[based on activity
“Explore the Life Cycle of Stars using data from the Sloan Digital Sky
Survey” by Jordan Raddick (Johns Hopkins University), Theresa Moody, and
Dr. Wil van der Veen (New Jersey Astronomy Center)]
To create a
HR diagram with star brightness vs. color, astronomers measure a star’s
brightness at two wavelengths, usually in the blue and yellow part of
the spectrum. By comparing the amounts of blue and yellow light
astronomers determine the stars color.
To fairly compare star
brightness we need to know how far away they are. However, for stars
grouped in clusters, all the stars in the cluster are at about the same
distance, so we can make fair brightness comparisons without knowing the
actual distance. In this investigation, we’ll make three HR diagrams:
one of “Field stars” (stars not in a cluster), one of a nearby “open
cluster”, and one of a “globular cluster” above the plane of our galaxy.
We will use data from the Sloan Digital Sky Survey, whose main goal
is to obtain spectra for millions of galaxies and quasars (and some
sars), but in the process, it acquires brightness and color data on
It uses a reflector telescope that is 2.5 meters in
diameter and has a very wide field of view: about 3 degrees (6 full moon
diameters) across. The telescope is actually stationary, and it takes
images as Earth rotates so the sky appears to move by the telescope
All Sloan Digital Sky Survey data is available on the free web
site called SkyServer.
Computer with spreadsheet
software (e.g. MS Excel) and WWW access, specifically to the SkyServer
website - http://skyserver.sdss.org
Part A. Diagramming People
[inspired by the activity “Life Cycle of Stars” from the NASA Ceres
at the set of images of people in the table:
1. Can you guess
the sequence of ages of each of them? Going by only these pictures, copy
the list of names and put a number “1” by the youngest, number “12” by
the oldest, and assign 2 though 11 by the others in sequence from
youngest to oldest.
2. What characteristics did you use in order to
make your guesses? What physical characteristic data would you like to
have in order to make the best attempt for a correct sequencing?
Now look at the “WHA” (weight-height-age) Diagram of people. How would
you describe the relationships of weight, height, and age as shown in
Part B. Getting Data from the Sloan SkyServer
Go to the SkyServer web site - http://cas.sdss.org/
on “Search” in the first column “SkyServer Tools”
3. Click on
“Search form” (third bullet)
4. Click on “Launch the Search Form
5. Fill in the following [ra and dec are for Palomar 5 globular
Show me “stars” in the region “around”
ra: 229.022 dec: -0.111 radius (arcmin): 4
objects: click “All”
Image Data: check “object IDs,” “RA and
6. Then click “Generate Query” button
Output Format: “CSV”
8. Then “Submit Query to SkyServer”
“Opening result.csv window,” select “Save to Disk”
1-9 for the other target regions for “Open Cluster” and “Field Stars”
| Palomar 5
|| - 0.11
|| Globular Cluster
| NGC 2420
| near NGC 2420
Part C. Make HR Diagrams with Spreadsheet
1. Open the CSV file with a Spreadsheet program, e.g. Excel
Make two new columns,
one with formula g - r [example: column
and one with simply copy of r [example: column K:
3. Fill down to last row of data by clicking on upper left cell
[J2] and Shift click on lower right cell [K1425 in this example]
and filling down. Select the two columns of data.
4. Click on Chart Wizard
5. Choose "XY (Scatter)"
6. Fill in Chart Title, e.g.
“HR Diagram for
Value (X) axis:
“Star Color (g-r) [labels the
Value (Y) axis:
“Star brightness (r)” [labels the
and click “Next” then “Finish”
7. Adjust the axes:
Reverse the Y-axis, since magnitude scale is reversed (brighter stars
have lower numbers). Set appropriate scales.
8. Repeat steps 1-8 for
the other target regions (Open Cluster and Field Stars)
Analyzing the Diagrams
What can you tell about the age of each
Why would part of the Main Sequence be missing from
one of the clusters?
: As stars age, their position on the HR
diagram changes. They are no longer placed on the Main Sequence. They
are now placed in the Giant and Supergiant groups.
How would a
cluster HR diagram look when the cluster first formed?
would this differ from a much older cluster?
: What happened
to all of the large, bright blue stars on the Main Sequence of Graph 2?
Further: Get FITS images of the target regions.
1. Go to Sky Server
Tools and find the Navigate tool on tools menu.
2. Enter the
coordinates for each of the target regions, and use zoom.
3. Click on
the quick look tool on the right hand side of the page, and then click
4. Under PhotoObj in the left column, click on FITS.
Sroll down to Corrected frames, and right click on the files next to
“g” and “r”. For each file, choose “Save As”, and change the file
extension to .fts.
6. For these target regions, the area is always
about 10x14 arcmin.