Size of 'OUR' Black Hole in galactic centre

[sirius0]

There´s also the 'outcast' star (http://www.sciencedaily.com/releases/2005/02/050222195319.htm). The process that formed it must take place very often in the universe.
This quote is from another thread about intergalactic stars.
Apparently it is similar to other threads so I am starting a new one with the below question.

Can the mass and velocity and other data gathered from the 'outcast' star (http://www.sciencedaily.com/releases/2005/02/050222195319.htm) that Argos refered to be used to calculate or infer the size of the Black Hole in the centre of our galaxy? it might be interesting to use as a check against other methods.[/URL][URL="http://www.sciencedaily.com/releases/2005/02/050222195319.htm"]
(http://www.sciencedaily.com/releases/2005/02/050222195319.htm)

[cbacba]

This quote is from another thread about intergalactic stars.
Apparently it is similar to other threads so I am starting a new one with the below question.

Can the mass and velocity and other data gathered from the 'outcast' star (http://www.sciencedaily.com/releases/2005/02/050222195319.htm) that Argos refered to be used to calculate or infer the size of the Black Hole in the centre of our galaxy? it might be interesting to use as a check against other methods.[/URL][URL="http://www.sciencedaily.com/releases/2005/02/050222195319.htm"]
(http://www.sciencedaily.com/releases/2005/02/050222195319.htm)


It would appear that using the outcast star would be a rather bad choice to attempt to determine the mass of the super massive condensed object at the galactic center.

However, there are a number of stellar objects very close in to the central object in fairly tight orbits, fairly elliptical ones too. Those have been observed for a number of years and have short enough orbital periods to actually show significant movement in the orbits permitting the orbits to be fairly well determined. The central mass has been determined from these to be about 300 million solar masses as I recall and appears to be rather small for something that sized mass were it made of standard matter in a somewhat common astronomical density. Hence, it is usually referred to as a super massive black hole.

While being of an incredibly tremendous mass compared to normal stars, this object is evidently fairly mediocre so far as super massive objects in the middle of galaxies, being perhaps 1/10 th the size of the one in the andromeda galaxy if I'm not mistaken.

While dust obscures the object in visible, xrays, IR and radio show a bit of what's going on in there. I just read that the object or something real close to it brightened up by a 1000 times around 50 years ago - evidently when a pluto sized object became an afternoon snack for it. Things could get quite interesting if some large star winds up becoming a main course. Note that there is a Chinese curse that is merely the wish that someone winds up living in interesting times.

[Tensor]

However, there are a number of stellar objects very close in to the central object in fairly tight orbits, fairly elliptical ones too. Those have been observed for a number of years and have short enough orbital periods to actually show significant movement in the orbits permitting the orbits to be fairly well determined.

Here (http://antwrp.gsfc.nasa.gov/apod/ap001220.html) is a short movie showing the motions of the stars near the central object. Here (http://www.astro.ucla.edu/~ghezgroup/gc/) is a good projection of the orbits of the stars.

[astromark]

As Tensor is telling you, There are stars dangerously close to the galactic core's black hole. To look outside the galaxy for a star that has been ejected and to use its motion to make relevant data on the size and density of that black hole seems an awfully big ask. When we can study material much closer and greatly effected by the massive gravity well.
Or have I missed the question here? :)

[cbacba]

Here (http://antwrp.gsfc.nasa.gov/apod/ap001220.html) is a short movie showing the motions of the stars near the central object. Here (http://www.astro.ucla.edu/~ghezgroup/gc/) is a good projection of the orbits of the stars.

Ugh, I seem to have something more now in common with the universe. I seem to be misplacing a few orders of magnitude here and there. I was thinking the most recent numbers for the core objects were 300 million and 3 billion Ms for the milkyway and andromeda objects. Those values look somewhat greater than most of the quick look up refs. I saw on a brief website search. Well, maybe I got the ratio roughly right.

