Beta

×

Welcome to the Slashdot Beta site -- learn more here. Use the link in the footer or click here to return to the Classic version of Slashdot.

Thank you!

Before you choose to head back to the Classic look of the site, we'd appreciate it if you share your thoughts on the Beta; your feedback is what drives our ongoing development.

Beta is different and we value you taking the time to try it out. Please take a look at the changes we've made in Beta and  learn more about it. Thanks for reading, and for making the site better!

Milky Way's Black Hole Wasn't Always Such a Wimp

Soulskill posted more than 2 years ago | from the you-wouldn't-like-me-when-i'm-angry dept.

Space 83

scibri writes "Sagittarius A*, the dormant supermassive black hole that lies at the center of our galaxy, was much more active not that long ago. Astronomers using the Fermi Gamma-ray Space Telescope have picked up some faint gamma-ray signals that suggest Sagittarius A* was emitting a pair of powerful gamma-ray jets like other galactic black holes as recently as 20,000 years ago (arXiv paper). If our black hole was more active in the past, it could explain why Sagittarius A* seems to be growing about 1,000 times too slowly for it to have reached its current mass of about four million solar masses since the Galaxy formed about 13.2 billion years ago."

cancel ×

83 comments

Sorry! There are no comments related to the filter you selected.

dormant black hole? (4, Interesting)

ZombieBraintrust (1685608) | more than 2 years ago | (#40160063)

What makes a black hole dormant? Lack of gamma ray jets... ?

Re:dormant black hole? (2)

Tanman (90298) | more than 2 years ago | (#40160121)

I guess the hulk moved on to make movies instead of crushing solar systems . . .

Can one black hole consumes another black hole? (1)

Taco Cowboy (5327) | more than 2 years ago | (#40162629)

If two black holes happens to be near each others, will they collide?

Or will one of the black hole swallow the other?

Will the bigger black hole swallowing up the smaller one, or will it be the reverse, the little guy biting chunks out of its bigger counterpart?

And when two black holes interact with each others, what will happen to the dimensions?

Will it create enough disruptions to the dimensions in the vicinity that temporary additional dimensions pop up here and there?

They Merge (2, Interesting)

Anonymous Coward | more than 2 years ago | (#40162847)

From the view of an outside observer, the larger (greater mass) black hole will appear to draw the smaller one in. In reality, they're drawn to each other, and eventually merge, like two drops of water meeting on a window pane. The singularities become one. Simulations suggest that the merger will radiate massive quantities of energy in the form of gravity waves, a major ringing of the bell.

The ringing quickly dies down, and the only evidence of the merger being 1) the changed direction of movement relative to other nearby objects and 2) major disruptions of whatever was orbiting the two holes, from stars down to the accreation disc gas.

Re:dormant black hole? (3, Funny)

Anonymous Coward | more than 2 years ago | (#40160227)

it's only dormant because Chuck Norris told it to relax

Re:dormant black hole? (0)

Anonymous Coward | more than 2 years ago | (#40164885)

I heard it was because Bruce Schneider had enough entropy in his store.

Re:dormant black hole? (4, Informative)

buchner.johannes (1139593) | more than 2 years ago | (#40160235)

What makes a black hole dormant? Lack of gamma ray jets... ?

Lack of gas and dust streaming in. The disk + torus the infalling gas produces while accreting produces all the radiation we see from black holes in active galactic nuclei (AGN). Another side effect are the jets that you can see in radio frequencies (although not in all AGN.

There is actually a gas cloud falling in in these decades, so we might see our black hole light up. http://www.nature.com/nature/journal/v481/n7379/abs/nature10652.html [nature.com]

Re:dormant black hole? (2)

cpu6502 (1960974) | more than 2 years ago | (#40160313)

I'd like to know where this black hole came from. Was there some random star floating through space, which died, and then it started gobbling up everything? Including our galaxy (which will eventually fall in). Or maybe the superblackhole was a previous galaxy from ~25 billion years ago that fell into itself?

