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Comment by kornork | Hacker News Reader

My physics isn't great, but why does it matter that these are white dwarfs?

The article says "researchers studied six white dwarfs whose _strong gravitational pull_ had sucked in shredded remnants of planets" and "if they stray near its _immense gravitation field_, they “will be shredded into dust, and that dust will begin to fall onto the star and sink out of sight.”

Isn't it the case that the gravitation field will be as strong as it was before the star died, or even weaker from blowing off its outer layers?

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White dwarfs are interesting because they make up almost all of the current or future outcomes of most stars in the milkyway. -> https://en.wikipedia.org/wiki/White_dwarf

The Roche-limit (the radius at which a satellite would be ripped apart) for the Sun-Earth system is at ~ 550000km. That's inside the current size of the Sun. So if the sun shrank a lot (white dwarfs are basically small, 'dead' stars which shrank to that size after the nuclear fusion stopped which had counteracted the attraction of the star's mass), Earth could actually be in such a low orbit and therefore fall apart due to the Sun remnant's attraction.

-> https://en.wikipedia.org/wiki/Roche_limit

Why would a decrease in the sun's radius cause a change in Earth's orbit? From those to articles it sounds like the mass of the sun will remain about the same, so the orbit should remain about the same. And less mass will cause the orbit to migrate outwards.

Shrinking of the star simply makes it possible to come within the roche limit without being inside the star. The inspiral would happen due to separate processes e.g. friction as the star sheds its envelope or due to interactions between multiple planets.

When the Sun enters the red giant phase, will the radius have expanded enough to consume the earth? I'm trying to remember if that was something my physics professor said years ago or if I heard it from someone less reliable and am merely attributing it to him...

Wikipedia says the sun is expected to expand out to roughly the orbit of Venus (consuming it and Mercury) and turning Earth into a blob of molten rock.

I think it may be that while the field is more or less the same overall, the star no longer occupies as much of it so planets can get closer in where the gradient is steeper (and the tides can then rip the planet apart, giving us the chance to see its contents)

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