Notice the colors of the stars

A multitude of blue background stars with 2 bright, reddish, star-like objects.
View at EarthSky Community Photos. | Jeremy Likness in Monroe, Washington, used a regular camera lens to capture this view of a bright red planet, Mars on January 8, 2023. Plus he captured the reddish star Aldebaran in the Hyades star cluster (part of Taurus the Bull). And this photo shows the tiny, dipper-shaped Pleiades star cluster, also in Taurus. Can you see the colors of the stars here? Thank you, Jeremy!

Tonight, go outside, and let your eyes adjust to the dark. Then note the subtle differences in the colors of the stars. Let’s explore some of the stars that you’ll see flickering against the black backdrop of night in winter. In fact, there’s a whole spectrum of star colors sparkling up there, from cool red stars to middle-range yellow stars to hot blue-white stars.

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The colors of the stars

First, look high overhead in the winter evening sky for a bright star with the name of Capella. Capella’s nickname is the Little She-Goat, and it lies in the constellation Auriga the Charioteer.

So can you spot Capella? Once you find it, notice that it’s a golden star. The fact is, a star’s color indicates its spectral type. More about spectral types of stars below.

Star chart showing the constellation Auriga with Capella and other objects labeled.
The bright star Capella in the constellation Auriga the Charioteer is overhead on winter evenings. To be sure you’ve found Capella, look for a little triangle of stars nearby. Capella is sometimes called the Goat Star, and the little triangle of stars is an asterism called The Kids.

Compare the different colors of the stars you see

Now try contrasting golden Capella with some of the stars in nearby Taurus the Bull. First, find the reddish star Aldebaran, the Eye of the Bull, and the bluish stars of the misty Pleiades cluster. Do you see the difference?

Two-pronged fork made with dots and lines, small dot cluster at top right.
Taurus the Bull contains 2 star clusters that are easy to spot, the Pleiades and the Hyades. Aldebaran appears as part of the Hyades cluster.

What about Sirius?

Sirius in the constellation Canis Major the Greater Dog is our sky’s brightest star, after the sun. It’s usually described as a white star.

So Capella is golden, and Sirius is white. But, by the way, Capella and Sirius often flicker deliriously when low in the sky. This effect has nothing to do with the colors of the stars themselves but rather is caused by Earth’s turbulent atmosphere. The twinkling effect is particularly prominent with the stars Capella and Sirius because they are so bright.

Sky chart showing Sirius, Canopus and Orion.
Sirius is the sky’s brightest star. You’ll always know it’s Sirius because Orion’s Belt – 3 stars in a short, straight row – points to it. Also, as seen from the latitudes like those in Florida, Texas or southern California, Canopus – the 2nd-brightest star – arcs across the south below Sirius on February evenings. From farther south on the sky’s dome, Sirius and Canopus cross higher in the sky, like almost-twin diamonds. Chart via EarthSky.

Next, check out Orion

Orion the Hunter, a prominent constellation in the winter sky, sports a noticeably red star and blue star. The red star is Betelgeuse marking one shoulder, while the blue star is Rigel marking the opposite knee.

Notice the shades of red and orange of Betelgeuse in the creative collage below.

Colors of the stars: Bright stars of Orion, all appearing bluish-white except for reddish Betelgeuse. There are many faint stars in the field against a black backdrop.
Orion the Hunter, captured by astrophotographer Alan Dyer, is a great place to see the different colors of the stars. Rigel appears in the lower right of the constellation. Contrast its bluish-white light with that of reddish Betelgeuse in the upper left. The fact is, most of Orion’s stars are hot blue-white stars. Image via Alan Dyer/ Used with permission.

The true colors of stars

And you don’t even have to know any star names, or any constellations. Just glance around the sky, and notice the subtle color differences in the stars.

It’s helpful to know that a star’s true colors are more apparent as the star climbs higher in the sky, moving above the turbulence of Earth’s atmosphere. So, if you have good eyesight and a dark, clear sky, you should be able to detect hints of color within the brighter stars.

And if you have difficulty discerning star colors with the unaided eye, look at the bright stars through binoculars. A useful trick is to put the star out of focus in your binoculars so the color will become more obvious.

Five pointed star filled with circles with varying shades of orange, red and green.
View at EarthSky Community Photos. | Soumyadeep Mukherjee in Kolkata, West Bengal, India, captured this photo of the colors of the somber red star Betelgeuse on October 14, 2021. Soumyadeep wrote: “This image shows the shades of Betelgeuse, as observed from Earth. The variation in the shades is what we call the ‘twinkling’ of a star. A star twinkles thanks to the atmospheric refraction [the same effect that causes a spoon in a glass of water to appear broken in two]. And the amount of twinkle varies due to many atmospheric factors. The effect becomes most prominent when the star is near the horizon as the light passes through more atmosphere than when near the zenith [at its highest in the sky].” Thank you, Soumyadeep!

Why do stars have different colors?

The light of a star reveals many things, including the stars’ surface temperatures. The yellowish color of Capella indicates a mid-range surface temperature, much like our sun. The red of Aldebaran is typical of the lower surface temperature of an older star, whereas the blue of the Pleiades reveals their high surface temperature and young age.

In fact, the surface temperature – or color – of a star determines its spectral class. On the HR diagram below, you can see the different spectral classes listed across the bottom of the chart with temperatures going from hottest to coolest. Also, it shows the colors of stars associated with each spectral class and temperature.

So what are the spectral classes of Capella, Aldebaran, Sirius, Betelgeuse, Rigel and the Pleiades? Capella is a G star. Our sun is also a G star. Both our sun and Capella shine with a golden light. Aldebaran and Betelgeuse are cool stars and appear as reddish stars. Aldebaran is a K type star and Betelgeuse is a M type star. Sirius is an A type star and appears white. Rigel and the stars of Pleiades are type B stars.

Chart showing groups of labeled stars of varying colors.
View larger. | A star that is blue or blue-white color, such as Spica at the upper left, has a high surface temperature. In contrast, a red-colored star (such as Antares and Betelgeuse at the upper right), has a lower surface temperature. Image of Hertzsprung-Russell diagram via Chandra/ NASA.

Bottom line: Winter is the perfect season for noticing the colors of the stars. Have you ever noticed them? By all means, go check them out tonight! And now you also can tell the temperature of a star by its color.

February 16, 2023

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