Natural History Magazine
With images as good as the Hubble Deep Field, one can gaze through time at the colors and shapes of galaxies.
Every now and then, a single photograph appears in the press that somehow forces you to take pause and reassess your place in the universe. In the 1960s, the first photograph of Earth from space reminded us that, as geologists had been telling us for some time, land masses do not have political boundaries drawn upon them—we were all together on “spaceship Earth.” Then there was the well-publicized photograph of Earth-rise over the barren lunar horizon taken by the astronauts of Apollo 8, the first manned mission to orbit the Moon. Earth looked small, fragile, and distant—just another orb out there in space.
For me, one of the Hubble Space Telescope’s recently released photographs, now known as the Hubble Deep Field, ranks among the world’s most profound images. It seems to have the right ingredients. It is unfamiliar. It is otherworldly. And it lures me someplace I have never been before.
What makes the Hubble Deep Field so special? The image grants the viewer a peek at a remarkably detailed subset of the billions and billions of galaxies in the universe, captured in a time line that spans from the first few billion years after the big bang, all the way to the present. Astrophysicists have been peeking at the universe with telescopes for 400 years, so the act of peeking itself is nothing new. But what the Hubble Telescope provides (by virtue of its above-the-atmosphere venue) is the highest resolution, and thus the clearest view, of the universe ever achieved in the history of optical telescopes.
Many ground-based pictures already exist of the seemingly countless galaxies in the outer universe, but in all cases the galaxies appear as undistinguished smudges. If you had bad vision you would encounter a similar problem when you looked at a lawn: you are told it is a lawn; you know in your mind it is a lawn; but all you notice is a sea of green, and you are not forced to think deep thoughts about what’s there. With good vision, however, the green lawn is revealed to be composed of multitudes of blades of grass. There are even insects crawling about. You are now forced to recognize the lawn to be a world unto itself.
The Hubble Deep Field is a small, specially chosen, random, boring patch of sky that covers less than one one-hundredth the area of the full moon. To be specially chosen, yet random, simply means that a random field was selected among all fields that: 1) could be monitored continuously by the Hubble Space Telescope, without the Sun or the Earth getting in the way; 2) was away from the plane of the Milky Way galaxy, where densely packed stars, gas, and dust clouds obscure our view of the rest of the universe; 3) was void of bright stars that might become over-exposed; and 4) was not coincident with clusters of galaxies that have already been cataloged.
The Hubble Deep Field, a full-color image created from 342 repeated exposures, was taken by the Hubble Space Telescope during a continuous stretch of orbits that spanned ten consecutive days, which represents far more observing time than is ever granted to an individual research project. In practice, long exposures are created by adding together many repeated, shorter exposures. With each added image, objects in the field of view become more and more pronounced against the background, which enables dimmer and dimmer objects to come into view. One of several reasons for this tactic is that if a hardware or software problem arises within a single image, then you still have all the rest of the images to add together, which may still enable you to accomplish your scientific objectives.
To observe with the Hubble normally requires that an astrophysicist write a detailed proposal that states and defends the scientific motivations of a project, the target objectives, and why the project must be accomplished from orbit rather than from the many available ground-based telescopes. The proposal is then reviewed and critiqued by a committee of peers and awarded observing time on the basis of merit. Often, more than twice as much observing time is requested than is available, so most proposals are awarded no time at all. But thanks to something called “director’s discretionary time,” Robert Williams, the Director of the Space Telescope Science Institute, was able to do what nobody else would have been permitted to do: point the telescope in a random place just to see what’s there.
At about 1⁄100 the area of the full moon, the Hubble Deep Field sampled only about 1⁄15,000,000 of the 41,000 square degrees of the entire sky. Even so, this single image reveals thousands galaxies. If we diligently count every one of them—from the large, bright ones, down to the small, faint ones—and then multiply the result by 15,000,000, we get a fast estimate for the total number of observable galaxies in the universe. When the Hubble Deep Field image was released, media accounts (based on the NASA press release that accompanied it) widely reported that there are five times as many galaxies in the universe than previously estimated, raising the count from 10 to 50 billion. Actually, even from fuzzy ground-based images, the estimates had already ranged from a lower limit of about 10 billion to an upper limit of about 100 billion, depending on how thoroughly you believed that the population of under-luminous dwarf galaxies had been counted. What the headlines should have said was that data from the Hubble Deep Field allow us to confidently raise the previous lower limit from 10 to 50 billion galaxies.
