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by Don Urban
One of the first deep sky objects I observed with my newly purchased telescope in 1988 was the magnificent globular cluster, M13 in the constellation Hercules. The third brightest in the sky behind NGC 5139 (Omega Centauri) and 47 Tucane, both of which are best seen from the southern hemisphere, M13 is always a pretty sight in any telescope.
Easily found and visible in the hours before midnight from the beginning of April until the end of September, M13 is a concentration of roughly 350,000 - 500,000 stars located 25,000 light years from Earth.
Many of the brighter globular clusters appear as small, concentrated circular patches of gray mist in small aperture (4" and under) telescopes at low power. But it is because of this appearance that they are so easily found. They stick out like the proverbial "sore thumb" next to individual stars in the eyepiece field of view.
To fully appreciate their beauty, medium to high power (125x - 250x) should be employed whenever possible. Using this magnification range, the eye can easily distinguish countless individual stars that comprise globulars.
An interesting fact about globular clusters is that, regardless of the number of stars they contain, the average distance between individual stars is between 3/4 and 1-1/2 light years. If an Earth-like planet was in orbit about any one of them, the night sky would be brilliant, with the combined light from stars shining as bright as several full moons. It would be next to impossible to see faint galaxies and nebulae through the blanket of stars overhead. And because most of the stars in a globular are of the same magnitude, they would appear as grains of sugar sprinkled onto a black piece of cloth or paper. It is also doubtful that, again due to their uniform proximity to one another, stars would be observed to ‘line up’ in patterns resembling constellations.
Almost all the globular clusters we can see reside above and below the plane of the Milky Way and contain very old stars. Estimates put their ages at 10 - 14 billion years, older in fact than recent estimates of the age of the Universe. Surely this paradox will be the subject of continued research for many years to come.
Most of the showplace globulars that amateur astronomers observe occupy positions in the sky that allows them to be seen between late Spring and late Fall. In addition to M13, Sagittarius is home to M22, M28, M54, M55, M69, M70 and M75. The undistinguished constellation of Ophiuchus contains just as many. Among them are M9, M10, M12, M14, M19, M62 and M107. Of the 110 objects in the Messier catalog, 29 are globular clusters.
One cannot write about these intriguing objects without making reference to the most magnificent of all, NGC 5139. Truly, it is a sight to behold and is an easy naked eye object shining at mag. 3.7 with a size of 3 times the Moon's diameter. In telescopes of 8" and under at low power, its million plus stars almost fill the entire field of view. In larger telescopes up to 24" in diameter, it takes your breath away. Stars appear as bright yellow pearls strung together on strands of golden thread, in definite patterns that resemble gently curving lines.
Lying at southern declination of -47.37 it would be visible from latitude +41.00 about 2o above the southern horizon were it not for the Earth's atmosphere and smog that perpetually blocks the view. Many amateurs living in dark sky areas of North Carolina and other states close to latitude +36o are routinely able to observe it.
So, the next time you travel south to Florida, be sure to pack a pair of binoculars in your suitcase. Better yet, take along your telescope. Best of all, make plans to attend the 1996 Winter Star Party next February 14th where you can observe NGC 5139 at 2:00am, 18o above the horizon!
