Random question about a hypothetical alien planet

[Arthur_Tuxedo]

I was thinking recently about superluminous stars that emit thousands of times more light than our sun, and wondered if that meant that their habitable zones could be in another star system. Would it be theoretically possible to colonize a planet outside of its own star's habitable zone but that gets its heat from a blue supergiant in the next system? If so, what would such a star look like to that planet's inhabitants? It would be so far away that it must only be a dot in the sky, yet a dot capable of lighting up the entire planet. Also, day / night cycles would be weird since the planet would not orbit its primary source of light and heat. Is this a clever setting for a sci-fi colony, or am I just misunderstanding how all of this works?

[Borgholio]

Even blue supergiants don't get THAT big. If our sun was replaced with one of the largest blue hypergiants we know of, such as the Pistol Star (150 solar masses) then the habitable zone would be out past the orbit of Jupiter or Saturn. A red hypergiant nearing the end of it's life will have a radius that actually goes out to Jupiter, so the habitable zone might even extend to Uranus or Neptune...but still well within the solar system.

[Iroscato]

Even with the most massive and powerful stars, the heat they generate would be so spread out across interstellar distances that I don't think you'd get any appreciable heat from them.

Not sure if even a supernova would keep the chill out of the air

[Borgholio]

Not sure if even a supernova would keep the chill out of the air

A Gamma Ray Burst would keep you nice and toasty.

[Arthur_Tuxedo]

What about WR-25? Appr. 6.3 million times brighter than the Sun, so unless I'm missing something, it should provide the same light and heat at 6.3 million AU's as the Sun does at 1 AU, which is about 99 light years.

[Borgholio]

Stars have to obey the inverse square law. This means that for every unit of distance away you travel from the star, the energy received drops by a factor of 4. So it's not linear. Being 6 million times brighter than the sun will only push the habitable zone out to Jupiter...maybe Saturn.

[Jub]

Even with the most massive and powerful stars, the heat they generate would be so spread out across interstellar distances that I don't think you'd get any appreciable heat from them.

Not sure if even a supernova would keep the chill out of the air

What if you were in the path of a high-frequency pulsar that splashed you with energy ever minute or so?

[Arthur_Tuxedo]

OK that's the part I was missing. Thanks for the clarification!

[Borgholio]

What if you were in the path of a high-frequency pulsar that splashed you with energy ever minute or so?

The energy released by a pulsar is fairly intense, but it's not enough to provide heat to keep an Earth-like planet alive unless you were so close that you'd be fried by the x-rays and gamma rays coming off the thing.

OK that's the part I was missing. Thanks for the clarification!

Np.

[Jub]

What if you were in the path of a high-frequency pulsar that splashed you with energy ever minute or so?

The energy released by a pulsar is fairly intense, but it's not enough to provide heat to keep an Earth-like planet alive unless you were so close that you'd be fried by the x-rays and gamma rays coming off the thing.

I'm picturing a planet once in the habitable zone that survived being pushed out by a star too small to go nova. Something around 1.8 earth masses, that still has an active core covered by a thick venus-like atmosphere, only less toxic. The Pulsar isn't exactly keeping it warm, so much as providing energy, which is in turn used to keep the planet warm.

[Eternal_Freedom]

More to the point, even if such a supermassive star were to have it's habitable zone in another solar system, that star would have such a wickedly short lifespan that no habitable ecosystem could develop on world's it's illuminating before it went kaboom.

And if the star is big enough to have a habitable zone several light-years away, it going kaboom will be a stupendous event. Since normal core-collapse supernovae can outshine whole galaxies for a week or so, I emphatically do not want to be within such a giant star's habitable zone when it happens.

[SpottedKitty]

More to the point, even if such a supermassive star were to have it's habitable zone in another solar system, that star would have such a wickedly short lifespan that no habitable ecosystem could develop on world's it's illuminating before it went kaboom.

Exactly — dwarf stars like ours have a friendly-to-life time of billions of years. Super-massive giants go through their entire lifetime in only a few million years, and don't stay on the stable part of the Main Sequence for long. Nowhere near long enough for evolution to get its toe in the door.

[Lord Revan]

I thought the Sun was a low end main sequence star not a dwaft?

In fact I though "dwarf star" referred only to stars that had gone nova but weren't massive enough to fully implode and become black holes.

[Borgholio]

Most dwarf stars are actually part of the main sequence...only white dwarves are in their own category, IIRC. A dwarf star is just one that is smaller than average. Our sun is pretty tiny compared to some of the behemoths out there.

[Purple]

So what is the largest possible star that has a lifespan long enough for life to be a real thing?

[Eternal_Freedom]

Sometihng not much bigger than the Sun frankly. IIRC, main-sequence lifetime is estimated by:

Lifetime of Sun/(star mass^3)

When the star mass is in solar masses anyway. So from looking at that (and the 10 billion year estimated lifespan of the Sun) we see that a 2 solar mass star will live for about 1.25 billion years. A 10 solar mass star will live for only 10 million years. Please note this is a very rough guestimate, but it gets the point across. Big stars do not live long.

[Iroscato]

Stars have to obey the inverse square law. This means that for every unit of distance away you travel from the star, the energy received drops by a factor of 4. So it's not linear. Being 6 million times brighter than the sun will only push the habitable zone out to Jupiter...maybe Saturn.

That's the one. You'd think after five years of being here I'd remember something as rudimentary as "inverse square law". Alas!

[Eternal_Freedom]

Hmmm...I am now tempted to try and work out just how bright this star would have to be to have a habitable zone in another star system.

Based on the assumption that we want roughly the same energy per square metre as we do at Earth distances (1.4 kW/m^2) and that the distance is, say, five light-years...I get a luminosity of ~4e37 W.

