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Far, far away from the Earth, there is a nebula. An enormous cloud made of 78% nitrogen, 20% oxygen and 2% other non-lethal materials. These gases swirl around a star, and the pressure around it is more or less 105 pascals. This star has several planets around it. In some of those planes, life sprouts. In a few, intelligent civilizations rise. They, each in their own way, develop flying machines. With these wonderful ship-like machines they can travel between worlds (no need to wear a helmet). The interplanetary sailors even tell tales about monstrous whales flying through the system.

In this world that only exist on my imagination, and now in yours, how many of those things are actually possible?

My main concern in this world is:

Could a star exist with this cloud similar to earth atmosphere around it where a person could breathe without much effort?

Near a planet the atmosphere could be thicker meanwhile in the interplanetary space could be a thinner atmosphere like in a high mountain. This cloud can be as big as you want, but the minimum size should be at least one AU. Because a star and a planet like Earth orbiting it should fit inside the cloud.

BONUS I: Assuming this system is possible; could a ship (like a zeppelin) navigate through worlds? The technology of those civilizations is between century XVIII and XIX.

BONUS II: Could it be possible for those worlds to have something like a continuous electromagnetic-pulse preventing them from develop electronic devices and forcing them to be in an eternal middle ages state? It may come from the star, or radiation from the nebula itself.

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    $\begingroup$ Unfortunately, atmospheric drag would make the planet fall into the star very quickly. The International Space Station orbits at 400 km altitude where atmospheric pressure is almost nothing (= about 100 nPa, one trillion times smaller that what the question asks for), and its orbit still decays by about 2 km / month and needs to be boosted up from time to time... $\endgroup$ – AlexP Oct 8 '19 at 11:39
  • $\begingroup$ What if the air spins with the planet? $\endgroup$ – Miguel NoTeimporta Oct 8 '19 at 11:44
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    $\begingroup$ then you have basically en.wikipedia.org/wiki/The_Integral_Trees. $\endgroup$ – ths Oct 8 '19 at 11:51
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    $\begingroup$ This feels like a question that should be sent to Randall as a what if. But I feel like the answer is "you get a bigger star" or "you get a black hole" $\endgroup$ – Trevor Oct 8 '19 at 17:48
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    $\begingroup$ @Trevor Thus it's perfect as a what-if question since the answer to those is basically always "Everyone dies and it's horrible the whole time" $\endgroup$ – Muuski Oct 8 '19 at 22:16
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Could a star exist with this cloud similar to earth atmosphere around it where a person could breathe without much effort?

Nope. The solar wind and radiation pressure and a combination of planetary and stellar gravity would either blow away or hoover up all the gas cloud in relatively short order. That's why there's a fairly hard vacuum between planets in our own solar system, after all.

You have two possibilities, I think. One, handwave physics away and create a fantastical world... Adam Roberts did this in Polystom. This isn't a bad solution, and does make a lot of other things in your setting straightfoward.

Alternatively, constrain the gas cloud somewhat, remove the requirement for it to fill all the space around the star and every planet and have a way for it to be replenished. Larry Niven did this in The Integral Trees, with a free-fall breathable gas torus in space around a neutron star, continually replenished by the destruction of an associated gas giant. You can't trivially fly through air from a planet to the gas ring, but the setting is a teeny tiny bit more physically plausible. Slightly. You wouldn't be able to seamlessly fly from a planet into the gas cloud, though.

Edit: there is a third option, which is to have a massive freefall habitat with artificial stars and maybe worlds in it. This is the approach Karl Schroeder took with his Virga setting, with a 5000 mile diameter sphere of air orbitting Vega. Again, you can't fly from a real planet into such a habitat without travelling through vacuum first, but it overcomes more of the physics issues that the exposed-to-space gas torus has.

Assuming this system is possible; could a ship (like a zeppelin) navigate through worlds? The technology of those civilizations is between century XVIII and XIX.

Flight through air in freefall or microgravity has some additional challenges not found in a gravity well (lift becomes an inconvenience instead of a necessity, and torque from propellers becomes more problematic, to name but two things), but there's no reason it shouldn't be possible.

Could it be possible for those worlds to have something like a continuous electromagnetic-pulse preventing them from develop electronic devices and forcing them to be in an eternal middle ages state? It may come from the star, or radiation from the nebula itself.

This is an entirely separate question, so I'm not going to answer it here. If you're interested in an anwer, ask it separately.

