Greetings, stargazers.
Most of us have heard the weather proverb “red skies at night, sailors delight; red skies at morning, sailors take warning.” I certainly enjoy watching a beautiful red sunset, but for those of us in southwest Colorado it often just means there is a fire in the area. Let’s hope the real monsoons kick in before too long.
The reason sunsets and light going through smoke looks redder than normal is the same reason most of the time the sky looks blue. Short wavelengths like blue light get scattered sideways much more easily than long wavelengths like red light.
A very simple demonstration I do in the classroom is shine a bright white flashlight through a small fishtank of water (without fish). When the water is clear, you cannot see the beam from the side, as a negligible amount of the light is scattered. I add a tiny bit of powdered milk to the water to make it slightly cloudy, but not opaque. When viewed from the side, the scattered light has a bluish tint. But looking straight through toward the flashlight, the light has a yellowish tint. When more cloudiness is added, the direct light gets noticeably redder until the beam is finally obscured.
The scattering of light from atom-sized particles that are much smaller than the wavelength of light is called Rayleigh Scattering. If the particles are much larger, such as dust grains or smoke particles, it is called Mei Scattering. For most of us, the detailed differences are unimportant.
The reddening of stars happens when their light must go through more atmosphere as the star gets close to the horizon. It also happens when passing through interstellar clouds of gas and dust before it gets to us. This effect is particularly noticeable for stars seen along the plane of the Milky Way where lots of interstellar dust clouds are concentrated. Some of those clouds are so thick that even visible red light is scattered away, leaving only infrared light that gets through. Instruments that are sensitive to infrared light, such as the new James Webb Space Telescope, are particularly useful for seeing stars through these clouds.
Because star color is the primary indication of a star’s temperature, and indirectly its size and distance it is important to be able to distinguish between a star’s unobscured color and a color that might be the result of passing through an interstellar cloud. If the details of a distant star can be determined using some other spectroscopic method, then the amount of reddening observed can be used to get information on the interstellar dust cloud the light is passing through.
Useful links
Rayleigh Scattering
https://en.wikipedia.org/wiki/Rayleigh_scattering
Perseid Meteor Shower
https://en.wikipedia.org/wiki/Perseids
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
The Perseid Meteor shower peaked last week. This is one of the best meteor showers of the year. Unfortunately for meteor watchers, the moon was full during the Perseid’s peak, Aug. 9 and was still very bright next few days washing out all but the very brightest meteors.
If you see social media notices of a total lunar eclipse, it is happening but won’t be visible in most of the Western Hemisphere.
On the mornings of the 11th and 12th, Venus and Jupiter were about a degree apart in the morning sky. Viewers should have no trouble finding them, as they are the two brightest things in the sky besides the sun and the moon. You can usually see Venus in the middle of the day, and this might be a time to try to see Jupiter as well. The easiest way to see it is to start watching at dawn, while it is very prominent, and then keep watching as the day gets brighter. Binoculars could help. I have seen Saturn during the day through a telescope, and Jupiter is much brighter.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.
