# Tag Info

6

Yours is a very, very interesting work. If you publish it it may happen that 3D diagrams will be used in the future. Please publish it. But this does not mean that there is a flaw in the H-R diagram. It is correct as it is: it displays Luminosity Vs. Color and is perfect at that. And then it happens that some extra relations appear: stars are not randomly ...

5

The wording provides a contrast with the Hayashi track phase that immediately precedes it, where the luminosity decreases by orders of magnitude with little change in effective temperature. You are correct though, the Henyey track for low-mass stars is not horizontal in the HR diagram, however it is a critical point that the luminosity does not change very ...

4

The Hertzsprung-Russell diagram is an observational tool. The axes are things that can be observed, or at least estimated reasonably well, for most stars (well, those with known distance anyway). We cannot in general measure the masses of stars - only those in some binary systems. The schematic that you show, with an arrow indicating that mass increases ...

4

The short answer is that the redshift of stars that are close enough to put into H-R diagrams (or "color magnitude diagrams", if we're being precise, given that you're talking about using colors) is so small that the effects are minimal. The correction to color you're thinking of (called "K correction") depends on the redshift: the ...

4

The redshift that is referred to is not a Doppler redshift, but a cosmological redshift. The difference is that the former is caused by the source moving through space, while the latter is caused by the "stretching" of the wavelength of the light as it travels through space. The cosmological redshift happens gradually along its journey, and hence it is a ...

4

The HR diagram has many forms. There is no one to one relationship between temperature, colour and spectral type that is true for stars of all surface gravities and metallicities. Spectral type depends on temperature primarily but spectral features can also be weaker or stronger at lower surface gravity or with changes in metallicity. For example, the ...

3

This is all absolutely true (pun intended). Finding absolute magnitudes is hard. For many types of star, we don't really know their absolute magnitude and so we don't really know their distance. For close-by stars we can get the distance by measuring the parallax (how far the star appears to move over a year due to the orbit of the Earth. Nearer stars ...

3

Where does the evolution actually stop? Is the Helium Core Flash the last thing a Sun-like stars experiences or does it in fact follow the rest of the tractory? Stars with masses below about 2 solar masses undergo a helium flash but their evolution does continue afterwards, as shown in the second diagram (Fig. 13.4). So no, the core flash is not the last ...

3

I will replace the previous answer to focus on the "subgiant" branch prior to red giant, rather than the pre-main-sequence or the "horizontal branch" of core helium fusion. Those are other times that the luminosity is constant, but this question is about the subgiant branch, which I missed before. The reason the luminosity is nearly constant on the ...

3

You can, in fact, differentiate between the crossings of the instability strip in certain types of variable stars. There have been quite a few evolutionary models devoted to this, and many, as well as scores of observations, show that the pulsational period $P$ of a Cepheid variable changes during each crossing. Look at, for instance, the period changes of ...

3

The answer is: both, but for different contexts. The HR diagram is used to display the properties of observed stars. A point on the diagram represents a star with a certain effective temperature and luminosity, which can be interpreted as being indicative of the stage of development of the star. Equivalently, the diagram can be expressed in terms of ...

2

What would the HR diagram look like if the axes were scaled linearly instead of logarithmically? This is a partial answer: Not so good. If things are well separated for clarity in a log-log plot, then they are going to be squished against the left side and bottom, and trends and behaviors will be hard to see in a linear plot thusly: import numpy as np ...

2

I think the issue you are going to have here is that the position of a star in the HR diagram - which amounts to saying what the (2 dimensional) spectral type is - does not just depend on its mass. The Vogt-Russell theorem says that the position of a star on the HR diagram depends on its mass, its composition and crucially, how its chemical elements are ...

2

I won't primarily explain the H-R diagrams, because I think focussing on some of the underlying physics is essential for understanding, especially stellar nucleosytheses. For simplification let's assume the star initially consists of nothing else than ordinary hydrogen and traces of carbon and nitrogen. Nuclear fusion of hydrogen forms helium; fusion of ...

2

With the Gaia Data Release 2, you can plot an HR diagram with a few million stars. From the Gaia website : The data can be accessed in the search tab, where you can specify the columns you want to display (phot_bp_mean_mag and phot_rp_mean_mag). There is a button at the bottom to export the results to a file on your computer. You can then plot one of the ...

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If your question is why the star starts moving up the red giant branch, it's in essence because of the behaviour of the surface opacity and the development of a substantial convective envelope in order to meet the surface boundary condition. It's basically the Hayashi track in reverse. We can say this because if you create a model of a star and ...

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For this, its important to understand how temperature increases in a star. Inside a stellar core, there are two forces that are balancing each other out to keep the star in equilibrium. The core pressure that is due to photons generated by chemical reactions inside the core forces the stellar atmosphere outwards (also called radiation pressure) while the ...

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You could plot $L$ vs $T$ and compare your model result to the measured/estimated $L$ and $T$ of main sequence stars? You have not explained or labelled what your diagram is, but it isn't $L$ versus $T$; I suspect that it absolute $V$ magnitude versus $B-V$ colour. The transformations between $L,T$ and $V$, $B-V$ are highly non-linear, especially at low ...

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With an absolute magnitude of zero and a colour (any colour) of zero, your stars are around spectral type A0 (by definition). The only thing that can confuse this is reddening/extinction. If there is appreciable reddening/extinction, then your stars could be more luminous and hotter (O/B stars), but there isn't any way to tell from your data.

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Try plotting absolute G magnitude (i.e. corrected for the fact tthat all the stars are at different distances) on the y-axis.

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Firstly, a HR diagram is not the same thing as a CMD. A HR diagram is luminosity (or absolute bolometric magnitude) against effective temperature or a reliable proxy for effective temperature. Assuming no "user error", then the information you are giving us is that this star looks "hot" with the 275-606 colour, but cooler in the 438-814 colour. There are ...

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Does the position of a star on a HR diagram depend on the magnitude system/color index used? Yes, of course. But note that HR diagram refers to a luminosity vs. effective temperature plot, while you're dealing with a color-magnitude diagram (or CMD). Why is this happening? First of all, you should check that it's not your fault. If you see unphysical ...

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The HR diagram is a plot of luminosity (or a proxy) versus temperature (or a proxy). $B-V$ is a proxy related to temperature, therefore to plot your point on a 2-d plot you obviously need the other axis information - the luminosity, or its proxy the absolute magnitude. This in turn needs an apparent magnitude and a distance to the star. It is also possible ...

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As the hydrogen supply in the core is exhausted, the pressure supporting against gravitational collapse lessens, and the core begins to collapse, which causes the inner temperature to rise. As a result, hydrogen in the less-processed regions outside the core starts to burn in a shell. Stellar models predict that, at this stage, there is a huge expansion of ...

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