Question about black holes

Pegasus · Post by **Pegasus** » Mon Oct 24, 2005 2:58 pm

I was left confused by the 23-Oct-05 APOD and its explanation. I was referred to this BB to find an answer. (Thanks Robert!) The APOD explanation talks about confirming the existence of a black hole at the centre of our galaxy by measuring the effect on a star that recently came within 17 light-hours of it.

My confusion is: if the black hole is so massive (and, if I understand correctly, I realise that "massive" in this case refers more to density than physical size), why wouldn't this star have been sucked completely into the black hole? I'm having trouble understanding how the gravitational pull of such a massive black hole wouldn't extend out that far to actually swallow the star? 17 light-hours seems like barely spitting distance!

I'm not looking for a crash course in astronomy physics (I don't think...) and I doubt I have enough knowledge to understand an in-depth answer. But I guess I wish the explanation had addressed this question for those of us who have too little knowledge to know the answer and too much knowledge not to ask the question.

Thanks for any help in understanding this.

Pegasus

Empeda · Post by **Empeda** » Mon Oct 24, 2005 3:31 pm

Don't know a massive amount about BH's myself but here's a penny-worth....

The black hole is huge is the sense of it's mass yes (millions of suns) but it's physical size is tiny- the gravitational pull is immense however...

If the star is travelling at a sufficient speed it won't be pulled into the hole but will orbit it (like the way that Mercury doesn't get pulled into the sun). Admittedly, it's speed must be pretty huge at that kind of distance.

I did a small research project on intermediate black holes in globular clusters, and the way the masses for these was estimated was by looking at the velocities of the central stars, which appear to be in an orbit. I suspect this is the same principle but on a much bigger scale.

William Roeder · Post by **William Roeder** » Mon Oct 24, 2005 5:49 pm

This is from memory

The Earth is 9 light minutes from the sun and takes a year to orbit the sun.

Mercury is less than 1/3 of that and takes something like 88 days.

Pluto is several light-hours out and takes 270 years.

The stars orbiting around the sm black hole are 17 light-hours out but take just 6 months to orbit. That's a lot of distance in very little time

BMAONE23 · Post by **BMAONE23** » Mon Oct 24, 2005 9:35 pm

Following Mr Roeders examples:
On average, Pluto is about 6 light hours away. the newly discovered Sedna varies from a minimum distance of approx 11 light hours to a maximum of 125 light hours and takes about 10,500 years to orbit the sun. The sun though only has 1 solar mass of gravity pulling on this object. The central black hole has a far greater influence on that matter around it.

So the 17 light hour sphere of influence would fit comfortably within our solar systems ORT cloud.

S. Bilderback · Post by **S. Bilderback** » Mon Oct 24, 2005 10:29 pm

The star 17 light-minutes from the black hole is most likely in a decaying orbit, tidal shifts in the star caused by the eminence gravity field of the BH stretching it out of shape causes it to slow down, as it slows it falls closer to the BH. I would estimate that the star will be torn apart and it's parts pulled into the BH in the very nears future - 100 years to a few thousand years.

Pegasus · Post by **Pegasus** » Mon Oct 24, 2005 10:34 pm

Thanks very much for all your replies!

Wow, I guess that sun must really be whipping around that black hole, to create enough motion to counterbalance the gravitational pull. Do all the stars near the centre of a spiral galaxy move that fast around the centre? I guess they must be or else they'd all get eaten by their galaxy's black hole, which I gather is assumed to be at the centre of most, if not all, galaxies. I guess the BH consists of the ones that didn't have enough counterbalance and were "assimilated."

Pegasus

makc · Post by **makc** » Tue Oct 25, 2005 7:07 am

S. Bilderback wrote:...100 years to a few thousand years.

same way I could "estimate" Earth age to be from 10k to 4.6B years. There's some stretch in calling it "estimation", don't u think?

S. Bilderback · Post by **S. Bilderback** » Tue Oct 25, 2005 12:20 pm

With out knowing the star's size, the rotational speed, or the eccentricity of the orbit, that's the best I can estimate. The chance of that star still orbiting in 10 thousand years is next to none, a .01% derogation in its orbit increasing exponentially on a six months orbit should give it a few hundred years; we'll have to watch to see how stable the orbit is.

harry · Post by **harry** » Sat Nov 19, 2005 8:09 am

The star 17 hours is very close to a BH and its life will be sucked out of it as we speak.

gordhaddow · Post by **gordhaddow** » Tue Nov 22, 2005 8:50 pm

Given that the galactic center is some 30,000 l-y away, it was likely gone some time ago.
Gord

harry · Post by **harry** » Wed Nov 23, 2005 6:45 am

Re:http://antwrp.gsfc.nasa.gov/apod/ap051023.html

This 17 hour star is been sucked in by the blackhole. Wow its destiny will be well observed sooner than we think. 17 hours must have its outer skin ripped apart and sucked into the blackhole. The only part remaining would be the high density core of the star.

Keep Cool

Empeda2 · Post by **Empeda2** » Wed Nov 23, 2005 12:32 pm

hmmm..surely the higher density part is more likely to be attracted to the black hole.?

justcallmebob · Post by **justcallmebob** » Wed Nov 23, 2005 10:04 pm

Hello, Im new to the site and haven't had a chance to post anything because I have been reading all the interesting posts, anyway, about black holes....I have the understanding that one could live to see the end of the universe..(in theory). It has been proven that the more gravity a person or object has then the slower time goes in said case. Now, the highest force of gravity known to man is at the edge of a black hole. If one were to live at the edge of a black hole and observe the sky then all of the stars would go by as a streak of light as time is slowed down and stars are speeding by. Would this not be true for a star at the edge of a black hole...given that it will be stretched to smithereens.....but wouldn't it mean that the life of that star would be prolonged much longer than what we expect?

S. Bilderback · Post by **S. Bilderback** » Thu Nov 24, 2005 1:58 am

That's our good friend "Relativity"

Time cannot stop completely, (that problem of dividing by zero again) But let's say that you were right outside an EH and time moved relativity much slower. As the rest of the light from the universe approached you, it would slow down also. In the end, the end of the universe would come to you relatively in the same amount of time. To see the end of the universe from a different "relative" point you would need a tool that worked faster than the speed of light.
Do you have one handy?

harry · Post by **harry** » Thu Nov 24, 2005 4:49 am

Hello Empeda

The 17 hour Core will be sucked in but! the core is of extreme density and wil hold together as one unit. The question is how close can this core get to the event horizin before it starts to break apart.