Dwarf Stars and their Doomed Planets
Who does not love a sky full of stars, twinkling stars! But what if one night there is not a single
star in the sky. It would be all black and pitch dark. There is a high possibility that
this hypothetical situation may come true and it might be hard to believe but even a star dies.
When a high-mass star is out of hydrogen to burn, it expands and becomes a red supergiant.
While most stars quietly fade away, the supergiant destroy themselves in a huge explosion, called a
supernova. Most stars take millions of years to die. It is observed that the stars that are
excessively big and massive live a shorter life than others in general. The death of massive stars
can trigger the birth of other stars. And since the sun is a star, it will become a white dwarf
when it will be dying surrounded by the decaying remnants of planets, asteroids, and comets. Now
you must be wondering that what is a white dwarf. A white dwarf is a dead star which is also known
as degenerate dwarf and is a stellar core remnant composed mostly of electron-degenerate matter.
They are hot and dense remnants of long-dead stars. They are the stellar cores left behind
after a star has exhausted its fuel supply and blown its bulk of gas and dust into space.
Degenerate matter has other uncommon properties. For example, the more giant a white dwarf is,
the smaller it is. This is because the more mass a white dwarf has, the more its electrons must come
together to maintain outward pressure to support the extra mass. We will soon have a complete video
explaining a white dwarf, so stay tuned to the channel. Also we have a playlist, “everything
about stars”, be sure to take a look into it. To help scientists understand our galaxy’s youth
and the birth of the solar system and possibly spill the beans to the world of how it all began,
we have NASA’s Wide-Field Infrared Survey Explorer (WISE) space telescope. WISE enables a great
variety of studies ranging from the evolution of protoplanetary debris discs to the history
of star formation in usual galaxies. It surveyed the sky in four wavelengths of the infrared band,
at an elevated sensitivity. WISE's main aim was to find objects that had not been captured before,
including very bright luminous galaxies, super cold stars, and nearby asteroids and comets.
So just like all the other times, astronomers discovered their first debris disk surrounding
a white dwarf by accident. Eric Becklin and Ben Zuckerman from the University of California,
Los Angeles, found it while searching for another unfamiliar object in the astronomical zoo: a brown
dwarf. A brown dwarf is a type of substellar object that has a mass between the most massive
gas giant planets and the least massive stars, approximately 13 to 80 times that of Jupiter.
They were first noticed in the year 1963 by an American astronomer Shiv Kumar,
who named them “black” dwarf. Later on an American astronomer Jill Tarter proposed the name
“brown dwarf” in 1975 even though they didn't appear brown in colour. They are also referred
as failed stars because they do not procure enough mass to trigger sustained nuclear fusion
of ordinary hydrogen into helium in their cores. Unlike stars brown dwarf never reach stable
luminosities by thermonuclear fusion of normal hydrogen. Despite their name they would appear
in different colours from our naked eyes depending on their temperature. The warmest are generally
either orange or red whereas the cooler brown dwarfs would possibly appear magenta to the human
eye. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.
WISE was able to spot this rare object because it surveyed the entire sky twice in infrared light,
observing some areas up to three times. Cool objects like brown dwarfs can be easily invisible
when viewed by visible-light telescopes, but their thermal glow stands out even in infrared light.
In addition, the closer a body, the more it appears to move in images taken months apart.
When a Sun-like star grows old and turns into a dim white dwarf, the amount of power it puts
out drops by a factor of more than a thousand. Becklin and Zuckerman realized that if some of
these stars have brown dwarf companions, then some white dwarfs should as well. And because
white dwarfs are so much smaller and fainter than Sun, it would be a lot easier to spot a
brown dwarf next to a white dwarf than next to a full-fledged star. The brown dwarfs are tiny with
diameters very much like Jupiter’s. But white dwarfs are only about one-tenth of that size,
with diameters roughly equivalent to Earth. Most of the objects appeared plain and boring in the
infrared. But one of the white dwarfs Becklin and Zuckerman targeted, Giclas 29–38 (G29–38),
stood out. G29 was shown to radiate substantial emissions far from expectations. The researchers
argued that this excess infrared light was radiated from a brown dwarf located awfully
close to the white dwarf that the light from the two objects mingled together. However,
later observations failed to confirm it as a brown dwarf. Becklin and Zuckerman suggested that G29–38
might have a dusty disk when they discovered its excess infrared light. But they knew that dust
cannot survive long in the vicinity of a white dwarf, so they were wary of this hypothesis. Then,
a different kind of observation of white dwarfs began to change people’s mind about
the possibility that these objects could harbour debris disks. The white dwarfs consist mostly of
¬carbon and oxygen covered by a thin atmosphere of the light elements’ hydrogen and helium. These
compact objects probably hold about half the mass of the Sun crammed into a volume not much bigger
than Earth, producing enormous densities and strong gravitational pulls near their surfaces.
