Life forms can survive as long as there is an
energy source, take it away and everything will die. Although humans discovered
numerous ways to spring out energy and thus life, no mean can ever equal the
most ancient, the most powerful and yet the most lasting source of energy ever,
Hydrogen fusion. Since the beginning of time, all kinds of stars have been
fusing Hydrogen into Helium releasing enormous amounts of thermal and light
energies feeding up the entire universe. Unfortunately, the universe will die.
It will happen very slowly, over billions of years, but it will happen. At
universal time scale, our sun will die in no time at all, and so will do all
the stars of all the galaxies in more or less time. Luckily, there will be
places that will exist nearly forever, and could be considered humanity last
home: the corpses of dead stars; white dwarfs. What are these strange and
mysterious things? And why are they our last hope for survival?
1. Once upon a time, white dwarfs were
stars
How long a star could live depends drastically on
its weigh. All in all, there are three types of stars: giant stars, medium
stars as our sun, and little stars. The really massive ones consume their fuel
astonishingly quick they end up in a violent explosion resulting in amazing
supernovas and thus giving birth to the mysterious black holes. Nevertheless,
these universal fireworks turn out to be rather rare as only 3% of the milky
way stars fall into the massive stars class. So, what about the other lot? Well,
the remaining 97% end up their existence as white dwarfs.
There are two ways this could happen. First,
really small stars, generally referred to as red dwarfs, heat up over trillions
of years, consuming all of the hydrogen in their core and become white dwarfs.
The other way is more interesting. Average-sized stars like the sun, fuse
hydrogen into helium in their core releasing great energy keeping the star in a
delicate balance against gravitational forces. When the star grows old, most of
the fuel will be used up, the star can no longer produce enough energy to pull
out gravity, the balance will break. This will cause the outer layers of the
star to shred apart in a spectacular planetary nebula. By the end of this
process, nearly half of the star’s mass will have disappeared. What remains is
a newly born white dwarf, the star’s corpse.
2. White dwarfs are very dense
White dwarfs, or degenerate dwarfs, not to be
confused with degenerate stars which are also called brown dwarfs, are in fact
among the densest bodies in the universe, surpassed only by black holes,
neutron stars and quark stars. But how on Earth could this be possible since
the star lost about half its mass before transforming into a white dwarf? Well,
one detail is missing. While their former selves were about 100 times their
diameter, white dwarfs are now as big as Earth, but still with half of their
former mass. This could only mean they are dense, very dense, in fact. It is
estimated that one table spoon of a white dwarf matter equals approximately one
metric ton, the equivalent of a car. Consequently, if you want to approach a
white dwarf, you will be instantly smashed, head down on the floor, not being
able to stand up ever again.
Such densities are possible because white
dwarfs, unlike other stars which are made of Hydrogen and Helium, consist of
mainly Helium 4-nuclei and electrons. This property makes white dwarfs as big
as Earth, as massive as the sun, and yet one million times denser than it.
3. White dwarfs are relatively rare
Though 97% of the milky way stars will
undoubtedly end up as white dwarfs, not much stars have arrived at advanced
stages of their existence yet. This
directly implies that they are rather rare and scarce. Astronomical
observations showed that only 8 out of the 100 closest stars to our solar
system are actually white dwarfs.
The nearest known white dwarf to Earth is
Sirius B, far with as much as 8.6 light-years. Sirius B is the smallest
component in the Sirius binary star system. When you look at it, Sirius B seems
like a faint little point of light near the huge Sirius A.
4. White dwarfs were first discovered in
1783
The discovery of white dwarfs is definitely not
a modern one; they were discovered rather very long ago. White dwarfs first met
human eyes back in 1783. It was in the triple star system of 40 Eridani. The trio consists of the relatively bright
star, 40 Eridani A, orbited by 40 Eridani B, which proved to be a white dwarf,
and 40 Eridani C, a red dwarf. Sirius B, the closest white dwarf to the sun,
was next to be discovered. In the nineteenth century, thanks to the precision
of, at the time, advanced measurement equipment, Friedrich Bessel noticed that
Sirius A was periodically changing its position. In 1844, he concluded that the
only explanation to this strange behavior, was the existence of an invisible
companion to Sirius A. It was until 31 January 1862 that Bessel predictions
were confirmed by the observation of a tiny little dot orbiting Sirius A. In
1917, Van Maanen’s Star named after its discoverer, was added to the growing
list. These first three discoveries are now called the classical white dwarfs.
5. White dwarfs host planets
Humans are social species, a man is always
surrounded by family and friends, even after his death, when they visit his
grave. Stars and planets are no different at all. While you might think that
since white dwarfs are dead star corpses, they cannot host planetary systems;
well, you are completely wrong.
