The temperature of a star is a fundamental property influencing its appearance, behavior, and evolutionary path. Stars generate energy through nuclear reactions in their cores. Understanding stellar temperatures and their variations allows scientists to determine a star’s composition, age, and ultimate fate.
Measuring Stellar Temperature
Scientists determine a star’s surface temperature by analyzing the light it emits. One primary method involves observing the star’s color, a direct indicator of its heat. Hotter objects emit light at shorter wavelengths, appearing bluer, while cooler objects emit light at longer wavelengths, making them appear redder. This relationship is described by Wien’s Displacement Law.
Another technique for measuring stellar temperature is spectroscopic analysis. When starlight is passed through a spectrograph, it is spread out into a spectrum, revealing dark lines caused by elements in the star’s atmosphere absorbing specific wavelengths of light. The presence and intensity of these absorption lines depend on the temperature of the star’s atmosphere, as different elements become ionized or excited at varying temperatures. By studying these spectral “fingerprints,” astronomers can precisely determine the star’s surface temperature and even the density of elements within it.
What Makes Stars Hotter or Cooler
A star’s mass is a primary determinant of its temperature. More massive stars possess stronger gravitational forces, which compress their cores to higher densities and temperatures. This increased pressure and temperature accelerate nuclear fusion reactions, causing these stars to burn through their fuel at a much faster rate. Consequently, massive stars are hotter and more luminous on their surfaces than less massive stars.
A star’s temperature also changes throughout its life cycle. Stars begin their lives on the main sequence, fusing hydrogen into helium in their cores. As a star exhausts its hydrogen fuel, it evolves into different stages, impacting its temperature. For example, a star like our Sun will expand into a red giant, where its outer layers cool even as its core heats up to fuse helium. Eventually, after shedding its outer layers, the core of such a star becomes a hot, dense remnant that slowly cools over billions of years.
The Range of Star Temperatures
Stars exhibit an enormous range of surface temperatures, from a few thousand Kelvin to tens of thousands of Kelvin. Astronomers classify stars into spectral types, denoted by letters O, B, A, F, G, K, and M, which correspond to decreasing surface temperatures. O-type stars are the hottest and bluest, while M-type stars are the coolest and reddest. Our Sun, for example, is a G2-type star.
Extremely hot stars, like blue supergiants, can exceed 10,000 Kelvin; Rigel, for instance, is around 12,100 Kelvin, and Sirius A is about 9,940 Kelvin. In contrast, cooler stars include red giants and red dwarfs. Betelgeuse, a red supergiant, has a surface temperature of about 3,100 to 3,300 Kelvin. Proxima Centauri, a red dwarf, is even cooler at around 3,050 Kelvin. Our Sun has a surface temperature of approximately 5,800 Kelvin.