Across the vast expanse of the universe, stars stand out as constant companions to Earth’s nighttime observers. But beyond their gleaming sparkles lies a fascinating spectrum of colors, each telling a unique story about the star’s composition, temperature, and stage in its life cycle. In this article, we delve into the science behind these star colors, offering a comprehensive understanding of what gives each star its distinctive hue.
While many casual observers might perceive stars as mere white or twinkling dots in the firmament, a closer inspection with a telescope—or even with the naked eye under ideal conditions—reveals a variety of colors of stars, from fiery red to icy blue. This colorful array is not just a matter of aesthetic intrigue. It serves as a window into the intricacies of stellar physics, offering insights into temperatures, ages, and even the potential presence of planets around these distant suns.
Historically, astronomers have sought to categorize and understand these stellar colors, realizing early on their importance in decoding the mysteries of the universe. The work of scientists such as Annie Jump Cannon, who helped develop the Harvard Classification Scheme, laid the groundwork for modern stellar classification. As we step further into this topic, we will elucidate how these classifications are linked with temperature, how different star colors correlate with a star’s life stage, and why understanding these factors is crucial for the broader comprehension of our galaxy and the universe beyond.
Observing Star Colors
The human eye is an intricate organ designed to adapt to various lighting conditions, allowing us to see in a wide range of environments, from the bright light of day to the dimness of night. This adaptability is achieved through the dual functionality of two types of photoreceptor cells in our retinas: rods and cones. The way these cells operate and respond to light plays a critical role in our ability to perceive color and discern details, especially under low-light conditions.
During the day, when light is abundant, the cones in our eyes are primarily responsible for our vision, termed photopic vision. There are three types of cone cells, each sensitive to different parts of the light spectrum: short wavelengths (blue), medium wavelengths (green), and long wavelengths (red). Together, these cones allow us to perceive the full spectrum of colors in well-lit conditions. However, as the light level drops and darkness prevails, the cones become less activated due to their higher threshold for light stimuli.
Enter the rods, which are far more sensitive to light than cones but lack the ability to discern color. In dim conditions, we typically use both rods and cones to see, known as mesopic vision. As the ambient light diminishes under a dark night sky, however, our eyes undergo a transition to what is called “scotopic vision”, where the rods take over from the cones entirely. While rods are excellent for detecting motion and seeing in low-light scenarios, they don’t contribute to color perception. This is why, after spending extended periods in darkness, our vision becomes monochromatic, dominated by shades of gray. This phenomenon is termed “night blindness” or “nyctalopia.”
Considering stars and their colors, this intricacy of our vision becomes especially relevant. Even though stars have distinct colors based on their temperatures and compositions, most of them appear white or twinkling to the naked eye. You will also notice that after extended periods, even the brightest and most vivid stars, like Betelgeuse, Arcturus, and Vega, lose their splendor and seem to appear closer to just an off-white or yellow tone as the night goes on, unless you do something to switch back to photopic or mesopic color vision, such as looking at a bright source of light or a very bright star/planet through the telescope. This is largely because the dim light from distant stars isn’t strong enough to stimulate our cones effectively without optical aid.
Your cones are also not stimulated by faint objects. This is why nebulae and galaxies appear gray through a telescope. There is simply not enough light to activate the cones. The same is true of faint stars. With the increased light-gathering power granted by a telescope or binoculars, progressively fainter stars can emit enough light to activate the eye’s cone cells, allowing us to see their true colors. Bright stars, on the other hand, appear even more vivid as the intensified light amplifies their inherent colors. This goes up the more aperture your instrument has. Likewise, very large telescopes can bring in enough light from distant nebulae and other objects to activate the cones and produce some hints of color, although not exactly vivid hues.
Generally, you can see the colors of stars down to about magnitude 2-3 with your naked eye. This is around 15 times dimmer than Arcturus, an orange giant that is among the most prominently colored stars to the naked eye. A telescope or pair of binoculars’ effective aperture gain over your eye (which we will assume has a 6mm pupil for simplicity) is therefore directly proportional to the color gains you’ll see. A pair of 50mm binoculars has a gain of about 5 magnitudes, or about magnitudes 6-7 for colors. An 8” telescope gains another two magnitudes (color limit ~mag 9), and a 20” gains another 3, resulting in a magnitude limit of around 12 for color in stars.
Coincidentally, magnitude 12 is around the brightness of the most prominent members of some globular clusters, so a 20” or larger instrument under a dark sky will start to show hints of yellow-orange in M13 and M3, blue-gold in M92, and so on, though a 30” or larger telescope is needed to make the colors really striking.
Why are there no Green or Purple Stars?
Understanding the colors of stars requires diving into the concept of black-body radiation. Black-body radiation pertains to the light and heat given off by an object that absorbs all radiation that falls on it, with no radiation reflected off or passing through. In simpler terms, it’s the spectrum of light emitted by a hypothetical, idealized object known as a “black body” based on its temperature. As it turns out, stars, being nigh-impenetrable balls of plasma emitting unimaginable amounts of light and energy, are really close approximations to a perfect black-body radiator. This means that their color spectrum, or the most prominent color they emit, is predominantly determined by their surface temperature and progresses from red-orange at low temperatures to white and blue at the hottest, much like a burning flame.