As you watch astrophysicists bandy billions, it may look as though we are clueless about the galactic contents of the universe. But when you consider that all numbers above a trillion (of which there are many) and all numbers below a billion, are not in the running, then the range in our ignorance is quite small.
For me, what is most striking about the Hubble Deep Field is the richness in morphology revealed in even the tiniest of galaxies. The photogenic spirals, kindred forms to our own Milky Way, show the characteristic central bulges and the knots of freshly made stars that dot the spiral arms. Each of these galaxies, however small they appear in the image, is its own collection of hundreds of billion of stars. There are other galactic forms such as elliptical and irregular galaxies. Though less photogenic, they too are part of the cosmic census.
The colors of galaxies are dominated by the colors of the most luminous resident stars. Bluish galaxies tend to have active areas where stars are forged; assortments of freshly made stars typically contain extremely hot (at least 20,000 kelvins), ultra luminous blue giants. Reddish galaxies contain relatively cool (3,000 kelvins), yet ultra luminous, red giant stars. In any galaxy, the absence of blue reveals the absence of stellar nurseries, which generally implies that the gas content (from which stars are made) was exhausted long ago.
But “long ago” is looking straight at us. On average, we expect the smaller galaxies to be farther away than the larger galaxies. And their light has been traveling longest in time to reach us. In other words, we see them not as they are, but as they used to be. As sedimentary deposits on Earth indicate a geological time line, distant objects betray a time long passed in the history of the universe. No new concepts here: Light from your elbow, provided it is where it belongs (hinged from your shoulder) is about a nanosecond (a billionth of a second) away in light travel time from your body’s light detector known as the retina. The Moon: about 1.5 seconds away. The Sun: 500 light-seconds. The nearest star: 4.1 light years. The beautiful spiral galaxy “M100”: 65 million light years. (Yes, voyeuristic residents of M100 could now be watching Earth’s dinosaurs go extinct.) Most of the galaxies in the Hubble Deep Field are billions and billions of light years away. The light we now see left their stars before single-celled life began on Earth. And in some cases, before Earth, itself.
With images of the quality of the Hubble Deep Field, one can even begin to test for evolutionary trends in galaxy colors. As stars are forged out of interstellar gas, less and less gas remains to create subsequent generations of stars. Eventually, stars stop forming. We would thus, on average, expect the more distant galaxies to be bluer than the nearby galaxies. A first estimate of the distances to these galaxies can be made from the Hubble data, but more reliable distances must be obtained from follow-up measurements. Until that happens, our evolutionary interpretation retains a level of uncertainty: are the smallest galaxies on the image small because they are “normal-sized” galaxies that happen to be far away? Or are they small because they are, indeed, dwarf galaxies that happen to be right in front of our noses? The real answer is likely to be some combination of these scenarios, with the distant galaxy description being the dominant of the two.
Let there be no misunderstanding: large, ground-based telescopes (such as the 10-meter Keck telescope in Hawaii) have detected galaxies as far away as the farthest galaxies in the Hubble Deep Field. Telescopes of the ten-meter class have over twenty times the light gathering power of the Hubble Space Telescope. But somehow, the relatively fuzzy, ground based images do not act as powerfully on my imagination. I am captured intellectually, but not emotionally. Only with sharp images am I viscerally reminded me that there are other worlds out there. Billions and billions of them.
In spite of the quality and beauty of the Hubble data, many scientific questions remain unanswered: Do we have the right to extrapolate what we learn from one postage stamp-sized region of the sky to the entire universe? How many more galaxies might have revealed themselves with an even longer exposure? How far away is the farthest galaxy? How soon after the big bang did galaxies form?
Many questions also remain that do not lend themselves to immediate scientific inquiry: Is there some undiscovered law of physics that will completely change our modern understanding of the cosmos the way Einstein’s theories of relativity redefined our understanding of the physical universe? On planets around stars in the galaxies of the Hubble Deep Field, are there life forms that are contemplating the universe the way we are? Or are they not paying attention because they are just looking for shelter, food, and sex, as does most life on Earth?
As I “galaxy-gaze” through time upon their diversity of colors, shapes, sizes, brightnesses, and structural detail, the boundary between knowledge and ignorance calls to me. When I reach for the edge of the universe, I do it knowing that along some paths of cosmic discovery, there are times when, at least for now, one must be content to love the questions themselves.