The stars in the centre of our galaxy. Our supermassive black hole IS in there, somewhere... (ESO)

Beyond Any Reasonable Doubt: A Supermassive Black Hole Lives in Centre of Our Galaxy

10 Dec , 2008 by

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One the one hand, this might not be surprising news, but on the other, the implications are startling. A supermassive black hole (called Sagittarius A*) lives at the centre of the Milky Way. This is the conclusion of a 16 year observation campaign of a region right in the centre of our galaxy where 28 stars have been tracked, orbiting a common, invisible point.

Usually these stars would be obscured by the gas and dust in that region, but the European Southern Observatory (ESO) in Chile has used its infrared telescopes to peer deep into the black hole’s lair. Judging by the orbital trajectories of these 28 stars, astronomers have not only been able to pinpoint the black hole’s location, they have also deduced its mass…

It has been long recognised that supermassive black holes probably occupy the centres of most galaxies, from dwarf galaxies to thin galactic disks to large spiral galaxies; the majority of galaxies appear to have them. But actually seeing a black hole is no easy task; astronomers depend on observing the effect a supermassive black hole has on the surrounding gas, dust and stars rather than seeing the object itself (after all, by definition, a black hole is black).

Yearly location of stars within 0.2 parsecs from Sagittarius A* orbiting the common, compact radio source (from a different research paper by A. Ghez)In 1992, astronomers using the ESO’s 3.5-metre New Technology Telescope in Chile turned their attentions on our very own galactic core to begin an unprecedented observation campaign. Since 2002, the 8.2-metre Very Large Telescope (VLT) was also put to use. 16 years later, with over 50 nights of total observation time, the results are in.

By tracking individual stars orbiting a common point, ESO researchers have derived the best empirical evidence yet for the existence of a 4 million solar mass black hole. All the stars are moving rapidly, one star even completed a full orbit within those 16 years, allowing astronomers to indirectly study the mysterious beast driving our galaxy.

The centre of the Galaxy is a unique laboratory where we can study the fundamental processes of strong gravity, stellar dynamics and star formation that are of great relevance to all other galactic nuclei, with a level of detail that will never be possible beyond our Galaxy,” explains Reinhard Genzel, team leader of this research at the Max-Planck-Institute for Extraterrestrial Physics in Garching near Munich, Germany.

Undoubtedly the most spectacular aspect of our 16-year study, is that it has delivered what is now considered to be the best empirical evidence that super-massive black holes do really exist,” Genzel continues. “The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”

Apart from being the most detailed study of Sagittarius A*’s neighbourhood (the techniques used in this study are six-times more precise than any study before it), the ESO astronomers also deduced the most precise measurement of the distance from the galactic centre to the Solar System; our supermassive black hole lies a safe 27,000 light years away.

A lot of information was gleaned about the individual stars too. “The stars in the innermost region are in random orbits, like a swarm of bees,” says Stefan Gillessen, first author of the paper published in The Astrophysical Journal. “However, further out, six of the 28 stars orbit the black hole in a disc. In this respect the new study has also confirmed explicitly earlier work in which the disc had been found, but only in a statistical sense. Ordered motion outside the central light-month, randomly oriented orbits inside – that’s how the dynamics of the young stars in the Galactic Centre are best described.”

Quite simply, the object influencing these stars must be a supermassive black hole, there is no other explanation out there. Does this mean black holes have an even firmer standing as a cosmological “fact” rather than “theory”? It would appear so

Sources: ESO, BBC


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mrbill
Member
December 10, 2008 8:03 AM
Rey
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Rey
December 10, 2008 2:47 AM

You know what, what’s even more amazing is that we are all inside this massive & dark “box” and yet, most ordinary humans are less likeky to be intereseted in the fact that currently, we all are moving at the rate of almost the speed of light but since no one notices it, no will believe it, except a selected few, such as readers of UT grin

LLDIAZ
Guest
LLDIAZ
December 10, 2008 10:27 AM

This black hole is 4 million times the size of the sun and I read that is small. If its true then how big can these monsters get I mean it grows by consuming and it hasn’t been proven that it ejects anything

hellkr
Member
hellkr
December 10, 2008 3:57 AM

ESO… ESO… VLT… ESO… and the picture is from Keck razz

latinquasar
Guest
December 10, 2008 4:34 AM

It’s a really amazing research. Finally we can talk about the supermassive black hole as a “fact” instead a “theory” smile

I’m sure this black hole will help us to better understand how our own galaxy works.

Mark
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Mark
December 10, 2008 6:26 AM

How big should the event horizon be for this black hole? We should be able to observe the black hole itself, more or less, by watching a star move in its orbit behind the BH. The star should disappear and then reappear when its trajectory reemerges from behind the black hole.

Does anyone know how the density of stars in this region compares to the density of stars near the center of Omega Centuri?

Mark
Guest
Mark
December 10, 2008 6:36 AM

Hmm, there’s something strange about that graphic showing the orbits of the various stars orbiting the BH … you can get a feel for where the BH would have to be for the trajectories of the orbits to be what they are, but that position of the BH is widely divergent for some of the different trajectories!

