by Ann » Sun Jan 21, 2024 7:29 pm
Chris Peterson wrote: ↑Sun Jan 21, 2024 2:32 pm
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?
We plotted (Fig. 1) the distribution of apparent rotational velocities (V sin i) as a function of the spectral type. Two stellar populations spinning at different rates can be identified: stars cooler than F7 generally rotate at angular speeds lower than 50 km/s, while hotter stars are often rotating faster than 100 km/s. Rotation is indeed competing with other physical processes, and is impacted by the interaction between the magnetic field of cooler stars and their protostellar environment, or by the development of strong stellar winds at higher effective temperatures.
So I guess you could say that stars like the Sun spin slowly compared with stars hotter than spectral class F7.
And interestingly, just as the Sun appears to be almost perfectly round, it also seems to be less active and more "benign" than most other stars of its type.
Phys.org wrote:
By cosmic standards the sun is extraordinarily monotonous. This is the result of a study presented by researchers from the Max Planck Institute for Solar System Research in the upcoming issue of Science. For the first time, the scientists compared the sun with hundreds of other stars with similar rotation periods. Most displayed much stronger variations. This raises the question whether the sun has been going through an unusually quiet phase for several millennia.
Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity.
To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable," explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication. A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki. The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are—and thus also how brightly the Sun shines.
The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong. "We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities."
Yes, but those G-type stars that vary so much in brightness compared with the Sun nevertheless display the same rotation period as the Sun. So apparently the Sun's rotation rate can't explain its mild behavior.
Ann
[quote="Chris Peterson" post_id=336430 time=1705847529 user_id=117706]
[b][color=#FF0000]Does the Sun spin "slowly"? In comparison to what?[/color][/b] (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.
[/quote]
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?
[quote][url=https://aa.oma.be/stellar_rotation]Astronomy & Astrophysics[/url] 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. [b][color=#FF0000]Rotation varies in time, it can be rapid or slow[/color][/b], [b][color=#0040FF]but it persists all along stellar life[/color][/b]. Ruled by the angular momentum conservation, rotation may lead to angular momentum, matter or/and energy transport between core and outer layers.[/quote]
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?
[float=right][img3="Fig. 1: Median V sin i (red horizontal bars) vs. spectral type, from M9 (left) to O4 (right). Values are taken from the catalog of Glebocki R. & Gnacinski P. 2005."]https://aa.oma.be/sites/default/files/VSINIM3_s.png[/img3][/float][quote]We plotted (Fig. 1) the distribution of apparent rotational velocities (V sin i) as a function of the spectral type. Two stellar populations spinning at different rates can be identified: [b][color=#FF0000]stars cooler than F7 generally rotate at angular speeds lower than 50 km/s[/color][/b], [b][color=#0040FF]while hotter stars are often rotating faster than 100 km/s[/color][/b]. Rotation is indeed competing with other physical processes, and is impacted by the interaction between the magnetic field of cooler stars and their protostellar environment, or by the development of strong stellar winds at higher effective temperatures.[/quote]
So I guess you could say that stars like the Sun spin slowly compared with stars hotter than spectral class F7.
And interestingly, just as the Sun appears to be almost perfectly round, it also seems to be less active and more "benign" than most other stars of its type.
[float=left][img3="Brightness variations of the Sun in comparison with the star KIC 7849521. Credit: MPS / hormesdesign.de"]https://scx1.b-cdn.net/csz/news/800a/2020/sunislessact.jpg[/img3][/float][quote][url=https://phys.org/news/2020-04-sun-similar-stars.html]Phys.org[/url] wrote:
By cosmic standards the sun is extraordinarily monotonous. This is the result of a study presented by researchers from the Max Planck Institute for Solar System Research in the upcoming issue of Science. For the first time, the scientists compared the sun with hundreds of other stars with similar rotation periods. Most displayed much stronger variations. This raises the question whether the sun has been going through an unusually quiet phase for several millennia.[/quote]
[quote]Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity.
To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "[b][color=#FF0000]The speed at which a star rotates around its own axis is a crucial variable,[/color][/b]" explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication. A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki. The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are—and thus also how brightly the Sun shines.[/quote]
[quote]The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. [b][color=#0040FF]While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation.[/color][/b] [b][color=#FF0000]Their fluctuations were typically about five times as strong[/color][/b]. "We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities."[/quote]
Yes, but those G-type stars that vary so much in brightness compared with the Sun nevertheless display the same rotation period as the Sun. So apparently the Sun's rotation rate can't explain its mild behavior.
Ann