Astrophysics

[hawklord2011]

I'm now obsessed by astronomy having bought a scope but of course have only managed four observing sessions in the last month. Well being a data and detail nerd I had to do something to fill my time so I bought 'Astrophysics is easy!'. I must say its a fascinating read and will really enliven my next brief sighting when the clouds clear. Best of all its not shy of including all the formulae and maths. Anyone fancy calculating the width of a black hole from its mass? Thoroughly recommend it, are there any other books people would recommend?

[cloudsweeper]

There's a lot of good stuff on the OU Openlearn programme. Galaxies, black holes. All free.

[coatesg]

On 03/02/2016 at 20:50, hawklord2011 said:

I'm now obsessed by astronomy having bought a scope but of course have only managed four observing sessions in the last month. Well being a data and detail nerd I had to do something to fill my time so I bought 'Astrophysics is easy!'. I must say its a fascinating read and will really enliven my next brief sighting when the clouds clear. Best of all its not shy of including all the formulae and maths. Anyone fancy calculating the width of a black hole from its mass? Thoroughly recommend it, are there any other books people would recommend?

Well the diameter of a black hole is zero as it's ahas infinitely small volume (by currently known physical law)- it's the event horizon that has radius 2MG/c^2.

There's nothing particularly special about the event horizon at the Schwarzchild Radius in physical termsas to what happens to an object - for very large black hole  a human could easily pass over the horizon without being killed by tidal forces (at least not until they got closer to the singularity), though it's a one way trip... 

 

[hawklord2011]

I think you are referring to the singularity there as having zero size. In the book the radius of a black hole is defined as the distance from the singularity to the event horizon which is called the Schwartzchild radius. The radius is calculated as 3xMass in solar masses as kilometers. So a star that leaves a remnant core of 5 solar masses after supernova could form a black hole with a radius of 5x3km or 15 km. Still pretty small! Newtons laws of gravity will start to break down at 3 x Sch Radius rather than at the event horizon which in this case would be at 3x15=45 km. So you'd feel a bit funny before you got to the event horizon!

[coatesg]

The Schwarzchild Radius is a sensible measure yes (and the "edge" of what an external observer can ever see), but this is only due to the gravitational effect of the mass within. 

While a black hole of the order of a few solar masses does have a small Rs, a black hole that is of the order of 10^8 solar masses has a substantially larger Rs! Tidal forces are proportional to M/R^3 - for exceptionally large masses, you could happily pass through Rs and (notwithstanding the weird optical effects and the probable presence of accretion discs and the like... ) you might be able to pass over to your doom without too much issue. For small black holes, you would be tidally shredded well before physical laws started to alter.

 

[hawklord2011]

Our own local black hole at the centre of the Galaxy seems to have an estimated mass of 4,000,000 solar masses so would be a fair size but still small on an astronomical scale.

[ollypenrice]

Kauffman's Universe is often used as an undergraduate text book but is aimed at American undergrads who may or may not be science majors, so it is very flexible in terms of its reader appeal. The maths is there if you want it. I don't think I've ever come across a better text book. The writing is first class.

Olly

Edit: I've always taken 'Black Hole' to describe the region within the event horizon and certainly not to mean the infinitessimal.

[hawklord2011]

Thanks Olly I'll certainly check that out

[nameunknown]

Black Holes have "recently" got a bit more complicated than the old "event horizon" and "singularity" model. In the old version you could drift beyond the point of no return and notice nothing odd, but the new models suggest that it is not quite so simple and the "event horizon" is a place where you might get fried. It is worth reading up on.

[George Jones]

On 2016-03-23 at 13:44, nameunknown said:

Black Holes have "recently" got a bit more complicated than the old "event horizon" and "singularity" model. In the old version you could drift beyond the point of no return and notice nothing odd, but the new models suggest that it is not quite so simple and the "event horizon" is a place where you might get fried. It is worth reading up on.

The firewall concept is still somewhat controversial. A non-mathematical (but a bit technical) is given at

http://www.scientificamerican.com/article/black-hole-firewalls-confound-theoretical-physicists/

There is also the possibility of infinite tidal forces at the inner horizon of a rotating black hole. Work by Amos Ori indicates that these might be survivable, because they would only be experienced for an infinitesimal time.