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I need a bit of help with this. I have made a universe where there is one star and one nearly dead star and the big star has either one or two living planets in orbit around it.

I don't know if its possible for a star to have a white dwarf and two living planets orbit it. Will the stars collide even if the second star is barely a star anymore?

Details:

  • the white dwarf should not be able to give up enough heat to warm up the planet with the sun but still close enough to the first planet so the main star and the dwarf star look like eyes in the sky. The dwarf star might not be visible on the second planet.

  • the first planet is either same size or slightly smaller than Earth. The second planet is bigger than Earth.

  • how far away are the two planets are from the main star, the dwarf and each other is still unknown.

EDIT; people have asked me what type of Star/sun I am asking about and I am not expert in stars in any way, but the sun is like ours while the other one is seen as a sun to a certain degree but dose not gives up enough heat to fry the planets.

I saw some correct me with the Brown Dwarf and said I might looked for a white dwarf. I looked at Wikipedia for the difference and I didn't understand anything that site told me. can someone tell me what the different is between a Brown Dwarf and a White Dwarf? I can tell you right now, I'm not looking for a death/dying sun/star since I don't want the second star to ever have been a sun.

also, sorry you all; I really like a lot of your answer but few of you talks way too complicated for me. I am unfortunately not that smart and have hard time following up what you are trying to say to me. I really wished I knew what you said but I can't figure out what you meant. if it is possible, can you make your answer for dummies?

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    $\begingroup$ en.m.wikipedia.org/wiki/Binary_star this might get you up to speed. I personally see no reason you can't have an old star-new star in theory. It's just gravitational pull. In practice it might be difficult, but either one of the stars entered or maybe one was siphoning stuff from the other to reduce the lifespan and increase It's own. $\endgroup$
    – Trioxidane
    Dec 15 '20 at 12:25
  • $\begingroup$ "I think they are called brown dwarf, correct me if I am wrong" you are wrong, brown dwarfs are stars that never ignited. your looking for a white dwarf. $\endgroup$
    – Topcode
    Dec 15 '20 at 12:57
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    $\begingroup$ I'd suggest any recent introductory astronomy text. OpenStax has a free downloadable one: openstax.org/subjects/science Bottom line is it's possible, but a lot depends on the precise sizes and positions of the stars. $\endgroup$
    – jamesqf
    Dec 15 '20 at 17:49
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    $\begingroup$ I wish to point out that the edits made to the post, while cleaning up spelling, have also changed the question's meritum. Originally the poster asked about a brown dwarf, rather than a white dwarf, incorrectly presuming it to be a dying star - without input from the poster we cannot be sure whether they meant a brown dwarf, a white dwarf or an actual dying star - which the white dwarf is not, it is a stellar remnant. Additionally the term "dwarf star" can be confusing, as it is generally used to describe small size main sequence stars which neither the brown dwarf nor white dwarf are. $\endgroup$
    – JANXOL
    Dec 15 '20 at 22:04
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    $\begingroup$ "I don't want the second star to ever have been a sun". In that case it seems like you're looking for a brown dwarf - a substellar object, a "failed star". A white dwarf used to be a star which then... "ejected its outer layers". A brown dwarf appears to fit your requirements. $\endgroup$
    – JANXOL
    Dec 16 '20 at 15:08
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Firstly, the term "star" is correct. The Sun is a star. And any star can be a sun (notice the lowercase) to planets in its system.

Then onto your system:

A brown dwarf is not a what you think it is. It is a substellar object that never was and never will be a star. It is commonly called a "failed star". It is too small to sustain hydrogen fusion. If you are looking for a dying star explicitly, you are looking for a red giant or red supergiant. Stars tend to get larger in diameter when they're nearing death. Alternatively, you could have a white dwarf which is a remnant of a dead star after it already "explodes". You can of course still use a brown dwarf, just be aware it is not a dying star.

In any case, what you are asking for is a binary system, which means it has two stellaror substellar object in it. That is indeed possible, regardless of the combination of bodies you select (white dwarfs have a "tendency" to siphon mass from certain stars which is whole phenomenon of its own, so I won't get into it). In binary system the two bodies orbit around a barycenter, location of which is determinant on the mass difference between them.

Binary 1Binary 2Binary 3

Here are some possible systems with two bodies of differing masses. If they are in a stable system, they will not collide.

Establishing orbits in binary systems can get somewhat complicated, for simplicity we could assume your planets also orbit around that barycenter and by making this assumption we can eliminate the possibility of planets colliding with the smaller star/dwarf.

Both stars will be visible from orbiting planets and they could easily be made to look like eyes, with exception of the part of the year when one star will pass in front of the other (from perspecitve of the planet). The only way to make the smaller star invisible from one of the planets would be to have that planet orbit on the same orbit as the smaller star, just at a phase angle of 180 degrees (which means they will always be on opposite ends of the orbit, with the larger star in between). I'd say this planet would not be very habitable though.

