A Black Hole Threw a Star Out of the Milky Way Galaxy | Hacker News Reader

A Black Hole Threw a Star Out of the Milky Way Galaxy

nytimes.com

148 pointswglb83 comments

(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 )

A web search says your approach (which I admit I don't follow) is off by a factor of 40 -- the Sun moves approximately 500k mph around the galactic center. A speed 8x slower than the star mentioned in the article.

The GP is talking about the sun’s apparent speed in Earth’s sky, which is dominated by Earth’s rotation. Another way of stating it:

The sun is 1 AU distant and appears to revolve around Earth once a day from our perspective so it has apparent speed of 6.28 AU / day or 2.4 million mph.

Their point is that unless this star was very close to us it wouldn’t appear to move quickly across the sky, because the sun has similar apparent velocity and looks stationary to our eyes.

yep!

"Quick" being relative to your expectations, of course.

If OP is right, and it moves six times faster than the sun, then it would go from one side of the sky to the other in just three nights, right? That's extremely fast for any star gazer. Faster than any planet we can see. Someone who was familiar with the night sky and who could see this star would notice its speed quickly.

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.

> 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.

Bingo

Yeah just trying to connect how this would look visually if this star were to fly across the night sky like a comet.

I think I had a better intuition for how fast this star would appear to move across our sky from the headline alone (which is just as well because I can't see the article) than how fast "tracers at the Knob Creek night shoot" might do.

Agree. Is Knob Creek a place from history, a place from a video game, a place from a movie, a place entirely made up?

I thought it was a whiskey

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.

It's less than 1% of c unfortunately [1111 miles per second]. (https://www.google.com/search?q=4m+mph+in+miles+per+second)

It's 0.6% c.

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.

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

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...

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...

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.

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!

Check out The World at the End of Time by Frederik Pohl. This is exactly what this is about!

Also _Against a Dark Background_ by Iain M. Banks. It describes a very, very old civilization with no stars close enough to even think about interstellar travel.

> If we were in the center of the void, we would see no stars in the sky with the naked eye. According to astronomer Greg Aldering, the scale of the void is such that "If the Milky Way had been in the center of the Boötes void, we wouldn't have known there were other galaxies until the 1960s."

Via https://en.wikipedia.org/wiki/Bo%C3%B6tes_void. Pretty eerie to think about.

The author of the paper was active on reddit and he said the only way to explain this is that there were initially two stars orbiting in very close proximity and there could not have been any planets to begin with.

I assume you mean planet's orbiting the star?

It's a fascinating problem. I've been thinking about it for about ten minutes now and I honestly don't know what would happen in this situation.

Ask Ethan had a great answer to this question "Can Stars Escape From The Galaxy, With Planets Intact?":

https://www.forbes.com/sites/startswithabang/2016/09/10/ask-...

So? What's the answer? The closest thing I can find is:

"Mira has a white dwarf companion, meaning that large gravitational "kicks" often aren't enough to unbind a solar system!"

Which still doesn't tell me whether a star could be ejected from the galaxy like this with its planets intact and orbiting it roughly the same way as before.

There's this sentence later in the article:

> While an interaction that happens too close to a planet could eject that as well, simulations indicate that's a rarity, and that most planets should remain intact.

But the way it's phrased, it could be read either way, with the most plausible reading for me being that the planets remain intact, but turn into rogue planets, no longer orbiting the star that's speeding away from them.

http://archive.is/6KYjo

>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.

According to the paper it's moving at 1700km/s (see also: https://www.wolframalpha.com/input/?i=4+million+miles+per+ho... ). 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.

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

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.

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

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

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

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.

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.)

relevant:

Factual explanation of the 3 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)

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.

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

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

I also don’t read it as not Sagittarius A*. I wouldn’t even expect a lay audience to know that name.

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

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".

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.

"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.

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.

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

Odd for sure, but not wrong.

Anyway there’s only one sun and many black holes.

I agree. Also, the average reader of NYT doesn't care or know about the giant black hole at the center of our galaxy and 'black hole' is enough for them to get the point of the story.

So when you say "not very accurate" you're refering solely to how it's not specifying that it's our black hole? As in, the headline is perfectly accurate except it should say "the black hole" instead of "a black hole", and that moves it to "not very accurate" for you?

But we are talking about Sagittarius A*.

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...

It's paywalled everywhere, not just for you. The easiest thing to do for almost any website that is paywalled is to just disable javascript through your browser: https://support.google.com/chrome/forum/AAAAP1KN0B0BYOQskiuG...

I've had pretty good success by disabling JavaScript.

Someone here also usually posts a link to the actual research being reported on, e.g.:

https://arxiv.org/abs/1907.11725

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

They introduced it like a year ago, so I stopped reading NYT since then.

I use Brave which is a fork of Chromium just like Chrome. Click Settings, Additional Settings, Privacy and Security, Site Settings, Javascript.

Then under Block click Add and type www.nytimes.com as the URL and click Add.

The site works fine for me then and is sooooo much faster to boot.

Since when are we supposed to read the articles? /s

Seriously though, I do not have a NYT subscription (my guess is only 10-15% of HN readers do), and I don't even bother trying to get around the paywall. I just go straight to the HN comments where you can find enough details to understand the story. This works fairly well for NYT articles because the headlines are usually clear (not clickbait) and you can assume the facts are generally accurate.

The point is that the comments will discuss the surprising or unusual parts of the story, and those are the ones I'd be interested in too. Then people also post the scientific article or links to other background info if you really want to dig.

If after reading the interesting comments, I want to comment myself, I'll go back and read the article to make sure I don't repeat something that was already written.

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

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

Since the sun isn't turning around the earth, I believe the sun is still where you point at.

Technically the Sun is moving as part of the arms of the Milky Way galaxy, which itself is moving through space on a collision course with Andromeda, so where you point at will be off.

This would be unnecessarily pedantic but not wrong* if not that the heart rotates on itself, so you are pointing at a 8 minutes old direction.

*All these discussion make no sense at all, as neither synchronous remote events nor the concept of what rotates around what are meaningful. This is not to say that we should not have this conversation, but to say that you are being pedantic without bringing anything to the table.

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.

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

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.

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

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.

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.

How do you know you have the correct (to the accepted precision) time?

GPS provides the correct time anywhere on the planet, it's a universal (as far as earth goes) clock, accurate to 10 nanoseconds (3m). The different reference planes that opposite sides of the planet (at the equator) gives you a precision of femtoseconds (micrometers at light speed) so makes no difference there.

The quality of GPS receiver of course is important, it may reduce your precision to to 100ns or even 1000ns, but it will give you the right time. Someone equidistant to two GPS synced clocks will see them both at the same time (within a microsecond)

UTC is a local clock on which other local clocks are synchronized.

Why do you have trouble understanding that knowing that light doesn't travel instantaneously?

By that definition there is no meaning to the word "now". If I'm five feet away from you, it takes time for light to travel between us.

Snark aside, using the phrase "now" to describe an in-motion event that is observed over 29000 light years is misleading and should rather say "is now observed as 29000 light years away".

> By that definition there is no meaning to the word "now".

Yes, exactly!

"Now" is an extremely useful concept on Earth, when we're all in approximately the same space going approximately the same speed. Now gets a lot done for us.

Now is not really a concept on the universal scale. "There is no meaning to now" is a great take-away from general relativity!

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

What a mean bully!

s/ully/ouncer

Yeet

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