BBC Focus Magazine

[scott fairbrass]

whilst watching wonders of the universe last nite brian cox showed a film of 6/7 suns at the centre of our galaxy all orbiting what is thought to be a blackhole. my question is do all galaxies in the universe have a blackhole at its centre in which all suns orbit?

[Shadowwolf]

All galaxies are to the best of my knowledge currently thought to have a central massive black hole, but as we don't know all galaxies and telling what is inside them can be difficult, it's hard to say with any certainty.

What was indicated with the six or seven stars shown was how their erratic orbits - sudden acceleration and looping tightly around an unseen object - point towards a significant source of gravity that can only be explained by being a super-massive black hole.

[scott fairbrass]

What was indicated with the six or seven stars shown was how their erratic orbits - sudden acceleration and looping tightly around an unseen object - point towards a significant source of gravity that can only be explained by being a super-massive black hole.[/quote]

he was trying to explain space time and how gravity bends space time and thats how a orbit is created. (or something along them lines)

[Shadowwolf]

Sorry but I did not see it, I've just seen similar animations made of multiple stills that show stars slewing around an otherwise empty point of space. This was evidence of a black hole and shown as such, no idea what Cox's aim was, the description just reminded me of something else.

[M Paul Lloyd]

Although a black hole is said to be at the centre of our Galaxy, as explained on this site
http://news.bbc.co.uk/1/hi/7774287.stm
we can only infer its presence from information such as can be found here
http://www-istp.gsfc.nasa.gov/stargaze/Sblkhole.htm
because the centre of our galaxy is obscured by all manner of dust, gas and stars.

So if a black hole (or any number of same) exists at the centre of our Galaxy well then it is altogether possible that they exist at the centre of most, if not all Galaxies.

However this does not preclude the possibility of a small galaxy being held together by a cluster of very massive stars without the need for black holes.

What you have to look for is a Galaxy that is brighter than usual for its size, one within which the amount of observed matter better matches is rotational velocity without the need for dark matter.

When it comes to describing space time being distorted by gravity I can think of no better, if rather simplistic, method than the Euclidean 'rubber' sheet model which features on this site
http://www.pitt.edu/~jdnorton/teaching/ ... index.html

Hope that is of some help?