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Old 01-11-2020, 04:38 AM   #1
GWJ
 
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Default [Space] star radius

For what is star radius (p. 104) in the Advanced Worldbuilding Sequence? I don't see it used anywhere
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Old 01-11-2020, 05:23 AM   #2
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Default Re: [Space] star radius

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For what is star radius (p. 104) in the Advanced Worldbuilding Sequence? I don't see it used anywhere
Sometimes, people just want to know how big the star is.
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Old 01-11-2020, 05:26 AM   #3
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Default Re: [Space] star radius

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For what is star radius (p. 104) in the Advanced Worldbuilding Sequence? I don't see it used anywhere
It's the actual radius of the star, which increases as the star grows brighter during its main-sequence lifetime.

It's not used anywhere in the star generation sequence itself, but it can be of interest to some users. I use it to figure out how large the star appears in the sky from each planet and moon, as an aid to setting scenes. In a Traveller campaign it would be important to interstellar travel, because the J drive in the Traveller setting only works if you are farther from the star than 100 times its diameter (and from each planet that 100 times its diameter).
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Old 01-11-2020, 05:48 AM   #4
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Default Re: [Space] star radius

Ok, another 2 quick questions I just wondering about

1. The most massive star in my solar system happened to be white dwarf. Should I still treat it as the main star, or replace stars and make it just a companion (it's 4-solar system) if it's not most massive anymore?

2. Should white dwarf have any planets?
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Old 01-11-2020, 06:24 AM   #5
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Default Re: [Space] star radius

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1. The most massive star in my solar system happened to be white dwarf. Should I still treat it as the main star, or replace stars and make it just a companion (it's 4-solar system) if it's not most massive anymore?
That's a bit of a tricky one. Everything's orbit around the former main star will have changed, moving outwards as the former main star's mass diminished. Distant companions might even have escaped. But I can't say what will have happened because it all depends on details that aren't available.

Furthermore, "main star" isn't really a physics thing or an astronomy thing, it's just a starting point for the random generator.

What I suggest that you do is make the surviving star that is now most massive the main star, and set the distance of the white dwarf's orbit around it to the old value of its orbit around the former main star times the mass of the former main star divided by the remaining mass of the white dwarf. Then increase all other orbits around the white dwarf by the same factor.

That's kind of arbitrary, but then it's a rather random process.

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2. Should white dwarf have any planets?
Planets have been detected orbiting pulsars, which are a kind of supernova remnant, so there is no reason to suppose that white dwarfs — which are merely red giant remnants — can't have planets of some sort. On the other hand the GURPS Space star system generation sequence is not going to be suitable for generating the planets in such a system. The problem is that a white dwarf is the low-luminosity remnant of a star that was once much more luminous¹ — a red giant or perhaps a supernova. The system that the GURPS Space star system generation sequence uses for working out what atmosphere a planet has is designed on the assumption that the blackbody temperature has been constant or steadily rising. In any system that has a white dwarf in it all the planets will once have been much more brightly illuminated and much hotter. The envelope of the red giant will have occupied the inner system presented drag that will have caused the inner planets to spiral in to their destruction, and the outer system will have been swept by expanding planetary nebulas. Gas giants might survive, but any terrestrial planets and moons will have been stripped of their atmospheres and probably most of their ices.

The system will most likely contain only gas giants and airless rocks. If it has been through a supernova perhaps even the gas giants will be reducing to their rocky cores.

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¹ Theoretically a red dwarf less that 0.5 M[sub]☉[/sub] will develop into a white dwarf without becoming a red giant, but (a) it will become a luminous blue dwarf first, and (b) that will take a very long time: the universe is too young to have any such any such white dwarfs in it.
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Old 01-11-2020, 10:22 AM   #6
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Default Re: [Space] star radius

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2. Should white dwarf have any planets?
Besides everything Agememos mentioned white dwarfs will trend towards being young stars. Sirius B is probably only 240 million years old and went off the Main sequence 120 million years ago after starting out as a B-class star of 5 Solar masses.

Big stars burn faster and stars that have burnt out have a certain likelihood of being formerly big stars.

You could have a white dwarf that used to be about the size of our Sun but it would have needed to have formed in the very early period of the Milky Way to have run through its' 10 billion year life span. Its' planets would be correspondgly old as well.

