February 23, 2023: After sundown, three bright planets and the crescent moon are easily visible. The bright winter stars of the Orion region are in the southern sky after sundown.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 6:35 a.m. CST; Sunset, 5:34 p.m. CST. Check local sources for sunrise and sunset times for your location. Times are calculated from the U.S. Naval Observatory’s MICA computer program.
Jupiter’s Great Red Spot’s transit times, when it is in the center of the planet in the southern hemisphere: 9:28 UT, 19.24 UT. Convert the time to your time zone. In the US, subtract five hours for EST, six hours for CST, and so on. Use a telescope to see the spot. Times are from Sky & Telescope magazine.
This evening step outside at about an hour after sunset. The stars of the Orion region of the Milky Way are in the southern sky. The flagship constellation is Orion. It is easy to identify with its three belt stars. Reddish Betelgeuse – meaning “armpit” – is to the upper left and bluish Rigel, a knee, is to the lower right.
Over a century ago, Henry Norris Russell and Ejnar Hertzsprung, along with their observing groups, determined fundamental stellar properties. One of those important factors is that temperature determines color. Unlike an artist’s interpretation of color, bluish stars are hotter than reddish ones. Colors are not vivid like a ruby or a sapphire. They are subtle and can be easily seen through a binocular.
Annie Jump Cannon cataloged over 350,000 stars into groups from their spectra, when starlight was passed through a prism and fractured into its component colors along with regions of less light from the elements in the outer regions of the stars. The spectral class refers to the chemistry of the star and uses the letters O, B, A, F, G, K, and M, from hottest to coolest, blue to red. (Type O stars are hottest and appear bluish in color, while type M stars are “coolest” and are reddish.)
Further, Russell and Hertzsprung independently graphed the stellar properties on a system that is today known as the H-R diagram. The x and y axes are labeled with interchangeable terms. Luminosity and absolute magnitude appear on the y-axis, while color, temperature, wavelength, or spectral class may be displayed on the x-axis.
The absolute magnitude is used to compared intrinsic stellar brightness. It is the brightness of a star if it were place 10 parsecs or 32.6 light years away. The magnitude system is a numerical rating of a star’s brightness. Lower numbers are brighter. At 10 parsecs our sun has a rating of 4.8, not visible from most urban and suburban backyards. In comparison, if Betelgeuse were at that distance, it would appear over five times brighter than Venus shines in tonight’s sky.
When the sun’s characteristics are plotted on the chart, it is not as bright as the named stars in the sky, but it is brighter than most of the dim stars that are near our solar system. Astronomers sometimes refer to our central star as a “typical star” or “garden variety star,” because many other stars are like it. It seems average compared to other stars.
Use a binocular to view winter’s Hertzsprung-Russell collection of stars. Some optical aid amplifies a star’s brightness as well as its subtle color. Here’s my tour of the winter stellar sampler.
- Spectral Class O: Alnitak – the eastern star in Orion’s belt, Zeta Orionis (ζ Ori, m = 1.7), 815 light years distant (l.y.);
- B: Rigel, Beta Orionis (β Ori, m = 0.2), 860 l.y.
- A: Sirius, Alpha Canis Majoris, (α CMa, m = −1.5), 9 l.y.
- F: Procyon, Alpha Canis Minoris (α CMi, m = 0.4), 11 l.y.
- G: Capella, Alpha Aurigae (α Aur, m = 0.1), 40 l.y.
- K: Aldebaran, Alpha Tauri (α Tau, m = 0.8), 70 l.y.
- M: Betelgeuse, Alpha Orionis (α Ori, m = 0.4), 500 l.y.
Here is today’s planet forecast:
Morning Sky
Two morning planets are immersed in bright morning twilight. Mercury is heading for its superior conjunction on March 17th and then the best evening appearance of the year during April. It rises about 30 minutes before the sun.
Saturn is climbing into the morning sky after solar conjunction a week ago. It rises less than 10 minutes before dawn.
Evening Sky
This evening Venus, Jupiter, and the crescent moon, 18% illuminated, are in the west-southwest after sundown. Brilliant Venus is nearly 20° above the horizon, with Jupiter 6.2° to its upper left.
The crescent moon, nearly 40° above the horizon, is about 15° to the upper left of Jupiter. It is showing earthshine on its night portion. This effect is reflected sunlight from Earth’s oceans, clouds, and land.
As the sky darkens further, Mars is high in the south above the bright stars of the Orion region of the galaxy.
The Red Planet marches eastward in front of Taurus. This evening it passes between Elnath and Epsilon Tauri (ε Tau), in the “V” of Taurus, opposite Aldebaran. Elnath and Epsilon are too far apart to fit in to the same binocular field of view, but use it to identify the starfield.
Mars is 10.6° to the upper left of Aldebaran and 6.4° to the lower right of Elnath.
During the next few weeks, look each clear evening as Mars approaches the Bull’s horns.
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