Happy monsoon season. I have been enjoying the nice clear mornings, followed by afternoon, dog-freaking thunderstorms, even if it puts a damper on evening observations.
If we could see the evening sky through the clouds, we would see that the brightest part of the Milky Way (the center) is above the southern horizon in the early evening. Several of the sky’s best open clusters and emission nebulae are visible there. Some of these you can see with your naked eye as faint fuzzy patches, but you will always benefit from the extra light that can be gathered through binoculars or a telescope.
Emission nebulae are glowing clouds of gas, usually associated with star forming regions, and have been the subject of some of the most remarkable Hubble photographs. I remember when the Pillars of Creation photo of the central portion of the Eagle Nebula (M16) was first released in 1995, stunning us with its detail and striking colors.
Although memorable, it should be pointed out that the colors in the famous picture are what are called assigned colors, or mapped colors, where each color represents emissions from a specific element. Besides being aesthetically appealing, these images are useful scientifically in that they show the distribution of those specific elements. It is worth discussing the background of some of these pictures and what has become known as the Hubble palette.
In the stellar nurseries such as the Eagle Nebula, the first stars to form are the biggest and the hottest. Besides visible light, these stars also emit lots of high-energy ultraviolet radiation that excites the atoms and molecules in the rest of the gas cloud. The low density gas cloud will then re-radiate, or glow, in the visible part of the spectrum. Each element has its own spectral fingerprint, and by looking at a wavelength of light in that fingerprint, the abundance of that element can be determined.
Most astronomical cameras are basically black-and-white cameras that detect all parts of the visible and infrared spectrum. The way color images are made is to take one image through a red filter, one image through a green filter and one image through a blue filter, and then combine them. Often a high resolution, unfiltered image is used to bring out detail and contrast.
Instead of just red, green or blue, it is possible to make a photographic filter that passes a much narrower wavelength band of light. The bands can be selected to let a very specific spectral line pass through, while blocking all other wavelengths. For example, the alpha line of the hydrogen spectrum is a very specific shade of red, with a wavelength of 656 nanometers.
For the Hubble palette, red has been assigned to ionized sulfur, green shows atomic hydrogen and blue represents doubly-ionized oxygen atoms. There are endless possibilities on how to combine different colors representing different elements, and often the assigned colors represent wavelengths that are outside the visible spectrum. But the Pillars of Creation picture set a standard for color mapping that is now widely copied, including by me.
This monthThe moon tonight (Saturday) is near the brightest part of the Milky Way. It is about halfway between bright Jupiter to the right and Saturn to the left. All three are great targets for either low or high magnification, and they are all bright enough to show good detail in telescopes with a very modest aperture. Tonight, the moon also happens to be very close to two good examples of emission nebulae – the Lagoon Nebula (M8) and the Trifid Nebula (M20). Both are relatively bright (M8 can be seen with the naked eye), but it would be much better to try to see those two when the moon is nowhere nearby.
The Perseid meteor shower is an August staple. These meteors are associated with comet Swift-Tuttle and can be seen for several evenings, with the peak being the night of Aug. 12-13. Unfortunately for meteor watchers, the full moon is on Aug. 15 and will be quite bright for most evenings before that.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at firstname.lastname@example.org.