This image, the M31 Deep Field, may not look like much. It doesn’t have the bold colors and dynamic composition of many Hubble images. Yet it’s one of my favorite images because of what we can see if we look a little harder. In fact it shows nothing less than the entire sweep of the cosmos from our “back yard” to the incomprehensibly vast depths of space.
The image is a composite of numerous long exposures with Hubble, staring at a patch of sky near the magnificent Andromeda Galaxy, a.k.a. Messier 31 (M31 for short). Sound familiar? Hubble has produced many “Deep Fields” starting in 1995, taking long exposures of small patches of the sky. Most of these fields were intentionally pointed away from anything relatively bright or nearby in order to explore the farthest reaches of space. In this case, Hubble was pointed toward one of our nearest neighboring galaxies, a near twin of our own Milky Way, to study the population of faint stars in its outer regions.
Wide-field ground-based images of M31 like the one above, made with a wide-field telescope at the great Palomar Observatory, mostly show the huge, bright disk of stars, gas, dust, etc., tilted to our line of sight. In addition there are a couple of small satellite galaxies accompanying M31. This is very much like what our Milky Way Galaxy would look like if we could fly far away and look back toward home. What this image doesn’t show well is the large, spherical “halo” of fainter stars that surrounds the galaxy and stretches to very great distances. Studying the stars in this halo was the reason for Hubble’s observations. The small green box shows the tiny size of the much more detailed Hubble image.
Unlike many of Hubble’s color images, this one was made using only two filters rather than the usual three, sampling the V and I bands. This was just what was needed for the science analysis to measure brightnesses and colors of the many stars in the field. A glance at the two images separately shows little difference. It is possible to construct a color composite from just two images, but the range of colors is not as varied as with three component images. When we apply color, composite the images together, and make some adjustments to brightness and contrast, the color pops out nicely.
Let’s zoom in on a few smaller pieces of the image.
The first few details show lots of stars with a range of brightness and colors and a smattering of galaxies of different types. In this case, the brightest stars are in our Galaxy, maybe a few thousand light-years away. The fainter stars scattered through the images are in M31, 2.5 million light-years away. And the galaxies, which are all roughly the same physical size as the Andromeda Galaxy, appear much smaller because they are vastly farther away: tens of millions, hundreds of millions, or even billions of light-years away, a mind-boggling range of distances in just one tiny piece of sky!
Other details from the image show galaxies of very different types, in this case a nicely formed spiral seen face-on, much like our Galaxy and Andromeda, but also very different ones, like the small, bluish dwarf that shows very little organized form and no obvious gas, dust or anything else other than stars.
Most amazing to me though is the final detail. It shows a star cluster just at the bottom edge of the Hubble image. This is a particular kind of object called a globular cluster, a large grouping of maybe hundreds of thousands of stars that formed together a long time ago and are still bound together by their mutual gravity. It looks for all the world like images of globular clusters in our own Galaxy that we’ve seen for many years. But this cluster is 2.5 million light-years away in the Andromeda Galaxy, about 100 times the distance to most similar objects in our Galaxy. Yet we clearly see individual stars thanks to the dramatic improvement in resolution provided by the Hubble Space Telescope.
The deep image focusing on the halo of M31 is visually subtle but conceptually mind-boggling, showing the vast sweep of the universe in a single view.
For more information about this and other Hubble images, visit HubbleSite.