Unprecedented 16-year-long study tracks stars orbiting Milky Way black hole

The Center of the Milky Way
This is the central parts of our galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope. By following the motions of the most central stars over more than 16 years, astronomers were able to determine the mass of the supermassive black hole that lurks there. Credit: ESO/S. Gillessen et al.

By watching the motions of 28 stars orbiting the Milky Way's most central region with admirable patience and amazing precision, astronomers have been able to study the supermassive black hole lurking there. It is known as "Sagittarius A*" (pronounced "Sagittarius A star"). The new research marks the first time that the orbits of so many of these central stars have been calculated precisely and reveals information about the enigmatic formation of these stars — and about the black hole to which they are bound.

"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, leader of the team from the Max-Planck-Institute for Extraterrestrial Physics in Garching near Munich.

The interstellar dust that fills the Galaxy blocks our direct view of the Milky Way's central region in visible light. So astronomers used infrared wavelengths that can penetrate the dust to probe the region. While this is a technological challenge, it is well worth the effort. "The Galactic Centre harbours the closest supermassive black hole known. Hence, it is the best place to study black holes in detail," argues the study's first author, Stefan Gillessen.

The team used the central stars as "test particles" by watching how they move around Sagittarius A*. Just as leaves caught in a wintry gust reveal a complex web of air currents, so does tracking the central stars show the nexus of forces at work at the Galactic Centre. These observations can then be used to infer important properties of the black hole itself, such as its mass and distance. The new study also showed that at least 95% of the mass sensed by the stars has to be in the black hole. There is thus little room left for other dark matter.

"Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. 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," says Genzel. The observations also allow astronomers to pinpoint our distance to the centre of the Galaxy with great precision, which is now measured to be 27 000 light-years.

To build this unparalleled picture of the Milky Way's heart and calculate the orbits of the individual stars the team had to study the stars there for many years. These latest groundbreaking results therefore represent 16 years of dedicated work, which started with observations made in 1992 with the SHARP camera attached to ESO's 3.5-metre New Technology Telescope located at the La Silla observatory in Chile. More observations have subsequently been made since 2002 using two instruments mounted on ESO's 8.2 m Very Large Telescope (VLT). A total of roughly 50 nights of observing time with ESO telescopes, over the 16 years, has been used to complete this incredible set of observations.

The new work improved the accuracy by which the astronomers can measure the positions of the stars by a factor of six compared to previous studies. The final precision is 300 microarcseconds, equivalent at seeing a one euro coin from a distance of roughly 10 000 km.

For the first time the number of known stellar orbits is now large enough to look for common properties among them. "The stars in the innermost region are in random orbits, like a swarm of bees," says Gillessen. "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."

One particular star, known as S2, orbits the Milky Way's centre so fast that it completed one full revolution within the 16-year period of the study. Observing one complete orbit of S2 has been a crucial contribution to the high accuracy reached and to understanding this region. Yet the mystery still remains as to how these young stars came to be in the orbits they are observed to be in today. They are much too young to have migrated far, but it seems even more improbable that they formed in their current orbits where the tidal forces of the black hole act. Excitingly, future observations are already being planned to test several theoretical models that try to solve this riddle.

"ESO still has much to look forward to," says Genzel. "For future studies in the immediate vicinity of the black hole, we need higher angular resolution than is presently possible." According to Frank Eisenhauer, principal investigator of the next generation instrument GRAVITY, ESO will soon be able to obtain that much needed resolution. "The next major advance will be to combine the light from the four 8.2-metre VLT unit telescopes – a technique known as interferometry. This will improve the accuracy of the observations by a factor 10 to 100 over what is currently possible. This combination has the potential to directly test Einstein's general relativity in the presently unexplored region close to a black hole."

Source: ESO

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Dec 10, 2008
Very impressive work. I did some calculations similar to this about a month ago, but the difference in scope... Wow. They must have had an absurd amount of data to analyse.

Dec 10, 2008
This will tell much about the mass of Sagittaraus A*. Hence, this will tell much about the Milky Way and of the origin (if combined will other data, etc).
This is a great piece of work, technology and dedication.
By the way, S2 completed a revolution in 16 years! How fast did it had to go. Its amazing that it still holds together.

Dec 10, 2008
Its amazing that it still holds together.

by the gravitational model billiard ball notion of celestial motion yes.

Dec 10, 2008
Is that just local to the centre of the galaxy, or does it apply broadly?

It would have to be local to the centre, since it refers to mass "sensed by the stars", therefore only relating to mass that affected the stars' orbits stronly enough to be measurable.

Dec 10, 2008
With a radial distance of 17 lighthours and orbit time of 15.56 years; I make that 3.7x10^4 m/s compared to the sun which is 2.2x10^5 m/s (from wikipedia) so quite slow unless someone wants to put me right.

