Do old stars revolve more slowly?

[cronus]

Stars and galaxies that have been around for billions of years must have slowed revolving a bit surely?

[laklak]

Ask Joan Rivers.

[hackenslash]

Stars and galaxies that have been around for billions of years must have slowed revolving a bit surely?

Stars rotate in a manner commensurate with conservation of angular momentum. Take our sun, for example. When it approaches the end of its life, it will swell into a red giant, which will cause its rotation to slow. Then it will collapse into a white dwarf, and it's rotation will speed up again (although this is also a function of mass, and it will lose a fair bit toward the end). That's why pulsars rotate so incredibly quickly, because they're super-dense and very small, with a lot of mass in a very small volume. The classic analogy is the ice-skater spinning with his arms out and then drawing them in, which increases the speed of rotation.

Galaxies are pretty much the same, although there are anomalies in the orbits of the outer stars, especially in smaller galaxies, because they actually orbit so quickly that, given the matter we can see in the galaxies, they should fly off into space. That they don't is the reason we postulate Dark Matter, because there must be more mass than we can see in order for them to remain in such fast orbits. The rotational rate of galaxies doesn't actually change very much because, again, it's a function of mass and conservation of angular momentum, and the mass of a galaxy remains fairly stable throughout.

[JimC]

Science is just a matter of putting the right spin on things...

[hackenslash]

[PsychoSerenity]

What about tidal forces? Moons drifting away and things slowing down because of that? Is that an inevitable result, or is it just a matter of coincidence that that's what we've got on Earth? Do other places have tidal forces that speed things up as moons drift in?

[hackenslash]

What about tidal forces? Moons drifting away and things slowing down because of that? Is that an inevitable result, or is it just a matter of coincidence that that's what we've got on Earth? Do other places have tidal forces that speed things up as moons drift in?

Tidal forces would only ever work the way they do here, because they're a result of friction and gravitational attraction. As the planet rotates, the moon attracts the ocean via gravity, and the resulting tidal bulge is subject to friction against the surface of the planet, which acts as a brake on the planet's rotation. Because angular momentum has to be conserved, the rotational momentum has to go somewhere, so it's transferred to the moon, pushing it away. It can only ever work like that, so no, there couldn't be a situation in which the moon drifted in with the rotation of the planet accelerating, because the tides couldn't accelerate the planet, they could only ever have a braking effect.

Without the moon, the oceans would rotate at the same rate as the planet, so there would be no braking effect.

[JimC]

What about tidal forces? Moons drifting away and things slowing down because of that? Is that an inevitable result, or is it just a matter of coincidence that that's what we've got on Earth? Do other places have tidal forces that speed things up as moons drift in?

Given 2 orbiting bodies that are also rotating, angular momentum can be transferred from the spin to the orbital motion, with the proviso that the total angular momentum of the system is conserved. However, the total kinetic energy (including rotational KE) of the system will decrease over time, as tidal forces convert some of this to heat.

Small bodies orbiting larger ones at relatively close distances will end up locked in with one face always facing the larger body because of the effect of tidal forces, as with our moon.