2025, February 21: Check Out the Winter Stars

February 21, 2025:  Winter’s bright stars gleam in the southern sky during late February.  Use a binocular to investigate their color and brightness.

Winter Stars
Chart Caption – 2025, February 21: Winter’s stars shine brightly from the southern sky during the evening hours.

by Jeffrey L. Hunt

Chicago, Illinois: Sunrise, 6:37 a.m. CST; Sunset, 5:32 p.m. CST.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Winter Stars in South

With a binocular, step outside on the next clear evening after the end of evening twilight, which occurs over 90 minutes after nightfall at the mid-northern latitudes.  The bright stars of the Orion region of the Milky Way shine brightly from the southern sky.  The flagship winter constellation is Orion, easily identified by its three belt stars.  Reddish Betelgeuse – meaning “armpit” – is to the upper left and bluish Rigel, a knee, is to the lower right.

Sirius, the night’s brightest star, is to Orion’s lower left.  The Dog Star, Procyon, and Betelgeuse are the vertices of the Winter Triangle.

Mars, Jupiter with Winter Stars

The largest canyon in the Solar System cuts a wide swath across the face of Mars. Named Valles Marineris, the grand valley extends over 3,000 kilometers long, spans as much as 600 kilometers across, and delves as much as 8 kilometers deep.
The largest canyon in the Solar System cuts a wide swath across the face of Mars. Named Valles Marineris, the grand valley extends over 3,000 kilometers long, spans as much as 600 kilometers across, and delves as much as 8 kilometers deep. (NASA)

The Gemini Twins, Castor and Pollux, are above Procyon.  Tonight, Mars is to their right.  The Red Planet is brighter than nearly all the starlike bodies in the sky at this hour except for Venus, low in the west, Jupiter, and Sirius.

The Jovian Giant is near rosy Aldebaran, marking the Bull’s eye.

Capella, meaning “the little she goat,” is highest in the sky, nearly overhead.

Stellar Properties

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 the binocular.

Solar Spectrum
Photo Caption – This high-resolution spectrum of the Sun is a single image cut into segments to fit into this photo. The dark lines indicate the sun’s composition. (Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF)

When starlight is passed through a prism and fractured into its component colors, dark lines appear along the spectrum indicating the stars’ compositions. Annie Jump Cannon cataloged over 350,000 stars into groups from their spectra.  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.)

The H-R Diagram.
Chart Caption – The Hertzsprung-Russel Diagram is a chart to compare stellar properties. (Credit: European Space Agency)

Further, Russell and Hertzsprung independently graphed the stellar properties on a system that is today known as the H-R diagram, in recognition of their fundamental stellar research. 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.

Tour Winter 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. 

On the next clear evening take a tour of winter’s bright stars.

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