It would also probably have needed to be a third generation star (child of two previous supernovas) to have rocky planets in spite of the early age of its' formation. This might be possible as supernovas may have gone off like popcorn in the very earliest time periods.
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Old 01-11-2020, 10:43 AM   #7
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Default Re: [Space] star radius

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The problem is that a white dwarf is the low-luminosity remnant of a star that was once much more luminous¹ — a red giant or perhaps a supernova.
I don't think white dwarfs can be supernova remnants. Also, note that the red giant will have affected planets orbiting other stars in the group if they're close by (the red giant was likely thousands of times brighter than the other stars, so if the planetary orbit was is more than a couple percent of the distance to the red giant, the red giant will have been providing more heat than the main star).
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Old 01-11-2020, 11:41 AM   #8
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Here is a bit from my Pyramid article Omicron Polypi. I did the analysis in 2006 but the article file is dated 2009; I assume it was published in that time range.

Over the course of its lifetime, Omicron Polypi B went through several stages with different luminosities. As a main sequence star and a subgiant, it had luminosity 4.5. When it became a giant, its luminosity was 25 times as high, or 112.5. Now, as a white dwarf, it has luminosity 0.001.

This drastically altered the conditions on all of its planets. But the changes were biggest on its third planet, Hector.

Hector is a large planet, orbiting 0.77 AU from its star. During Omicron Polypi B’s early life, this gave it a blackbody temperature of 462 K, which made it a greenhouse planet. But when Omicron Polypi B became a giant, Hector’s blackbody temperature rose to 1,032 K, well into the chthonian range. The intense heat drove away Hector’s atmosphere, leaving it with only a trace of gases replenished by solar wind. At the same time, much of Hector’s crust melted. Then Omicron Polypi B diminished to a white dwarf, dropping Hector to its current blackbody temperature of 56 K. Normally this would make it an ice world, but with its atmosphere already driven off, it has nothing to form ice with. Instead it becomes an anomalously cold rock world. The other planets that orbit Omicron Polypi B had comparable, but less extreme histories because of their lower mass.
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Old 01-11-2020, 12:26 PM   #9
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Pulsar planets are most likely captures (they are too close to have formed at their current radius, and the loss of mass during a supernova would have had any surviving planetary cores migrate outwards rather than inwards). The few planets we have found around white dwarfs are likely captures as well.

With the age of the Milky Way being ~13 billion years, there was plenty of time for supernova cascades (chains of supernovas that lead to star formation, which leads to supernovas, whuch lead to more star formation) to occur during the first billion years. 100+ solar mass stars burn through their fuel quickly and explode in less than 100,000 years, creating the shock waves that would lead to further stellar evolution. Even if you had 10 million years between supernova cascades, the Milky Way could have had dozens of supernova cascades before everything calmed down (10,000s of supernova per year during the supernova cascades).

After the first billion years, there would have been more than enough deuterium and helium-3 having been consumed to slow down the process. Metals would have formed planets, either rogues forming in interstellar space or bound planets forming around smaller stars. As the first white dwarfs formed around two billion years after the end of the supernova cascades, they would have captured rogue planets or stolen bound planets. Some of them would be 10 billion years old now.
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Old 01-11-2020, 02:15 PM   #10
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Default Re: [Space] star radius

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I don't think white dwarfs can be supernova remnants.
Yes, that's a mistake on my part.

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Also, note that the red giant will have affected planets orbiting other stars in the group if they're close by (the red giant was likely thousands of times brighter than the other stars, so if the planetary orbit was is more than a couple percent of the distance to the red giant, the red giant will have been providing more heat than the main star).
Quite right. I meant to imply that, and should have been more clear.

The ideal thing to do would be to calculate the maximum luminosity and radius of the star at its red giant maximum, remove everything that is inside it, generate the whole system by calculating all the planets' and moons' black-body temperatures from combined insolation at their closest approach to the red giant, add the effect of the immense solar winds during the formation of the planetary nebula. Then alter the orbits, calculate the luminosity of the white dwarf, calculate the new values of insolation and black-body temperature, adjust the atmosphere types and masses to account for anything that condenses or freezes out, and calculate the new surface temperature.

Unfortunately GURPS Space doesn't provide most of the tools for that. And even if you figure out how to do it yourself the results are pretty crude.
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