I'll go ask the Vorlon.
He's probably old enough to remember.

Re:dormant black hole? (0)

ZombieBraintrust (1685608) | more than 2 years ago | (#40160371)

The black hole is our galaxy. Everything falls to the center. When there is enough mass boom star. When more mass boom black hole.

Re:dormant black hole? (4, Informative)

buchner.johannes (1139593) | more than 2 years ago | (#40160401)

I'd like to know where this black hole came from. Was there some random star floating through space, which died, and then it started gobbling up everything? Including our galaxy (which will eventually fall in). Or maybe the superblackhole was a previous galaxy from ~25 billion years ago that fell into itself?

To my knowledge it is currently unknown how those massive black holes (millions of solar masses) form originally. We know they form very early in the universe (1Gyrs after the big bang, our universe is ~14Gyrs old). Do they come from many stars? Were stars in those early times extremely massive? Is there some way of growing black holes very fast?
Those are open questions in Astrophysics ... you are welcome to join in :)

We know that merging galaxies should combine their black holes but also grow them (more gas infall) -- but nobody knows how two black holes merge ( https://en.wikipedia.org/wiki/Binary_black_hole#The_final-parsec_problem [wikipedia.org] ).

Re:dormant black hole? (1)

sociocapitalist (2471722) | more than 2 years ago | (#40160911)

Where G = Gajillion?

Re:dormant black hole? (1)

SJHillman (1966756) | more than 2 years ago | (#40161099)

K = Kilo = Thousand
M = Mega = Million
G = Giga = Billion

Go hand in your geek license.

Re:dormant black hole? (0)

Anonymous Coward | more than 2 years ago | (#40163683)

even if you didn't know that, you could tell from context

our universe is ~14Gyrs old

that it meant billion. unless you actually think our universe is 14 gajillion years old, in which case go back to school.

Re:dormant black hole? (1)

sociocapitalist (2471722) | more than 2 years ago | (#40164115)

Was a joke...

Re:dormant black hole? (0)

Anonymous Coward | more than 2 years ago | (#40164383)

My understanding of the problem is this: we're pretty sure that galaxies formed around supermassive black holes. We see galaxies at high redshifts (i.e. in the early universe), so those supermassive black holes must have formed very quickly, within about 1 billion years of the big bang. But we can't explain how a black hole that big could have formed that quickly.

Option A: massive stars formed in the early universe, went supernova very quickly, and the black holes left over from those supernovae grew to supermassive size very quickly by gobbling up the surrounding gas. The trouble with this is that there's a limit on how fast a black hole can grow without the radiation from the accretion disk blowing away any other infalling gas. For a black hole to grow to supermassive size soon enough after the big bang, it must have started off pretty big in the first place, which means it must have come from a star (and supernova) much bigger than those we see today. There are arguments that this was possible in the early universe because of the lower metal content of the gas that formed the stars ... which I don't understand, but I get the impression that they're in the "maybe it's enough, maybe it isn't" range.

Option B: black holes formed in the early universe (again, from supernovae), and grew by eating each other. This gets around the growth-rate problem above (since another black hole is too massive to be blown away by radiation from the accretion disk). The catch is that it's pretty unlikely for black holes to merge. If you throw two black holes roughly towards each other, 99.9% of the time they end up in a fairly stable orbit around each other. There might be some mechanism which makes these orbits unstable (follow the parent's link to the "final parsec problem") - but these are still speculative, rather than observed.

Btw, I'm an astronomy student, but working in a different sub-field. The above is what I gleaned (probably with some distortions) from an open lecture given by Joss Bland-Hawthorn at the University of Sydney, who *does* work on this sort of thing.

Vorlon is a mere child (1)

rssrss (686344) | more than 2 years ago | (#40161481)

Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn.