Fuck a duck. For comparison, thats 11 orders of magnitude greater than the Sun.

[Purple]

What would be the minimum habitable distance from this thing?

[Eternal_Freedom]

I don't know the formula to work that out I'm afraid, I'm just doing a back-of-an-envelope guestimate based on having the same energy per square metre.

However, this hypothetical star has a luminosity that is an order of magnitude greater than the Andromeda galaxy, so I think we can assume it cannot exist.

[Borgholio]

What would be the minimum habitable distance from this thing?

To expand on what EF said, stars bigger than about 150 solar masses will literally tear themselves apart due to the amount of energy they create.

[Purple]

Well yea, but like say it was an artificial construct made by say enclosing a bunch of stars inside some sort of strange insane apparatus. It can happen.

[Eternal_Freedom]

Um, that basically requires god-like technology. Especially since you'd need something like ten galaxies worth of stars to do it. There's no point for such a construct. Especially since before very long (astronomicaly speaking) one of those stars will supernova and utterly fuck up your apparatus. I seriously have no idea how you would begin to cram so much mass into such a small volume.

Hell, you'd be well over the Schwarzchild limit and the whole thing would collapse into a truly monstrous supermassive black hole. One that your habitable planet is now orbiting. At five light-years. Which is bad.

Hell (again)m just getting something to orbit something that massive at a (comparatively) short distance is an impressive feat, it would have to be moving very quickly indeed.

[Simon_Jester]

It's worth remembering that any exotic, ultra-intense stellar light source (pulsars, blue hypergiants, etc.) will be producing most or all of its energy as hard radiation. Bombarding a planet with that much of that sort of radiation virtually guarantees that if it ever supported life, it won't do so for very long.

I would expect that the ionizing radiation will cause the very atmosphere to become radioactive, strip away anything like an ozone layer, and render the planet uninhabitable in short order.

Basically it's like saying "well, what if instead of using the Sun to light up the Earth, we just detonated bombs on the surface of the Earth all day long, one after another?" Sure, the bombs could release enough energy to warm the planet the way the Sun does. But in the process they'd cause so much devastation and damage that no one could live on it.
______________

Also, Borgholio's math is off, though his top-level conclusion (no way can even the brightest star-like objects ever seen provide enough light for a planet at a range of several light-years) is correct.

Stars have to obey the inverse square law. This means that for every unit of distance away you travel from the star, the energy received drops by a factor of 4. So it's not linear. Being 6 million times brighter than the sun will only push the habitable zone out to Jupiter...maybe Saturn.

Er, not quite. What it means is that if you increase the distance by a factor of X, the amount of energy you get from the star decreases by a factor of X times X.

If the Sun were 25 times brighter, it would appear as bright around the orbit of Jupiter as it does now around the orbit of Earth. Correspondingly, at the orbit of Earth it would appear about as bright as it now appears at an orbital radius of 0.2 AU... which means that the Moon would look like a hotter version of Mercury, and the Earth would look like a hotter, bigger version of Mercury.

If the sun were in fact six million times brighter, then the habitable zone would be about, oh... 1000 times the square root of size farther away. Which is, what, 2400 AU out? That's still nowhere near the next solar system, though; it's only about twelve light-days.

[One light-day is about 200 AU, and one light-year is of course roughly 365 light-days, so one light-year is over seventy thousand AU]

[Napoleon the Clown]

https://en.wikipedia.org/wiki/Hertzspru ... ll_diagram Handy reference-sheet for the kinds of stars out there.


The sun on the small end of what stars can attain, but it's bigger than most of the stars out there right now. That's largely because most stars are in the red dwarf range, bottoming out at around .08 solar masses. The bigger the star, the shorter (and more violent) the life.

The only way I can see you having another "star system" become habitable due to an enormous star is if you had a very large brown dwarf, or maybe the smallest possible red dwarf, in orbit of one of these monster stars. And then the planet that can support life would be relatively near the brown dwarf, likely fated to become tidally locked if it hadn't happened already. I can't be bothered to crunch any numbers on how this could be accomplished and the system still be stable, but I see no immediate reason why it couldn't happen. Exceedingly unlikely to happen, but vaguely possible.

Downside here, though, is such a system wouldn't stick around for long, even if the orbits were all stable. Eta Carinae is maybe the biggest star we know of, sitting at somewhere in the region of 150 solar masses. We think. There's a ton of hot gas around it, obscuring accurate measurements of the orbit of the baby star nearby and the oscillation of Eta Carinae itself. It's tossing off obscene amounts of matter all the time, and a few centuries back it had a massive temper tantrum that astronomers of that era took notice of.

Regardless of exact sizes needed, it would definitely have to be a star far more massive than our sun in the center of this system. And as I mentioned above, bigger means a much shorter life. The core of a 2 solar mass star is bigger than the core of our sun, but it isn't twice as much mass. And fusion is happening in there are a fantastically higher rate. So while it has more fuel, and can fuse heavier elements than our sun, it's going to burn through that fuel so much faster that it's going to die very, very young. Each "stage" of fusion lasts around half as long as the one before it, until you hit iron and then fusion stops entirely. And that is... not pretty.


So to answer your question, Arthur: If you consider a brown dwarf in orbit of a super/hypergiant to be a parent star of an object big enough to be pulled into a roughly spherical shape, it's technically possible to have a star provide a goldilocks zone for another star. But you're playing fast and loose with definitions, and the system will not last long enough for life to arise at all, much less evolve into anything approaching complex. It took roughly a billion years for life to even arise on Earth, and the system I describe would have an absolute maximum life expectancy in the millions of years range, with it being quite probably less due to how much mass enormous stars like to throw into space causing the orbital system to become... questionable.