Stepping outside on an April evening, you’ll notice many more bright stars in the western sky than in the east. Those bright stars in the west are the stars of winter, still high in the west at dusk because winter officially ended about a month ago.
Orion, the Hunter, remains well placed for observing. As you watch him set in the west, his three-starred ‘belt’ seems parallel to the horizon, with the brighter stars Betelgeuse and Rigel above and below the belt, respectively. Extend a line to the right from the belt. This line points to Aldebaran, which marks the eye of Taurus, the Bull. As the sky darkens, notice the dim V-shaped cluster right under Albebaran, marking the Bull’s face. Go back to Orion’s belt, and this time extend a line to the left. In this direction, Orion’s belt points at Sirius, the Dog Star, which is the brightest star we ever see at night. Forming an almost equilateral triangle with Sirius and Betelgeuse is Procyon, the Little Dog Star.
Standing over Orion as he sets in the west are the Twins of Gemini. Look for two stars of roughly equal brightness and less then 5 degrees apart. (Three fingers held together at arm’s length is about five degrees). These stars are Castor and Pollux, marking the twins’ heads. To the right of Gemini are stars in the form of a pentagon–the stars of Auriga, the Charioteer. The brightest star in Auriga is Capella.
April is the last full month to observe this swath of brilliant stars; they begin leaving the evening sky in May. Not only does this set of stars include six of the twelve brightest stars at night, but it also includes at least one example of each spectral class of star. Astronomers classify stars into seven spectral classes in order from hottest to coolest: O, B, A, F, G, K, and M. “Oh Be A Fine Girl/Guy, Kiss Me” is the most common phrase we use to remember the order.
Astronomers learn what a star is made of by producing a spectrum of its light and noting which wavelengths are absorbed. Black lines across the continuous rainbow spectrum indicate particular wavelengths absorbed, which correspond to particular gasses in the star.
In the 1890 Draper Catalog of Stellar Spectra, Williamina Fleming divided stars into fifteen categories from A to O based on the presence of hydrogen lines in the stars’ spectra. Only much later did astronomers figure out the more interesting relationship between a star’s spectrum and its temperature. Annie Jump Cannon of Harvard eliminated redundant categories and merged others, leaving us with the seven modern categories in the current order.
The hottest stars are bluish and the coolest stars are reddish, with pure white stars in between. This may sound counter-intuitive at first. If you think about it, however, you probably know that ‘white hot’ is hotter than ‘red hot’, and that the hottest flames are bluish.
In the sky tonight, the three stars of Orion’s belt are O stars. Rigel is a ‘B’ star, while Betelgeuse is in the coolest ‘M’ class. Sirius, the brightest star in the night sky, is an ‘A’ star. The Little Dog Star Procyon is an F star. Capella in Auriga is a G star (as is our Sun). The bull’s eye, Aldebaran is a K star. At first glance, all stars look white to us, since starlight is scarcely bright enough to stimulate the color-detecting cones of our eye. When a bright M star is close to a bright O or B star, however, you often can see the difference by contrasting the two. I invite you then, to contrast Betelgeuse and Rigel in the sky and see if you notice a difference in color.
We often describe G stars such as our Sun as yellow. The Sun’s rays even look yellowish when we accidentally glimpse it through, say, a canopy of leaves. However, sunlight is in fact white. The yellow in the Sun’s rays is an illusion created by our atmosphere.
The Sun emits light of a wide range of wavelengths, including all colors of visible light. This light interacts with the molecules of gas making up our atmosphere. A molecule of gas, though, is much smaller than the wavelength of visible light. As a result, the shorter the wavelength of the light, the more likely that the light will be absorbed or redirected after interacting with an air molecule. This is called ‘scattering‘.
We often use the mnemonic device ROY G. BIV to teach the order of the colors in the rainbow: red, orange, yellow, green, blue, indigo, and violet. (By the way, we rarely use ‘indigo’ in any other context; Isaac Newton slipped it in because he really wanted seven as opposed to six colors). This is the order from longest wavelength (red) to shortest (violet). Therefore, our atmosphere scatters violet and blue light much more than the other colors–this is why our sky is blue. Although violet light is even more scattered than blue light, the sky does not look violet to us because our eyes are less sensitive to violet light than to blue light.
Now that the heavy rains have passed, we’re in for a spate of clear weather this week (of April 20). I invite you to enjoy the clear blue sky and watch a sunset or two. And of course, keep watching after sunset as all those bright stars appear in the west. As you learn to appreciate them, the colors in the sky become as fascinating as those in the flowers on the ground.