Look at the almost circular trajectory of one the stars. The BH would have to be near the center of that orbit right? But the center of that orbit is nowhere near the where the BH would have to be for some of the oblong shaped orbits … can anyone explain this?

Mark
Guest
Mark
December 10, 2008 7:09 AM

Never mind about the non-common-“center of orbits” comment … look at figure 16 of the original paper :

http://arxiv.org/PS_cache/arxiv/pdf/0810/0810.4674v1.pdf

They have all 28 orbits plotted there along with the point they are orbiting. The circular orbit is more oblong than it looks on the keck graphic.

Essel
Member
Essel
December 10, 2008 7:29 AM
Some questions that come to my mind: – Is the position of Sgr A stationery from our point of observation? From the trajectories plotted for various starts it appears somewhat unlikely. The super-massive black hole itself is possibly spinning as well as wobbling. – Some of the starts come very close to Sgr A, much closer than 0.2 parsecs – say even 0.05 parsecs. Would it not create tremendous tidal forces to tear apart these stars? – For a 4 million solar mass monster, the region appears too calm to believe its existence. Have we noticed any violent event in these 16 years? – Is it possible that there is no super-massive black hole at all, what we… Read more »
Usman
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Usman
December 10, 2008 7:50 AM

Wait a moment wasn’t this thing confirmed by NASA some years ago? What’s new this time?

Mr. Obvious
Guest
Mr. Obvious
December 10, 2008 7:54 AM
Mark… Sag A, while very massive is quite small. Smaller than a planet. So the likelyhood of seeing it in front of a star at this point isn’t likely. Also, having everything lined up perfectly so we could see it wouldn’t happen very often. Most of the orbits the stars are taking around the black hole are extremely elliptical. Several trajectories take the stars pretty close (relatively) to the black hole, while others are still quite far away… even at their closest point. Finally… there will be slight variations in the track as we see things, since these orbits are tracked over a decade in time, and the galaxy isn’t perfectly stable, and our position changes during this… Read more »
John Mendenhall
Member
John Mendenhall
December 10, 2008 8:16 AM

“Rey Says:
December 10th, 2008 at 2:47 am
You know what, what’s even more amazing is that we are all inside this massive & dark “box” and yet, most ordinary humans are less likeky to be intereseted in the fact that currently, we all are moving at the rate of almost the speed of light but since no one notices it, no will believe it, except a selected few, such as readers of UT “

Mark
Guest
Mark
December 10, 2008 9:25 AM

Stefan Gillessen one of the authors of the paper emailed me back about the possibility of watching a star disappear behind the BH …

“the chances for that to happen are very low. The apparent diameter of the black hole is 10 micro-arsec only.

(I.e. we have not observed that and it would be a big coincidence if we would)”

Thanks to Mr Obvious too.

kymmie_cat
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kymmie_cat
December 10, 2008 10:13 AM

Thank you for providing the links. This research helps people wrap their minds around an invisible object that has great influence on our universe. One of my favorite quotes has always been that somethings have to be seen to be believed, while others have to be believed to be seen. My late father would have enjoyed reading this. We discussed black holes many years ago, when the “theory” was based mainly on previously unknown indicators (radio signals they emitted and strange orbital behaviors).

John Mendenhall
Member
John Mendenhall
December 10, 2008 10:17 AM
Rey Says: December 10th, 2008 at 2:47 am “You know what, what’s even more amazing is that we are all inside this massive & dark “box” and yet, most ordinary humans are less likeky to be intereseted in the fact that currently, we all are moving at the rate of almost the speed of light but since no one notices it, no will believe it, except a selected few, such as readers of UT ” What? Earth and the solar system are not traveling anywhere near the speed of light relative to anything local, including our galaxy, the SMBH at its center, Andromeda galaxy, the Local Group, the Virgo Cluster, and anything else closer than halfway across the… Read more »
John Mendenhall
Member
John Mendenhall
December 10, 2008 10:19 AM

And we’re also not part of the SMBH at the center of the Milky way. Neither are those stars that are orbiting it.

hiro
Guest
hiro
December 10, 2008 12:04 PM

R = GM/c^2 ~ several million km across. It can be observed if we have a telescope at some distance 550 AU from the sun.

hickninja
Guest
hickninja
December 10, 2008 12:16 PM

Keep in mind that the almost circular orbit you see is actually a more elliptical orbit, but we are viewing it at at angle so it looks circular. Draw an ellipse on a piece of paper with the two foci marked, and then tilt the paper until it looks like a circle. The actual focus is now very close to the edge of this circle.

tacitus
Member
December 10, 2008 1:16 PM
Is the position of Sgr A stationery from our point of observation? From the trajectories plotted for various starts it appears somewhat unlikely. The super-massive black hole itself is possibly spinning as well as wobbling. Well, any star orbiting the black hole will influence its position somewhat, just as Earth makes the Sun wobble a tiny amount, but give that the black hole is 4 million solar masses in size, then I highly doubt we can detect that wobble from this far away with today’s technology. (By comparison, the Sun is only 333,000 times the mass of the Earth and we are not yet able to detect wobbles caused by Earth-sized planets around stars much closer to us… Read more »
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