During the early evening hours of winter, the stars that shine from the southern sky are a sampler of the sky’s brightest stars.
Step outside on a clear February evening. You’ll see a great congregation of bright stars in the southern sky. From the mid-northern latitudes, this region contains nearly one-third of the bright stars that can be seen anywhere from our planet and half of the bright stars visible from northerly locations.
You might see Sirius twinkling wildly low in the sky or rosy Betelgeuse gleaming from Orion’s shoulder.
While unusual in the stellar population of our Milky Way Galaxy, they are a sampler of star temperatures as displayed by their colors.
In astronomy, star color (planets are not included) indicates temperature.
Over a century ago, Henry Norris Russell and Ejnar Hertzsprung, along with their observing groups, determined fundamental stellar properties. These groups developed a classification system for stars through the chemical analysis of the spectra of stars (Annie Jump Cannon) and the relationship between the average stellar brightness and length of time it takes for variable stars to go through a period of brightness change (Henrietta Levitt).
One resulting achievement of the research is a chart known as the Hertzsprung – Russell Diagram (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 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.)
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.
The bright stars that gleam in the night sky of Earth are unusual. They are much brighter than our sun and they shine across long distances.
The H-R diagram is used to determine the ages of star clusters, like the Pleiades star cluster – to the upper right of Aldebaran on the chart above – and the Hyades star cluster – the check-mark pattern immediately to the right of Aldebaran. Seemingly stars in a cluster form at about the same time, but they are not the same size or mass. More massive stars burn through their plasma fuels faster than their less massive siblings. Consequently, the more massive stars’ stellar characteristics change sooner than the fuel-efficient stars. By looking at the location of all the stars plotted on the chart, the cluster’s age can be estimated.
Stars are neither sapphire blue nor ruby red. Notice the subtle colors on the star chart at the top of this article. The colors of the stars’ names have been set to the approximate color our eyes perceive.
Take a binocular or small telescope outside 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 our tour of the winter stellar sampler. Use the chart above.
- 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.
As you observe the stars beginning with the O spectral class, which spectral classes show a distinct difference of color?
For more about the planets during February 2021, see this article.
July 6, 2021: In less than a week, brilliant Venus passes Mars in the west-northwestern sky after sunset. This evening the two planets are 3.8° apart. Venus is over 18° to the lower right of the star Regulus.
July 1 – July 7, 2021, the waning crescent appears in the eastern sky. Early in the viewing period, the moon is among the dim stars of Pisces. As the week progresses, the moon wanes and moves farther eastward, appearing near Taurus.
July 5, 2021: Our planet Earth reaches its farthest point in its yearly trek around the sun. Our seasons are not related to Earth’s distance from the sun. Coincidentally, the moon is at its farthest point from Earth today.
July 5, 2021: Venus continues to close in on Mars in the west-northwest after sunset. In a week Venus passes the Red Planet.
July 4, 2021: The Venus – Mars conjunction is eight days away. This evening Venus moves to within 5° of the Red Planet.