As a black body (in our case, a star) heats up, the color of light it emits shifts. It starts with red, moves to orange, then yellow, and finally blue-white as it gets hotter. But what about green or purple? If stars progress from red to blue in terms of color as they heat up, why don’t we see any prominent green stars? As a star’s temperature increases and its peak emission approaches the green part of the spectrum (around 500 nanometers in wavelength), it also emits significant amounts of red and blue light. This combination of blue, green, and red light appears white to our eyes. Thus, stars that might otherwise appear green end up looking whitish. Their green emissions get “drowned out” by the other colors, making them seem more neutral.
Purple is a combination of red and blue light that is essentially made up by our brains when we see an object that strongly reflects or emits towards the fringes of the visible spectrum but has little in the way of yellow or green light. Stars can’t do this because if they emit a lot of red and a lot of blue, there would also be a lot of emission across the middle of the spectrum – again, a white star. However, there’s an exception for brown dwarfs. While not technically stars because they can’t sustain hydrogen fusion, some brown dwarfs can appear purplish due to absorption by certain molecules in their atmospheres.
Harvard Stellar Classification
Delving into the spectrum of star colors, we can categorize stars based on their spectral class and the corresponding color we observe. This is based on the Harvard classification system created by astronomer Annie Jump Cannon. The Harvard system uses letters to classify temperatures and thus colors, and then a letter to classify where in that range the star is – a G0 star, for instance, is very close to an F9 star in coloration. Our sun is a G2 star, at the upper end of the temperature range of a G star, and is almost bright enough to be an F. These two digits are followed by a Roman numeral to classify the type of star (either main sequence “dwarf” stars, giants or subgiants, or unusually small subdwarfs). So our Sun is a G2V star, while Betelgeuse, a red supergiant in the constellation Orion, is a class M2Ia star.
Stellar classification and colors provide a method to our cosmic madness, helping astronomers determine a star’s temperature, age, luminosity, and more. Let’s explore this spectrum and the celestial examples within each category.
Stellar classification, in order from hottest to coolest, goes as follows:
|T (Brown Dwarf)||800-1,200|
|Y (Brown Dwarf)||<800|
However, unsurprisingly, these letters don’t really track directly with real-world colors. Let’s take a look at the broad color categories that stars fall into and how this tracks with temperature and spectral classification.
Crimson to Light Orange-Red (L5-M8, C, S)
- Carbon Stars: Often exhibiting a deep red or nearly crimson hue, these red giant stars have an atmosphere rich in carbon. Their color arises from the carbon compounds and molecules in their outer layers, which absorb light in the blue and green parts of the spectrum, letting only the red wavelengths dominate.
- Other unusual red giant/supergiant stars can occasionally be so cool as to fall into class L or the lower fringes of M, often during the final phases of their lives as they rapidly eject large clouds of matter and lose energy (thus heat) in the process.
- Ultracool Red Dwarfs: These are among the smallest and coolest stars. They typically have temperatures too low to appear bright to our eyes, but they are nonetheless plentiful in the universe.
The smallest red dwarf stars, the coolest red giants, and the most massive of the brown dwarfs (“failed stars” which burn deuterium and not hydrogen) constitute the lower bounds of the M and upper L spectral classes. The star TRAPPIST-1, an ultracool red dwarf home to multiple Earth-sized planets, is an example of a star in this temperature range.
The red supergiant star NML Cygni is of class M9-L1. However, the star sheds mass into a dust cloud that masks much of its spectrum and virtually hides it from view, making precise measurements of some of its physical properties difficult.
Orange to Deep Yellow (M7-K4)
- “Red” Dwarfs: Despite being termed ‘red’, many of these stars are closer to orange in color, especially those towards the warmer end of the class.
- Red & Orange Giants: As stars exhaust their hydrogen, they evolve, becoming red or orange giants or even supergiants if they are of sufficient mass. They expand and cool, which gives them their distinct hues.
The nearby star Proxima Centauri is a pretty typical example of a red dwarf. Red dwarfs are among the most common stars in the Universe and can live for trillions of years on account of their extremely slow burning of hydrogen fuel.
Betelgeuse is an M2-class red supergiant star, nearing the end of its lifespan. Aldebaran is a K5-class orange giant that is similar to our Sun in mass but is further evolved, nearing the apex of the red giant stage.
Yellow to Yellow-White (K3-G0)
- “Orange” and “Yellow” Main Sequence Stars: Paradoxically, what we term “orange” stars often appear deep yellow, while “yellow” stars can be closer to yellow-white in appearance.
- Yellow Giant Stars: These are somewhat rare, representing a transitional phase in the evolution of certain stars. They’ve left the main sequence and are on their way to becoming red giants.
Our very own Sun is a G2-class star that appears as a yellow-white star. Alpha Centauri consists of a G2 star (A) like our Sun and a K1 star (B), which both appear a similar shade of yellow-white (our own planet’s atmosphere tints our Sun a bit yellower than it really is).
