Download LET`S MAKE A PORTRAIT OF A GALAXY Abstract

LET’S MAKE A PORTRAIT OF A GALAXY
Veselka S. Radeva
EAAE Summer School Working Group (Bulgaria)
Abstract
The goal of the exercise is for students to learn about the different types of galaxies,
about the Hubble diagram for galaxies, and about some of the characteristics of
galaxies. Students will determine the level of flattening and the size of several elliptical
galaxies, and the size of several spiral galaxies. Students will have to make
measurements and calculate the size of galaxies using very simple formulae. In this
exercise students will use the astronomical software and the images provided by the
Discovery Space project and the National Astronomical Observatory, Rozhen, Bulgaria.
INTRODUCTION
• Definition of the term “Galaxy”
A galaxy (coming from the Greek kyklos galaktikos, representing the Milky Way) is a
large gravitationally bound system of stars, interstellar gas and dust, unseen dark matter,
and possibly dark energy. Typical galaxies contain 10 million to one trillion, or more
stars, all orbiting around a common centre of gravity. Most galaxies are several
thousand to several hundred thousand light years in diameter and are usually separated
from one another by distances on the order of millions of light years.
• The Hubble Sequence
In 1925 Edwin Hubble created a
system of galaxy classifications
called the “Hubble Sequence” or
“Tuning Fork Diagram”. The
classification separates galaxies
into:
- Elliptical Galaxies
- Spiral Galaxies
- Barred Spiral Galaxies
- “Irregulars”
And then divides each one up by
shape.
Figure 1. The Hubble Sequence
• Elliptical Galaxies
These galaxies are very different from spiral galaxies in shape and stellar population,
and range from objects that are almost spherical, to those which are almost flat. Those
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which are spherical are classed as E0 galaxies, and those which are flat are classified as
E7, with the types in between being given the numbers E1 to E6. Elliptical galaxies
contain mostly Population II population stars and little or no gas or dust. Star formation
is not taking place in them. Elliptical galaxies can be the largest galaxies in the known
universe (called giant elliptical) or can be very small galaxies (called dwarf elliptical).
• Lenticular Galaxies
Lenticular galaxies look like spiral ones, thus their abbreviation is S0. However, their
structure is quite different: stellar formation stopped long ago, because the interstellar
matter was used up. Therefore, they consist of old population II stars only, or at least
chiefly. From their appearance and stellar contents, often they can hardly be
distinguished from ellipticals observationally.
• Spiral Galaxies
Spiral galaxies have a flat, disk-like shape. At the centre of the disk is a ball-shaped
distribution of stars referred to as the nucleus. The nucleus of the spiral galaxy and the
halo of globular clusters surrounding the galaxy contain mostly old stars - called
Population II type stars. The spiral arms contain a mixture of old and new stars Population I type stars. There is also gas and dust within the spiral arms. And it is
within this gas and dust that new star formation is taking place. Spiral galaxies are
further classified as Sa, Sb, and Sc. This classification is based on the appearance of the
galaxy. The Sa type galaxies have a large central nucleus and tightly wound spiral arms.
In contrast the Sc type galaxies have a small nucleus and loosely wound spiral arms.
• Barred Spiral Galaxies
The second type of galaxy classified by Hubble are barred spiral galaxies. Their
characteristics are very much like spiral galaxies except that instead of the spiral arms
emanating from a spherical nucleus, the spiral arms emanate from a bar. This bar is an
extension emanating from the spherical nucleus. The stellar populations are
predominantly the same as those within spiral galaxies. Once again barred spirals are
labelled according to the appearance of the arms and the central bulge, and so we see
labels such as SBa, SBb, and SBc, as well as intermediate types such as SBbc, and so
on.
• Irregular Galaxies
As the name suggests, “irregular” galaxies are those galaxies which have no specific
form, and so the group contains a very diverse selection of objects. In fact, there are two
types of irregular galaxies. Type I are usually single galaxies of peculiar appearance.
They contain a large fraction of young stars, and show the luminous nebulae which are
also visible in spiral galaxies. Type II irregulars include the group known as interacting
or disrupting galaxies, in which the strange appearance is due to two or more galaxies
colliding, merging or otherwise interacting gravitationally. Type II galaxies appear to
contain a large amount of dust.
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• Main characteristics of galaxies
The main characteristics of galaxies are very different and can vary within large
boundaries. The characteristics are very difficult to determine and many galaxies have
their characteristics undetermined. Table 1 shows the limits of variation of galaxy
characteristics compared to the same characteristics of the Sun.
Table 1. Galaxy characteristics
Type according to Hubble
Elliptical
Spiral
Irregular
6
13
9
12
8
10 – 10 МSun
10 – 10 МSun
10 – 1010 МSun
Characteristics
Mass
Diameter
1/10 kpc – 100 kpc
106 – 1012 LSun
5 kpc – 100 kpc
109 – 1012 LSun
1 kpc – 10 kpc
107 - 1010 LSun
<100
100 - 300
50 - 150
Spectral class
K
F-K
A-G
Contents of the star population
old
Old and young
Old and young
Contents of interstellar matter
<0.1 %
1 – 10%
15 – 20 %
Luminosity
Speed of rotation (km/s)
Figure 2. Images of the galaxy NGC1032
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PEDAGOGICAL INSTRUCTIONS
Educational goal:
To introduce students to the main types of galaxies, to the Hubble’s galaxy diagram,
and to the galaxies’ main characteristics.
