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PostPosted: Sat Apr 13, 2019 1:43 pm 
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I know that stars somewhere btw 1-2 solar masses will go through a giant phase and then peter out into a white dwarf.

But there's a diagram that sort of looks like a toddler scribbled on a wall...

here: https://en.wikipedia.org/wiki/Stellar_e ... ack_1m.svg

... and it shows a star spiking in luminosity and then dropping back down again quickly, then wobbling its way back up.

My question is whether the star's radius is always proportional to its changing luminosity. Does it shrink in size during the "red clump" stage as listed on the diagram?

Also there's no sense of time associated with that diagram, so I'm not sure how long any of those sub-phases of the giant phase actually last.

One last interesting (to me) note: that a white dwarf is apparently the hottest phase of a star's life, if I'm reading that diagram correctly.


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PostPosted: Sat Apr 13, 2019 7:55 pm 
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This summary I wrote about stellar evolution may help! http://evildrganymede.net/rpg/world/stellarevol.pdf

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PostPosted: Sun Apr 14, 2019 11:05 am 
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Extremely helpful, thanks! The story of our sun entering its giant phase always focuses on the "it will engulf the earth!!!!" bit, so I was confused to see that, say, Arcturus only has a radius of ~ 0.11 AU.

But that means Arcturus is some way along the horizontal branch, right? And already blasted its inner planets.


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PostPosted: Sun May 05, 2019 2:20 pm 
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Follow-up questions:

Main sequence stars get hotter as they age, right? And the number after the spectype is a general indicator of temperature? Does that temperature change enough over a star's lifetime that our sun was at one time a G5V? And before it reaches its subgiant stage, will it be a G1 or G0?


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PostPosted: Sun May 05, 2019 8:11 pm 
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My understanding is that stars get brighter during their tenure on the main sequence, but not hotter. The surface temperature stays the same, so the output of radiation per square metre of surface area stays the same, but the stars get bigger, their surface area increases, and their total luminous output increases.


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PostPosted: Sun May 05, 2019 11:46 pm 
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The number does change over a star's main sequence life, yes. So a solar mass star could start as a G5 V and end up as a G1 V over its 10 billion year main sequence life span. More massive stars can go from one spectral type to another (about 10 'numbers') but do it more quickly because their MS lifespans are much shorter.

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PostPosted: Tue May 07, 2019 11:09 am 
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Thanks for the info, fellas!


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PostPosted: Wed Jan 01, 2020 3:58 pm 
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I thought I might resurrect this thread to keep some of this info all in the same place.

Also, hello and happy new year to you all! Today being a puttering-around kind of day, it naturally made a lot of room for science thoughts, and I can't find a satisfying answer anywhere else.

I know from various things I've learned here and elsewhere that a star will slowly increase its luminosity during its main sequence.

And I know that luminosity is determined by a formula that I'm not going to reproduce here that involves temperature and radius.

So my question is: what changes during the main sequence? temperature, radius, or both?

Thanks in advance, and I hope you all have a terrific 2020.


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PostPosted: Wed Jan 01, 2020 8:07 pm 
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Both to some extent — a star's spectral type does change a bit through the course of its main sequence life — but mostly radius. They get bigger while not changing their colour/temperature very much.


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PostPosted: Fri Jan 03, 2020 1:33 pm 
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Thanks for the quick response!


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