Planning on a cross-galactic holiday? Think again...

[VT-16]

The Beeb

Black hole confirmed in Milky Way
By Pallab Ghosh
Science correspondent, BBC News

Core of the Milky Way galaxy, taken with Nasa's Spitzer space telescope
The Milky Way was tracked from an observatory in Chile

There is a giant black hole at the centre of our galaxy, a study has confirmed.

German astronomers tracked the movement of 28 stars circling the centre of the Milky Way, using two telescopes in Chile.

The black hole is four million times more massive than our Sun, according to the paper in The Astrophysical Journal.

Black holes are objects whose gravity is so great that nothing - including light - can escape them.

According to Dr Robert Massey, of the Royal Astronomical Society (RAS), the results suggest that galaxies form around giant black holes in the way that a pearl forms around grit.

'The black pearl'

Dr Massey said: "Although we think of black holes as somehow threatening, in the sense that if you get too close to one you are in trouble, they may have had a role in helping galaxies to form - not just our own, but all galaxies.


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 exist
Professor Reinhard Genzel
Head of the research team

"They had a role in bringing matter together and if you had a high enough density of matter then you have the conditions in which stars could form.

"Thus the first generation of stars and galaxies could have come into existence".

The researchers from the Max Planck Institute for Extraterrestrial Physics in Germany said the black hole was 27,000 light years, or 158 thousand, million, million miles from the Earth.

"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," said Professor Reinhard Genzel, head of the research team.

"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."

Observations were made using the 3.5m New Technology Telescope and the 8.2m Very Large Telescope (VLT) in Chile. Both are operated by the European Southern Obsevatory (Eso).

This might possibly be old news, as I've heard about it before. Guess it's actual confirmation this time.

[Qwerty 42]

I thought it was theorized that all galaxies have black holes, which leads to the formation of galaxies themselves, because that gives them a massive source of gravity at their center. I could be wrong, of course.

[Zac Naloen]

This would make sense for Galaxies like the milky way and Andromeda which are vaguely similar to a solar system in their structure, but what about galaxies with irregular formations?

[Solauren]

Multiple smaller black holes / early stages of galactic formation.

[open_sketchbook]

Four million times as massive as the sun? Jebus, that's not something one can really ignore. How big would that be, then, size-wise?

[Junghalli]

How big would that be, then, size-wise?

IIRC, as it's a black hole, the mass itself would be in a single point. I don't know how big the event horizon would be.

If you actually wanted to go to the other side of the galaxy, it shouldn't be difficult to go around it.

[VT-16]

Here's the thing, it didn't look all that big. The closest of the orbiting stars are only five times further away from it, than Jupiter is from the Sun, at the closest.

[Executor32]

The Schwarzschild radius is estimated to be approximately 22 million kilometers, or slightly less than half that of Mercury's orbit.

[Ryan Thunder]

How big would that be, then, size-wise?

IIRC, as it's a black hole, the mass itself would be in a single point. <snip>

I thought it merely had to be dense enough. Of course, any mass will become a black hole if its crushed to a mathematical point, because it will have infinite density.

[Sriad]

In one of those neat little tricks physics plays, the radius of a black hole can be found exactly (well, to 3 significant digits which is exact enough for casual use) by the proportion 1.48×10^-27 meters/kg, or 2.95 km/solar mass. So the diameter (not radius) of a 4 million solar mass black hole would be 23.6 million km; about 12 times as big as the sun and extending ~1/5 of the way to Mercury.

[Executor32]

Observations using VLBI were reported in the September 4, 2008 issue of Nature that set a size of 37 +16/-10 µas on the instrinsic diameter of Sgr A*. Given the distance to Sgr A*, 25,900 +/- 1,400 ly, this yields the figure of ~44 million km that I quoted earlier, with error margins of +~22/-~14 million km. According to the article it's smaller than expected, so it's surmised that a good portion of the detected mass is part of the accretion disc, rather than the black hole itself.

[Sriad]

Observations using VLBI were reported in the September 4, 2008 issue of Nature that set a size of 37 +16/-10 µas on the instrinsic diameter of Sgr A*. Given the distance to Sgr A*, 25,900 +/- 1,400 ly, this yields the figure of ~44 million km that I quoted earlier, with error margins of +~22/-~14 million km. According to the article it's smaller than expected, so it's surmised that a good portion of the detected mass is part of the accretion disc, rather than the black hole itself.

Can you post the relevant part of the article? A black hole's Schwarzschild radius is a completely accurate mathematical function of its spin and mass. Unless it's spinning at like 80% C, it should be ~22 million km in diameter, not 44. Part of the reported radius might be accretion disk, or maybe it IS spinning that fast, or maybe some stupid intern mistook radius for diameter. Either way, I'd like to know.

The other possibilities are that math doesn't work, or I'm bad at it. I don't THINK either of those are true, but I'd like to verify it.

[starslayer]

I thought it merely had to be dense enough. Of course, any mass will become a black hole if its crushed to a mathematical point, because it will have infinite density.

Once a given amount of mass has been compressed below its Schwarzschild radius, it is a black hole. At that point, nothing can stop it from collapsing down to a singularity.

Can you post the relevant part of the article? A black hole's Schwarzschild radius is a completely accurate mathematical function of its spin and mass. Unless it's spinning at like 80% C, it should be ~22 million km in diameter, not 44. Part of the reported radius might be accretion disk, or maybe it IS spinning that fast, or maybe some stupid intern mistook radius for diameter. Either way, I'd like to know.

The other possibilities are that math doesn't work, or I'm bad at it. I don't THINK either of those are true, but I'd like to verify it.

Unfortunately, it's behind Nature's pay wall. After skimming it, Executor32 is misusing their figures; those measurements aren't of the event horizon and photon sphere, they're of Sgr A*, the radio source associated with the black hole. The article says that the apparent size of the event horizon should be larger than the 37 µas observed size of Sgr A* due to lensing; the physical horizon itself should only be about 10 µas across. The conclusion of the authors is that this means that Sgr A* is centered not on the black hole itself, but instead on some part of the accretion disk, which is a very interesting result.

I also double checked your math, using R_sch = 2GM/c^2, and the mass in kilograms. You got it right.

[Executor32]

That's what I get for reading that article when I was dead tired.

Was my conversion correct, though? It's been a while since I've really used geometry.

[Ryan Thunder]

I thought it merely had to be dense enough. Of course, any mass will become a black hole if its crushed to a mathematical point, because it will have infinite density.

Once a given amount of mass has been compressed below its Schwarzschild radius, it is a black hole. At that point, nothing can stop it from collapsing down to a singularity.

Oh, right. Yeah, I forgot how it worked when I said that.