
A Black Hole Threw a Star Out of the Milky Way Galaxy - wglb
https://www.nytimes.com/2019/11/14/science/stars-black-hole-milky-way.html
======
jcims
(Edit: To get an idea what this would look like if you were to watch this star
moving across the sky like a comet)

The sun's apparent diameter in the sky is roughly a half a degree. Over the
course of an hour, it will move approximately 15 degrees across the sky, or
rougly 30 apparent diameters. The actual diameter of the sun is ~860,000
miles, making the 'apparent velocity' of the sun across the sky nearly 26
million miles per hour or ~6 times faster than this beast.

I mention that to apply wet socks to any notion of a bullet star flinging
across the sky like tracers at the Knob Creek night shoot. It's fast yet, but
at that scale, doesn't look it.

(for the uninitiated -
[https://www.youtube.com/watch?v=h11uEvGc7u8](https://www.youtube.com/watch?v=h11uEvGc7u8)
)

~~~
chkaloon
The 15 degrees comes from the Earth's rotation, right? Not sure what you're
trying to compare here. If I spin in a circle at 1 second per revolution, I
guess I can make the sun travel about 2.2 billion miles per hour using your
numbers.

~~~
shantly
> any notion of a bullet star flinging across the sky like tracers at the Knob
> Creek night shoot. It's fast yet, but at that scale, doesn't look it.

That would appear to be what the poster is trying to compare, there.

~~~
jcims
Bingo

------
mberning
That ended abruptly. And on a bit of a grim tone. How many college students
leave home and never return, not even once. Oh well, at least it wasn’t
written in the New Yorker where it would have required a 10k word “slice of
life” preamble before even discussing the star. “Dr. Li always enjoyed feeding
pigs on the weekend...”

As for the star, I wonder what type of relativistic effect a person orbiting
it might experience. I assume 4 million miles per hour is an appreciable
fraction of C.

~~~
mirimir
It's 0.6% c.

~~~
simonh
Wow, that’s actually really moving! I wonder how close the star got to being
shredded in the process. The gravitational forces must have been pretty
powerful.

~~~
gsaga
It's just freefalling. So there won't be any significant forces on it unless
it gets very close.

~~~
simonh
Well that's the questions really isn't it, how close did it have to get and
what was the gravity gradient? Too close and tidal forces would strip off the
star's upper layers, or tear the whole star apart. We detected that happening
not long ago.

[https://www.sciencealert.com/we-ve-just-caught-the-
immediate...](https://www.sciencealert.com/we-ve-just-caught-the-immediate-
aftermath-of-a-black-hole-tearing-apart-a-star)

------
Tepix
Fascinating. I wonder if any planets survived this "flinging manoeuvre".

This star is also covered in the latest episode #159 of the Interplanetary
Podcast: [https://www.interplanetary.org.uk/post/159-andrew-rader-
beyo...](https://www.interplanetary.org.uk/post/159-andrew-rader-beyond-the-
known)

~~~
bloak
Imagine being an astronomer in a society living on a planet orbiting an
intergalactic star like that: no proper stars in the sky, just smudges of
distant galaxies. It might be difficult to discover the "Copernican principle"
in such circumstances.

~~~
SiempreViernes
There'll be plenty of quasars to act as stars, the big problem is they will be
_very_ hard pressed to measure any exosolar distances at all.

This particular star is not that big too, so over its lifetime of 1Gyr it'll
make it pretty far in the local group. For fictional purposes you could
therefore imagine a solar system being sent from one galaxy into another one,
that'll make for some interesting astronomical history!

------
neonate
[http://archive.is/6KYjo](http://archive.is/6KYjo)

------
wruza
>the seriously blinding speed of four million miles an hour

That’s ~600km/s. The Sun's orbital speed around the Galaxy: ~200 km/s.
Distance from BH to “suburbans”: 400ly or 3.8x10^15km. Time to escape: 200k
years.

~~~
Tepix
According to the paper it's moving at 1700km/s (see also:
[https://www.wolframalpha.com/input/?i=4+million+miles+per+ho...](https://www.wolframalpha.com/input/?i=4+million+miles+per+hour)
). I prefer "20.4 light seconds per hour" :-) It's also 0.56% the speed of
light.

The escape velocity of the Milky Way is 500km/s.

~~~
wruza
I divided by 1.8 instead of multiplying by 1.6 and messed it all up. Got it
only now, thanks for the correction!

------
pfdietz
I believe some of the stars very near our galaxy's central black hole have
been measured at ~3% of the speed of light, at periapsis. They are
gravitationally bound, though, so their total energy (after subtracting out
the gravitational potential energy) is less than this star.

------
SiempreViernes
As far as I can tell the big deal is they are completely sure it came from Str
A*.

It is also moving too fast to be easily explainable, but for now I would bet
on this simply being a very extreme event. The set of hyper velocity stars is
rather small at the moment, something like ~20 in total, so I don't think
there is any reason to believe we have an unbiased sample.

Amusingly they do use it to measure the position of the _sun_!

Preprint at:
[https://arxiv.org/abs/1907.11725](https://arxiv.org/abs/1907.11725)

------
biddlesby
Did the black hole impart any energy to the leaving star? It sounds more like
there were two stars each under the other's influence and the black hole
removed one star, freeing the other to go off on an adventure

~~~
raducu
The author said on /r/space that was the case -- this was a binary system with
the stars orbiting very close to eachother.

~~~
ars
He was asking if the velocity was given by the black hole, or if it was
already there from the binary star orbit.

I can't figure out any way a black hole can give velocity (relative to the
black hole) - any increase in velocity would be balanced by a decrease as the
object leaves.