Remnant of Galactic Formation (0)

Anonymous Coward | more than 2 years ago | (#40162919)

Much as the primordial gas cloud of our solar system created enough density at the center to light up as a star, the much more massive cloud of material that is our galaxy was likely to collect a clump at/near the center that would pass the Schwartzchild limit and collapse as a black hole. It might not have had all that much time to shine like a star before collapsing, or it may have been a number of stars merging within the crowded galactic core. After the collapse, those stars that were about to merely merge with the center mass instead were instead ripped into a huge accreation disc, and feed the powerful polar jets we used to call quasars

Re:dormant black hole? (-1)

Anonymous Coward | more than 2 years ago | (#40160289)

What makes a black hole dormant?

Welfare checks.

Re:dormant black hole? (0)

Anonymous Coward | more than 2 years ago | (#40165523)

A black hole doesn't become dormant. It only becomes terribly boring once it's eaten everything around it.

Too Much Space Turkey? (2, Funny)

Scarletdown (886459) | more than 2 years ago | (#40160071)

Just more evidence that eating turkey makes you sleepy. It would appear that our galaxy's super massive black hole ate too much space turkey, and now it is having a nice long nap.

Re:Too Much Space Turkey? (0)

Anonymous Coward | more than 2 years ago | (#40160483)

Or it has cleaned up as much matter as it can in its surrounding areas and it takes a bit more effort for mass that is farther away?

Re:Too Much Space Turkey? (2)

Zocalo (252965) | more than 2 years ago | (#40160637)

Maybe it's just contemplating the celstial equivalent of a wafer thin after-dinner mint.

I'd imagine the after effects might be a little more colourful than with Mr. Creosote though...

Re:Too Much Space Turkey? (1)

RaceProUK (1137575) | more than 2 years ago | (#40165097)

Just one wafer-thin star!

*nom*

KABOOM!

Weight of a teaspoon amount (1)

andydread (758754) | more than 2 years ago | (#40160075)

so at 4 million solar masses how much would a teaspoon of that stuff weigh?

Re:Weight of a teaspoon amount (4, Insightful)

fuzzyfuzzyfungus (1223518) | more than 2 years ago | (#40160147)

I get the impression that concepts like 'volume' start to get a little tricky once you pass the event horizon...

Re:Weight of a teaspoon amount (4, Informative)

Colonel Korn (1258968) | more than 2 years ago | (#40160257)

The singularity itself? A teaspoon of singularities would have infinite weight. Maybe you mean everything inside the event horizon? In that case calculate the Schwarzschild radius (2Gm/c^2) of 4 million solar masses, then get the density [4 million solar masses /(4/3 pi r^3)] and multiply by the volume of a teaspoon. I think the density of everything inside the event horizon for that big of a black hole is actually pretty low.

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40160347)

Sooooo....4?

Re:Weight of a teaspoon amount (1, Informative)

sanosuke001 (640243) | more than 2 years ago | (#40160549)

4x10^6 solar masses = 7.95568 × 10^36 kilograms
Schwarzchild radius = 1.1804758431349163 x 10^10 meters
4/3 pi r^3 = 6.89064573 × 10^30 m3


mass / volume = 1.19612658 × 10^-27 kg / m3
1 tsp = 5.0 × 10^-6 m3
1 tsp of Sagittarius A* = 5.9806329 × 10-33 kg

Re:Weight of a teaspoon amount (3, Informative)

FrootLoops (1817694) | more than 2 years ago | (#40160905)

Your mass/volume ratio is way off, though the other three are correct. It should be...

mass / volume = 1.155 * 10^6 kg/m^3
1 tsp = 4.929 * 10^-6 m^3
1 tsp of Sagittarius A* = 5.693 kg

So, it's pretty heavy, but eg. neutron stars are far, far heavier. This black hole is far denser than the sun, which has about 6.94 g per tsp [wolframalpha.com] .

Re:Weight of a teaspoon amount (2)

chadenright (1344231) | more than 2 years ago | (#40162191)

Wolfram Alpha disagrees with both of you. [wolframalpha.com] It gives the result as 1.1 kg.

