INTERLUDE 26-1 A Stunning View of Deep Space
As we probe deep into space we are looking far back in time. Telescopes are time machines, and astronomers are historians. Now, thanks to the Hubble Space Telescope, we have the deepest, most detailed optical view of extragalactic space yet obtained. Called the Hubble Deep Field, it is shown here in approximately true color. It was assembled from nearly 300 exposures taken during more than 100 orbits over the course of 10 consecutive days, for a total exposure time of about 100 hours. All the data were combined to make this one spectacular image. (Figure 24.24 shows a smaller view of the same field.) The field is in a region of the sky near the Big Dipper that happens to lie in a direction perpendicular to the congested plane of the Milky Way.

Virtually every blob of light in this image is a galaxy. (A few clear exceptions are local stars in our own Milky Way, such as the bright star with the artificial spikes at the bottom left of the image.) In all, approximately 1600 galaxies of all types can be counted in this one image. Hundreds of these galaxies are so faint that they have never been seen before. The image extends to 30th magnitude, making the Hubble Deep Field the most sensitive photograph ever taken. (Sec. 17.4) The image is "deep" in the sense that it allows us to see very faint objects. The word deep should not be construed to mean that this image goes beyond the realm of ground-based telescopes. Hubble does not see significantly farther into the universe than any other large telescope, but it does see more sensitively; hence it is able to pick up light from very dim objects and often to resolve them better than ground-based instruments.

The Hubble Deep Field covers only a tiny part of the entire sky. The field of view shown here is just 2' across. To map the entire sky to this depth would take Hubble nearly a million years! One way to put this image into a larger perspective is to think of it as a core sample, much like the sample a geologist might take on Earth. By examining the dirt and rocks in their samples, geologists try to reconstruct the history of events that occurred as Earth evolved. Likewise, by studying the galaxies in the Hubble Deep Field, astronomers seek to understand better the history of the universe.

 The image shows many galaxies of many ages. Disentangling young, distant galaxies from old, nearby ones is tricky, but as a general rule of thumb (and perhaps a little counterintuitively, given that we have gone to some lengths to stress that distant objects are characterized by large redshifts) the blue objects are probably the farthest away (since we see their stars in their blazing youth), whereas red objects tend to be closer (we see them by the light of their old, reddened stars).

Of particular interest are the small, faint smudges of blue light in the image; they may well be the faraway protogalaxies that oßriginally gave rise to the galaxies we see today. If this is so, then some of the faintest galaxies in this image existed when the universe was a mere billion years old, eons before the birth of our solar system. Only spectroscopic observations can tell if the dim smudges are truly distant. The big ground-based telescopes in Hawaii, Chile, and on Kitt Peak are better suited than Hubble to determine redshifts, and this work is now under way. The second image shows the same field of view at left but with some redshifts marked—note the enormous spread in distances (see Table 25.1) evident in even the small sample of redshifts measured so far.

Although the Hubble Deep Field covers but a speck of the sky, astronomers consider it to be representative of the typical spread of galaxies in space. This is another way of stating the cosmological principle—statistically, the universe on the largest scales should look pretty much the same in all directions. Extrapolating the contents of this new image over the entire sky, astronomers then estimate that the total number of galaxies in the observable universe is approximately 40 billion. Contrary to sensationalized news accounts, this is not an astonishingly larger number of galaxies than previously thought. Rather, it is midway between the 10 billion and 100 billion galaxies that astronomers for decades have reasoned should exist in the observable universe.