
Supernova-propelled white dwarf found zooming through Milky Way at 5 million mph - clayt6
http://www.astronomy.com/news/2018/05/snapshot-second-gaia-release-results-so-far
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HyperTalk2
The shared link immediately redirected me to a fake malware page upon loading
for the first time.

[https://i.imgur.com/wOJ0b67.png](https://i.imgur.com/wOJ0b67.png)

Please blacklist astronomy.com links from being shared on this site.

Also, perhaps consider shadowbanning clayt6 because judging from his account
history he's apparently only here to spam astronomy.com links.

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mkempe
That speed (5 million mph = 2,235,200 m/s) is not quite 1% of the speed of
light (299,792,458 m/s).

The article is about a large Gaia data release [1] not just the speed of one
astronomical object. Maybe change the title (currently "Supernova-propelled
white dwarf found zooming through Milky Way at 5 million mph ").

[1]
[https://www.cosmos.esa.int/web/gaia/dr2](https://www.cosmos.esa.int/web/gaia/dr2)

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Jadex1
But is it the speed of dark?

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cetico
And I used to worry about asteroids..

Looking forward to the Hollywood movie in which heroes have to stop a STAR
from hitting Earth.

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lewis500
Armageddon 2: Starmageddon. Bruce Willis has to land on the surface of a star,
drill down into it and release a hydrogen bomb. The fusion reaction will cause
the star to blow up even though it’s already full of fusion. He’ll wear a suit
made out of diamonds and have a spaceship made of diamonds, and an Aerosmith
rendition of twinkle twinkle little star will play as the star explodes and
diamonds rain from the sky. Now seeking story credit for this script.

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sarkhan
Your script is missing how Bruce managed to escape exploding asteroid.

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civilitty
Or how he managed to get to it. Any start exploding close enough for non-FTL
technology to be a reasonable mode of transportation would also turn the Earth
into dust or a molten ball of rock.

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krapp
No problem... it turns out we reverse engineered a primitive FTL drive from
the debris at Area 51, so we stick it onto a Space Shuttle, but it also turns
out to be the only thing powerful enough to blow up the star, so it has to be
detonated, sacrificing the ship.

However,the explosion also creates a wormhole that Bruce Willis bounces back
into after blowing the star up, which lets him land safely back on Earth in a
cool pose like Iron Man at the end.

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phyzome
My intuition is having trouble coming to grips with the notion of accelerating
a fluid object to that velocity via an explosion.

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autocorr
Since white dwarfs have around the mass of the sun in a size comparable to the
Earth, the matter in them is quite exotic. Astronomer's have speculated that
these hypervelocity stars (moving at a few tenths of a percent the speed of
light in the referenced studies case[1]) are shot out like a slingshot. The
original system is a binary, so they are already under extreme circular
gravitational acceleration in their orbit. When the companion explodes in a
core-collapse supernova, it loses that source of acceleration and moves in a
straight line like a slingshot. The Gaia satellite is able to measure the
velocity vector of the star very well, and show that it points back to an old
supernova remnant. The scenario isn't controversial, but it does give quite a
good example that this is how they come about.

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Steel_Phoenix
This makes me wonder about the nature of gravity. How much of the
gravitational pull from the companion would disappear? How fast? It seems like
a reduction in gravity would be immediately followed by a massive energy
release. It's impressive that the remaining star is moving away fast enough to
avoid being destroyed in the resulting blast.

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whatshisface
The reduction in gravity isn't due to anything special, the matter in the
"anchor" star is just leaving to go somewhere else. (In all directions.)

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Steel_Phoenix
That makes it seem like gravitationally, the anchor star would just be
expanding. My intuition thinks the remaining star gets slammed by debris
before the direction and strength of the anchor dissipates enough to launch it
away, though my confidence in my stellar intuition is low.

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whatshisface
Your intuition contains some truth: according to Gauss's law, the gravitation
of the expanding nova shell _shouldn 't change at all_ until it passes by its
surviving companion. The matter expanding away from the companion will
gravitate less, but that will be canceled out by the increased attraction to
the matter expanding towards the companion. Really, we're just assuming the
hot, dense, and small dwarf can sustain the licking it gets when the remnant
hits it. The smallness helps it get hit by less material, moreso if it's far
away. It's gravitationally bound so it won't shatter - the most you could do
to disassemble it would be to boil off its matter by raising it all to escape
velocity. It takes a _lot_ of energy to do that.

