Stellar temperature is primarily determined by a star’s surface heat, known as the photosphere temperature, because this is the layer from which light is radiated into space. A star’s surface temperature dictates the color it appears, providing a visual cue to its thermal output. Comparing temperatures allows scientists to classify stars and understand the physical processes that govern their existence and evolution.
Defining Stellar Temperature
Astronomers classify stars based on their surface temperature using the spectral classification system, which sorts stars into seven main groups: O, B, A, F, G, K, and M. This sequence places stars in descending order of heat, with O-type stars being the hottest and M-type stars being the coolest. A star’s letter is further subdivided by a number from 0 to 9, where zero is the hottest within that class. Analyzing a star’s spectrum allows scientists to determine its surface temperature with high precision.
This classification correlates directly with a star’s color. The hottest O and B-type stars emit light at shorter wavelengths, causing them to appear blue or blue-white. Moderately hot A and F-type stars appear white or yellow-white as their peak light emission shifts toward the middle of the spectrum. Cooler stars, such as K and M-type stars, emit most of their light at longer wavelengths, making them appear orange or deep red.
The Sun’s Temperature Benchmark
The Sun is a G-type star with the specific spectral classification of G2V, placing it in the middle of the stellar temperature range. This classification indicates the Sun is a yellow main-sequence star. Its surface temperature, the photosphere, is approximately 5,772 Kelvin (roughly 9,930 degrees Fahrenheit). This surface temperature serves as the standard benchmark for comparison with other stars.
It is important to distinguish the surface temperature from the Sun’s core temperature. The core, where nuclear fusion occurs, reaches nearly 15.7 million Kelvin. However, the surface temperature determines the star’s color and is the property used in the OBAFGKM classification system. Stars hotter than the Sun are any star with a photosphere temperature exceeding 5,772 Kelvin.
Specific Examples of Hotter Stars
Stars in the A, B, and O spectral classes are all hotter than the Sun. Sirius A, the brightest star in the night sky, is an A-type star with a surface temperature near 9,900 Kelvin, appearing brilliant white. Rigel, a B-type supergiant in the constellation Orion, has a surface temperature of about 12,000 Kelvin and shines with a blue-white hue.
The absolute hottest stars belong to the O-class, with surface temperatures ranging from 30,000 Kelvin up to over 50,000 Kelvin. These are massive, rare stars that radiate intensely in the ultraviolet spectrum, giving them a vivid blue appearance. For instance, the Wolf-Rayet star WR 102 has an estimated surface temperature exceeding 200,000 Kelvin. This means its photosphere is over 34 times hotter than the surface of the Sun.
The Physics Behind Stellar Heat
The primary factor determining a star’s temperature is its initial mass. Stars are spheres of gas held in balance by the inward pull of gravity and the outward pressure from nuclear fusion. A more massive star exerts a stronger gravitational force on its material, which highly compresses the star’s core.
The intense compression drives the core to higher densities and temperatures than in a lower-mass star. These core conditions force hydrogen atoms to fuse into helium at a faster, more energetic rate. The resulting surge of energy production creates greater outward pressure, leading to a higher surface temperature. This explains why O-type stars, which can be 15 to 100 times more massive than the Sun, are hotter. The rapid rate of fusion means these massive stars consume their fuel quickly, resulting in shorter lifespans.