The planetary system around a white dwarf is
inherited from its progenitor star. Observations made by NASA’s Spitzer Space
Telescope confirmed that nearly all the white dwarfs in the milky way are
emerged in a dust cloud. This could have two explanations: It is either the
disintegration of some comets passing by the white dwarf, thanks to its
gravitational forces, or the debris of the closest planets that could not
afford the great explosion made by the dying star. Other observations stated
that at least 15% of the observed white dwarfs are indeed orbited by planets.
These planets are thought to be the ones that survived the explosion phase of
the star, but lost however some of their outer layers.
6. Life is unlikely on white dwarfs
Life around white dwarfs is very unlikely, but
possible. Let us not forget that white dwarfs were once medium-sized stars that
exploded, destroying all the surrounding planets rendering them no more than
debris and dust. Further, even if we found a planet that is still intact, it
must be very close to the white dwarf, probably as close as Mercury is to the
sun, or even less, and that’s because white dwarfs are small, and more especially
dim, that is not strong enough to nourish farther planets with energy. This
proximity may subject planets to strong tidal forces that could render them uninhabitable
by triggering the greenhouse effect, in other words, it would lock one side of
the planet to permanent day, and the other to perpetual night.
However, life can flourish at the edges of
these two zones. Moreover, white dwarfs, unlike the red ones, are quite stable.
So, if find a white dwarf matching all of these requirements, who knows, it may
become humanity’s last shelter before the end of time.
7. White dwarfs shine much longer than
other stars
White dwarfs are somewhat sterile, in the way
that they have no energy source since they do not have Hydrogen atoms to fuse
into Helium. But, this fact could not stop them from being hot, very hot
actually. White dwarfs are about 40 times hotter than our sun, ranking among
the hottest objects in the universe. But It is not incredibly active. The heat
of a white dwarf in trapped inside its core, but because its energy is not
replaceable, it cools down, gradually, over many trillions of years. But, why
does it take a white dwarf this much to cool down?
This has two explanations: First, the white
dwarf is small. Consequently, they have an extremely tiny area to radiate heat
from, making them very slow. In addition to that, the degenerate matter that
makes up the white dwarf is very opaque, meaning light does not escape easily
from it. These two reasons make of white dwarfs the last objects that will
shine in a dying universe.
8. Some white dwarfs are metal-rich
The fact that most of the white dwarfs show
heavy metal lines in their spectra continues to elude the brightest
astronomers, and it is now considered an enigma to be solved, since metals -the
name attributed to elements heavier than Helium- must gravitate towards the
core of the white dwarf and not in its atmosphere. Now it is believed that this
abundance of metals might come from the debris of planets that used to orbit
the white dwarf progenitor. This is still a speculation to be confirmed later.
9. White dwarfs can re-ignite and
explode
A white dwarf alone may not cause any harm at
all because of its stability. This stability is guaranteed by the low mass of
the object, forbidding the Hydrogen atoms to fuse into Helium. However, put it
besides another one, and wait for a variety of phenomena to occur including
novae and Type la supernovae.
There is a limit in mass for white dwarfs to
remain white dwarfs, it is called the Chandrasekhar limit. The mass of an
isolated white dwarf cannot exceed this limit. However, if a white dwarf
happens to be in a binary system, it can take material from its neighbor star
and step over the limit. The extra mass gained from the companion may cause the
white dwarf to start a fusion process, that is called re-ignition of a white
dwarf. In a stellar body as hot and as dense as a white dwarf, the fusion may
attain drastic speeds causing a Type la supernovae explosion that annihilates
the star.
10.
White
dwarfs can also die
As said before, white dwarfs undergo no fusion
reactions providing they do not gain any extra mass. They will cool down very
slowly, over an estimated 100 billion billion years, ten billion times longer
the universe has ever existed. That’s why it is believed that white dwarfs are
humanity’s last home. But the time will come when white dwarfs reach the lowest
levels of heat, and finally cool down completely, stars would have been shut
down, galaxies would have been evaporated, and only then when the first white
dwarf will become a black dwarf, a frozen object that emits no light at all.
Black dwarfs are stars’ final form.
White dwarfs can avoid this dark
transformation, but then face a more violent and a more aggressive fate than
the former. If they happen to be in a binary system, white dwarfs can be
cannibalized by the companion stars. They will be consumed slowly over many
years until no sufficient mass remain. The residual object will then orbit the
cannibal star, now the host star of the system, as a mere Helium or Diamond
planet.
By Yacine GACI
Ressources:
- http://www.astronomytrek.com/10-interesting-facts-about-white-dwarf-stars/
- https://en.wikipedia.org/wiki/White_dwarf
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