Yet another stellar corpse
Snow White and the seven dwarves come racing to my thoughts every time I hear the term ‘white dwarf stars’. It’s interesting how certain words or phrases can trigger different associations. While “Snow White and the Seven Dwarves” is a whimsical fairy tale, the mention of white dwarf stars actually refers to a stage in the life cycle of stars, where they have exhausted their nuclear fuel. These stars are incredibly dense and compact, showcasing the fascinating diversity of our universe. This diversity of the universe is so wide-ranging that it continues to amaze scientists and astronomers, who are constantly discovering new celestial objects or even what happens after the death of a star.
There are three main stellar corpses as we know them: neutron stars, black holes, and white dwarf stars. Their aftermath depends on their masses prior to losing nuclear fuel. When stars of solar masses between 0.07 to 10 reach the end, they are believed to become white dwarfs.
Imagine a star that is so dense that a teaspoon of its matter would weigh as much as an elephant. A star that is so small that it could fit in your hand, but has the mass of the Sun. A star that is so hot that it can ignite its companion and create a spectacular explosion in the sky. This is a white dwarf, the final stage of most stars in the universe.
White dwarf stars are so-called because of the white color observed in the first few that were discovered. They are distinguished by a modest luminosity, a mass similar to the Sun’s, and a radius similar to Earth’s. They are extremely dense. White dwarfs are very hot when they are born, with temperatures over 180,000 degrees Fahrenheit. That’s hotter than the surface of the Sun! But they gradually cool down over billions of years, until they become black dwarfs that emit no light or heat. However, the universe is not old enough for any black dwarfs to exist yet.
There are two types of white dwarfs, namely, carbon-oxygen white dwarfs and neon-oxygen white dwarfs, which again depend on the mass of the star that produces them. Carbon-oxygen white dwarfs are more common, as they come from stars that never burned carbon in their cores. Neon-oxygen white dwarfs are rarer, as they come from stars that are massive enough to fuse carbon into neon and oxygen.
Helium is created when hydrogen nuclei combine due to the pressure of a few octillion tons of gas pressing down on the core. Nuclear fusion. The star is kept from collapsing on itself thanks to the constant emission of thermonuclear energy from this process. When a star like our Sun runs out of hydrogen fuel in its core, it starts to fuse helium into carbon and oxygen. This makes the core shrink and heat up, while the outer layers expand and cool down. The star becomes a red giant, a huge balloon of gas that can swallow planets like Earth. The red giant is like an old man who has grown fat and weak. Eventually, the red giant loses its outer layers to the stellar wind, a powerful stream of particles that blows away from the star. The core is exposed, and it becomes a white dwarf, a tiny ball of carbon and oxygen that is held together by the pressure of electrons. The white dwarf can be thought of as an old man who has shrunk and become frail.
While solitary white dwarfs fade gradually, a white dwarf that orbits another star is highly explosive. Hydrogen is sucked from its companion and spills onto the surface of the white dwarf through a gaseous bridge. And when the hydrogen builds up, it eventually reaches a flash point in terms of temperature and density. An enormous quantity of energy is released in a nova as the entire shell of recently obtained fuel violently fuses. The white dwarf suddenly bursts with the brightness of 50,000–100,000 suns before gradually fading back into obscurity.
FUN FACT: More than 97% of the stars in the Milky Way, including the Sun, are not massive enough to become anything other than white dwarf stars when they end their lives
