APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

[Chris Peterson]

Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.

Well... in comparison with those egg-shaped B and A-type stars like Achernar, Regulus and Altair? Unlike the Sun, B- and A-type stars are young.

And how about young M-type dwarfs that are highly magnetic and have tremendous outbursts? Perhaps they spin fast, too?

Astronomy & Astrophysics wrote:

Rotation is a property shared by most celestial bodies, including stars. Stars take birth in the core of molecular clouds from the infall of spinning matter driven by self-gravity. Rotation varies in time, it can be rapid or slow, but it persists all along stellar life. Ruled by the angular momentum conservation, rotation may lead to angular momentum, matter or/and energy transport between core and outer layers.

Don't know how to read that. Does it mean that different stars are born with different angular momentums that persist throughout their (main sequence?) life times, or does it mean that the rotation of one and the same star may change over time?

I would take a stellar system to be essentially closed, meaning that its net angular momentum doesn't change over time. There are scenarios where it wouldn't be closed, like a trinary system that ejects one member, or a system that another star passes very close to. But generally, and certainly in the case of the Solar System, essentially closed.

Of course, a protostar with jets could eject material from the system, which would change the net angular momentum.

But taking a "simple" system like our own, almost all of the angular momentum of the presolar nebula ends up in the star that forms, which (combined with the star's mass) defines the rotation rate (keeping in mind that it isn't a rigid body so doesn't have a single rotation rate), and I'd not expect that to change much over time.

[johnnydeep]

Ann wrote:

Astronomy & Astrophysics wrote:

Rotation is a property shared by most celestial bodies, including stars. Stars take birth in the core of molecular clouds from the infall of spinning matter driven by self-gravity. Rotation varies in time, it can be rapid or slow, but it persists all along stellar life. Ruled by the angular momentum conservation, rotation may lead to angular momentum, matter or/and energy transport between core and outer layers.

Don't know how to read that. Does it mean that different stars are born with different angular momentums that persist throughout their (main sequence?) life times, or does it mean that the rotation of one and the same star may change over time?

I think it means that due to convection of layers of material over time to lower or higher depths within the star, the spin can change, like a spinning skater pulling in their arms to speed up. But the total angular momentum of the star remains constant.