The main difference between a white dwarf star and a main sequence star lies in their stage of stellar evolution and energy production:

1. Main Sequence Star:

• A main sequence star is in an active phase of its life, where it generates energy through nuclear fusion.
• Specifically, hydrogen atoms fuse into helium in the star's core, producing thermal energy that balances the inward pull of gravity.
• This energy is radiated from the star's surface, making it luminous.
• Main sequence stars maintain hydrostatic equilibrium, where the outward pressure from fusion balances gravitational collapse.
• The Sun is an example of a main sequence star.
• Main sequence stars come in a range of masses, colors, and luminosities but share the common characteristic of fusing hydrogen in their cores.

2. White Dwarf Star:

• A white dwarf is a stellar remnant formed after a star has exhausted all its nuclear fuel.
• It no longer supports nuclear fusion; its energy emission is from residual thermal energy and slowly cooling over time.
• White dwarfs have a very high density and a compact size similar to Earth but contain a mass comparable to the Sun.
• They have a core typically composed of carbon and oxygen, with thin outer layers of helium and sometimes hydrogen.
• The star's structure is supported by electron degeneracy pressure rather than fusion pressure.
• White dwarfs represent the final evolutionary stage for medium and low mass stars after they shed their outer layers (often via a planetary nebula phase).

In summary, a main sequence star actively fuses hydrogen in its core to produce energy, while a white dwarf is a dense, compact stellar remnant with no ongoing fusion, slowly cooling after fusion has ceased.