Your references are quite neat. I was afraid the movie was that animation someone did showing the whole orbital paths based upon the decade of data but the one you ref'ed was the actual images.

I almost forgot that the reason for going to the second page was the estimated orbit diagram after seeing those Keck images at 3.2um comparing the laser/natural star AO systems. I'm not sure why such a big difference - other than maybe ref. 'star' brightness and perhaps lower control loop time delays but the images are both incredible.

[Cougar]

I seem to be misplacing a few orders of magnitude here and there. I was thinking the most recent numbers for the core objects were 300 million [solar masses]... for the milkyway... Those values look somewhat greater than most of the quick look up refs. I saw on a brief website search.
Well, just a couple of orders of magnitude. :o This NASA site (http://science.nasa.gov/headlines/y2000/ast29feb_1m.htm) puts the Milky Way's central black hole at around 3 million solar masses -- well less than 1% the mass of the galaxy.

[Peter Wilson]

I was thinking the most recent numbers for the core objects were 300 million and 3 billion Ms for the milkyway and andromeda objects...Those are the same numbers lodged in the attics of my mind ;)

[cbacba]

Those are the same numbers lodged in the attics of my mind ;)

My recollections likely came from F. Melia (AZ U.), either in book or lecture a couple of years back - maybe I can blame it on him. I can't seem to locate my copy of his book at the moment to even check. In several webpages, it looked like andromeda ranged for 10 or 30 up to 140 Ms so it's evidently no done deal. 30E6 vs 3E9 - they all add up to the same number (12) so what the 'hey'. Besides compared to 1E120 - vacuum energy density descrepancy? - it's inconsequential.

So far as the size goes for that little blob in sagitarius - seems like Melia thought it might be as small as 40 AU or was that 400 AU?

Gut feel makes me think that these critters may really be more along the same lines of being far more massive than initially assumed. Perhaps even large enough to start to explain those radial velocities out in the disk.

[cbacba]

Those are the same numbers lodged in the attics of my mind ;)

My recollections likely came from F. Melia (AZ U.), either in book or lecture a couple of years back - maybe I can blame it on him. I can't seem to locate my copy of his book at the moment to even check. In several webpages, it looked like andromeda ranged for 10 or 30 up to 140 Ms so it's evidently no done deal. 30E6 vs 3E9 - they all add up to the same number (12) so what the 'hey'. Besides compared to 1E120 - vacuum energy density descrepancy? - it's inconsequential.

So far as the size goes for that little blob in sagitarius - seems like Melia thought it might be as small as 40 AU or was that 400 AU?

Gut feel makes me think that these critters may really be more along the same lines of being far more massive than initially assumed. Perhaps even large enough to start to explain those radial velocities out in the disk.

[astromark]

Do not think as if all were orbiting the central point. Its the gravity of the whole galaxy that is holding it together. We are far enough away from that central mass not to need to ever worry about it. Those stars shown in those links are in what appears to be stable orbital positions. There orbital distances though not large are stable. Chaotic night skies and impossible radiation levels would be reason enough to stay away.

[dirty_g]

According to Astronomy now....... or was it the sky at night...... one of my mags they say the size of our galaxies SMBH is about the same as the distance from the sun to pluto. So not HUGE (In galactic terms)!!! But its influence is obviously a bit more.

[antoniseb]

one of my mags they say the size of our galaxies SMBH is about the same as the distance from the sun to pluto.

That is calculated based on the mass determined by the orbits of the stars which are very close to it. So the mass is reliable, but the actual diameter of the event horizon of this object is given assuming that our theories about gravity are correct (so far there is not any reason to doubt them, they are supported by *many* diverse observations). My point is that we have not imaged the SMBH directly or its disk, or the distorted light from objects immediately behind it. So, the original poster's question about ejected stars was actually a good one. If we find enough of them it might prove to be a way of getting a statistical indirect way of measuring the diameter of the object.

