- 4451 - SUN - becomes a White Dwarf? - In a couple billion years, our Sun will be unrecognizable. It will swell up and become a “red giant”, then shrink again and become a “white dwarf”. The inner planets aren’t expected to survive all the mayhem these transitions unleash, but what will happen to them? What will happen to the outer planets?
------------------------------------- 4451 - SUN - becomes a White Dwarf?
- Our Sun is about 4,600,000,000 years old.
It’s firmly in the “main sequence” now, meaning it's fusing hydrogen into
helium and releasing energy. But even though it’s about 330,000 times more
massive than the Earth, and nearly all of that mass is hydrogen fuel, it will
eventually run out.
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- In another five billion years or so, its
vast reservoir of hydrogen will suffer depletion. As it burns through its
hydrogen, the Sun will lose mass. As it loses mass, its gravity weakens and can
no longer counteract the outward force driven by fusion. A star is a balancing
act between the outward expansion of fusion and the inward force of gravity.
Eventually, the Sun’s billions-of-years-long balancing act will totter.
-
- With weakened gravity, the Sun will begin
to expand and become a “red giant”. The
Sun will almost certainly consume Mercury and Venus when it becomes a red
giant. It will expand and become about 256 times larger than it is now.
-
- The inner two planets are too close, and
there’s no way they can escape the swelling star. Earth’s fate is less certain.
It may be swallowed by the giant Sun, or it may not. But even if it isn’t
consumed, it will lose its oceans and atmosphere and become uninhabitable.
-
- The Sun will be a red giant for about one
billion years. After that, it will undergo a series of more rapid changes,
shrinking and expanding again. But the mayhem doesn’t end there. The Sun will pulse and shed its outer layers
before being reduced to a tiny remnant of what it once was: a “white dwarf”.
-
- This will happen to the Sun and almost all
stars that host planets. Even the long-lived red dwarfs (M-dwarfs) will
eventually become white dwarfs, though their path is different.
-
- Astronomers know the fate of planets too
close to the stars undergoing these tumultuous changes. But what happens to
planets further away? To their moons? To asteroids and comets?
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- Practically all known planet hosts will
evolve eventually into white dwarfs, and large parts of the various components
of their planetary systems—planets, moons, asteroids, and comets—will survive.
-
- There’s lots of observational evidence for
this. Astronomers have detected planetary debris polluting the photospheres of
white dwarfs, and they’ve also found compact debris disks around white dwarfs.
Those findings show that not everything survives the main sequence to red giant
to white dwarf transition.
-
- Previous research had shown that when
asteroids, moons and planets get close to white dwarfs, the huge gravity of
these stars rips these small planetary bodies into smaller and smaller pieces. The authors observed three white dwarfs over
the span of 17 years. They analyzed the changes in brightness that occurred.
Each of the three stars behaved differently.
-
- When planets orbit stars, their transits are
orderly and predictable. Not so with debris. The fact that the three white
dwarfs showed such disorderly transits means they’re being orbited by debris.
It also means the nature of that debris is changing.
-
- The unpredictable nature of these transits
can drive astronomers crazy—one minute they are there, the next they are
gone. As small bodies like asteroids
and moons are torn into small pieces, they collide with one another until
nothing’s left but dust. The dust forms clouds and disks that orbit and rotate
around the white dwarfs.
-
- The simple fact that we can detect the
debris of asteroids, maybe moons or even planets whizzing around a white dwarf
every couple of hours is quite mind-blowing.
These systems can evolve rapidly, in a matter of a few years. The fate of these systems is far more
complex than we could have ever imagined.
-
- During the 17 years of observations, all
three white dwarfs showed variability.
The first white dwarf (ZTF J0328?1219) was steady and stable until a
major catastrophic event around 2011.
The system underwent a large collision event around 2011, resulting in
the production of large amounts of dust occulting the white dwarf, which has
since then gradually dispersed, though leaving sufficient material to account
for the ongoing transit activity, which implies continued dust production.
-
- The second white dwarf (ZTF J0923+4236)
dimmed irregularly every couple of months and displayed chaotic variability on
the timescale of minutes. These
long-term changes may be the result of the ongoing disruption of a planetesimal
or the collision between multiple fragments, both leading to a temporarily
increased dust production.
-
- The third star (WD 1145+017) showed large
variations in numbers, shapes and depths of transits in 2015. This activity
concurs with a large increase in transit activity, followed by a subsequent
gradual re-brightening, adding that the overall trends seen in the brightness
of WD1145+017 are linked to varying amounts of transit activity.
-
- But now all those transits are gone. But astronomers have also found
planetesimals, planets, and giant planets around white dwarfs, indicating that
the stars’ transitions from main sequence to red giant don’t destroy
everything. The dust and debris that astronomers see around these white dwarfs
might come from asteroids or from moons pulled free from their giant planets.
-
- For the rest of the Solar System, some of
the asteroids located between Mars and Jupiter, and maybe some of the moons of
Jupiter may get dislodged and travel close enough to the eventual white dwarf
to undergo the shredding process.
-
- When our Sun finally becomes a white dwarf,
it will likely have debris around it. But the debris won’t be from Earth. One
way or another, the Sun will destroy Earth during its red giant phase. Whether or not the Earth can just move out
fast enough before the Sun can catch up and burn it is not clear, but if it
does the Earth would still lose its atmosphere and ocean and not be a very nice
place to live.
-
-
April 29, 2023 SUN
- becomes a White Dwarf? 4451
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