Re:Weight of a teaspoon amount (1)

FrootLoops (1817694) | more than 2 years ago | (#40162283)

I was doing the calculations originally suggested by Colonel Korn. Presumably the Schwarzschild radius isn't completely accurate here, and the "four million suns" mass figure is of course not very precise.

Still, the answer I was replying to was off by around 33 orders of magnitude (which it got modded up for...) whereas mine was only off by a factor of 5--and I'm not sure how precise WA's figure is.

Re:Weight of a teaspoon amount (2)

drerwk (695572) | more than 2 years ago | (#40162637)

http://www.wolframalpha.com/input/?i=Schwarzschild+radius+of+4+million+solar+mass+ [wolframalpha.com] = 7 million miles
http://www.wolframalpha.com/input/?i=Sagittarius++A*+radius [wolframalpha.com] = 13 million miles
Seems Sag A* radius is based on observed angular size of the radio source.

Re:Weight of a teaspoon amount (1)

sanosuke001 (640243) | about 2 years ago | (#40184065)

ahh, yes. I used google and it didn't like my order of operations. mass/vol = 1 154 562.33 kg/m^3 redone. oh well.

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40160731)

M = 4 million solar masses = 8e36 kg
Schwarzchild radius: r = 2 G M / c^2
Density = M / (4/3 pi r^3)
  = 3/32 pi^-3 c^6 G^-3 M^-2
  = 1.2e5 kg m^-3
  = 120 tonnes per cubic metre

So it's still denser than anything you could reasonably encounter on Earth, but it's not very dense by the standards of white dwarfs, neutron stars, etc.

Note, btw, the M^-2 dependence in the final formula. This means that as the mass of a black hole increases, the density inside its Schwarzchild radius decreases (because that radius increases even faster).

Re:Weight of a teaspoon amount (2)

Thiez (1281866) | more than 2 years ago | (#40160789)

> The singularity itself? A teaspoon of singularities would have infinite weight.

No, it wouldn't. Black holes have a finite weight.

Re:Weight of a teaspoon amount (3, Insightful)

Anonymous Coward | more than 2 years ago | (#40160857)

> The singularity itself? A teaspoon of singularities would have infinite weight.

No, it wouldn't. Black holes have a finite weight.

Singularities consume no space, so you can fit an infinite number of finite weight singularities in a teaspoon. Hence infinite weight

Re:Weight of a teaspoon amount (1)

geoffball (1195685) | more than 2 years ago | (#40165185)

> The singularity itself? A teaspoon of singularities would have infinite weight.

No, it wouldn't. Black holes have a finite weight.

Singularities consume no space, so you can fit an infinite number of finite weight singularities in a teaspoon. Hence infinite weight

The mass is finite. The volume is 0. D=M/V. The density is infinite.

Re:Weight of a teaspoon amount (3, Informative)

osu-neko (2604) | more than 2 years ago | (#40160915)

> The singularity itself? A teaspoon of singularities would have infinite weight.

No, it wouldn't. Black holes have a finite weight.

Right, but an infinite number of singularities will fit in a teaspoon (or any volume, for that matter).

Re:Weight of a teaspoon amount (1)

Anonymous Coward | more than 2 years ago | (#40161377)

You've touched on one of astrophysics' proverbial elephants in the room.

There is no physics-based reason to believe there is a "singularity" at the center of a black hole, only a place where the math breaks down to a division-by-zero. In reality, some physical constraint would prevent further collapse, in the same way that degeneracy pressure halts the collapse of less massive stars.

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40162897)

I thought everyone knew about this. It's as if world renowned physicists like Leonard Susskin aren't having free lectures on youtube.

Haven't read Susskind, yet (1)

Fujisawa Sensei (207127) | more than 2 years ago | (#40163097)

I haven't read Susskind yet, but I have read General Relativity by Dirac.