This intense gravity ensures that any element heavier than hydrogen or helium falls through
the atmosphere and out of view in quite a short period of time. Yet surprisingly, the atmosphere
of some white dwarfs is full of heavy elements such as calcium and aluminium. Starting about
a decade ago, astronomers with their large telescopes and high-powered spectrographs
found more and more of these “polluted” white dwarfs with “clutter” in their atmospheres.
It seemed highly possible that this pollution could be coming from dead planetary systems,
if the disks were dense enough to survive for long periods and had dumped some of their material
onto the white dwarfs recently. G29–38 is one of these polluted white dwarfs, and that made many
astronomers believe that its excess infrared light came from a debris disk. By 2005, most astronomers
were convinced that the extra infrared radiation coming from G29–38 was the sign of a dusty debris
disk, but some of us were still dissatisfied. Then late in the year 2006, the Spitzer Space Telescope
ushered in a new era for white dwarf science. Spitzer is a small telescope by modern standards,
but it’s a powerhouse. It was designed to study the early universe infrared light
and we can proudly say that it is the first telescope to see light from a planet outside
our solar system and not only that but Spitzer also made important discoveries about comets,
stars, exoplanets and distant galaxies. Its main mirror measures just 33 inches in diameter. But
unlike the much larger Hubble, Spitzer collects light at infrared wavelengths. What it lacks
in size, it makes up for with advanced infrared detectors that are hundreds of times more powerful
than any that had ever flown in space before. Spitzer does not provide us with the most
dazzling wall-art images of disks; this is not its specialty. It is best at dissecting light and most
of the light it analyses has wavelengths within a few times than that of the width of a human hair.
When we first saw the spectrum of G29–38, we felt strangely queasy mainly because of two reasons,
first being that it confirmed what we already thought we knew that dust explained the extra
infrared light coming from this dead star. And second, when we spotted that ordinary glove-shaped
bump in the spectrum which sent chills down our spine as if we had seen a ghost. Therefore,
one of the astronomers referred to the G29–38 spectrum as the “signature of the angel of death.
Now imagine it is the year 180,000,000, give or take a few million. The Sun as usual has been
burning through its hydrogen fuel for nearly five billion years now, gradually growing bigger and
brighter in the process. These changes are about to cause a catastrophe or even worse if we could
give it a word on Earth. But it is just a taste of the cosmic disaster that awaits our solar system.
The situation becomes foreshadowing and the growth of sun causes an increasing flood of
ultraviolet radiation to hit the atmosphere. As water evaporates from the oceans, it helps
the atmosphere retain heat, which further increases the evaporation rate. A runaway
greenhouse effect soon turns Earth’s surface into a broiling, inhospitable place that more or less
resembles Venus. Life as we know it or at least on the place that we know it comes to an end.
But maybe this was not enough because the situation keeps getting worse.
As the Sun continues to burn through its hydrogen, a vast ball of helium builds up in its core.
Eventually, this spent fuel disrupts the Sun’s structure. At an age of 12.2 billion years or so,
the Sun turns into a red giant, cooling down and swelling up. It expands to a radius of about 110
million miles which is big enough to swallow our planet along with Mercury and Venus. And thus,
the sun will swallow Earth completely. As it does so, all the planets, asteroids, comets,
and so on that orbit the Sun march outward into new orbits, farther from their enormous
red stellar host. In the meantime, the Sun’s evolution thoroughly shakes up all the comets,
asteroids, and Kuiper Belt objects. Many of them vaporize in the intense heat. This causes these
rocky objects to split apart and release dust. Other bodies find their orbits destabilized as
the planets rearrange ¬themselves. These objects fly around the future solar system in a frenzy.
Ultimately, the Sun blows off roughly half its mass in violent winds and pulsations.
And, in the process, it will stop spinning. As the Sun’s rotation slows, forceful tides drag
Earth into the bloated star. Then, once the Sun blows off the last of its outer envelope,
all that is left of it is a weird almost strange and intense ball of spent nuclear fuel — a white
dwarf. As the remaining of the planetary system continue to destroy, the pounding of the white
dwarf becomes more sporadic. The WISE mission is turning white dwarf debris disks from a
small collection of oddball objects into keys to understanding the evolution of planetary systems,
including the one we call home. Maybe white dwarf debris disks tell a brutally scary story. Maybe
white dwarf is nothing, but another discovery or maybe white dwarf is a warning. Whatever
it is if you are smart you'll pay attention. So, what do you think about these dwarf stars?
And what about the dead planetary systems around them? Do let us know your thoughts and ideas in
the comment section below. Also don’t forget to like the video and watch our “ Stars playlist”.