It’s only been about ninety years since the then new 100 inch Hooker telescope was installed at the Mount Wilson observatory in California. It was the largest telescope built to that point and would reveal whole new dimensions of human cosmic understanding. It resolved the Andromeda Nebula to the point that individual stars within it could be viewed and one of those stars, a ceheid variable star provided the evidence that Astronomer Edwin Hubble would use to determine that Andromeda lay beyond the Milky Way galaxy, and indeed was a galaxy unto itself.
Hubble’s discovery of galaxies beyond the then known universe of the Milky Way expanded human perception of the universe from a mere 100,000 light years to billions of light years. Just a few weeks ago, in January of 2011, the Hubble Space Telescope’s(HST) deep field camera captured a time exposure revealing the most distant galaxy detected to date. “The object appears as a faint dot of starlight in the Hubble exposures. It is too young and too small to have the familiar spiral shape that is characteristic of galaxies in the local universe.” (NASA) The galaxy is 13.2 billion years old and dates to a period just 500 million years after the big bang. In fact, with the HST, mankind has learned more about galaxies in the past twenty years than the sum knowledge accumulated since Edwin Hubble’s discovery in the 1920’s.
Over the years, astronomers have cataloged the literal billions of galaxies out there in the cosmos. As a part of the cataloging process, each new galaxy is classified by type. As might be expected, Edwin Hubble was first to establish a system of galactic classification sorting them into three categories: ellipticals, spirals and barred spirals. Spiral galaxies exhibit the flat disk pin wheel appearance most often associated with galaxies. Elliptical galaxies have a more spherical shape, and barred ones most resemble a small disk shaped center with long arms trailing far out into space.
Later on a few new galactic configurations were added to Hubble’s original designations. Lenticular galaxies are those which fall somewhere in between elliptical and disk or spiral shaped ones. They are more like a flattened out elliptical, lacking any spiral structure. They contain mainly old stars and not a lot of gas and may also exhibit dark bands of dust. Another galactic classification is the “irregular” type. Irregular galaxies, as the name implies, don’t conform to any specific shape and might appear to be large star clusters. Since they lay in intergalactic space beyond the Milky Way, they are clearly not star clusters and are distinctly identified as galaxies, albeit, irregular ones.
Another classification of galaxies has nothing to do with their shape but the goings-on within them. They are known as active galaxies. They generally have a spiral structure, but rotate much faster than any inactive spirals. Like quasars, the distinctive feature of these more violent galactic formations are plasma jets extruding from their centers and perpendicular to the plane of the disk. The plasma is made up of high intensity cosmic X and gamma radiation and charged particles. The source of this energy in both cases, quasars and active galaxies, is believed to be super massive black holes.
Unfortunately, unless one owns a pretty good telescope, galaxies, save one, are all but invisible to the naked eye. All the bright twinkling light observable in the night sky are stars and nebulae within the Milky Way galaxy. But if you get away from city lights, high in the atmosphere where the air is thin, the Andromeda galaxy is visible in the northern hemisphere most of the year and a truly awesome sight. It covers three degrees of the night sky, the equivalent of six moons side by side. With the unaided eye it appears just as a fuzzy streak of light, but will become well resolved just with a pair of binoculars. For the more avid, telescope equipped, amateur astronomer using the right filters, this spiral class galaxy is simply breath taking.
Of course, the adage “if you’ve seen one you’ve seen them all,” really doesn’t apply to galaxies as they come in all sorts of shapes, sizes and colors. Perhaps, “once you’ve seen one you’ll be eager to find others to contemplate,” would be a quaintly appropriate aphorism in the case of galaxies, and maybe even lead to a new and profoundly interesting hobby.
Incidentally, that most distant galaxy recently captured by HST’s deep field infrared camera has yet to be classified in terms of any of the classical categories, as it is simply too faint to determine much about it, beyond the fact that it is the oldest object yet observed in the universe. We await a new generation of equipment and astronomers to resolve this newest galactic discovery, which may one day be assigned the designation “ptoto-galaxy.”