According to Dirac's, the singularity will never actually occur because of time and space dilation. The stellar matter will accelerate towards the singularity but never actually reach it. And never is an appropriate term since according to Hawking the black holes have a finite lifetime and will eventually evaporate according to Quantum Mechanics.

Will get to Susskind as soon as I finish a couple of Penrose books.

Re:Haven't read Susskind, yet (1)

spiralx (97066) | more than 2 years ago | (#40165609)

I would thoroughly recommend this book [amazon.co.uk] by Susskind, for both the content and the most awesome title for a popular science book ever.

Re:Haven't read Susskind, yet (1)

Fujisawa Sensei (207127) | more than 2 years ago | (#40228969)

Thanks, that's actually the one on the list.

Re:Weight of a teaspoon amount (0)

NemoinSpace (1118137) | more than 2 years ago | (#40163169)

Statistically speaking almost no-one knows this. At its extremes these limits are often interpreted wrongly. - Mostly by people that have never built anything.
In most "physical" problems, an answer approaching 0 means you are about to waste your time.
An answer of infinity is supposed to let you know you have NO idea of WTF you are doing.
Everything in between is actually pretty interesting. Mathematical models are things of inherent beauty and most of it in theoretical physics is above my pay grade.

OTOH the singularity is handy when the "scientific" "crowd" starts becoming overly smug with the "religious" "nuts".

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40164395)

If there were a physical constraint that could prevent further collapse under a black hole's event horizon (i.e. when the escape velocity is greater than c), couldn't you use it to violate relativity? Build a rod out of matter compressed to the point where this physical constraint takes over. Tap one end of it. The sound wave travelling through it has a speed which depends on the strength of the force which resists compression - which in this case, would be greater than c. So the sound wave reaches the other end of the rod faster than light.

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40165027)

In the conditions where you can create this super-dense material, time and length don't have their normal meaning. And what the hell are you going to tap it with?

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40161151)

density is mass per volume or mass/volume if a singularity has no volume ( = 0 ) then density is undefined (unknowable)

Re:Weight of a teaspoon amount (5, Funny)

Anonymous Coward | more than 2 years ago | (#40160367)

Don't know, none of our "teaspoon on a rope" measuring devices have been successfully pulled back out past the event horizon.

On an unrelated note, we need more interns.

Re:Weight of a teaspoon amount (1)

Tarlus (1000874) | more than 2 years ago | (#40161307)

so at 4 million solar masses how much would a teaspoon of that stuff weigh?

That depends on how much the teaspoon itself weighs. =)

Re:Weight of a teaspoon amount (0)

Anonymous Coward | more than 2 years ago | (#40170087)

But you need to remember, "There is no spoon" then all the answers will be revealed!

More Of A Neutron Star Question... (0)

Anonymous Coward | more than 2 years ago | (#40162777)

The singularity (theorized to be) at the center of a black hole is a mathematical point in space/time, and so of infinite density. As we "step back" from the singularity, the average density of the matter/energy racing by quickly decreases, and would depend on how much crap is accreating during the measurement. Once we're within the outer (Schwarzschild) horizon, we (again, theoretically) speed up beyond the speed of light, only to find things getting really backed up close in. We hit the inner (Cauchy) horizon, which I'll naively label a standing "shock wave", and go subluminal the rest of the way in. Calculating the density at that "shock wave" might be an interesting exercise for a post-grad/doc.

The singularity would hold the vast majority of the mass within the event horizon, and thus when deposited in your teaspoon (or hell, on the tip of a pin) would weigh in at 4 million solar masses. Your question works better when scooping up a bit of a white dwarf or neutron star, two stellar remnants that are in fact solid at their surfaces.

white dwarf: 4.95 metric tons (5.4 short tons)/teaspoon
neutron star: 18.3 metric tons (20.1 short tons, near the surface)/teaspoon

Don't go into black holes (-1)

Anonymous Coward | more than 2 years ago | (#40160081)

You never know what type of STDs you'll have when you leave.