[dirty_g]

sorry i said a load of rubbish and remembered the article badly. I looked it up in Astronomy Now (April 2007 issue, page 83) the actual answer according to Doctor Alan Longstaff of The Royal Greenwich Observatory is that the Radius of it is a mere 10 million Kilometeres (for comparison Mercury orbits at 60 Million Kilometers from the sun). Thus it is too small to see. The closest we have gotten is to see stars orbiting at 17 light hours from galactic centre and THAT is about three times the distance of Pluto from the Sun (I remembered pluto was in the article somewhere). Apparently the fromula to calculate the radius of any black hole in meteres is this 2GM/c2; where G is the gravitational constant (6.67 * 10-11 Nm2kg2) M is the mass in kilograms and c is the speed of light in meters per second. (I am not about to pretend I understand that bit at all) hope that helps!!

[mugaliens]

As Tensor is telling you, There are stars dangerously close to the galactic core's black hole. To look outside the galaxy for a star that has been ejected and to use its motion to make relevant data on the size and density of that black hole seems an awfully big ask. When we can study material much closer and greatly effected by the massive gravity well.
Or have I missed the question here? :)

The task is formidible, yes, but not impossible. See this month's edition of Scientific American where an article features the techniques astronomers are using to determine which stars originated within our galaxy, which did not, and the relative positions and motions of those stars, today.

[Peter Wilson]

Apparently the formula to calculate the radius of any black hole in meters is this 2GM/c2...
What I find interesting about that is that it implies a maximum possible density in the universe, because the radius of a BH increases linearly with mass, but density goes as mass/r^3, so the density decreases as a BH's mass increases.

There is a minimum size to BH creation, however, as the only known mechanism to create one is through collapse of a large star. Thus, there is a minimum mass of BH that can be created, and at creation, this minimum-mass BH corresponds to its maximum density.

The velocity of light forms a natural speed-limit in the cosmos, while a combination of factors creates a natural density limit.

[cbacba]

What I find interesting about that is that it implies a maximum possible density in the universe, because the radius of a BH increases linearly with mass, but density goes as mass/r^3, so the density decreases as a BH's mass increases.

There is a minimum size to BH creation, however, as the only known mechanism to create one is through collapse of a large star. Thus, there is a minimum mass of BH that can be created, and at creation, this minimum-mass BH corresponds to its maximum density.

The velocity of light forms a natural speed-limit in the cosmos, while a combination of factors creates a natural density limit.

I'm not sure what the max size limit is now thought to be. Note that it's not a foregone conclusion that the object has to be as small as the schwartzchild radius as that is a theoretical concept and doesn't necessarily have to be real. I think they've isolated it down to 90 AU or perhaps 40 AU and perhaps it's been verified down to a few million Km by now.

As for the basics, it seems like a multimillion solar mass bh only has to have the density of water to be within the schwartzchild raduis - hence forming the event horizon.

While it's a stretch to assume the laws of physics work inside the event horizon, if they do, then there's nothing to keep the matter inside from continuing to collapse down to the singularity point in the middle. Perhaps there's quantum factors that ultimately limit that from happening which aren't part of the regular GR secenario.

Note though that while we know there is a small unbelievably heavy object there, we don't know exactly what it's nature happens to be. There are alternatives to BHs in theory for collapsed objects - like ECO and MECO, as well as variations of BHs such as those spinning ones which may not have an event horizon. Plus, we are assuming that objects that massive are collapsed objects and are not subject to yet some other conditions we're not aware of at present that prevent them from collapsing further like the notion of a quark star or perhaps something beyond that.

Hopefully, additional study will lead us to a few answers on this as it undboutedly will create more questions that need to be solved.

[sirius0]

Thank you all for so many interesting replies!
As I think about them I realise that there is a background thought to my question that has come forward.

Surely an ejected star; being completely out of the space-time and direct influence of the MBH, has in Hawkings language "information" that other stars in direct orbit may or may not have.

For example what if we found that MBHs have a larger (or smaller) size by calculation compared to those closer etc. I mean it is only by checking everything that we will find more endless quests for knowledge.