Download P10263v1.2 Lab 8 Text

Physics 10263 Lab #8:
The Milky Way Galaxy
Introduction
In this lab, we’ll extend the capabilities of Starry Night
and take advantage of the Internet to learn about coordinate
systems and our home galaxy, the Milky Way.
Because we live in a galactic disk, our view of the night sky
is profoundly affected, in obvious and subtle ways. For
example, the billions of distant, unresolved stars in the disk
of our galaxy stretch around us in a ghostly band that rings the
sky. We call this the Milky Way. The dust from the disk that
surrounds us substantially limits our ability to look for
objects outside of our own galaxy, particularly in the direction
of the surrounding disk.
Because of this, Astronomers thought that our galaxy was the
only large object in the Universe until the first half of the
20th century. Other galaxies were too faint or too obscured by
clouds of gas and dust for us to examine closely to determine
their distances. Even after other galaxies were discovered
outside of our own, we still had trouble seeing objects outside
our own galaxy in the direction of the disk. Less than 10 years
ago, a huge, faint companion spiral galaxy to our own, Malin 2.
It hadn’t been seen previously due to the fact that it is
blocked by the interstellar medium in our galaxy and the fact
that it has a very low surface brightness.
The structure of the Milky Way also affects our observations
of objects within the galaxy. For example, even though we know
that stars are more or less uniformly distributed around the
Milky Way ring, there are places where the star counts vary by
factors of hundreds or even thousands. This is due to gas and
dust obscuring our line of sight so that only the brightest and/
or nearest stars are visible. Without taking this into account,
our theories about the structure and dynamics of the Milky Way
would be woefully incomplete.
The gas and dust itself has many interesting properties worth
exploring. Photometry and spectroscopy of these clouds of gas
and dust and varying wavelengths reveals properties such as
mass, size, temperature, composition and motion. Often, the
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thickest clouds along our line of sight harbor forming stars
that can only be seen with very long wavelength observations,
using radio light that is less prone to scattering by dust and
gas.
Because the sky looks so different near the ring of the Milky
Way, we often use a Galactic coordinate system to find our way
around the sky. Much like the Earth’s equator marks 0 degrees
latitude on the Earth while the Earth’s north and south poles
mark +90 and -90 degrees latitude, we consider the Milky Way in
the night sky to mark the galactic equator. So the galactic
equator makes a great circle spanning the entire sky, parallel
to the faint band of the Milky Way itself.
Objects observed at low galactic latitude (near the galactic
equator) tend to be obscured or otherwise affected by the
presence of gas and dust along one’s line of sight. Conversely,
objects near high galactic latitude (near either the North or
South galactic poles) are often easily seen, even if they are at
enormous distances (like distant galaxy clusters).
Step 1
Launch Starry Night, and set the time to 8pm tonight. Use
the Options Tab in the sidebar to turn on planets and
constellations along with labels for each. Answer the questions
on your worksheet about Jupiter and Saturn in order to
familiarize yourself again with the Starry Night controls.
Step 2
Next, let’s take a look at a couple of useful coordinate
systems Astronomers use to find their way around the night sky.
Use the Options tab on the sidebar, and look under “Guides” and
“Celestial Guides”, then turn on “Grid” and “Poles”. This
produces a faint grid on the sky that is the Earth-based
celestial coordinate system. Use the zoom feature to zoom all
the way out so you can see the whole celestial sphere.
Further down the options sidebar, under “Local View”, turn
off “Local Horizon”. Now scroll around so that the North
Celestial Pole is centered on your screen and zoom in until you
only see one or two stars in your field of view. What is the
name of the brightest star near the North Celestial Pole? Do
the same thing for the South Celestial Pole. For the star
closest to the South Celestial Pole, write down its name and the
name of the constellation in which it is found.
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The equatorial coordinate system’s gridlines are based on the
latitude and longitude grid of the Earth’s coordinate system.
Our galaxy, however, does not naturally align itself in Earth’s
coordinate system. In fact, our solar system is tipped about 60
degrees with respect to the plane of the Milky Way galaxy’s
disk. Now turn off the equatorial guide and activate the
Galactic Guide. This shows you a spherical grid of another
coordinate system.
In the Galactic coordinate system, the equator is defined by
the plane of the Milky Way. The North and South Galactic Poles
lie along an imaginary line perpendicular to the plane of the
Milky Way’s disk. Find the HIP designations of the two
brightest stars near the North Galactic Pole, also known as
30 Com and 31 Com.
Step 3
For the next part of the lab, we’ll look at some maps of the
night sky in galactic coordinates generated by another program
and placed on a web page. Quit Starry Night (don’t save any
settings). Open up an Internet Browser using a shortcut on your
desktop. Now load the following URL:
http://personal.tcu.edu/dingram/phys10263/lab8.html
Follow the instructions there and answer the appropriate
questions on your worksheet.
Essay
Instructions for your essay are located on the web page.
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Lab #8 Worksheet
Name:
Home TA:
Step 1
In which constellations are Jupiter and Saturn?
_______________ and __________________
Write the names of the three brightest stars in each
constellation, with Bayer designations Alpha, Beta and Gamma
(turn on star labels and “show info” on the bright, named stars
to find this information).
_______________, _______________, _______________
_______________, _______________, _______________
Write the name of a bright star (with a proper Arabic name)
close to zenith at 8pm tonight (in the options tab on the
sidebar, under Guides, under Alt-Az Guides, select “Poles” to
show the zenith on the sky).
____________________
What constellation is this star close to the
zenith a part of?
___________________
Step 2
What is the name of the brightest star near
the North Celestial Pole?
What is the name of the star closest
to the South Celestial Pole?
__________________
_________________________
What is the name of the constellation in which
the South Celestial Pole is found?
______________
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What are the HIP numbers of the two relatively
bright stars next to the North Galactic Pole?
____________,____________
In what constellation are these stars found?
_________________________
Step 3
Which type of stars are more concentrated toward
the disk of the Milky Way galaxy?
__________
Which of these two distributions of stars is younger?
_________
Explain your answer in the space below, based on what you’ve
learned in the lecture portion of the class.
Explain why most visible galaxies appear to be located outside
the disk of the Milky Way galaxy.
Essay
Complete your essay on your own paper and attach it to the
lab worksheet when you are finished.
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