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Main-sequence stars are fascinating components of our universe. These celestial bodies spend the majority of their lifecycle in a stable state. The secret to their long-lived stability is the process of nuclear fusion, where hydrogen atoms in the star's core are converted into helium. This process releases a tremendous amount of energy, illuminating the star brightly.
Think of main-sequence stars as the "middle age" phase of a star's life, where they burn steadily and maintain a relative equilibrium between the inward pull of gravity and the outward push of thermal pressure. An excellent example of a main-sequence star is our very own Sun. It has been in this phase for approximately 4.5 billion years and will continue for about the same duration.
Main-sequence stars are characterized by their mass and brightness, which depend on conditions like temperature and chemical composition. These stars range from smaller, cooler red stars to larger, hotter blue stars.

When a main-sequence star exhausts the hydrogen fuel in its core, it undergoes dramatic transformations, evolving into a giant or supergiant. This transition occurs as the core contracts under gravity, increasing the temperature and causing outer layers to expand dramatically.
In this phase, the star becomes significantly larger and more luminous, though cooler at the surface. Giants and supergiants are much bigger than main-sequence stars and can be several times larger than our Sun. While giants may tower several times the Sun's size, supergiants take this to an extreme, growing several hundred times larger.

• Giants are stars that have expanded to several times their original size.
• Supergiants are even larger, with significantly higher luminosity due to their vast outer layers that become more extended.

Despite their massive size and brightness, the life of giants and supergiants is relatively short-lived compared to the stable existence of main-sequence stars.

After a giant or supergiant star sheds its outer layers, what remains is the core—a white dwarf. These stellar remnants are incredibly dense and represent the final evolutionary state for stars that are not massive enough to become neutron stars or black holes.
A white dwarf no longer has the capacity to perform nuclear fusion, making them unable to produce energy like their younger selves. Consequently, they are much smaller and less bright compared to main-sequence stars, giants, or supergiants. Despite their dimness, white dwarfs can still be very hot due to the heat left from their former fusion processes.
White dwarfs are composed primarily of electron-degenerate matter and have a mass comparable to the Sun while being only about the size of the Earth. Over billions of years, a white dwarf will gradually cool and fade, eventually becoming a cold, dark black dwarf, though this stage is theoretical as the universe isn’t old enough for any to have formed yet.