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ifAndOnlyIf
When things really get going with outer space civilizations, the idea of
weaponry and conflict is going to take some surprising and horrifying turns.

We bicker about islands and aircraft carriers, nuclear arsenals and orbital
ballistics, and something like this would rend the entire planet limb from
limb in the blink of an eye, without breaking a sweat.

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cfadvan
Manipulating a white dwarf to kill a planet is like using a hydrogen bomb to
kill an ant.

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ifAndOnlyIf
Given that a typical white dwarf is thought to occupy a volume comparable to
the volume of earth-like planets, it strikes me as precisely the sort of
object one might select to destroy a planet like earth in a single stroke.

From wikipedia:

    
    
      The estimated radii of observed white dwarfs 
      are typically between 0.8% and 2% of the 
      radius of the Sun; this is comparable to the 
      Earth's radius of approximately 0.9% solar 
      radius.
    

[https://en.wikipedia.org/wiki/White_dwarf](https://en.wikipedia.org/wiki/White_dwarf)

Catapults, naval cannons, carpet bombing cities to hit one obscure factory.
War is littered with stories of absurd mismatches in economies of scale.

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cfadvan
The problem is while it has a volume comparable to Earth, its mass is
comparable to Sol! It would take an unbelievable amount of energy to move
something so massive from its original position to a collision course with
another body. With a fraction of that energy you could pummel a planet with a
swarm of tens of thousands of giant asteroids for _years_.

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Black-Plaid
The article also talks about a black hole accreting at a rate of 1% every
million years that is 'currently' 20 billion solar masses.

It is 12 billion ly away, which means, if it really does continue getting 1%
larger every million years it's actually ...
143,719,397,330,047,128,616,744,826,647,657,321,145,325,176,127,225,856
billion solar masses at this moment. (if the compound interest calculator is
to be believed)

I really hope that's not true.

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CarVac
This reminds me of black hole merger recoil, in which a merger emits a
gravitational wave with net linear momentum, sending the resulting black hole
in the opposite direction at comparable speeds to this.

[https://en.wikipedia.org/wiki/Binary_black_hole#Black-
hole_m...](https://en.wikipedia.org/wiki/Binary_black_hole#Black-
hole_merger_recoil)

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hinkley
It’s like the machine at a batting cage that shoots the baseballs.

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arrel
When they say 5 million mph, is that relative to the center of the Milky Way?

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KindOne
5,000,000 MPH = 8,046,720 km/h = 0.007455824656 in light speed

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sriku
> ...the Gaia satellite – which precisely measured the positions, motions,
> colors, and luminosities of over a billion Milky Way stars ...

Can any physicists here explain how the coordinate system is constructed here?

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iaw
I occasionally wonder about whether there are any objects out there traveling
at a measurable fraction of c, could something even get through the atmosphere
without disintegrating near those speeds?

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krapp
Would something moving that quickly and carrying that much energy be more
likely to disintegrate in the atmosphere, or to disintegrate the atmosphere?

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pjc50
Both, I suspect. At that speed, there's no point in talking about "flow"
around it, it would simply compress all the atmosphere hit into a disc of
plasma. The object would be disintegrated, but all the momentum has to
transfer to the plasma disc instead. It would produce an impressive crater.

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perl4ever
I wish people would put high speeds in terms of miles/sec or km/s, because I
know what light speed is in both, and I have a vague idea of orbital/escape
speeds as well.

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mannykannot
5,000,000 / 3,600 = 5,000 / 3.6 ~= 1,000 miles/sec.

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perl4ever
5000 / 3.6 = 2500 / 1.8 = 1250 / 0.9 ~= 1,250 miles/sec

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hoodoof
Could this have planets?

Well not planets, but my question is more general - could a star moving this
quickly have a planetary system - obviously this one would not if it got shot
out of a supernova.

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hinkley
Anything it passed that had a delta-v of less than the escape velocity of the
white dwarf will get hoovered up.

But most of what it passed would just scatter like bowling pins.

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nabla9
That's 972 km/s. Escape velocity from the milky way is 537 km/s. That star is
leaving the milky way.

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placebo
2235.2 km/s

