One of the most breathtaking wonders of stargazing is the range of colors the night sky puts on display. These different hues give each constellation a distinct character and serve as visible proof of the temperature differences between stars.
Many of summer’s brightest celestial objects, such as the brilliant Vega rising in the east-northeast at sunset, glow with a blue-white light.
But the sky also offers other striking shades: the reddish Antares, the yellowish-white Altair, and the orange Arcturus dominating the sky high to the south.
Antares star region in scorpius, very close to the milky way. Credits: Shutterstock
Seeing these colors is easiest with the brightest stars, due to how the human eye works. The cone receptors in the retina do not respond to faint light, which is why dimmer stars appear white. When brightness increases, the cones activate and reveal the star’s true color. With a telescope or a steady pair of binoculars, the experience becomes even richer and more complete.
Colors Through Contrast
One of the most striking ways to appreciate stellar colors is through visual contrast. Arcturus, for instance, is easy to find by following the arc of the Big Dipper’s handle toward the southeast. Henry M. Neely (1879-1963), former president of the Hayden Planetarium in New York, famously said: “Follow the arc to Arcturus and speed to Spica.”
Spica shines with a distinctly blue-white tint. If you quickly shift your gaze from Arcturus to Spica, the contrast between their orange and blue hues becomes vivid and striking.
Another rewarding method is observing a double star made up of two stars with contrasting colors. The most famous example is Albireo in the constellation Cygnus, also known as the Northern Cross, which marks the swan’s beak. Even with a small telescope or a steady pair of binoculars, Albireo splits into two points of light with a gorgeous contrast: one yellow-orange and the other deep blue. At magnifications of 18x to 30x, the sight is truly magical.
The double star system Albireo in the constellation Cygnus. Source: Shutterstock
Hot Stars and Cool Stars
Antares is often described as reddish, but it is technically a yellow-orange star with a surface temperature of around 3,000 Kelvin, similar to that of an old incandescent light bulb. Class M stars share a similar temperature range.
Human eyes evolved to make the most of the Sun’s radiation spectrum, which sits at a middle ground in terms of temperature and color. Stars that are much hotter or much cooler radiate more intensely in the ultraviolet and infrared ranges, respectively.
Starlight closely resembles what physicists call blackbody radiation, the electromagnetic energy absorbed and emitted perfectly by an idealized body. The hotter a star is, the more energy it emits, and its emission peak shifts toward the shorter, bluer end of the spectrum.
