Stardate:06:29:99
Tonight at midnight, the nebulae rule the skies. Two very bright, large clouds of gas and dust will be an easy find nearly due south at this time all this week. These are the Lagoon Nebula and the Trifid nebula.
Near these colorful clouds lies an open cluster; a rich family of companion stars, huddled together in the near vacuum of space. Its name is M21. All of these objects are just barely too dim to see with the unaided eye, but are an easy find with only a pair of binoculars, although the view using a telescope will be brighter, larger and more detailed.
Perhaps, long ago, in the days before streetlights, city lights and billboards, our ancestors may have been able to view these fantastic objects without optical aid. What legends, dreams and myths may have been conjured in an effort to explain their strange appearance is now long clouded by the mists of time!
We begin our journey across the light years at M8. It is also known by the names of NGC 6528, and, more descriptively, the Lagoon Nebula. This dusty nebula will be found around seven degrees east of south and a little over 25 degrees above the horizon at midnight this week. It spans over a degree and a half across the sky, making it appear three times further across as does the Moon. In reality, it stretches 60 light years across. This distance is tremendous. It represents 560,000 billion kilometers, or 350,000 billion miles. To cover the same distance, you would have to travel around the world over 14 billion times. Traveling around the world once a second, it would take over 440 years to complete this futile journey.
Glowing a deep, bright pink, the Lagoon Nebula resembles a splotch a red paint with an off center white core. Its colors are produced by the emission of red, violet and blue light by the hydrogen within the nebulas mass. These emissions are caused by the hydrogen being excited by ultraviolet radiation from the hot young stars within the nebula.
About 1 1/3 degrees from the Lagoon Nebula we find the Trifid Nebula. This is also a dusty nebula which is also known as NGC 6514, or more simply, M20. It is about 60% dimmer than M8, and spans half a degree across the sky. This nebula lies almost directly above the Lagoon nebula, just 1/4 of a degree further to the south. The Trifid Nebula appears to be divided into three pieces by dark lanes of dust. This is what gave rise to its name. Nearly 40 light years across, this nebula is also multicolored, much like the famous Great Nebula in Orion. This is because there are two different aspects of the light from this nebula. Like the Lagoon nebula, M20 is also heated by hot young stars within its central region. However, the Trifid Nebula is less concentrated than M8, so there are areas far from these hot stars which are only dimly light by them.
Near these young hot stars, the gases of the Trifid Nebula glow bright red, but far from these stars, all the color we see is reflected by the dust in the nebula. Dust is, for the most part, larger than hydrogen gas, and thus reflects a longer wavelength; hence the blue color on this side of the nebula.
Our third hop of the evening brings us just 1/4 of a degree away from where we last were. Just a quick turn nearly due south brings us to M21, otherwise known as NGC 6531. Appearing 1/2 as wide across as M20, it is nearly as bright as the Lagoon Nebula. M21 is not a dusty cluster as are M8 and M20. It is, instead, an open cluster, giving the appearance of many small white bees swarming in a tight sphere.
As you ponder how much dusting there is to be done in your house, imagine how much there is to do in just our first two objects and their great lanes of interstellar dust alone.
Clear skies, and good viewing.
"Understanding is joyous" - Carl Sagan
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