# A completely alien sky

• Writing: Input Wanted

## Main Question or Discussion Point

How far in the Milky Way would one have to travel to land upon a planet where the night sky would have zero familiar stars? Basically, I'd like to set up a world somewhere so far from our section of the galaxy that the only things that are the same are distant galaxies (which I'm assuming would be visible from anywhere in our galaxy).

Thanks!

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jedishrfu
Mentor
You don't have to go far. I went to New Zealand and everything looked totally different. It took me awhile to realize the constellations I knew were upside down.

I imagine if you went to the planet orbiting Proxima Centuri there would be enough variation and thats 4.5 LY away.

I think you'd need to look at the stars in the well-known constellations determine their distances and then move away a comparable distance.

http://www.space.com/3380-constellations.html

I guess I need to clarify. I want this to be a planet that is still in our galaxy but so far away from Earth that were a human to be instantly transported there, they would not be able to figure out *where* they were in relation to Earth without a lot of observation and an advanced level of astronomical knowledge. I doubt Proximia Centauri would cut it because, even though it's 4.5 Ly from us, it would still have a lot of the same stars and we could use those as reference points to figure out that we'd landed 4.5 Ly from Earth.

If this isn't possible without leaving the galaxy, let me know and I'll work on that aspect of my plot development. :)

Thanks!

Note that stars are dots of light.
You could easily be in a position where you do see a lot of stars visible to Earth, but cannot identify them - because on Earth, they are identified by constellations consisting of both distant and nearby stars. Move a modest distance, and the distant stars are still there, but nearby stars aren't - they are replaced by different stars in different patterns, and that prevents identification of the distant stars.

Bandersnatch
Move 2000 ly in any direction, and 99% of stars we see here will be completely invisible to a naked eye.

If you want closer, install one of the popular planetarium software (e.g. Celestia, Digital Universe), which lets you fly away from the Sun and see how the sky is changing. You'll just stop where you think the sky is garbled enough to be unrecognisable.

If they spectrally analyze stars you'd have to go a good way. If you have a star with a known spectrum and you knew how to determine the center of the galaxy the rest would be an exercise in navigation. (I read a story where the ship was lost until one of the crew noticed that the big star over yonder was Betelgeuse. They were quickly oriented. )

jedishrfu
If they spectrally analyze stars you'd have to go a good way. If you have a star with a known spectrum and you knew how to determine the center of the galaxy the rest would be an exercise in navigation. (I read a story where the ship was lost until one of the crew noticed that the big star over yonder was Betelgeuse. They were quickly oriented. )
How special is Betelgeuse around Milky Way? For example, figuring out whether a red star is Betelgeuse or Antares.

Staff Emeritus
2019 Award
I think it depends on who your traveler is. Similar top a previous comment, if you plop someone who has spent his whole life in NYC to Australia, he will have no idea what planet he's on. If you took a professional astronomer with tools and reference books, you would have to go pretty far.

Half of the brightest stars in the sky are within about 75 light years. Move by that much and most of the sky is now different (particularly if the new planet has a different tilt). Rigel, Betelgeuse, Deneb and Antares won't have moved much, but which four stars are they?

jedishrfu
Mentor
I guess I need to clarify. I want this to be a planet that is still in our galaxy but so far away from Earth that were a human to be instantly transported there, they would not be able to figure out *where* they were in relation to Earth without a lot of observation and an advanced level of astronomical knowledge. I doubt Proximia Centauri would cut it because, even though it's 4.5 Ly from us, it would still have a lot of the same stars and we could use those as reference points to figure out that we'd landed 4.5 Ly from Earth.

If this isn't possible without leaving the galaxy, let me know and I'll work on that aspect of my plot development. :)

Thanks!
You could make it simple and say the traveller is transported to the other side of the galaxy. Everything will be different then and its easy on the reader to comprehend.

You could make it simple and say the traveller is transported to the other side of the galaxy. Everything will be different then and its easy on the reader to comprehend.
And it´s fairly easy to verify, enjoying the panorama of Milky Way. From Earth:
https://commons.wikimedia.org/wiki/File:ESO_-_Milky_Way.jpg
Note:
1) It is easy to spot the bright centre of Milky Way if you know what to look for.
2) The Magellanic Clouds are big and distinctive - anywhere in Milky Way.
If Magellanic Clouds are not where they should be - the bands both sides of Milky Way - then you are not in Milky Way.
If you have found the centre of Milky Way and found and identified the Magellanic Clouds, you know which sector of Milky Way you are in. Then the next step is identifying closer pointers.

