Here's why:

* Nuclear Fusion: Main sequence stars are powered by nuclear fusion in their cores. This process fuses hydrogen into helium, releasing immense amounts of energy in the form of light and heat.

* Temperature Gradient: The core of a main sequence star is extremely hot, and the temperature gradually decreases as you move outwards. This creates a temperature gradient that drives the outward flow of energy.

* Surface Temperature: The surface temperature of a main sequence star varies depending on its mass. Larger, more massive stars are hotter than smaller, less massive stars.

Examples:

* Our Sun: A G-type main sequence star, with a surface temperature of about 5,500° Celsius (9,932° Fahrenheit).

* Rigel: A blue supergiant, a massive main sequence star with a surface temperature of around 12,100° Celsius (21,832° Fahrenheit).

While the surface temperatures of main sequence stars vary, they are all hot enough to emit significant amounts of light and heat.