The color of a star provides important information about its temperature, composition, and stage of evolution. Stars come in a range of colors, from cool red dwarfs to hot blue giants and everything in between. In this article, we will explore the different star colors and identify which is considered the second coolest after red.
The color of a star depends primarily on two factors – its surface temperature and the elements it contains. Hotter stars appear bluer or white, while cooler stars take on an orange or red hue. The coolest stars with surface temperatures under 4,500 Kelvin are classified as red dwarfs. So which star color comes next in terms of coolness? To answer this, we’ll need to take a closer look at star temperatures and the categories used to classify them.
Star Color and Temperature
The surface temperature of a star determines the color it emits via blackbody radiation. Here is a general overview of star colors and corresponding temperatures:
| Star Color | Temperature (Kelvin) |
|---|---|
| Blue | Over 10,000 |
| White | 7,500 – 10,000 |
| Yellow | 6,000 – 7,500 |
| Orange | 5,000 – 6,000 |
| Red | Under 5,000 |
As this table shows, the coolest stars with temperatures under 5,000 K appear red. So which color occupies the next range up? Orange stars, with surface temperatures between 5,000-6,000 K, are the second coolest.
Star Classifications
Astronomers also categorize stars by spectral type or class based on their temperature and other properties. The major spectral classes from hottest to coolest are O, B, A, F, G, K and M. Each of these letters correlates approximately with a certain temperature range and color:
– Class O (over 30,000 K) – blue
– Class B (10,000 to 30,000 K) – blue-white
– Class A (7,500 to 10,000 K) – white
– Class F (6,000 to 7,500 K) – yellow-white
– Class G (5,200 to 6,000 K) – yellow
– Class K (3,700 to 5,200 K) – orange
– Class M (under 3,700 K) – red
So based on this spectral classification scheme, K-type orange stars are the second coolest, coming after M-type red dwarfs.
Common Orange Star Types
There are a few subclasses of orange K-type stars we can look at to get a better sense of the second coolest stellar color category:
– K0-K4 – Slightly orange stars with surface temperatures around 5,000 K. An example is Epsilon Indi.
– K5-K9 – Noticeably orange stars with temperatures from 4,000-4,900 K. Examples include 61 Cygni A and Tau Ceti.
– K10-K14 – Deep orange subdwarfs with metallicities lower than the Sun. Groombridge 1830 is a K10 orange subdwarf.
So middle K-class stars from K5 to K9 represent the archetypal orange dwarfs that fit our definition of the second coolest star color next to red.
Why Orange Stars are Cooler than Yellow or White
Cooler stars shift down the visible light spectrum from blue/white to red as their blackbody radiation curves peak at longer (redder) wavelengths. A star’s blackbody curve is directly linked to its surface temperature – the hotter the star, the bluer and more intense its peak emission.
As stars age and cool over billions of years, their emissions shift gradually from the blue end to red end of the spectrum. Orange stars like our K-class dwarfs represent an intermediate stage in stellar evolution. Their surface temperatures of 4,000-5,000K are not as hot as F, A, or B type stars, but still hotter than the coolest M dwarf red stars.
Unique Properties of Orange Stars
Here are a few key facts about K-type orange dwarf stars that make them distinct:
– Smaller than Sun-like stars – Usually between 0.5-0.8 solar masses with radii 0.7-0.9 times the Sun. This makes them dimmer as well.
– Long stable lifetimes – Orange dwarfs can burn hydrogen for 15-30 billion years, giving rise to some of the oldest stars in the galaxy.
– Capable of hosting planets – Orange stars are cool enough to allow orbiting rocky planets in the habitable zone. Kepler-442b is one potential habitable planet orbiting an orange dwarf.
– Make up about 12% of main sequence stars – Orange dwarfs are the third most common stars after red dwarfs (74%) and yellow dwarfs like our Sun (7%).
So in summary, orange K-class dwarfs represent a cool, long-lived, and abundant category of stars that populate the galaxy. Their intermediate status between hotter and cooler stars give orange dwarfs their uniquely colorful radiation.
Conclusion
Based on an analysis of star classifications and temperature rankings, orange dwarf stars emerge as the clear second coolest category after red dwarfs. With surface temperatures ranging from 4,000-5,000 K, orange stars produce light dominated by longer wavelengths for a distinctive warm color signature. Spectrally categorized as K-type, these mature stars are smaller, stable for billions of years, and very common around the galaxy. So the next time you look up at the night sky, see if you can spot the orange glow of one of these cool second-ranked stars shining brightly among the multitude of celestial bodies.