EDIT: After the poster clarified their requirements I am adding this summary:

"I don't want the second star to ever have been a sun" - That means you want the secondary star to be a brown dwarf

As such, you have a binary star system with the primary star being similar to our sun and a secondary star being the brown dwarf. Such a system can exist and the two will not collide. Taking into account the mass difference between the dwarf and the primary star they will likely orbit like you can see in the second picture above. This system can have planets. The stars will be visible from the planets, though brown dwarf emits very little, if any, visible light, so it might be a bit difficult to spot on bright sky (it should reflect the light of the primary star though). Both stars will be visible from both planets, with exception of periods where larger star passes in front of the other (and obstructs vision).

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    $\begingroup$ Having the brown dwarf and the planet always on opposite sides of the sun is an unstable configuration — it would last a few centuries (if that much) but the planet would slowly change its orbit. $\endgroup$
    – Peter Shor
    Dec 15 '20 at 13:17
  • $\begingroup$ Fair point. In this case yes, the only time when one star would be invisible would be when one of them passes in front of another. $\endgroup$
    – JANXOL
    Dec 15 '20 at 13:23
  • $\begingroup$ This poster wants a visual image of "close enough to the first planet so the sun and the dwarf looks like eyes in the sky", so a nice close binary like the first image here, with the smaller star being a "failed" brown dwarf even though it is 50-60% as large diameter as the main star. (maybe 10% of the mass) $\endgroup$
    – PcMan
    Dec 15 '20 at 13:36
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The distances between two stars in a binary system can vary greatly.

Two stars can orbit each other with a separation only a few times as great as their diameters.

In fact, two stars can be so close together that they actually touch each other.

https://en.wikipedia.org/wiki/Contact_binary[1]

Other stars orbit each other at vast distances. The system of HD134439 and HD134440 has a separation of about half a light year between the two stars.

https://en.wikipedia.org/wiki/HD_134439/HD_134440[2]

And the separations of other binary stars vary between those two extremes.

A planet in a binary star system can either orbit one of the stars in an S-type orbit or orbit both of the stars in a circumbinary or P-type orbit.

https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems[3]

The closer the two stars are to each other, the more likely it is that any planets in the system will orbit both of the stars in P-type orbits. The farther the two stars are apart, the more likely it is that planets will orbit either one star or separately around each of the stars in S-type orbits.

It is perfectly possible as far as orbital dynamics are concerned to have a binary system where there are planets in close S-type orbits around each of the stars and other planets in distant P-type orbits around both of the stars.

Each star, or close binary pair of stars, has a circumstellar habitable zone around it where the temperatures of orbiting planets would permit surface water to be liquid.

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone[4]

Because of the narrowness of circumstellar habitable zones, there very probably could not be habitable planets in both the S-type orbits and the P-type orbits. So writers of stories with one or more habitable planets in a binary system have to choose whether those planets orbit one of the stars in S-type orbits and the other star is much more distant than the planets, or the planets orbit both of the stars in P-type orbits several times as far away as the stars are from each other.

If two stars in a binary system appear to have the same angular diameter as seen from a planet, either they have the same physical diameter, and are the same distance wawy, making the planet have a P-type orbit around both, or have greatly different diameters, with the smaller star being closer to the planet, close enough to appear as large as the farther star, thus making the planet have to orbit the smaller star in an S-type orbit.

Of course the two stars do not need to have the same apparent diameter as seen from their lanets, whether those planets are in S-type orbits around one star or P-type aorbits around both stars.

There are many binary or multiple star systems where the stars are separated by such vast distances that from a planet orbiting one star the other star would appear as a dimensionless dot of light, a very bright star, instead of like a sun with a visible disc.

There are limitations on the types of stars which can be within a system with habitable planets. Most binary or multiple star systems have stars of the same age because they formed together. Only a tiny minority of binary or multiple star systems should contain stars of greatly different age which happened to capture each other long after they formed.

More massives stars have more nuclear fuel, but they have to consume that fuel much faster. The increase in the rate at which fuel is burned is much greater than the increase in fuel reserves, so more massive stars use up their fuel sooner than less massive stars.

A star needs to shine relatively steadily in the main sequence stage of its development for billions of years in order for a planet with life to develop conditions - such as an oxygen rich atmosphere - suitable for the survival of humans or other beings with similar environmental requirements.

Stars which have passed out of the main sequence stages of their development - stars such as red giants and supergiants, white dwarfs, neutron stars, and black holes - are likely to have damaged, and possibly totally destroyed, any once habitable planets which once orbited them. And if those stars are close enough to other stars in their systems they might have damaged and even totally destroyed any planets which orbited those other stars in the system.

And there are other requirements for stars to have habitable planets. Unless a writer writes about a dead and lifeless planet, or one with only primitive lifeforms and no large plants or animals, or one where life has a totally alien biochemistry, they need to limit the types of stars in their fictional star system to types of stars compatible with planets habitable for humans and beings with similar requirements.

So a writer of stories set on planets of other stars should seek out and read Habitable Planets for Man, Stephen H. Dole, 1964, 2007 to study the various requirements, including stellar type, for a habitable planet.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[5]

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"Star" and "Sun" are often used interchangeably, so that doesn't really matter.

For the main question; Yes, it is possible to have two stars close together, and also to have one of them being nearly 'dead' with the other still going strong. It might be extremely rare, but there is no real reason why it would be impossible.

Now, as to a number of planets orbiting just one of said stars; that is a different story. Any planets would orbit both stars together, either in a big circle around both, or in some kind of figure-8 pattern looping in between them

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