You might also consider that the number of stars you see in the sky roughly depends on how far you are from the center of the galaxy. Asimov made a point of how few stars were in the night sky of one of the most distant worlds (maybe Haven?) in the Foundation trilogy.

DHF
Just make sure your plane is in relatively the same position in the galaxy. You can move it thousands of light years but you want it to be about the same distance from the central bulge and in between the arms of the Galaxy. This is important because much like solar systems, the Galaxy also has a habitable zone. move too close to the center and its likely your planet would be sterile from the radiation. if you put it in the arms instead of in between, its likely your planet would suffer frequent bombardment from dust and derbies.

This is important because much like solar systems, the Galaxy also has a habitable zone. move too close to the center and its likely your planet would be sterile from the radiation. if you put it in the arms instead of in between, its likely your planet would suffer frequent bombardment from dust and derbies.
I find that very interesting. For one, I don't recall it being a consideration in the Petigura et al estimate of billions of habitable worlds in the Milky Way. Do you know how close to the galactic center radiation becomes a clear and present danger?

It also brings to mind having read that the sun weaves in and out of the galactic plane (I think it was in a review of Randall's Dark Matter and the Dinosaurs). I am not sure of the time scale for the shifts, but that might have bearing on the original poster's question.

(Edit for clarification: Petigura et al estimated 40 billion Earth-like planets in habitable zones of Sun-like stars.)

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Could you reduce the threat from radiation by putting some nebulae between you and the galactic center?

Could you reduce the threat from radiation by putting some nebulae between you and the galactic center?
No, nebulae are far too thin to block any amount of that. They're essentially invisible to high energy radiation.

No, nebulae are far too thin to block any amount of that. They're essentially invisible to high energy radiation.
Wads up plans for a stellar refuge and throws it away.

Wads up plans for a stellar refuge and throws it away.
Lets be more specific about what the threat is. From my understanding, when the black hole eats something, an accretion disk forms and pumps out x-rays (and gamma?). We need to know how intense that radiation is and the max the life can theoretically take.

Another problem is that the center of the galaxy is chaotic and mostly old dying stars and huge new ones. The stars are densely packed so would have a hard time holding onto planets due to gravitational forces of the other stars. If the plants don't get torn away or right, they'll likely migrate constantly. There are not many ingredients for planets because there are lots of big stars moving around very fast so they vacuum the area pretty thoroughly.

These big stars also blow up a lot, so having a biosphere there for billions of years would be dumb luck. Supernovas can sterilize many nearby systems when that densely packed. Like the black hole, they spit out gamma and X-rays which both destroy life and strip atmospheres.

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Not to fret, I had an interesting thought. Intelligence probably wouldn't evolve there, but there is no reason it might migrate there. Any species with the ability to migrate and colonoize would have the observational and math skills to predict far ahead of time when the galaxy poses a threat and protect themselves.

How much more than the existing level of cosmic rays could Earth atmosphere stop without problems?

How much more than the existing level of cosmic rays could Earth atmosphere stop without problems?
A supernova would remove our atmosphere from a distance of about 30ly, a gamma ray burst would destroy it from 10K ly.

It's not the cosmic rays, those come from deep space and are fairly constant. it's the once-in-a-while catastrophic events that will just sterilize anything that got started.

How far from Sun would a supernova have to be to match the Carrington Flare of 1859?

I'm sure if you calculate the power of the flare and look up the average luminoscity of a supernova you could calculate it but I'm. It sure t would matter to you. A flare is a slow moving group of charged but low energy particles. These get trapped in our magnetosphere and are funneled to the poles, that produced enough electromagnetic flux to bring down telegraph wires, but I doubt it harmed anyone or had much effect on the atmosphere. Supernova shockwaves are moving at relativistic speeds and have are high energy.

There are no star systems anywhere near our own Sun that is a likely candidate for a supernova, it's very very unlikely.
A direct hit on Earth of a major flare from the Sun could definitely pose problems for communication systems, and could happen any time.

There are no star systems anywhere near our own Sun that is a likely candidate for a supernova, it's very very unlikely.
A direct hit on Earth of a major flare from the Sun could definitely pose problems for communication systems, and could happen any time.
The good news there is that a CME could take off in any direction, so the odds of it hitting Earth are rather low.