B Thank you for understanding. Have a great day!

[lomidrevo]

The pop. II stars, including halo, bulge, and globular clusters, were all formed in one fell swoop, roughly all at the same time and so are considered one population. Their metallicities don't change. The thin disk on the other hand is comprised of a significant amount of gas, and so star formation is continually ongoing. As heavier elements are continuously being produced and mixed into the gas, the metallicity of the disk stars is continuously changing; as you mention.

Yes, that makes sense to me. However, below is a snapshot of a table from a textbook I am reading, Carroll&Ostlie. The metallicity is expressed here as iron-to-hydrogen ratio of the observed star compared to the Sun's value [Fe/H]:
$$
[Fe/H] = \log_{10} \left [ \frac{(N_{Fe}/N_H)_{star}}{(N_{Fe}/N_H)_{sun}} \right ]
$$
Halo: from -5.4 to -0.5
Bulge: from -2.0 to +0.5
(Thick disk: from -2.2 to -0.5)

That is what I had in my mind, when mentioned changing and overlapping of metallicities in my previous post. The declared range for Halo stars is quite wide, isn't it? Indeed, I am not sure how accurate or up-to-date are the data in the table, when browsing I found slightly different values from different sources, eg. this one.

But what is also interesting, the authors mention in the textbook, that correlation between age and [Fe/H] may not be 100% reliable. They suggest that some astronomers rather use [O/H] or [O/Fe] ratios. Probably it is not easy to define stellar populations with such a variety of parameters :)

If you talk about Population II stars, do you mean stars with [Fe/H] very close to -5?

 

[Tom.G]

(warning! Some Devil's advocacy here.)

I am not sure I understand the situation you described. What do you mean by apparent gravity? You and the object would experience the same change of the gravitational acceleration. If you were at rest relative to the object before the change, you would be at rest to the object after the change of the gravitational field.

Yes, because for an increase in acceleration to be due to an actual increase in gravity, that would require magic to suddenly increases the mass of the object you are on so if you feel acceleration you know you are changing velocity. (emphasis added)

I was referring to the "true speed" phrase. As an example if you are on an asteroid in free space with no other objects, and experience a change in force holding you to the asteroid. Knowing that a change in gravity is not a viable assumption, a likely conclusion would be that there was an outgassing acting as a jet to change the asteroid course. With no other objects available for reference, then there is no 'speed' (velocity) either before or after the course change, yet something changed as shown by the delta acceleration that was felt.

Can somebody define what changed if it was not 'speed'?

 

[lomidrevo]

(warning! Some Devil's advocacy here.)I was referring to the "true speed" phrase. As an example if you are on an asteroid in free space with no other objects, and experience a change in force holding you to the asteroid. Knowing that a change in gravity is not a viable assumption, a likely conclusion would be that there was an outgassing acting as a jet to change the asteroid course. With no other objects available for reference, then there is no 'speed' (velocity) either before or after the course change, yet something changed as shown by the delta acceleration that was felt.

Can somebody define what changed if it was not 'speed'?

Yes, there would be some change of speed. Initially, the 3 reference frames of you, the asteroid and the gasses were all at rest, relative to each other. Assuming, that you and the asteroid remain in relative rest, after releasing the gasses, you could measure the relative velocity between your frame of reference and the one of released gasses.

Btw. the outgassing itself (more typical for comets), would require some source of additional thermal energy, so the space couldn't be empty anyway -> additional frame of reference to compare with, maybe? [emoji846]

 

[alantheastronomer]

The declared range [of metallicities] for Halo stars is quite wide, isn't it? That is what I had in mind, when mentioning changing and overlapping of metallicities...

Yes that's basically because they're all from the same population. Unlike terrestrial physics, because we're limited by our observations and can't tailor our experiments to what we're trying to determine, some areas of astronomy are unfortunately imprecise. Measured iron abundances vary widely from globular cluster to globular cluster, and even within the same cluster. These variations are not errors of measurement, they're what's found in nature. They show that all these structures are all of the same population, and they're only differentiated by their kinematics. It also shows that the thick disk was formed concurrently and not formed by larger abundance disk stars kicked up out of the galactic plane.

The authors mention...that correlation between age and [Fe/H] may not be 100% reliable. They suggest that [we] rather use [O/H] or [O/Fe] ratios.

Yes this was some time ago and they made a good suggestion. Iron alone is just produced in supernovae, while oxygen is formed (along with carbon and nitrogen) in lower mass stars that die as planetary nebulae and white dwarfs. Using all three abundance markers together then gives a more accurate picture of abundance evolution. Ages are still mostly determined by finding a population's turn off point on the main sequence, but this can still vary by as much as a billion years!

 

[Tom.G]

On star metallicities, see: https://phys.org/news/2018-08-stars-rebirth-home-galaxy.html

 

[fresh_42]

Thread closed.

As we apparently discuss a complete different topic now, it is time to end this one. If you wish to go on discussing metallicity please create a new thread with that topic.