Re:Don't go into black holes (-1)

Anonymous Coward | more than 2 years ago | (#40160329)

"You never know what type of STDs you'll have when you leave."

This is like Hotel California. You can check in any time, but you can never leave.

Incomplete data (0)

Anonymous Coward | more than 2 years ago | (#40160211)

Is that +/- the 27.000 years that light would have to travel in order for us to observe the jets stopping?

Re:Incomplete data (0)

jmorris42 (1458) | more than 2 years ago | (#40160761)

Obviously. Gamma rays obey the light speed limit just like everything else.

What I'd like to know is how much gamma ray action the Earth sees when those jets are fired up. We now have evidence that gamma rays influence climate so would it be a good or bad thing if it lit up again? Then we need to be asking if we can determine if/when it will start back up.

Re:Incomplete data (1)

Charliemopps (1157495) | more than 2 years ago | (#40160965)

well, since the gamma rays are practically pointing directly perpendicular to the rotation of the galaxy, they wont come anywhere near us. Even if it were pointing directly at us, it would take them at least 26,000 years to reach us. Granted, we wouldn't know they were coming until we were fried.

Re:Incomplete data (1)

drerwk (695572) | more than 2 years ago | (#40161003)

The jets should be perpendicular to the accretion disk which should mostly be co-planar with the disk of the galaxy. So, the jets will not point at us. If some oddball star or mass or whatever that is in a highly out of galactic plane orbit gets sucked in, we should still be ok as there is quite a lot of dust between us and Sag A*.

Re:Incomplete data (3)

TapeCutter (624760) | more than 2 years ago | (#40162379)

We now have evidence that gamma rays influence climate

Speaking of incomplete data - No, we have evidence that gamma rays leave a vapour trail in a cloud chamber since that's how we detect them in an atom smasher. A few people have taken this fact and speculated that gamma rays affect the climate by seeding clouds, there are even a couple of books about the idea. Only problem is, their speculation does not not fit everyone else's observations. However you will find the 'Iris theory' presented as fact in the opinion pages of the wall street journal. Why? - Because on top of their complete lack of evidence, the people pushing this idea just happen to think it 'proves' Earth's climate is self regulating so we don't have to worry about regulating our emmissions.

Re:Incomplete data (1)

bill_mcgonigle (4333) | more than 2 years ago | (#40176501)

so would it be a good or bad thing if it lit up again?

The oscillation of our solar system's orbit brings us above and below the plane of the galactic ecliptic every 200,000 years or so and that's when massive speciation seems to happen.

So, hang on for that long and you won't need Red Bull to give you wings.

Re:Incomplete data (0)

Anonymous Coward | more than 2 years ago | (#40160955)

Careful, you might set off yet another idiotic relativity "discussion".

Asterisk (4, Funny)

Megane (129182) | more than 2 years ago | (#40160373)

It wasn't always such a wimp, but then it got caught doing steroids, so it had to have an asterisk after its name.

I dare you (5, Funny)

Anonymous Coward | more than 2 years ago | (#40160413)

Oh sure, it's easy to call it a wimp from way out here on the outskirts of the galaxy. But I bet you wouldn't call it a wimp if it were right in your face!

Re:I dare you (0)

rubycodez (864176) | more than 2 years ago | (#40163343)

Chuck Norris got in its face. He told it to quit sucking so much, or he'd tear it a new asshole, reach inside, grab its singularity and turn it inside out. That's why we now have a quiet black hole in our galaxy.

WWKKT? What would Kim K. think? (1)

Impy the Impiuos Imp (442658) | more than 2 years ago | (#40160769)

Is anyone else disturbed that such an incredibly major change happened only 20,000 years ago?

This could be worse than an ice age.

Re:WWKKT? What would Kim K. think? (5, Insightful)

osu-neko (2604) | more than 2 years ago | (#40160947)

Is anyone else disturbed that such an incredibly major change happened only 20,000 years ago?

