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# Quite a nice Hertzsprung-Russell diagram

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Nice one Cath!

I've often wondered, though, why it is plotted as it is. Did HNR and EH both decide on the temperature going from hotter to colder,and, if so, why?

I suspect it's because they used colour, which is obviously directly related to temperature. However it is the colour they would measure directly, usually as two filters subtracted - e.g. (red - blue), and then associate it with temperature when they worked out the calibration.

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Oh heck, it makes me feel positively sea sick!

Sorry, that wasn't my intention lol

Just thought I'd see what it looks like with the temperature axis reversed.

The colours are a bit off but that can be fixed.

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I am not sure of there is a "correct" way of plotting to define a rule if it should be increasing or not , as in, is it recommended by bodies such as IUPAP or IUPAC that define such things. I don't think there is a standard to say it must be increasing or decreasing left to right, though I may be wrong, interesting point, as long as it is clear I guess.

One issue however that I do see everywhere, and it may seem like nitpicking, but actually it isn't, in terms of providing unambiguous information are the unit labels on the axis, what that graph is actually saying on the horizontal axis is Temperature in Kelvin squared, what it should say is Temperature/K. It is one of the most common errors made in all the sciences and you see it everywhere, so it has become an accepted way of doing things, but technically it is incorrect. Units follow the same rules of algrebra as numbers. Once you know that rule, there is only one correct way that is not ambiguous

When you start adding multiplication factors like 10^5 on the axes it can get confusing, because what did they really intend ?? divided by, multiplied by. Usually you can figure it out from the context, so it is not that big a deal. Still, it is an interesting fact that bodies such as IUPAC/IUPAC have been around to define such standards and the are often ignored by many, though some journals/books will ask for such things to be corrected before publication, depending what sticky fellows are on the editorial board

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Just thought I'd see what it looks like with the temperature axis reversed.

A simple change, but that makes it much easier to read for me. Just shows how used I am to an X-axis which increases from left to right.

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Red giants form at the end of the main sequence of lower mass stars and then end up shedding planetary nebula and the core collapses to form a white dwarf (supported by electron degeneracy pressure).

Super giants are the result of higher mass stars after they leave the main sequence - because of the higher masses the outcome is a violent collapse and rebound as a supernova. They could be red, blue or anywhere in between, depending on mass.

In both cases when the core runs out of fuel to 'burn', the equilibrium between gravity and the thermal pressure will no longer be sustained and the collapse will occur.

Really the characteristics of all stars are essentially determined by their mass and their age (therefore their position along their life-cycle). Stars change very little in mass over their lifetime until the final stages, strangely. In some cases, a star can accrete matter, say from a binary companion, for example a dormant white dwarf could be rejuvenated by accretion from it's neighbour via the Roche lobe.

Bigger stars burn brighter, hotter (bluer) and have a shorter life. If the core collapse is above the Chandresekhar limit then a supernova and neutron star is the outcome (supported by neutron degeneracy pressure).

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Nice diagram by the way. Where would Eta Carinae be on there relative to Betelguise I wonder. I could work it out but I'm off out now, maybe later

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Nice diagram by the way. Where would Eta Carinae be on there relative to Betelguise I wonder. I could work it out but I'm off out now, maybe later

Eta Carinae by all accounts is not an easy one to pin down on the HR diagram.  Sometimes described as a Luminous Blue Variable but has qualities making it an extreme example of the class.  During its eruptive outbursts and mass loss it makes transits of the HR diagram, it seems to skate around the instability strip.

There is a journal on all this insanity here :

http://inspirehep.net/record/820939?ln=en

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