12 days of Christmas for Astronomers

 On the first day of Christmas my truelove gave to me….

A black hole that swallows street lights

2 Adjuster screws and …..

3 Red LEDs

4 Dew strips

5 Parfocal rings!!!!

6 Colour filters

7 Sisters sparkling

8 Mounts a-moving

9 Guidescopes guiding

10 Tripods tripping

11 Lenses looking

12 APOs pointing

Black Holes are Misunderstood

 Despite the myths, the gravitational pull of a black hole is caused by its mass and, for distant objects, is no greater (initially) than that of the star that collapsed to form it. It doesn't suddenly develop a stronger gravitational pull simply by changing from a star to a black hole.  Obviously as it swallows more matter, its mass increases and so does its gravitational pull.

The scary bit about a black hole is that you can’t see it, because it’s hiding behind its “event horizon”; the invisible sphere, surrounding the black hole, from which light cannot escape. So you could accidentally stray into its gravitational field without knowing it’s there, until you suddenly feel yourself being strongly pulled towards it!

The radius of the event horizon (on formation) is very much less than the radius of the star that collapsed to form it. While the radius of the Sun is 7 x 10⁸ m, that of a Schwarzschild black hole of 10 solar masses is only  3 x 10⁴m. It’s only when you get closer to the black hole than the radius of the original star, that the weird stuff happens.

Distant objects are no more strongly pulled towards a black hole than towards a star of the same mass.

When a star collapses to form a black hole, it loses mass due to the energy squeezed out, according to Einstein’s mass/energy relationship. (E=mc²).

The minimum mass required of a star to become a black hole is about 6 solar masses. Less than that, it becomes a neutron star, and if much less it becomes a white dwarf.

We could not create a black hole accidentally on Earth because there is nowhere near enough mass in our neighbourhood to be able to do so. The Sun contributes 99% of the mass of the solar system, and the nearest massive objects, outside the solar system, are in the constellation Centaurus, 5 light years away.

Maybe some of the Centaurus constellation stars, at that distance, could potentially merge at some time in the distant future, and become a black hole, but at that distance would exert no bigger gravitational pull, on us, than they do now.

You can all calm down now.            

Olbers' Paradox

This states that if the universe was infinitely large, there would be a star along every possible line of sight and the sky would at all times be uniformly as bright as the sun. (Day and night; no dark bits!)

This argument presupposes that an infinitely large universe would contain an infinite number of bright objects. That is not necessarily the case.

According to currently prevailing theories, the universe is expanding at an increasing rate due to SPACE expanding. The total mass/energy in the universe has been estimated and is therefore finite. It is not believed to be increasing in mass/energy. Rather the total mass/energy density is decreasing.

Were the "finite" universe, as we see it today, to expand infinitely, in space terms, there would be no increase in the number of bright objects, and the sky would appear pretty much as it does today, albeit with some moving about. In fact, I believe (as many do) that it will become dimmer.

We would then have an infinite universe without a star on every possible line of sight.

Hence Olbers' paradox would appear to be flawed.