White dwarf stars are so named because they are small and white in color, and correspond to the final stage of evolution of stars with similar mass to the Sun.
Historically, the term white dwarf dates back to the early 20th century, when astronomers discovered very small but very bright stars of white color. They are called "dwarf" because these stars are significantly smaller than the Sun, even though their intense brightness might lead one to think they are larger. The first to clearly identify this type of star was the astronomer Williamina Fleming while observing the star 40 Eridani B, soon followed by other similar observations. At the time, it was intriguing: how could such a small object shine so brightly? It required a better understanding of physics and stellar evolution to solve this mystery. The name stuck, stylish and simple: a star small as a dwarf, bright and white like a tiny diamond lost in space.
A white dwarf is simply what remains of a star after it has burned almost all of its fuel. It is super dense: imagine the mass of the sun compressed into a ball barely larger than the Earth. As a result, these stars are small (hence the term dwarf), extremely hot, but not very bright because they have lost their outer shell. Their hyper-heated surface emits a blinding white light (that's why it's called white), which will gradually fade until it completely extinguishes over billions of years.
Stars like our Sun are called yellow dwarfs: they are larger than white dwarfs and burn their hydrogen steadily. Next to them, red giants are true swollen giants, much larger but significantly less dense. The blue giants, on the other hand, are massive and bright, true blazing stars that live fast and die young. Brown dwarfs are even more different: too small to initiate real nuclear fusion, they remain stuck between giant planets and small stars. In comparison, a white dwarf is a small and compact celestial body, very hot, but destined to cool down slowly until it disappears into the shadows.
The white color of a white dwarf is directly linked to its very high surface temperature: often over 10,000 °C, which is extremely hot! The hotter an object is, the more it appears blue or bright white, as opposed to cooler stars, which are more red or orange.
Paradoxically, despite their brightness and heat, these stars are called dwarfs because they are very small on a stellar scale: often comparable to the size of the Earth. So, you have an incredibly dense object, shining like crazy, but ridiculously small compared to other stars, hence the term "dwarf."
In short, called white dwarfs, these compact stars derive their name from a direct link between their small size, their very high density, and their typically white color, revealing their extreme heat.
Did you know that the first white dwarf discovered was Sirius B? This companion star of Sirius A was identified in 1862 by Alvan Graham Clark.
White dwarfs are so dense that the atoms on their surface are compressed into a state of exotic matter called degenerate electron matter.
White dwarfs can shine for several billion years before cooling down and becoming objects called black dwarfs, which remain theoretical so far because the universe is not yet old enough to produce them.
The fate of our Sun is to become a white dwarf in about 5 billion years, after its red giant phase.
The lifespan of a white dwarf before it becomes a black dwarf is extremely long, lasting up to hundreds of billions, or even several trillion years—much longer than the current age of the universe!
A typical white dwarf is roughly the size of Earth while maintaining a mass close to that of the Sun. Its density is therefore extreme, resulting in a gravity that is significantly higher than that of Earth.
In the current state of knowledge, no white dwarf is close to or poses a danger to Earth. However, our sun will become a white dwarf in about 5 billion years; by that time, life as we know it will have long since disappeared.
White dwarfs are remnants of stars that have exhausted their nuclear fuel. Red dwarfs are small, cool stars that are still undergoing fusion, while red giants are advanced stages of stars that have significantly increased in size by consuming their hydrogen.
No, a white dwarf cannot become a normal star again. It has already consumed all of its nuclear fuel and is slowly cooling down to become a black dwarf after several billion years.
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