A star's temperature fluctuates based on the physical processes within the star. The nuclear fusion inside a star's core produces energy, which radiates outward. As a star ages, the fuel in the core gets used up and the star cools. The core will then contract due to gravity. As a result, the temperature inside the core can increase and trigger more fusion.
The Main Sequence and Beyond
A star spends most of its life in the main sequence, during which it burns hydrogen fuel. A star with a mass around that of the sun can burn hydrogen with a surface temperature around 6,000 K for around 10 billion years. Hydrogen fuel is replaced by helium, which requires a hotter temperature for nuclear fusion. As the remaining hydrogen is used up, the star develops a hydrogen shell surrounding the helium core. Fusion continues in this hydrogen shell as the star expands. The net result of this process is a decrease in surface temperature. Hence, a star cools as it deviates from the main sequence and becomes a red giant. This cooling process can last for 1 billion years. A star like the sun can cool to a surface temperature of roughly 4,500 K during this period.
During the red giant phase, the core of the star eventually reaches 100,000,000 K, which is the temperature required for helium fusion. The helium fusion in the core and continued fusion of the hydrogen shell results in an increase in surface temperature. This phase is often called the Horizontal Branch. During this phase, the surface temperature of a sunlike star can reach 9,000 K. This phase can last 10,000 years.
The End of a Star's Life
As helium is consumed in the core, it is replaced by carbon and oxygen. It can have both a hydrogen and helium shell during this time. Again, the star expands and the temperature cools. As the helium shell burns, a stellar wind blows the outer layers of the star away, leaving behind the star's hot, massive core. This core increases in temperature as the star becomes a planetary nebula; a core surrounded by dissipating gases. The core eventually dims and cools, and the star becomes a white dwarf. The star's core continues to burn, but fusion of the heavier elements does not occur. During this time, the surface temperature of the star can range from 5,000 to 30,000 K, depending on its size and core temperature.
High-Mass and Low-Mass Stars
High-mass stars have higher temperatures and shorter main sequence lifetimes than low-mass stars. For example, a star with a mass 10 times that of the sun could have a main sequence temperature of 20,000 K but a lifetime of only 20 million years. As these stars age, they burn increasingly heavy elements in their cores. However, once the core becomes predominantly iron, nuclear fusion stops. At this point, the star will collapse on itself due to gravity and then explode in a supernova. On the other hand, some stars are not even massive enough to enter the red giant phase. These stars are called red dwarf stars. Their main sequence lifetimes can be greater than the current age of the universe. The surface temperature of a red dwarf can range from 2,500 to 4,000 K.
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