All About White Dwarf Stars
When a star reaches the end of its life, it explodes into a brilliant light which is called a supernova. Supernova is the most humongous explosion seen by humans and each blast is a super-powerful and ultra-bright explosion.
As per a new study, our Sun will morph into giant cosmic jewels after a few billion years. Like most of the stars in our Milky Way Galaxy, the Sun would finally collapse into an exotic object called the white dwarf star.
When a faint star of low or intermediate mass reaches the endpoint of its evolution, it is referred to as a white dwarf star. The name white dwarf was given to this object owing to the low luminosity and white color of the first few of the white dwarf stars discovered.
Let us look closely at the white dwarf definition in this article and learn about white dwarf size, and what is winter white dwarf hamster.
Accurate White Dwarf Definition
White dwarfs are a specific kind of star who possess as much matter as the Sun but are packed into a much smaller size which can be compared to the size of Earth. The first white dwarf discovered in history was in 1862 by Alvan Clark who was a telescope maker. He called this white dwarf “Sirius B”
Key Features of White Dwarf
Most white dwarfs are supposed to be composed of oxygen and carbon.
In stars which are like the Sun, there is a balance between the inward gravitational pull and outward push by the high-temperature hydrogen which is fusing with helium at the center of the Sun and releasing a huge amount of energy in the process.
There is no nuclear fusion in white dwarfs. White dwarfs are supported by forces that oppose gravity known as "electron degeneracy pressure".
The temperature inside a white dwarf is more than 100,000 Kelvin as per NASA. Despite such sweltering temperatures, white dwarfs are not so luminous due to their small size.
The typical white dwarf radius ranges between 0.8% to 2% of the Sun's radius.
White dwarfs are approximately the same size as Earth and recent research from scientists have found the smallest and densest white dwarf which is 4,300 kilometers wide (slightly bigger than the moon).
White dwarfs have very high densities of around 109 kg/m3. Due to its high density, the gravity inside white dwarfs is 100,000 times more than that on Earth.
The theoretical upper limit of white dwarf mass is 1.4 solar masses (called Chandrasekhar limit). Beyond this mass, there is immense electron pressure in the star that it would collapse into an even denser object called a black hole or neutron star.
The heaviest known white dwarf is 1.2 solar masses and the lightest white dwarf weighs as little as 0.15 solar masses.
A peculiar property of a white dwarf is that the more its mass, the smaller is its size.
A NASA team of international astronomers used a Spitzer space telescope and TESS (Transiting Exoplanet Survey Satellite) to report the first intact white dwarf planet forming a close orbit around a white dwarf star. The name given to this Jupiter-like planet is WD 1856 b and it is orbiting the white dwarf named WD 1856+534. The white dwarf planet is seven times larger than the white dwarf star.
The Appearance of a White Dwarf
A white dwarf star looks like any other star and appears as a tiny dot of light. Astronomers are able to distinguish a white star from other stars by two means:
White dwarf stars are very faint stars so we are able to see only the ones which are close by. These nearby stars appear to move relative to their neighboring stars.
Most of the light of a white dwarf star is emitted in the blue part of the spectrum.
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How do Stars Become White Dwarf?
White dwarfs denote the evolutionary endpoints of low or intermediate stars like the Sun. How exactly a star becomes a white dwarf star is dependent on the mass of the star but all the stars which have a mass less than 8 times that of the Sun (which includes 99% of stars) are bound to become white dwarfs eventually.
In general, stars fuse hydrogen at the center with helium till the point hydrogen runs out. Very massive stars take only a million years to reach this stage but for some stars like our Sun, the hydrogen can last more than 10,000 million years.
When this fusion has produced enough helium, it starts sinking into the center of the star, releasing heat in this process. Due to this, the internal balance of the star is disturbed and it starts bloating into a red giant.
If the star is extremely massive, the heat in the center will grow eventually to a level that will start fusing helium into oxygen and carbon. This star can then enjoy a stable period, though much shorter this time as the carbon and oxygen eventually sink to the center.
If the star is unable to gain the temperature that can burn oxygen and carbon into heavier elements, then they will keep accumulating in the center of the star till the time helium runs out.
This will produce a white dwarf of carbon and oxygen.
The electron pressure inside the white dwarf is because of quantum mechanical effects as a large volume of electrons are squeezed into the smallest possible space. This stops gravity from suppressing the star's core any further.
It is the pressure of the electrons and not the energy at the core which supports the white dwarf.
