by Jay Reynolds Freeman
I slept as late as I could. After I got going, I bought dinner for the friends who had done the same for me earlier in the week. My list of early-evening objects was cleaned out, so we had time for a leisurely meal. I arrived at the Onizuka Visitor Center late in twilight. For a few hours, I had nothing much to do but chat and look at the sky. Several staff members expressed opinions about the most impressive and spectacular celestial sight visible from Mauna Kea. There was general consensus what that was, and I agreed. The most spectacular cosmic view from Mauna Kea, and the finest deep-sky target I have ever looked at, is our own galaxy, not piecemeal, through telescopes, but as a single object, seen with the naked eye.
I was primed to ponder the view of the Milky Way from southern locations, by the experience of a fellow graduate student years ago. He came home from an observing run on a Chilean mountaintop, where Sagittarius culminates straight up, and couldn't rave enough about the spectacle. I could see what he was talking about from central California, knowing where to look and what to expect. Yet the view from far enough south to see well beyond Scorpius is unmatched: The Milky Way is a classic edge-on spiral, with central bulge and dark lane. In visible wavelengths, it looks rather like M104, but from our close-up, insider's viewpoint here on Earth, it spans the sky, horizon to horizon, and then some.
There are many decent photographs, montages, or synthesized images that show the entire Milky Way, or at least the central portions. My web search turned up several nice ones, including
http://www.astropix.com/HTML/D SUM S/MILKYWAY.HTM
http://www.nofs.navy.mil/projects/pmm/universe.html
http://adc.gsfc.nasa.gov/mw/milkyway.html
There are lots more. Look at one to understand what I saw.
The vast star clouds that obscure the heart of the galaxy are bisected by a straight dark lane, that passes north of the teapot and north of the Greater Sagittarius Star Cloud, then crosses the southern portion of the fishhook of Scorpius. That dark lane bifurcates on both sides of the galactic center; offshoots extend north of the plane. They begin far to either side, and widen out as they get closer to Scorpius and Ophiuchus. Thus the star clouds there appear framed by two long, moderately narrow "V"s of darkness, whose points lie in northern Aquila and in eastern Centaurus.
Stardust is visible on the galactic north side of the offshoots for much of their length. It defines their northerly boundaries. The northern dark offshoot, the Cygnus Rift, is well-known to northern amateurs. Some of us are car-crazed California kooks disguised as amateur astronomers: We refer to the long, curved streak of Milky Way at its northern boundary as the "Off Ramp". The wide end of the southern offshoot is visible from mid-northern latitudes, west of Scorpius, enough so that some native Americans described the combined complex of all these bright and dark areas as "the hands of Father Sky", but the full symmetry of the pattern is not apparent unless you can see all the way to alpha and beta Centauri.
From a site as dark as Mauna Kea, the vast amounts of dark matter in or near the galactic plane stand out readily against the glow of stars beyond. Most of the naked-eye star clouds located close to the line of sight to the center of the galaxy are relatively nearby. The actual galactic center is highly obscured. One web page I cited shows views in several wavelengths, including some that make it past the dust: They give an entirely different picture, one that is in some sense more realistic. Yet it is dark clouds that provide the specific character of the naked-eye view of our galaxy. The dark lanes themselves are the largest such areas that are clearly defined. The Coal Sack, a big blot at the southeast border of Crux, is probably the most famous cloud, but northern observers should not miss the Pipe Nebula, in southern Ophiuchus. It resembles a conventional smoker's pipe, with the bowl at the east and the stem running east-west. This nebula is about as long as the lid of the teapot of Sagittarius is wide, and is clearly visible to the naked eye from a dark site.
Bright star clouds that lie at least a little away from the direction to the center, are related to the large-scale structure of the galaxy. Most of these clouds appear where our line of sight passes along the axis of a major spiral arm, so that we see many of the bright stars within it. Stand close to a curving hedge row and you can understand this phenomenon. When you look at right angles to the shrubbery, you can perhaps see through it, but the view along the row, looking into it as it bends, is dense with leaves. So it is with stars in the arms. The eye of the knowing observer can ferret out the spiral structure of our home galaxy, even from deep within it.
Other physical features of the galaxy are detectable in the sky, if you know what to look for. The area from Scorpius through Centaurus contains many blazing suns of early spectral type. These young blue-white giants comprise one, or perhaps several, physically associated groups, located in places where substantial star formation has taken place in relatively recent times. My doctoral thesis research obtained a point measurement of the temperature, density, and velocity vector of the interstellar gas, close to the solar system, but before beginning to experience the local effects of the Sun. When I corrected the observed velocity vector for the motion of the Sun through the local group of stars, the interstellar gas appeared to be flowing outward from that vast collection of young giants. Perhaps past supernovae among them had caused the gas to expand outward, as within a swelling balloon or a growing bubble. How strange to find the placid tidewaters of the solar system subtly lapped by long swells from distant, powerful disturbances, hundreds of light years away, storms beyond the far horizon of the outstretched sea of space. As a kid, I dreamed of travel to the stars, yet it seemed perhaps that they had come to me, instead.
Presently alpha Indi rose, and then the Peacock Star, so I placed my telescope for a good view of the southeast horizon and picked up the last few objects on my primary lists. Yet these were generally nondescript. I logged them all, double-checked and re-checked that I hadn't forgotten any, then drove down the mountain. It had been a rather light session -- only 58 objects viewed -- but I was glad for the chance to relax under the night sky. Back in Hilo, I moved the telescope inside. On the next afternoon, I disassembled and packed it for return to the mainland, and mailed myself a box of dirty laundry and heavy telescope hardware, to lighten my baggage for the trip home. Yet there was one night's observing yet to come: I had planned to spend the final evening of my vacation with my 14x70 binocular.