Deep field images never disappoint, do they? For the novice astronomer, however, these images need quite a lot of interpretation. I usually start by identifying different types of objects in the image, and help students figure out how to identify them. The spikey objects are stars (objects like our Sun) in the foreground. These stars are bright enough that the bits of light deflected by the telescope itself are noticeable. (This is a gloss on diffraction, of course, but it’s enough for early in the semester!) A student might notice that the diffraction spikes.
The smaller, fainter objects are nearly all galaxies. This gives me the opportunity to mention what a galaxy actually is: a conglomeration of billions or trillions of stars, plus dust and gas. I’ll ask them to estimate the number of galaxies in this image, and accept any number that is in the thousands. So then I ask them to think about how many STARS are represented in this image—including the stars in the galaxies!
Next, I’ll point out that galaxies come in several shapes. I’ll point out a nice face-on spiral (there is a particularly nice one just above center on the right), and then an edge-on spiral (there are lots!). Then I’ll point out a smudgy elliptical galaxy, like the one near the center of the image.
I’ll mention that those are the two basic shapes of isolated galaxies, and then I’ll draw students’ attention to the gravitationally-lensed galaxies that appear as arcs along concentric circles around the big elliptical near the center of the image.
I’ll often ask students to notice the colors of the galaxies. At the beginning of the course, it’s too much to ask them to understand WHY more distant galaxies are redder, in general. But I ask them to notice that the galaxies shown have colors, and that distance is one of the things we can figure out from those colors.
Finally, I circle back to the most accessible big WOW of these kinds of images: a feel for the size of the universe. The image took 12.5 hours to build from lots of shorter images, and “zooms in” on a section of sky as small as a grain of sand held at arm’s length. In order to get a sense of the scale of the universe, students must imagine wallpapering the sky with grains of sand…and then replacing every single grain with an image like this one, including its thousands of galaxies.
Supporting material in the texts and online: This image provides a great opportunity to talk to students about “What an Astronomer Sees”--how astronomers draw meaning out of an image by closely examining shapes, colors, and relationships. You might reference this image in Chapter 1, as a final “zoom out” from the Figures that develop the universal address. Other connected material appears in:
The Light and Telescopes chapter, where diffraction makes an appearance (these are particularly beautiful diffraction spikes, if you are into that sort of thing!).
Evolution of High-Mass Stars, where it first becomes possible to discuss gravitational lensing.
Hubble Law, where the connection between redshift and distance is made concrete.
Formation and Evolution of Structure, which connects this image to the larger mission of JWST to find the first stars and galaxies, and finally answer these questions about how the universe has changed since very early times.
The Learning Astronomy by Doing Astronomy Workbook, Activity 32: Hubble Deep Field North is a great activity for introducing the ideas that could be explored by astronomers in the newer James Webb image.
Two related Astronomy in Action videos: Galaxy Shapes and Orientation and Expanding Balloon Universe.