The Colors of Stars

Introduction

Starlight ranges from fiery reds to icy blues, showing various colors that reveal the unique traits and processes of these celestial objects. But what exactly affects the color of each star?

A star's temperature primarily determines its color. Stars with higher surface temperatures emit bluer light, while cooler stars emit redder light. If we look closely enough, we can see this in a candle flame or bonfire. The center of the flame always appears white or bluish, while the outer layer appears yellow, orange, or red.

In this article, we explore the fascinating realm of star colors, examine what other factors affect that, classify stars based on their temperature, and see where our most loved star in our Solar System—the Sun—stands in this chart.

What Affects the Color of Stars?

Star Temperature and Distance from Earth

The color of a star is mostly influenced by two key factors: its surface temperature and its distance from Earth.

Surface Temperature: Stars emit light across a spectrum of colors, with hotter stars appearing bluer and cooler stars appearing redder.
Wien's Law describes this relationship, which states that hotter objects emit shorter-wavelength (bluer) light, while cooler objects emit longer-wavelength (redder) light. For example, blue-white stars like Sirius have surface temperatures exceeding 10,000 Kelvin. In comparison, red stars like Betelgeuse have temperatures around 3,000 Kelvin.

Our Sun generally gives off a clear white light because it burns at 6,000 Kelvin. This whiteness is more evident from further out into space. However, from Earth—a measly 149.6 million kilometers from the Sun—the visible light looks richer, appearing yellow or gold to the human eye, because we're getting flashes of the burning process from our position.

Distance from Earth: A star's distance can also affect its apparent color, as observed from Earth. Stars appear dimmer and redder as their distance from Earth increases because gas and dust in space block or scatter their light.

This phenomenon, known as interstellar extinction, causes distant stars to show a redder hue compared to their actual color.

Pictures taken 60 seconds apart showing how stars can change colors. Credit: Amanda Cross

Stellar Classsifications: OBAFGKM

No, the title is not someone sitting on the keyboard, though it may seem like it at first glance. These are actually the letters for different star types or stellar classifications if you will.

Right now, we classify most stars under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M. We assign these classes based on the stars' temperature and color. Based on this chart, the hottest stars, which appear blue to us from Earth, are class O, while the coolest ones in colors like red and orange are class M. 

Blue Stars

Stars that appear blue often exceed the surface temperature of 10,000 degrees Celsius. These high temperatures cause them to emit light with shorter wavelengths, which our eyes perceive as blue. Blue stars are often massive and young, as they burn through their nuclear fuel at a rapid rate, emitting intense blue light in the process. Examples of blue stars include Rigel in the constellation Orion and Spica in the constellation Virgo.

White Stars

Stars that appear white have surface temperatures ranging from 7,500 to 10,000 degrees Celsius. These temperatures produce balanced light emission across the visible spectrum with no dominant color. White stars are often intermediate-mass and in the middle stages of their lifecycle. Examples of white stars include Sirius, the brightest star in the night sky, and Vega, one of the brightest stars in the northern hemisphere.

Yellow Stars

Stars that appear yellow typically have surface temperatures ranging from about 5,000 to 7,500 degrees Celsius. Yellow stars are often of intermediate age and mass, transitioning from the hotter, bluer stage of their evolution to the cooler, redder stage. Examples of yellow stars include our Sun, which emits a whitish-yellow light, and Capella, one of the brightest stars in the constellation Auriga.

Orange Stars

Stars that appear orange have surface temperatures ranging from about 3,500 to 5,000 degrees Celsius. Orange stars are generally cooler and less massive than yellow or white stars, with a longer lifespan. Examples of orange stars include a supergiant star in the constellation Orion, Arcturus, one of the brightest stars in the northern sky, and the brightest star in the constellation Boötes.

Red Stars

Stars that appear red usually have surface temperatures lower than those of other colored stars, typically ranging from about 2,500 to 3,500 degrees Celsius. Red stars are often cooler and less massive than stars of other colors. We commonly find them in the later stages of their evolution, such as red giants or red dwarfs. Examples of red stars include Betelgeuse, a red supergiant in the constellation Orion, and Antares, a red supergiant in the constellation Scorpius.

Green Stars and Purple Stars

Green stars are quite rare, and we do not typically observe them in the night sky. In fact, no stars emit a pure green light. The perception of green stars may occur under specific conditions, such as atmospheric effects or unusual interactions with a telescope's optics. The same can be said about purple stars, which may appear to glow purple because of some combinations of blue and red colors, but true purple stars do not exist in nature.

How is the Sun Classified?

This star classification chart places our most important star, the Sun, as a G-type main-sequence star. We often call it a yellow dwarf, although its actual color is closer to white. As a main-sequence star, the Sun is in a stable phase of its life cycle. This classification helps astronomers understand the Sun's place among the billions of stars in the universe and provides insights into its structure, behavior, and evolutionary path.

Can Stars Change Colors?

Yes, stars can change colors over time due to various factors:

Temperature Changes: A star's surface temperature primarily determines its color. As a star ages, its temperature may change due to internal processes. For example, a young star might start off as a hot, blue giant but eventually cool down as it exhausts its nuclear fuel, causing it to become a cooler, red giant.

Evolutionary Stage: Stars go through different stages of evolution, each of which can affect their color. For instance, as a star exhausts its hydrogen fuel and transitions into a red giant, its surface temperature decreases, causing it to emit more red light. Similarly, a star may shed its outer layers and expose its hot core during the later stages of its life, causing it to appear blue or white.

Composition: The chemical composition of a star can also impact its color. Stars with different compositions emit different wavelengths of light, which can affect their perceived color. For example, a star with a high concentration of heavy elements may appear bluer than a star with fewer heavy elements.

Interactions with Other Objects: Stars can interact with other objects in their vicinity, such as companion stars or surrounding dust and gas clouds. These interactions can influence how the star emits or scatters light, potentially altering its color. For instance, if a star orbits another star closely, the pull of gravity between the two may affect their shapes and temperatures, leading to color changes.

Conclusion

To conclude our exploration of different star colors and why they are different in the first place, here is what we can take from this article:

1. A star's surface temperature and its distance from Earth primarily influence its color. Hotter stars emit bluer light, while cooler stars emit redder light. Additionally, stars appear dimmer and redder as their distance from Earth increases.
2. We classify stars based on their temperature using the OBAFGKM system. O-type stars are the hottest and appear blue, while M-type stars are the coolest and appear red. Our Sun, classified as a G-type main-sequence star, emits a whitish-yellow light.
3. Depending on their surface temperatures, stars exhibit a wide range of colors, from blue and white to yellow, orange, and red. Blue stars are typically young and massive, while red stars are cooler and less massive, often found in the later stages of their evolution.
4. Stars that appear green or purple are rare and typically not observed in the night sky. We may perceive green stars under specific conditions, but true purple stars do not exist in nature.
5. Stars can change colors over time due to temperature changes, evolutionary stages, and interactions with other objects nearby. 

References

1. O’Connell (27 March 2023). "Magnitude and Color Systems." Caltech ASTR 511.
2. Wikipedia - "Stellar Classification"
3. NASA - "Star Types"
4. NASA - "Star Basics"
5. LCO - "Magnitude and Color"
6. LCO - "Apparent Magnitude"