Stars are magnificent, wondrous bodies. When you look up in the sky, you will see them twinkling in various colors. Some tend to be pale and reddish. Others seem to have a bright whitish or bluish color. But, have you wondered why they come in different colors?
Well, think about a campfire. The blue-white region of its flame is its hottest part, while the orange-red area is the coolest. Then, have you ever tried putting some salt or other chemicals to play with and alter the flame’s color? Magnesium Sulfate or Epsom salts will give the flame a white color. Boric Acid will turn into green. And, Strontium Nitrate will cause the fire to change to red. To put it simply, various elements burn, causing different colors at varying intensities.
The color of the stars pretty much follows the same principle. Two standard reasons are causing changes in color: temperature and age.
Same with flames, cooler stars appear red to orange, warmer ones range from yellow to white, while the hottest stars emit blue light. As a star age, it starts to release various chemicals that burn at varying temperatures.
The red dwarf stars are the coolest stars in the Universe. They are also the smallest with some only about 9% of the Sun’s radius and 7.5% of Sun’s mass. Red dwarf’s surface temperature averages around 3,500° Kelvin or 3226.85°C. While there are other colors mixed within, red is the majority and appears to our eyes.
The same color is what you see on red giants. These stars have exhausted all their hydrogen and inflated many times their initial size. Their radius is relatively more massive and has a surface temperature of 5,000° Kelvin or 4726.85°C. Red giants are still cooler as their luminosity is scattered throughout their large surface area. Betelgeuse and UY Scuti are some of the red supergiants.
On the other end lies the white and blue stars, which are around three times of Sun’s mass. They burn at extreme temperatures starting at 10,000° Kelvin or 9726.85°C. Some hypergiant stars may reach a surface temperature of 40,000° Kelvin or 39726.85°C. Rigel, the Orion’s brightest, is a sample of a blue-white supergiant.
So, stars exist in a range of colors, from red, yellow, yellowish-white, white, blue-white, and blue, with red being the coolest and blue being the hottest.
A star’s color is also an excellent tool in determining its age. The process is called stellar classification, which we employ to see where a star is on its lifespan, is it young, middle-aged, or near of its doom. There are several stellar classifications available, though, the most renowned is the Hertzsprung-Russell Diagram. It associates a star’s absolute magnitude, luminosity, and surface temperature to find it’s age.
The diagram has two axes. The x-axis displays temperatures in Kelvin. It has no negative values, meaning there is nothing colder than 0°or -273.15°C. Using the H-R Diagram, the hottest stars are on the left, while the coolest ones are on the right.
On the other hand, the y-axis refers to the luminosity and absolute magnitude, as compared to the brightness of our Sun. That means that the ‘1’ is equivalent to the Sun on the H-R Diagram. Stars with negative exponents, such as 10^-2, are 100 times less bright than the Sun and are around 10,000 times less dim in terms of absolute magnitude. Those who have higher exponents are more luminous and have greater absolute magnitudes.
Through that, stars are given their specific spectral types divided into seven categories with an acronym of OBAFGKM.
O – Blue
B – Blue/White
A – White
F – White/Yellow
G – Yellow
K – Orange
M – Orange/Red
O stars are the hottest, while M stars are the coolest. To make it easier to remember, you can remember the mnemonic, “Oh, Be A Fine Girl/Guy, Kiss Me.”
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