You should also ask yourself, however, is this really necessary? With the ability to trivially fly up into "space" and build whatever vast floating structures you want, or visit other worlds (are there asteroids in your gas cloud?) and mine or farm them as you see fit, there's plenty of scope for a sprawling low-tech civilisation to last for quite a long time.

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No to the star, Yes to the travel

A star cannot exist in that type of atmosphere for many reasons. Not the least of which is heat. Temperatures range into the millions of degrees, which destroys any chance of a stable atmosphere. Furthermore, any particles in the air would result in continual dust explosions.

This of course does not mention issues with gravity (air does not "sit" in an ethereal plane; the gravitational force of the star and planets would pull the atmosphere toward themselves), nor drag, nor electromagnetism, nor radiation, nor any number of other issues.

That being said, if we hand-wave over these issues and instead simply accept the premise where we have a "space" which is more like an atmosphere between planets, then yes, I suppose a ship could "sail" between planets in one of many different mechanisms. When you have a "space" which is more atmosphere than space, then you have enough particles to breathe, push against, create lift, etc.

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    $\begingroup$ The atmosphere would also make it possible for sound to travel from the star to planets; and stars are actually quite loud! At Earth's distance, the Sun would be over 100 decibels. $\endgroup$ – Skyler Oct 8 '19 at 19:34
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    $\begingroup$ @Skyler According to cdc.gov/nceh/hearing_loss/what_noises_cause_hearing_loss.html 100 decibels is the noise level of a car horn from 16 feet away. $\endgroup$ – Muuski Oct 8 '19 at 22:29
  • $\begingroup$ This is a very good answer too, i've given the check to @StarfishPrime just because their is longer than your answer. Too bad i can't give it to both of you. Thanks for your answer! $\endgroup$ – Miguel NoTeimporta Oct 9 '19 at 7:56
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Standard stars forms from the collapse of hydrogen nebulas. The nebula self-gravity concentrates matter in the center. Friction eventually generate enough pressure to start a nuclear fusion process and the star ignites.

What if we have a nebula made with air? Let's assume that it is spheric and have 1 astronomic unit radius (i.e. $1.5 \times 10^{11}\,\text{m}$) and that each cubic meter of air weights $1.2\,\text{kg}$.

This means that all that air weights this:

$$ 1.2\,\frac{\text{kg}}{\text{m}^3} \times \frac{4}{3} \pi (1.5 \times 10^{11} \,\text{m})^3 = 1.6964 \times 10^{34}\,\text{kg}$$

The Sun mass is:

$$\text{M}_\odot = 1.989 × 10^{30}\,\text{kg}$$

So, converting all the air mass in solar masses we have:

$$ 1.6964 \times10^{34}\,\text{kg} \times \frac{\text{M}_\odot}{1.989 × 10^{30}\,\text{kg}} = 8528\,\text{M}_\odot $$

With all that 8528 solar masses compressed in a volume of 1 AU radius, the result is pretty much obvious now:

$$\text{BLACK HOLE}$$

So, sorry. What you want simply is not possible.

Quoting from this article:

[...] if the CCSN [core-collapse supernova] mechanism lacks efficacy and fails to revive the shock and continued accretion pushes the PNS [protoneutron star] over its maximum mass. In this last channel to a stellar-mass BH, there is no electromagnetic (EM) signal other than the disappearance of the original star.

It is way beyond the maximum mass, so it will collapse directly to form a black hole.

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  • $\begingroup$ You don't get a black hole just from mass... you get a black hole from mass that cannot fuse to release energy. As this nebula collapsed, it would eventually begin to fuse despite being mostly nitrogen and oxygen. However, it wouldn't take all that long, relatively speaking, before it all fused to Iron, and a supernova collapsing to a black hole became inevitable. $\endgroup$ – Monty Wild Oct 9 '19 at 5:42
  • $\begingroup$ @MontyWild Yes, it might glow as a very odd star by a very short time. But even the most massive known star R136a1 has only 315 solar masses with something like 30 solar radius. Also, this thing have no appreciable quantity of hydrogen to serve as a decent long-lived nuclear fuel, and even it had, it would be very far from enough to prevent collapse. So, I think that the most probable outcome is collapsing directly to a black hole in a very short time. $\endgroup$ – Victor Stafusa Oct 9 '19 at 6:03
  • $\begingroup$ @MontyWild Technically you can get a black hole from just mass; in fact, a sufficiently large low-density region will form a black hole. But I don't think 1 AU of 1 atm gas is enough. $\endgroup$ – Yakk Oct 9 '19 at 13:33

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