Investigation 2-3:

Browsing The Universe

There is a myriad variety of celestial objects. Astronomers delight in describing, classifying, and naming them, but also grapple with trying to explain why they look the way they do.


* The instructions for this investigation have been adapted for use with SalsaJ software.  The Hands-On Universe Image Processing software (HOU-IP 2.0) formerly used with A Changing Cosmos is no longer supported by the US-HOU project. 

Part I: Browse

2.19. Using each of the files, browser1 through browser7, use and familiarize yourself with the following Salsa J functions:

• Open the image (file folder icon or “Open” in File menu).

• Use Zoom (in the Process menu) or Zoom icon (magnifying glass in Tools Palette) to enlarge the image.

• Go to Image menu, then "Lookup Tables" to change colors.

• Adjust "Brightness and Contrast" (also called Min/Max) settings to change contrast, brightness and improve the appearance of the image. This can be done in two ways: clicking on the brightness and contrast icon on the toolbar (half-filled circle) or by going to Image menu, choosing "Adjust," and "Brightness/Contrast."

SalsaJ screenshot

a.    Get or create worksheets on which to write a detailed description of the appearance of each object. See sample worksheet attached to the bottom of this page.

b. For each object, make a hypothesis about what type it is and why it looks the way it does. The following are a few questions to think about.  They may not all apply to each object, and you may choose other questions to explore.
    Is it solid or gaseous?
    Why is it dark or bright in certain areas?
    Are we looking at it from a side view or top down view?

c. Pick your favorite of the images, select the best color palette for the image with Look Up Tables and then adjust with the Min and Max tool. Then record your settings for the best display of this image. Optional: Print out or save your image (both options are under the File menu). If your printer is a black and white one, you probably should use the grey or igrey palettes.

Part II: Galaxy Features

If you are fortunate enough to view the sky from a place far from city lights—the mountains, desert, or a remote area—you may see the largest and most beautiful sky object visible without a telescope: a large cloud-like band where there are many more stars than anywhere else in the sky. It is called the Milky Way, and astronomers have studied it, concluding that we live in a galaxy—a huge collection of billions of stars—that we call the Milky Way Galaxy. If we could go outside our galaxy, it would look like this:

In the 20th century, astronomers discovered other galaxies than our own, some larger than our Milky Way Galaxy, many smaller. There are different types of galaxies. Our own looks like a spiral galaxy. Spiral galaxies have a lot of dust and gas with stars forming in them. The famous Orion Nebula is a star-forming region in our own galaxy. Galaxies NGC 4636 and NGC 4697 in the “Galaxy Atlas” on the next page are not like spirals. They are simply a lot of stars clumped into the same region of space with no measurable interstellar dust or gas and no new stars forming now. They look like ellipses, so astronomers call them elliptical galaxies. Finally, there are peculiar galaxies. They are not spiral or elliptical.Sometimes galaxies crash into each other. The two peculiar galaxies in this unit, NGC 2146 and NGC 3034, are interesting because they have an enormous amount of dust and gas, so many stars are being born in them right now.

2.20. Describe and categorize eight galaxies. Get or create a worksheet like the sample at the bottom of this page. With your computer, open images galaxy1, galaxy2, etc. one at a time. [All images came from the Leuschner Observatory which is operated by the Astronomy Department of the University of California at Berkeley.]. On your worksheet, draw a quick sketch of the galaxy and compare it to the ones in the Galaxy Atlas. Decide whether it is a spiral galaxy, an elliptical galaxy, or a peculiar galaxy and record that on the worksheet. Then identify different features—see if any of the ones described below are present. Change the Brightness/Contrast, Minimum and Maximum settings to better bring out its features. Changing color palettes is often very helpful, too.

 Useful Terms
 1.    Galaxy nucleus:  Almost all galaxies have a nucleus. It is the bright central part of the galaxy. Galaxy nuclei are made of millions of stars and tons of dust and gas (if available). There is reason to believe there might even be enormous black holes in the center of galaxy nuclei.
2.  Foreground Stars:  You know what stars look like. They are the bright points of light in your image. Foreground stars are ones inside our own galaxy that lie between us and other galaxies. They are not part of the galaxy in the image. We have so many stars in our Milky Way galaxy that all of the images in our collection include foreground stars.
3.    Spiral Arms:  These are the features that give spiral galaxies their name. Only spiral galaxies have them. They are spiral shaped regions of dust, gas, and stars where star formation is occurring.
4.    Bar:  An interesting feature in many spiral galaxies is a bar running through the middle of the galaxy nucleus. While there are many theories about why this feature forms, astronomers are not completely sure why they do. There are many things in astronomy that are not known.
5.    Ring:  Similar to the bar, except that this looks like a ring around the galaxy nucleus in some spiral galaxies. Like galaxy bars, astronomers are not 100% sure why the rings form or why they form in some galaxies and not in others.
6.    H II Regions (pronounced “H 2 Regions”):  Areas of star formation. Young, hot stars heat the dust and gas around them, causing the dust and gas to radiate light. These appear as faint balls of light. Elliptical galaxies do not have H II regions because there is little dust and gas in these galaxies. HII Regions are made up of ionized hydrogen, the nuclei without its electron.
7.  Dust Lanes:  Dark bands of dust that block the light from a galaxy. If you look closely at the two peculiar galaxies, you will see that both have dust lanes.
8.  Companion Galaxies:  A galaxy that orbits around another galaxy the way the Earth orbits the Sun. These galaxies can interact with their parent galaxy and change the parent galaxy’s appearance.