Dec 11, 2008
The radial distance is between 17 lighthours and 5.5 lightdays (http://jumk.de/as....shtml). So the velocity is between 3.7*10^4 m/s and 2.9*10^5 m/s (based on the assumption that your further calculations were correct).
This site also stated that it isn't dangerousely close. When it will have a distance of less than 16 lightminutes (about 2 times the distance to the moon), it will have a problem. Then it is bye bye S2
I completely agree that this speed isn't as high as I expected. My apologies for first stating something without making the proper calculations :-)

Dec 11, 2008
This one has got me vexed. The difference in the speeds of the the two stars around the galactic centre should be roughly the square root of the proportion of their respective radii (ignoring for simplicity's sake the fact that S2 has a very eccentric orbit)

This means the square root of 27,000 light years divided by the rough mean of 4.8 light days, which gives a factor of 1433.

Someone correct me and put me out of my misery.

Dec 11, 2008
this sounds right IF the two stars always have the same position relative to each other (like when you place two dots on the spokes of a bicycle wheel. One complete revolution of the first dot will automatically mean that that is also true for the second dot. Hence, the velocity of the second dot will be much larger than for the first dot).
You're not sure whether this is the case and therefore you can't make that statement (the statement would also mean that we would make a complete revolution in just under 16 years.

I think youre statement will be true for other stars in the spiral arm. But these stars are so close to the centre that my guess is that they don't (necesserily) have the same angular speed.

Dec 11, 2008
I was just using the old school physics formula, which if I remember correctly is F = v^2 / r. Or in other words velocity is proportional to the root of the radius.

For example Neptune is roughly 100 times further from the Sun than Mercury and the proportion of their speeds is about ten times as well, (ATW)

Dec 11, 2008
I think I might have misled you by not making it clear that it's the object nearest the centre that is travelling the fastest - that's to say S2 should be travelling roughly 1433 times faster than the sun.

Dec 11, 2008
To correct the old formula I quoted above
a = v^2 / r
a itself is inversely proportional to the square of the radius, so 1 / r^2 is proportional to v^2 / r.
i.e. v is inversely proportional to the square root of the radius.

I hope that makes it clearer rather than foggier!

Dec 12, 2008
Im not certain about one thing. I don't think the example of Mercury and Neptune is similar to these stars. The orbits of the planets are somewhat "stable" and and have a fixed orbit around the sun. If (for example) Neptune would go faster, it would leave the solar system. If Mercury goes slower, it will plunge into the sun.
The last example is what will happen to S2. It isnt a clean orbit. It will be sucked up by Sagitarrius A*. I think that is the reason why your theory (which makes sense, it must be said) doesn't hold up in this case. It goes to slow to maintain a clean orbit around that black hole.
Let me know what your ideas are.

Dec 12, 2008
I'm surprised no-one has spotted what was wrong with my reasoning before now. It's something of a schoolboy howler. Embarrassingly, I'd only gone and clean forgot about the rest of the Galaxy. All 6 x 10^11 solar masses of it, which completely dwarfs the mass of the black hole at the galactic centre. And a fair proportion of that mass would be much closer to the Sun and therefore add to the strength of the gravitational pull. This much greater pull would require the Sun to have a much faster speed to keep in orbit. I'd imagine you'd need an advanced mathematical model or supercomputer to fully work out what that speed would actually be, but I can now easily see why the Sun moves as fast as it does when compared to S2.

Dec 12, 2008
That is a fair comment. I have to admit I also didn't take that into account. We considered everything like a spinning wheel where only the center counts.
That is also the reason why it will take S2 so long before becoming a part of Sagitarrius A*. I thougth there was something wrong as well but couldn't really put my finger on it.
Thanks Smiffy!

Dec 15, 2008
Btw, this is a nice link (from wikipedia):
What Smiffy observed is exactly what is the main problem for all scientists:
In 1959, Louise Volders demonstrated that spiral galaxy M33 does not spin as expected according to Keplerian dynamics,[1] a result which was extended to many other spiral galaxies during the seventies.[2] Based on this model, matter (such as stars and gas) in the disk portion of a spiral should orbit the center of the galaxy similar to the way in which planets in the solar system orbit the sun, that is, according to Newtonian mechanics. Based on this, it would be expected that the average orbital speed of an object at a specified distance away from the majority of the mass distribution would decrease inversely with the square root of the radius of the orbit (the dashed line in Fig. 1). At the time of the discovery of the discrepancy, it was thought that most of the mass of the galaxy had to be in the galactic bulge, near the center.
And this is exactly what shagrabanda calculated, that the velocity of S2 is almost the same as what the velocity of the sun.
So Smiffy, it isn't just all the matter in the Milky Way that causes this effect but it is the main reason why people expect the presence of dark matter

Dec 15, 2008
Thanks for pointing that out, Thecis. I had already known about the galaxy rotation problem but for some reason understood that the dark matter surrounded the galaxy, rather than permeating the entire disk, as Wiki makes clear. (Possibly because the dark matter I have always read/heard as being described as a halo.)

I'd was always puzzled about how any configuration of DM could have a 'constant speed' effect on the stars outside the disk but this new insight (to me) just makes it much worse. Seems even more preposterous.

If DM is the same as ordinary matter as far as gravitation goes then it should pretty much form similar distribution patterns as the ordinary matter - in which case why should it have an effect on speeds at all?

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