This could be worse than an ice age.

No. If, 20,000 years ago, it was much more active, it proves living in a galaxy with an active nucleus is not a problem. What it means is, if it becomes more active again, we don't really have anything to worry about -- we've been living with the "problem" for most of five billion years and gotten along just fine...

Re:WWKKT? What would Kim K. think? (1)

Kaenneth (82978) | more than 2 years ago | (#40168781)

Unless there is some unknown thing about the radiation stopping that allowed civilization to develop.
Like a lower rate of mutations allowing humanity to become genetically stable.
Sounds like a good Sci-Fi premise.

Re:WWKKT? What would Kim K. think? (2, Funny)

SJHillman (1966756) | more than 2 years ago | (#40161147)

Al Gore went back in time 20,000 years (and you thought he only invented the Internet) to begin the process of Galactic Cooling so counteract the effects of Global Warming.

Re:WWKKT? What would Kim K. think? (1)

TapeCutter (624760) | more than 2 years ago | (#40162417)

Surely he would have un-invented chads while he was there?

Galactic superwave theory (1)

Paul Fernhout (109597) | more than 2 years ago | (#40162145)

http://www.google.com/search?q=galactic+superwave+theory [google.com]

One example:
http://www.etheric.com/LaViolette/Predict.html [etheric.com]
"Subsequent concurrence (1998): In 1988, when presented with Dr. LaViolette's Galactic explosion hypothesis, astronomer Mark Morris dismissed the idea as having no merit. However, in 1998 after ten years of observation, Morris was quoted as saying that the center of our Galaxy explodes about every 10,000 years with these events each lasting 100 years or so."

Imagine if you were to go outside one night and the sky suddenly lit up as bright as day and stayed that way for 100 years!

Maybe matter falls towards the galactic core, but interacts with the core to produce shock waves that push it away again, to form some sort of resonant process that happens every 10,000 years?

Relativity... (0)

Anonymous Coward | more than 2 years ago | (#40161067)

What does 20000 years ago mean, given that there 26000 is a light-year distance between us an Sagittarius A*?

Is that 20000 + 26000 = 46000 years, since otherwise we would still see the jets for the next 6000 years?

Correction:Relativity... (0)

Anonymous Coward | more than 2 years ago | (#40161091)

What does 20000 years ago mean, given that there is a 26000 light-year distance between us an Sagittarius A*?

Re:Relativity... (1)

amRadioHed (463061) | more than 2 years ago | (#40161355)

It means exactly what it always does. 20,000 years ago. You're trying to be too clever with the speed of light thing, when we observed it is when it happened as far as we're concerned.

quieted by mass (4, Interesting)

Shavano (2541114) | more than 2 years ago | (#40162321)

It was explained in my astrophysics class that when a black hole reaches a certain mass that whole stars pass inside the event horizon before being torn up by tidal force. Then the singularity no longer has a big accretion disk and the radiation emitted by infalling matter is trapped within the event horizon. So it goes quiet.

Re:quieted by mass (2)

dotancohen (1015143) | more than 2 years ago | (#40164309)

It was explained in my astrophysics class that when a black hole reaches a certain mass that whole stars pass inside the event horizon before being torn up by tidal force.

Then the singularity no longer has a big accretion disk and the radiation emitted by infalling matter is trapped within the event horizon.

So it goes quiet.

Who was your lecturer? Pick a mass, any reasonable mass for a black hole, and calculate it's Schwarzschild radius. Now add the diameter of a star, any star, to that radius and calculate the gravity field at that distance from the center of the black hole. Do you notice the difference? That tidal force will tear any star apart. It will tear iron apart.

Furthermore, it is very unreasonable to assume that the only object orbiting the black hole is the single ingested star. Most (all?) black holes have large quantities of mass orbiting them in accretion disks, the ingested star will of course disturb matter in the accretion disk. I will leave to your own imagination why the ingested star's influence on the accretion disk will be anything but 'quiet'. As to the assertion that afterwards "the singularity no longer has a big accretion disk" it is likely that the ingested star leaves more of its own material in the accretion disk than that which it drags down with it. Hint for calculating it: stars rotate.