Activity:
1. Students will learn how to process astronomical images of elliptical and spiral
galaxies with two astronomical computer programs:
LTImage and Avisfitsviewer (http://www.sky-watch.org/data library.htm).
2. Students will process the images of elliptical galaxies in order to calculate the level
of flattening of the galaxies.
3. Students will process astronomical images of elliptical and spiral galaxies in order
to calculate the diameter of the galaxies.
Practical preparation:
1. The teacher chooses astronomical images of 5 elliptical and 5 spiral galaxies
(http://www.discoveryspace.net; http://www.astro.princeton.edu/~frei/catalog.htm).
2. The teacher finds information about the distance to these galaxies from the Internet.
3. The teacher learns how to use the astronomical programs for processing images.
Organization of the lesson:
Steps:
1. The teacher introduces the different types of galaxies, the Hubble diagram and
the galaxy characteristics. As a supplement to the lesson for galaxies, the teacher
can use the information in the Introduction section.
2. The teacher divides the class into 6 groups. 5 groups are going to process the
images of different types of galaxies. The 6th group will look for information in
Internet about the characteristics of every galaxy.
3. At the end of the lesson students fill in Table 2 together.
Table 2. Results from the processing of the images
Galaxy
Distance
Diameter
Flattening
(for elliptical galaxies)
1
2
3
4
5
4
Type
Image
Instructions for students:
o Problem 1. Calculating the flattening of a galaxy
We will calculate the flattening of an elliptical galaxy using the following formula:
e=
(a 2 − b 2 )
f
=
a
a
where
а – the large semi-major axis of the ellipse
b – the small semi-minor axis of the ellipse
f – focal length
Figure 3. Elements of an ellipse
•
•
•
If the eccentricity of the ellipse is е = 0, the ellipse is a circle with radius r = а = b.
If е = 1, the ellipse is flat and becomes a line with length 2а.
When e is close to 0, the galaxy is of type Е0, while if e is close to 1, the galaxy is
of type Е7.
o Problem 2. Calculating the size of a galaxy
1.- Calculating the diameter of a galaxy in
pixels.
When you know how many arcseconds there are
in one pixel, you can calculate the diameter of a
galaxy in arcseconds (you can obtain the
necessary information from the astronomical
program that is used for processing the image).
The “thickness” of the galaxy NGC1032 is 180
pixels – Figure 4.
2.- Calculating the diameter of a galaxy:
Step 1. Using the image data library from skywatch.org, we can easily calculate any of the
visible dimensions of the galaxy. Here, we will
measure the “thickness” of NGC 1032.
Step 2.1. With the help of the LTImage software,
the required dimension is measured in pixels.
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Figure 4. The diameter of a galaxy in
pixels
Knowing the pixel scale of the image (0.28 arcseconds per pixel), we are able to work
out how “thick” NGC 1032 is in arcseconds:
180 pixels × 0.28 arcseconds/pixel ≈ 50 arcseconds
Step 2.2. Now all we need is to turn these 50 arcseconds into light years. It is an easy
task as we know r, the distance to NGC 1032: 117 000 000 ly.
Our circumference’s perimeter is:
C = 2 × 117 000 000 ly × π = 234 000 000π ly
An arcminute is 1/60th of a degree (remember that there are 360 degrees in a circle).
An arcsecond is 1/60th of an arc minute. Therefore, there are 1,296,000 arcseconds in a
circle! (1,296,000 = 360 degrees × 60 arcminutes × 60 arcseconds).
And if ‘x’ is the required dimension, then: 50 / 1 296 000 = x / 234 000 000π, and
x = 28 347 ly
We have just calculated that the diameter of the galaxy NGC 2032 is 28 347 light years.
Conclusion:
During this lesson students gain knowledge of galaxies and obtain skills for working
with astronomical programs for processing images. Thus, they calculate some of the
most important characteristics of galaxies (their size) and the way professional
astronomers determine these characteristics. Students will find information in Internet
about each of the galaxies and will fill in Table 2. Together students will make a portrait
of each galaxy.
References
Николов Н., Радева В., Илиева Е., Астрономия, Педагог 6.
http://www.sky-watch.org/
http://www.discoveryspace.net/
http://www.seds.org/messier/galaxy.html
http://www.schoolsobservatory.org.uk/
Images from Zsolt Frei’s “Galaxy Catalog” www.astro.princeton.edu/~frei/catalog.htm
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APPENDIX 1
IMAGES OF GALAXIES STUDENTS ARE GOING TO PROCESS
Galaxy
Distance
Ly
NGC 266
188.106
NGC 278
26.106
NGC 524
130.106
NGC 1161
27,9.106
NGC 2776
28,4.106
NGC 3193
60.106
NGC 3998
14,1.106
NGC 4995
30.106
NGC 7479
105.106
NGC 7331
46.106
NGC 6946
10.106
M51
37.106
M81
12.106
M101
27.106
Diameter
ly
Flattening
(for elliptical
galaxies)
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Type
Image