Stars are classified based on their temperature and brightness using a classification system known as the stellar classification or spectral classification. The two most commonly used parameters for classification are the star's surface temperature and its luminosity or brightness.
Temperature: Stars' temperatures can range from a few thousand degrees Kelvin for cooler stars to tens of thousands of degrees Kelvin for hotter stars. Temperature is a crucial factor in determining the color and characteristics of a star. Astronomers use the star's spectrum, which is the distribution of light at different wavelengths, to determine its temperature. The spectrum of a star is obtained by analyzing the starlight and identifying the absorption lines or emission features present.
Luminosity/Brightness: The luminosity or intrinsic brightness of a star is a measure of the total amount of energy it radiates per unit of time. Brightness is influenced by both temperature and size. However, to compare stars of different sizes, astronomers use a standardized measure called the absolute magnitude. It represents the brightness of a star if it were placed at a fixed distance from Earth. The absolute magnitude allows astronomers to compare stars as if they were all at the same distance from us.
Based on temperature and luminosity, stars are classified into different spectral types using the Morgan-Keenan (MK) system, also known as the Harvard spectral classification. The MK system categorizes stars into seven main spectral types, labeled with letters: O, B, A, F, G, K, and M. Each spectral type is further divided into subclasses (0-9) based on more detailed temperature differences.
Here is a breakdown of the spectral types and their characteristics:
O-type stars: These are the hottest and most massive stars, with surface temperatures exceeding 30,000 Kelvin. They appear blue in color and are very luminous.
B-type stars: They are slightly cooler than O-type stars but still very hot, with temperatures ranging from 10,000 to 30,000 Kelvin. They appear blue-white in color and are also quite luminous.
A-type stars: These stars have temperatures between 7,500 and 10,000 Kelvin, and they appear white in color. They are less massive than O and B stars but still relatively bright.
F-type stars: With temperatures ranging from 6,000 to 7,500 Kelvin, F-type stars appear yellow-white. They are smaller and less luminous than A stars.
G-type stars: These stars have temperatures between 5,000 and 6,000 Kelvin. Our Sun, classified as a G-type star (specifically G2V), belongs to this group. They appear yellow in color and have moderate luminosity.
K-type stars: With temperatures ranging from 3,500 to 5,000 Kelvin, K-type stars appear orange in color. They are smaller and dimmer than G stars.
M-type stars: These are the coolest and least massive stars, with temperatures below 3,500 Kelvin. They appear red in color and are known as red dwarfs. They are the most common type of star in the universe.
This spectral classification system provides astronomers with a useful way to categorize stars based on their temperature and brightness, allowing them to study and understand the vast diversity of stellar objects in the universe.