WK04_Star_Cluster_Distance
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School
Indiana University, Bloomington *
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Course
A105
Subject
Astronomy
Date
Dec 6, 2023
Type
docx
Pages
4
Uploaded by DrIron12403
A105 – Week 4 Exploration
Name
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Distance to a star cluster
You will be able to
Estimate the brightness and temperatures of stars from an image of a star cluster.
Use measurements of the brightness and temperature of stars to determine the distance to a
star cluster.
What’s the idea?
The parallax method can be used to determine the distance to a star, but only if it is not too far from
us (beyond few hundred light years the parallax is too small to measure). What to do if it is more
distant? There is one specific method that allows us to determine the distance to stars in a star
cluster. Now, star clusters are small (few light years) in comparison to how far they are from us
(thousands of light years). So, it is helpful to think of all the stars in a cluster as being at
the same
distance
from us.
How can we determine this distance, the goal of this exploration? First, if we were to look at the HR
diagram containing the stars of some cluster, we expect it to have the main sequence: hot, luminous
stars left and top, and cooler, less luminous stars right and bottom. That’s because when a cluster
forms, there will be a range of stars with different temperatures in it. Typically, to make an HR diagram
we need to know the true brightness of stars because just the apparent magnitude also depends on
the distance. However, for stars in a cluster, that are all
equally distant from us
, it is safe to assume
that a star that
looks brighter
than some other star in a cluster is truly brighter (
more luminous)
than
that other star. In other words, for a cluster we can produce an
HR-like
diagram (main sequence and
all) based on the
apparent
brightness of stars. What has this to do with the distance? Well, if the
cluster is far, then all of its stars will appear fainter, and if it is closer, then they all look brighter. Here is
how our HR-like diagrams would look for clusters at different distances. (Note the main sequence is
cut off in the right diagram.)
Distance is indicated at the top. Brighter stars are up. See how the main sequence slides downwards
(gets fainter) for more distant clusters.
So, to summarize, by establishing the apparent brightness of the main sequence we can (only in the
case of clusters) estimate its distance. Now, let’s actually do that.
Instructions
Go to the star cluster web tool at
https://astro.sitehost.iu.edu/ala/clusters/index.html
.
The star cluster web tool allows users to explore the properties of many star clusters in our sector of
the Milky Way galaxy.
We'll use this tool to measure the brightness and temperatures of stars in the
star cluster Messier 21 (or M 21).
To select it, click on the “Select Cluster” button on the web tool and select “m21” from the pull-down
menu. A color image of M 21 will appear in the image box.
Notice that you can move the image around to place any star adjacent to the measuring guide on the
right side to estimate the star’s color and apparent size. In an image, the apparent size of the dot is
related to a star’s apparent
brightness
, not its actual size.
Our goal is to measure the brightness and temperatures of stars that are
members
of the M 21.
However, not all of the stars in the image belong to the cluster. Some may lie closer or further away
than the cluster itself and just happen to be in the same direction as the cluster. So, only the stars that
are
circled
in the image below are known to be the members of the cluster M 21.
Task: Determine the brightness and temperature of each of the
member
stars by comparing each one
with the measuring guides for color and brightness in the web tool. Record your measurements on the
brightness/temperature grid provided on the next page by placing (select and drag) the cross symbol
in the box corresponding to each star's temperature and brightness.
To get full credit you need to measure at least 15-20 stars!
← Stars appear dark here because
this is a “negative” (inverted) image.
Makes it easier to see things stars.
Try to measure stars in some order (top to bottom, left to right) so you know which ones you did. It
may be helpful to print the image and mark it off once you have measured and recorded each its
brightness/temperature grid.
After the designated stars have been measured, compare your main sequence to those on page 1
and estimate the distance to M 21.
What is your estimate of the distance to M 21 in light years? We need
your
estimate, not the value
from the web.
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Now look the cluster up on the web to learn more about it properties, and answer the following
questions.
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Toward which constellation does M 21 lie?
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What is the approximate age of M 21?
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About how many stars does M 21 contain?
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Extra credit (0.5 pts). How do you think that the procedure used by the astronomers may differ from
what you did?
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