuss or
even test with students.
4 Instructions
1. Assign students in groups of two or three. Have the group choose a
HUDF cutout. The conversations that ensue when working as part of
a team are an essential part of science.
2. The students are then are to count the galaxies. Before counting,
the students need to consider:
a. Be Random - The PDF files are listed in numerical order, which
means that human nature may lead them to choose the first
image in the first PDF file. It will not make for good statistical
sampling, if everyone does HUDF cutout #00. Choose a
random number between 0 and 99.
b. What to count – In this activity, students need to count the
galaxies of their HUDF cutout. Galaxy is defined on the
Amazing Space website as
A galaxy is a collection of stars, gas, and dust bound
together by gravity. The smallest galaxies may contain
only a few million stars, while the largest galaxies have
thousands of billions of stars. Galaxies are classified or
grouped by their shape. Round or oval galaxies are
elliptical galaxies and those showing a pinwheel structure
are spiral galaxies. Others are called irregular because
they do not resemble elliptical or spiral galaxies.
As a visual guide to the three main galaxy shapes, there is a
graphic organizer available at this link:
http://amazing-space.stsci.edu/resources/organizers/galaxy.php
For the HUDF, most of the galaxies will not be the big beautiful
ones seen on posters. The HUDF is dominated by small and
irregularly-shaped galaxies. The reason for this will be
discussed during the Hubble National Teach-In.
c. What not to count - The HUDF does contain some stars. The
bright ones are easily identifiable by the plus sign (“+”)
shaped diffraction spikes emanating from their center. Some
of the faint stars are indistinguishable from galaxies. Our
advice is to ignore anything with diffraction spikes, and count
the rest as galaxies.
5 One question we get is: “How small of an object in the image
qualifies as a galaxy?” It can be difficult to assess whether a splotch
is a small galaxy or just one piece of a larger galaxy. For this
exercise, those decisions are up to the students. In fact, working
through such questions is where the process of science really
happens. Anyone can count, but it requires some thought to
determine what to count.
3. Have students record their observation on the recording sheet. If
possible, have students count galaxies in additional HUDF cutouts.
6 Data Submission
Submit your data via email by April 17, 2015. Basic instructions are on the
Hubble National Teach-In web page.
http://hubble25th.org/go/Teach-In
Please provide the grade level, school, and zip code. You may provide a
teacher’s name, but please do not send us any student information.
Here is an example email:
To: [email protected]
From: [email protected]
Subject: data for Nationwide Galaxy Count
Here is the data from Miss Marple’s 7th grade class at Christie Middle
School. Our zip code is 13579.
HUDF cutout #47 galaxy count 73
HUDF cutout #73 galaxy count 47
…
We will compile and combine the data from all participants and present
the results during the Hubble National Teach-In on April 24, 2015.
7 Student National Galaxy Count Data Table
HUDF Cutout #
Galaxy Count
What to count
In this activity, you are counting the galaxies in a HUDF cutout.
Galaxy is defined on the Amazing Space website as
A galaxy is a collection of stars, gas, and dust bound
together by gravity. The smallest galaxies may contain
only a few million stars, while the largest galaxies have
thousands of billions of stars. Galaxies are classified or
grouped by their shape. Round or oval galaxies are
elliptical galaxies and those showing a pinwheel structure
are spiral galaxies. Others are called irregular because
they do not resemble elliptical or spiral galaxies.
What not to count
The HUDF does contain some stars. The bright ones are easily identifiable
by the plus sign (“+”) shaped diffraction spikes emanating from their
center. Some of the faint stars are indistinguishable from galaxies. Ignore
anything with diffraction spikes, and count the rest as galaxies.
8 Teacher Nationwide Galaxy Count Data Table
00
34
68
01
35
69
02
36
70
03
37
71
04
38
72
05
39
73
06
40
74
07
41
75
08
42
76
09
43
77
10
44
78
11
45
79
12
46
80
13
47
81
14
48
82
15
49
83
16
50
84
17
51
85
18
52
86
19
53
87
20
54
88
21
55
89
22
56
90
23
57
91
24
58
92
25
59
93
26
60
94
27
61
95
28
62
96
29
63
97
30
64
98
31
65
99
32
66
33
67