~~~
phaedrus
Not velocity - momentum is what is conserved. So the black hole could have
absorbed the equal and opposite momentum, but being more massive, not gained
much velocity. Or, a hypothetical scenario in separating the two it could have
flung them in opposite directions, in which case it would be net zero
momentum. (But I don't know if - and kind of doubt whether - that is a
possible outcome of a three-body interaction.)

~~~
glaberficken
relevant:

Factual explanation of the 3 body problem:
[https://en.wikipedia.org/wiki/Three-
body_problem](https://en.wikipedia.org/wiki/Three-body_problem)

And the 1st book of an (excellent) si-fi triology:
[https://en.wikipedia.org/wiki/The_Three-
Body_Problem_(novel)](https://en.wikipedia.org/wiki/The_Three-
Body_Problem_\(novel\))

------
greglindahl
The headline ("A Black Hole Threw a Star Out of the Milky Way Galaxy") isn't
very accurate. The black hole in question is the super-massive black hole in
the middle of our galaxy, and like most galaxies, there's only one of them.
The article does go on to explain that, which is great, but it would be nice
if the headline writer hadn't dropped that essential fact.

~~~
Simon_says
What’s not accurate? When I read “a black hole”, I don’t assume we’re talking
about Sagittarius A*, just a black hole.

~~~
greglindahl
... that's exactly what's inaccurate. It's not just some ordinary black hole,
it's the unique one in the Milky Way galaxy.

~~~
StuffedParrot
...all definite articles can also be indefinite. The title is absolutely
correct.

~~~
shele
Remind me when you "see a sun rising in the morning" next time. Using an
indefinite article here is noticed by the reader and will pose the question
"Oh, is there one more?". In language rules are sometimes not as absolute as
"all definite articles can also be indefinite".

~~~
afiori
This is not a proper comparison, as the relevant property is not of something
being the only member of a category, but of being the already mentioned one.

For the sun we suppose the person speaking knows of THE Sun. For the
supermassive black hole at the center of the galaxy we don't.

This can be easily shown as "The Black Hole Threw a Star Out of the Milky Way
Galaxy" is clearly misusing the definite article.

~~~
ncallaway
"The Black Hole at the Center of the Milky Way Threw a Star Out of the Galaxy"
would probably be an alternate way to write the headline using the definite
article.

Agree that _just_ an article swap leaves the headline worse off.

~~~
205guy
I agree this is a better headline. In the original, I was left wondering if it
was possible for a black hole outside of our Galaxy to eject a star from ours.

~~~
afiori
That should be impossible (unlikely), but it could have been a smaller black
hole around the galaxy.

------
notimetorelax
Sorry for the offtopic post. I recently see a high number of posts linking to
nytimes.com. Every time I'm not able to read the articles because they are
paywalled. I'm accessing them from Europe, is the experience different from
the US? Or is the majority of HN have NY Times subscription?

As an alternative, I found an article by CCN:
[https://edition.cnn.com/2019/11/12/world/milky-way-black-
hol...](https://edition.cnn.com/2019/11/12/world/milky-way-black-hole-star-
scn/index.html)

~~~
bildung
You can block Javascript and cookies just for nytimes.com. Suddenly the site
is fast, too!

------
zzzeek
when they say "it's currently 29000 light years away from earth", doesn't that
mean that was its position 29000 years ago ?

~~~
chubbyrabbit
Yes. However, it's like pointing at the sun and say "That's not the sun, the
sun was there 8 minutes ago".

~~~
gdubs
I’m listening to Carlo Rovelli’s book, “Reality is not what it seems”, and I’m
_still_ hung up on the concept of the “extended now”. The way he describes it,
everything being relativistic, there’s no such thing as an “objective” point
of view. So, if I understand his take on your example, we’re not seeing the
sun as it was 8 minutes ago. It simply takes 8 minutes for the sun’s “now” to
reach us.

~~~
prab97
That reminds me of a very important aspect of distributed systems (computer
science) - there is no global clock!

~~~
isostatic
UTC is a global clock. If I show the time in Sydney and the time here in UK,
Sydney will be 150ms behind to me, and 150ms ahead to a viewer in Sydney, but
I know the distance therefore I know if the clocks are in sync.

The time dilation between the two places is on the order of
femtoseconds/second, a millionth of a clock cycle of a cpu.

~~~
pbhjpbhj
Sounds to me like you could have equally started that post with "UTC is not a
global clock".

~~~
isostatic
It's a global clock. UTC is the same time in Miami as it is in Singapore.

The accuracy of the delivery of that timesource is 10ns in theory, and upto
1000ns in practice.

~~~
pluma
CET and EST are also the same time in Miami as in Singapore. That's just how
time zones work. The distinction with UTC is that it isn't tied to any
particular physical location, it's not the "time zone" for anywhere.

But UTC is ultimately defined by consensus. We need a reference clock and we
need to be able to measure or estimate our skew in order to sync to it.

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samirillian
Anybody else think of Lovercraft when you read this? And wonder, but what if??
(it's actually some battle between elder gods)

------
Izmaki
What a mean bully!

~~~
croon
s/ully/ouncer

------
RandomInteger4
Yeet

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novaRom
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