# Why is a white dwarf hotter than a red giant star?

Apr 13, 2018

A white dwarf has a higher surface temperature than a red giant star.

#### Explanation:

A red giant star is a star which has a mainly Helium core which is not hot enough to start fusion reactions. Hydrogen is being fused in a shell around the core. The Hydrogen fusing shell caused the outer layers of the star to expand greatly.

To put a red giant into perspective, when our Sun becomes a red giant it will swell to about the size of the Earth's orbit. So, the core of a red giant will be very hot - tens of millions of degrees. The more distant surface will be a relatively cool $3 , {000}^{\circ}$K. This gives it a red colour.

The white dwarf stage comes after the red giant stage. A white dwarf is the collapsed core of a star. It is mainly Carbon and Oxygen. It isn't hot enough to start Carbon fusion. It is still however very hot, with a surface temperature of $3 , {000}^{\circ} - 30 , {000}^{\circ}$. This makes it white.

As a red giant still has active fusion reactions taking place, it is very luminous. A white dwarf on the other hand has no internal power source and slowly radiates away its heat, cooling and getting redder in the process.