Observe the sky’s colorful stars

Professional and amateur astronomers post amazing photos of the sky. A colorful, star-filled Milky Way is often shown rising above a landscape, and photos from the Hubble Space Telescope reveal amazing and vivid colors in galaxies and nebulae.

As beautiful as these pictures are, they also can be misleading because they don’t truly represent what the night sky looks like to the naked eye. The sky is filled with color, but it is subtle and takes effort to perceive. The human eye is an amazing organ — but it has limitations and can’t record the saturated colors that cameras capture using long exposures.

But there is one place celestial color is easy for our eyes to tease out: in the stars. Although a casual observer might think that all stars are white, they actually come in an array of yellow, red, white, and blue hues.

Eyes on the universe

Packed into our eyes are hundreds of millions of photoreceptors called cones and rods. The cones are sensitive to red, blue, and green. Green is the color that the eye is most receptive to in bright light.

In the dark, the rods take over. Millions more of these highly sensitive receptors exist than their brethren, the cones. Rods, however, don’t distinguish colors, but are sensitive to low light and therefore allow us to see in dark conditions. So, in darkness, the rods take over and we lose most of our bright-light perception, making it more difficult to distinguish colors.

Vega and friends

You can test your color acuity on a variety of bright stars. Start with one that appears prominently each summer night: Vega (Alpha [α] Lyrae) is currently high in the sky. 

This is an amazing star and offers a wonderful way to start your color journey. For one thing, the star glows at almost exactly 0 on the magnitude scale. For decades, this brilliant luminary was used as a standard reference star for photometric calibrations. On a summer night in 1850, Vega became the first star other than the Sun to be captured photographically. In another first, two decades later, American astronomer Henry Draper captured Vega’s spectrum on a glass plate. And if you are willing to wait about 14,000 years, Vega will become the Northern Hemisphere pole star, displacing Polaris.

Due to its brilliant white color, Vega is referred to as a beautiful diamond and even “the Arc-light of the sky.” Sharp-eyed observers might detect a tinge of blue in Vega, a color that is easier to see through a telescope. Take Vega a bit out of focus and the blue will become more apparent.

Not far from Vega is a real color challenge. At magnitude 4.7, Epsilon (ε) Lyrae is near the limit of naked-eye visibility under all but the darkest skies. Known as the Double Double, the star splits into two pairs of binary stars through even a small telescope. Victorian astronomers loved recording the delicate colors of stars; one astronomer recorded these pairs as greenish-white and bluish-white, while an English observer claimed to see yellow and red in them. Whether these descriptions were imaginative or real is still debated.

Arcturus

The summer sky is also graced by the beautiful star Arcturus (Alpha Boötis). This star slightly outshines Vega as the fourth-brightest star in the nighttime sky and the brightest sun north of the celestial equator. The ancient astronomer Ptolemy called it “slightly red.” Since then, its color has been described as warm amber, yellow, ruddy red, and orange.

The indefatigable astronomy popularizer Garrett P. Serviss wrote, “When [Arcturus] has not risen far above the horizon, [it] is a yellowish red, but when the star is near the mid-heaven the color fades to light yellow.” This color change is likely explained by Arcturus shining through an ever-thinner atmosphere as it rises. In 1852, an astronomer claimed that Arcturus “nearly lost its color!” And on the night of Oct. 5, 1858, the ruddy color of Arcturus was ever-so-slightly dimmed when the densest part of Comet Donati’s tail passed in front of it. What a sight that must have been.

Our Sun is classified as a yellow-white G-type star, while Arcturus is a K-type star 25 times the size of the Sun. Arcturus is aging and moving into the red giant stage of stellar evolution, which creates its orange hue. The orange glow of Arcturus falling onto a photocell turned on the lights for the opening of the 1933 Chicago World’s Fair.

Stars of the dogs

Not far from Arcturus is the small constellation Canes Venatici the Hunting Dogs. The brightest star in this constellation is Cor Caroli, which was named for King Charles II of England. This is a wide double star. The colors have been described as light yellow and blue, pale olive and blue, and even pale copper. Like most observers, however, the great English astronomer John Herschel only saw two white stars. What colors do you see?