Galaxy Atlas

Spiral Galaxies
NGC 4321
  NGC 4321 (M100)
 NGC 2841
NGC 5194 (M51)
T.A. Rector and Monica Ramirez?NOAO/AURA/NSF
 Elliptical Galaxies
NGC 4636
Courtesy Digital Sky Survey
NGC 4697
Daniel Verschatse - Observatorio Antilhue
Peculiar Galaxies
NGC 2146
M 82
Mark Westmoquette (University College London), Jay Gallagher (University of Wisconson-Madison), Linda Smith (University College London), WIYN/NSF/NASA/ESA

Part III: Image Data vs Image Display

a. Pick any image of the ones you have opened, and open it twice. For example, if you open browser6 twice, you will get two windows titled browser6:1 and browser6:2. Choose an appropriate zoom size so that you can put the two windows side by side on your computer screen.

2.21 With progressively larger and larger values for Zoom, at what zoom value do you clearly discern the individual picture elements—pixels—as little squares? Within each square, does the color or shading vary? And if you Zoom the image even more, does that color within each pixel change?

b. About the Image Contrast—Min/Max tool: This controls the shading (or the coloring). If you set the palette color as Grey, pixels dimmer than Min will be black and pixels brighter than Max white. Everything in between will be a shade of grey. The software assigns shades of grey or colors across the range of brightness between the Min and Max. Changing the values to make the range narrower brings out more detail in the parts of the image whose brightness is within that narrower range.

c. Cursor Information: The (x,y) coordinates and the value (brightness) for the cursor’s position are displayed in the Pixel Coordinates area of the Yellow Task Bar. Each (x,y) pair of coordinates identifies a specific location—picture element—or pixel for short.

2.22 What are the dimensions of your “Display Region” (in pixels of the image displayed without scrolling)? Find the (x,y) coordinates of the bottom left corner of the window and then the (x,y) coordinates of the top right corner of the window. What are the window’s dimensions?

There are two ways to do this. One way to calculate display region is to look at the X and Y coordinates of the bottom left-hand corner and the top right-hand corner. Place your cursor over the bottom left-hand corner until the X value = 0. The Y value listed is the height of the display region. Then, place cursor in the top right-hand corner until the Y value = 0. The X value listed is the width of the display region.

The second way to do this is to access Image  in menu, then click on scale. Make sure X Scale and Y Scale are both set to 1. The Width and Height listed are the dimensions of display region.

2.23 How do the dimensions of the Display Region change when you change the Zoom Factor to other values? Since you have two windows of the same image open, you can easily compare different zoom values by zooming in one image and leaving the other the original size.

2.24 Does the position image data (x,y coordinates in the status bar) of a particular star or feature on your image change when you change zoom value?  Change zoom factor; find the star or feature; click the cursor on it, and read “x,y” in the Yellow Task Bar.

2.25 Does the brightness data (Counts in the Pixel Coordinates area) change when you change thes image display functions with the Min/Max tool?
  • Go to Image, then "Adjust," and "Brightness/Contrast" to bring out detail in dimmer parts of an image. You may need to adjust Min/Max to enhance the features you are interested in. Did you note the better detail in the lower part of the image?
  • You can return to original Min-Max settings by selecting “Reset”  in the "Min-Max" tool.  Click on the brightness and contrast icon on the toolbar (half-filled circle) or go to Image menu, choose "Adjust," and then "Brightness/Contrast."  Then, click on the button that says "Reset."

d. With Min/Max and Brightness/Contrast adjusted in one image and original values in the other, the two images look very different.

e. From Image menu, go to "Lookup Tables" and select "Show LUT". With the same image in two windows, you can compare changes in settings. The LUT window only refers to the active image, the one with its title bar highlighted. The LUT bar shows the range of colors in the active window and shows the relationship between the colors brightness in Value.

2.26 When you change the Min-Max setting, does relationship of colors and brightness shown in the LUT table change? If so, how?

Alan Gould,
Dec 10, 2015, 9:10 AM
Alan Gould,
Dec 10, 2015, 9:10 AM