- F & A-Class Stars: These stars are hotter than our Sun and emit a pure white light. They are more massive and have shorter lifespans compared to Sun-like stars.
Procyon is a bright F-class main sequence star, while Polaris is an unusual F-class giant star nearing the end of its life.
- A-Class Stars: These stars are much hotter than our Sun and die young, with the least massive A-class main sequence stars living less than 1 billion years, hardly enough time for planets to condense in orbit around such a star.
Sirius A, the brightest star in our night sky, is an A0-class star with a blue-white glow. Vega is also an A0 main-sequence star but lies further away and is less overwhelming in brightness, thus appearing a bit bluer.
Sky Blue (B6-B0)
- B-Class Stars: These are among the hottest and most massive stars. They have short lifespans, burning brightly and exhausting their fuel rapidly.
Rigel, at the western foot of the constellation Orion, is a B-class star.
Deep Blue (O, WR)
- O-Class Stars: The hottest O-class stars venture into deep blue territory. They are rare but extremely luminous. Most of their light is actually emitted in the ultraviolet.
- Wolf-Rayet Stars: These are highly evolved, massive stars that are shedding their outer layers at a rapid rate, and they can often appear in deep blue hues.
Iota Orionis – just south of the famed Orion Nebula – is a notable O-class giant, as is Meiass at the head of Orion. HDE 226868, the star being swallowed by the black hole Cygnus X-1, is also an O-class giant. Zeta Puppis is a notable Wolf-Rayet star.
Other Colorful Stars You Can See With a Telescope
- Albireo (β Cygni)
Arguably one of the most beautiful double stars in the sky, Albireo is a favorite among stargazers. Located in the constellation Cygnus, the Swan, it presents a striking color contrast. The brighter star appears amber or gold, while the dimmer companion shines a deep azure blue. This stellar duo offers a stunning visual when observed through even a modest telescope or a pair of binoculars.
- Almach (γ Andromedae)
Situated in the constellation Andromeda, Almach is another splendid double star that can be spotted even from light-polluted skies at an apparent magnitude of 3.1. A telescope with at least 50x magnification splits this pair. The primary star is bright and golden, whereas the secondary star is a lovely blue, providing a breathtaking color contrast.
- Antares (α Scorpii)
Antares is a bright orange supergiant star, one of the most prominent in the night sky at magnitude 1. Its deep red hue is particularly striking. Close to Antares, a telescope reveals a faint, deep-blue companion star on a steady night, offering a striking color contrast.
- Acrab (β Scorpii)
Also in Scorpius and readily seen with the naked eye at magnitude 2.5, Acrab is a binary star system where both components are hot, blue-white B-class stars. At 13.5 arcseconds of separation, a telescope with a modest magnification—say, 50x or more—reveals both electric-blue suns.
- Gamma Delphini
This is a delightful magnitude 3.9 double star in the constellation Delphinus, the Dolphin. The primary star shines with a golden-yellow hue, while its companion presents a contrast with its pale blue tone. As of the time of writing (2023), the stars are about 9 arc seconds apart, easily split at 75x or more magnification. The two stars are in a highly eccentric orbit, however, and are slowly closing in on each other. When the binary nature of Gamma Delphini was observed by William Herschel in 1779, the two were about 12 arcseconds apart. It will take thousands of years for them to complete one orbital cycle, however, owing to their separation; at their closest, the stars are about as far apart as the distance from the Sun to Pluto, and at their furthest, they are about 15 times more distant than that.
- Hind’s Crimson Star (R Leporis)
A carbon star, R Leporis is famed for its deep ruby-red appearance. Carbon stars are red giants, in which carbon molecules in the atmosphere scatter blue and green light, allowing the red light to dominate. Located in the constellation Lepus, R Leporis provides an impressive view for those with a telescope, especially when it’s at its brightest during its variable cycle. R Leporis ranges from magnitude 5.5 (modestly apparent with the naked eye under dark skies) at its brightest to magnitude 11.7 at its dimmest, meaning it’s best spotted with a telescope.
- Izar (ε Boötis)
Another brilliant double star, Izar, is located in the constellation Boötes. It is easy to see with the naked eye, at magnitude 2.7 and due north of Arcturus. With a telescope, observers can discern a bright orange primary star and a fainter blue secondary star. Separated by only a few arc seconds, it takes a steady night to split this pair, though they are within range of even a small telescope. The color contrast makes Izar a treat for stargazers.
- La Superba (Y Canum Venaticorum)
A standout among carbon stars, La Superba is aptly named. La Superba shines with a rich, deep red hue, making it one of the reddest stars visible to the naked eye when it shines at its peak brightness of magnitude 4.9, which is modestly apparent under a suburban or darker sky, but too dim to reveal its color without optical aid. As the star pulses, it brightens and dims, reaching magnitude 7.3 at its faintest. Even a small telescope or binoculars can reveal this star’s ruby-red color, regardless of where it is in its cycle.