Re:quieted by mass (1)

dotancohen (1015143) | more than 2 years ago | (#40164321)

Just a followup: let us assume the largest black hole that we can imagine, ingesting the smallest star that we can imagine, in the interest of minimising the tidal forces on the star. In this extreme hypothetical case of no tidal force on the star until it contacts the event horizon, you will agree that the star will have also drag down with it minimal material from the accretion disk, and certainly not all of it. Now imagine what a star passing through the remaining material does to the distribution of that material. You can see that even in this extreme example of the star going "quietly" the resulting effect on the accretion disk is anything but quiet.

Re:quieted by mass (1)

Shavano (2541114) | more than 2 years ago | (#40190461)

That depends on what you mean by quiet. A star getting ripped apart by a smaller black hole would emit more radiation.

Re:quieted by mass (2, Informative)

Anonymous Coward | more than 2 years ago | (#40164425)

The Schwarzchild radius is proportional to the mass M. The tidal effect is proportional to the derivative of the gravitational field, which is proportional to M / R^3. Setting R to be the Schwarzchild radius, to measure the tidal effect at this point, we find that it is proportional to 1 / M^2. So the more massive a black hole is, the smaller the tidal effect at its event horizon - and for a sufficient black hole mass, the tidal effect must be insufficient to break up a star.

Calculating the mass at which this happens is left as an exercise for the reader. ;-)

Re:quieted by mass (1)

dotancohen (1015143) | more than 2 years ago | (#40164563)

The Schwarzchild radius is proportional to the mass M. The tidal effect is proportional to the derivative of the gravitational field, which is proportional to M / R^3. Setting R to be the Schwarzchild radius, to measure the tidal effect at this point, we find that it is proportional to 1 / M^2. So the more massive a black hole is, the smaller the tidal effect at its event horizon - and for a sufficient black hole mass, the tidal effect must be insufficient to break up a star.

Calculating the mass at which this happens is left as an exercise for the reader. ;-)

Thanks. I address this in my followup post, posted as a reply to self. And I also left the "insufficient tidal effect" BH mass as an exercise for the reader, for any arbitrary definition of insufficient!

Re:quieted by mass (1)

Shavano (2541114) | more than 2 years ago | (#40190437)

For a star the size of the sun, at 90 million solar masses, a star like the sun would stay intact as it fell through the event horizon. \

Re:quieted by mass (1)

Shavano (2541114) | more than 2 years ago | (#40190493)

The most massive black holes ever observed are over a billion solar masses. At 90 million solar masses, the sun could hold together at least with respect to gravity vs. tidal force. If you were observing this happen, you would see the sun getting dimmer and redder as it fell toward the event horizon, and at some point I think it would just go dark. You wouldn't see a flash or flare because a big piece of matter just disappeared from the observable universe without making a splash.

Re:quieted by mass (1)

dotancohen (1015143) | more than 2 years ago | (#40192907)

You are correct, the post that you replied to did not account for galaxy-core supermassive black holes. However, the followup post (rely to self) addresses this.

What we call Black Holes.... (0)

Anonymous Coward | more than 2 years ago | (#40166281)

Wouldn't it be ironic what we call Black Holes are really Wormholes...

Check for New Comments
Slashdot Login

Need an Account?

Forgot your password?
or Connect with...

Don't worry, we never post anything without your permission.

Submission Text Formatting Tips

We support a small subset of HTML, namely these tags:

  • b
  • i
  • p
  • br
  • a
  • ol
  • ul
  • li
  • dl
  • dt
  • dd
  • em
  • strong
  • tt
  • blockquote
  • div
  • quote
  • ecode

"ecode" can be used for code snippets, for example:

<ecode>    while(1) { do_something(); } </ecode>