The real prize in this constellation is the extravagantly named star La Superba (Y Canum Venaticorum). It glows at magnitude 4.8 and is intensely ruby red. The Jesuit astronomer Angelo Secchi applied the name in 1860 while studying this star’s carbon-rich spectrum. The prevalence of carbon in the star’s atmosphere absorbs blue light and creates its red color. To further enhance the color, view La Superba with a small telescope at low power, slightly out of focus.

The Scorpion’s prizes

Low in the southern summer sky is the star known as “the rival of Mars”: Antares (Alpha Scorpii). The brilliant red of Antares stands out among the blues and whites of the surrounding stars in Scorpius. Antares is part of an expanding cluster of massive stars. This puffed-up giant of a star has a surface temperature of about 5,500 degrees Fahrenheit (3,040 degrees Celsius), which is about half as hot as the Sun.

At nearly 12 billion years old, Antares has evolved into a red supergiant star. Even if you have trouble seeing some star colors, as I do, Antares is an easy exception. Through a medium-size telescope, Antares resolves into a beautiful binary system. The fainter companion is a hot star that appears bluish to most observers. Some have even described this star as purple or green. The star’s color is further enhanced because of the contrast with ruddy Antares.

Not far from Antares is the upper claw of Scorpius, marked by Acrab (Beta [β] Scorpii). With modest telescopic magnification, Acrab splits into two beautiful stars. Here lies another color challenge: To most observers, the primary star appears white and the fainter star pops out as distinctly blue. Others have claimed to see pale yellow and green. Whatever you may perceive, Acrab is intense and colorful.

In the Dipper’s handle

Before the summer night slips away, turn toward the northwest and find the Big Dipper. The middle star in the Dipper’s handle — Mizar (Zeta [ζ] Ursae Majoris) — has long been a favorite showpiece at star parties.

Telescope operators ask people to look at Mizar with just their eyes: “How many stars can you see?” A few visitors will say they can see a second faint star near Mizar; binoculars will easily separate these two stars. At low power in a telescope, Mizar appears white while its companion, Alcor (80 UMa), looks vivid blue. Or does it? The 19th-century astronomer Reverend T.W. Webb recorded seeing the pair as white and green.

However, there are no green stars. So why might Alcor appear that way? The pupil is only fully open when the eye is fully dark-adapted. Under this condition, the cones that give us our color vision have stopped working and the rods have taken over, reducing our ability to see subtle shades.

Artists have long known that contrasting colors can increase the boldness of a painting, and this effect works with stars, too. Rods are most sensitive to blue light. When looking at a bright white star such as Mizar, the contrast can enhance the blue color of a star like Alcor. Experiments have shown that at times our perception of green will dominate over blue. This may explain why many observers have recorded shades of green in double stars.

The summer showpiece

Before the night is over, find Vega again. To the southwest, you’ll see Altair, the brightest star in Aquila the Eagle. Completing the Summer Triangle is Deneb in the tail of Cygnus the Swan. The head of the Swan, Albireo (Beta Cygni), lies roughly in the center of the Triangle. This is the double star that many observers target at summer star parties.

Albireo is a great example of how contrast can enhance star colors. To the unaided eye, it appears a dull white. But through a telescope, Albireo resolves into a double star that most observers see as gold and sapphire. The color variation is due to the difference in the surface temperatures of these two stars. The yellow primary is a relatively cool 7,500 F (4,150 C). The secondary, on the other hand, has an incredible surface temperature of 23,000 F (12,760 C) and radiates visible light mostly in the blue part of the spectrum.

Stargaze a while

Throughout the year, the parade of colorful stars continues. In the fall, look toward Cepheus the King and Auriga the Charioteer, and in winter, the constellation Orion the Hunter hosts copper-hued Betelgeuse and brilliant blue Rigel. It’s tempting for us to test our eyes and telescopes to view nebulae and distant galaxies. But don’t forget to take time for the subtle and colorful points of light that glow softly in the night sky.