Conclusion
Low and intermediate-mass stars, at the end of their evolution, collapse into white dwarf stars. White dwarf stars have low luminosity and are of the size of Earth having 100,000 times more gravity than the Earth. The mass of the white dwarf is comparable to that of the Sun (1.2 to 1.4 solar masses). White dwarfs, unlike other stars that are supported by gas pressure, are supported by electron degeneracy pressure.
FAQs on White Dwarf
1. What is a white dwarf in simple terms?
A white dwarf is the incredibly dense, hot core of a star that is left behind after a low-to-medium mass star, like our Sun, has exhausted its nuclear fuel. It has a mass comparable to the Sun but is compressed into a size similar to the Earth, making it one of the densest forms of matter in the universe.
2. How is a white dwarf formed from a star like our Sun?
The formation of a white dwarf marks the final evolutionary stage for most stars. The process involves several key steps:
- First, the star runs out of hydrogen fuel in its core and swells into a red giant.
- It then begins to fuse helium into carbon and oxygen.
- Eventually, this fuel also runs out, and the star's outer layers drift away into space, creating a beautiful structure called a planetary nebula.
- The hot, dense, and gravitationally stable core that remains is the white dwarf.
3. What is the importance of electron degeneracy pressure in a white dwarf?
Electron degeneracy pressure is the single most important factor preventing a white dwarf from collapsing under its own immense gravity. Since nuclear fusion has stopped, there is no outward thermal pressure. Instead, a quantum mechanical principle takes over. The electrons are packed so tightly that they cannot be squeezed any closer, creating a powerful outward pressure called electron degeneracy pressure that perfectly balances the inward pull of gravity.
4. How does the size of a white dwarf compare to other celestial bodies?
A white dwarf demonstrates an extreme state of matter. While its mass is typically about 0.6 to 1.4 times that of our Sun, its volume is only about the size of the Earth. This means a teaspoon of white dwarf material would weigh several tons on Earth. Compared to a neutron star, it is much larger and less dense.
5. What is the difference between a white dwarf and a neutron star?
While both are stellar remnants, white dwarfs and neutron stars differ in their formation, composition, and the force that supports them:
- Formation: White dwarfs are formed from the collapse of low-mass stars (less than 8 times the Sun's mass). Neutron stars are formed from the supernova explosions of more massive stars.
- Support Against Gravity: White dwarfs are supported by electron degeneracy pressure. Neutron stars are supported by the more powerful neutron degeneracy pressure.
- Size and Density: A neutron star is far smaller and denser than a white dwarf, packing more mass into a sphere just a few kilometres in diameter.
6. What is the significance of the Chandrasekhar limit for white dwarfs?
The Chandrasekhar limit is a critical concept in stellar evolution. It defines the maximum mass that a stable white dwarf can have, which is about 1.4 times the mass of the Sun. If a white dwarf accumulates more mass than this limit, for instance by pulling matter from a companion star, the electron degeneracy pressure fails. This triggers a runaway nuclear reaction that completely destroys the star in a massive explosion known as a Type Ia supernova.
7. Why does a more massive white dwarf have a smaller radius?
This counter-intuitive relationship is a direct consequence of how white dwarfs are structured. A more massive white dwarf has a stronger gravitational force pulling it inward. To counteract this immense gravity, the electron degeneracy pressure must be even higher. This requires the electrons to be squeezed into a smaller volume, resulting in a more compact star with a smaller radius. Therefore, as you add mass to a white dwarf, it shrinks.
8. What is the difference between a white dwarf and a black dwarf?
The difference lies in their stage of evolution. A white dwarf is the hot, newly formed remnant of a star that still glows brightly from residual heat. A black dwarf represents the hypothetical final state of a white dwarf after it has radiated away all its heat and cooled down to the temperature of the cosmic background. It would be a cold, dark, and invisible stellar remnant. However, the time required for this cooling process is longer than the current age of the universe, so no black dwarfs are believed to exist yet.
9. Can planets orbit a white dwarf, and what would conditions be like?
Yes, planets can and do orbit white dwarfs. Some planets can survive their star's red giant phase and remain in orbit. Conditions on such a planet would be extreme. The habitable zone, where liquid water could exist, would be incredibly close to the white dwarf. Any surviving planet would likely be tidally locked, with one side in perpetual daylight and the other in permanent darkness.
10. What are some famous examples of white dwarf stars?
Several white dwarfs have been discovered, and some are relatively close to us. Key examples include:
- Sirius B: The companion star to Sirius A, the brightest star in our night sky. It was one of the first white dwarfs ever discovered and is a crucial object in the study of stellar evolution.
- Procyon B: The faint companion to the bright star Procyon.
- 40 Eridani B: A white dwarf that is part of a triple-star system and is notable in science fiction as the location of the planet Vulcan from Star Trek.
