Greetings, stargazers.
Emission nebulae are the subject of many of the most spectacular astrophotographs published in the last decade. Although there are many subcategories of nebulae, they all consist of interstellar gas and dust. These “extra-dense” nebular regions between the stars are a better vacuum than a cryogenically cooled laboratory vacuum chamber on Earth can produce. But even at such low pressures, the volume of space involved is so enormous that collectively there is enough material for us to see easily, even if only a few of the atoms or molecules in the clouds happen to be emitting light.
The most common element in the universe is hydrogen, and that is also the most common type of glowing interstellar gas. When hydrogen gas is irradiated with ultraviolet light from nearby stars, the gas will absorb and reradiate that light. Most of the energy is reemitted in the (nonvisible) ultraviolet, which is largely blocked by our atmosphere. However, a significant fraction of the energy comes to us in the wavelengths of visible light, which is what is most often recorded in photographs.
Useful links
Lagoon nebula
https://en.wikipedia.org/wiki/Lagoon_Nebula
Vera C. Rubin Observatory
https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory
Astronomy picture of the day
An Astronomer’s forecast for Durango
http://www.cleardarksky.com/c/DrngoCOkey.html?1
Old Fort Lewis Observatory
http://www.fortlewis.edu/observatory
hakes_c@fortlewis.edu
This is a good month to see nebulae. The only requirement is dark skies. Because their light is spread out, and not like the pinpoint of a star, nebulae are most easily seen when the background sky is as dark as possible. That means away from city lights on a night with no moon. Although these nebulae appear pinkish in photographs, they are always pale gray through binoculars or the eyepiece of a telescope. That is just a limitation of the color sensitivity of our eyes.
The teapot of Sagittarius is a good reference for finding nebulae in the southern summer sky. That constellation is in the direction of the center of the Milky Way, which is in the southern sky after sunset. Rather than “the archer” as seen by ancient astronomers, the brighter part of Sagittarius is the asterism known as the teapot. The teapot is slowly pouring its contents on the southern horizon and the Milky Way is the steam coming up out of the spout.
These are a few of my favorite emission nebulae. They are most often identified by their Messier number. These numbers are from the list Charles Messier compiled in the late 18th century of fuzzy things in the sky that were not comets. At least one can be seen with the naked eye, and many more make great targets through binoculars or wide-field telescopes. Of course, larger scopes allow us to see even more.
The Lagoon Nebula, or M8, is visible to the naked eye less than a fist width to the right of the topmost star of the teapot. It is a large star-forming region about 4,000 light-years distant. I have seen this object show up in photographs using a hand-held cellphone, but the most recent image by the new Vera C. Rubin Observatory is spectacular.
Less than one degree to the north of the Lagoon Nebula is M20, the Trifid Nebula. This one is much smaller and includes a prominent reflection nebula. Rather than the pink of an emission nebula, a reflection nebula is sky blue. Just as sunlight scattering off our atmosphere sends blue light toward our eyes, starlight scattering off the interstellar clouds does the same thing.
About 10 degrees, or a full fist width to the north of the top star in Sagittarius is M17. This nebula goes by a variety of names, including the Swan Nebula, the Omega Nebula, the Checkmark Nebula or (a new one for me) the Lobster Nebula. It has a distinct structure that is obviously wide open to interpretation.
A couple of degrees to the north of M17 is M16, the Eagle Nebula. One part is known as the Pillars of Creation, from the famous Hubble telescope photo. Instead of natural light, the Hubble team assigned colors to filters that isolated specific elements in the image. The red channel is light from sulfur atoms, green is from hydrogen and blue is from oxygen.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.
