# What would the night sky look like if Earth orbited an intergalactic star?

If the Earth and its host star were located exactly in-between the Milky Way and Andromeda would the night sky be completely void of light?

Would stars be visible to the unaided eye?

So there could be a civilization that exists in which the night is completely void of light? …that would suck

• – ProfRob Feb 13 '15 at 9:02
• Good responses to this question focuses on galaxies and objects of the same star system, but they don't mention other stars. Are stars so rare in intergalactic medium that it's unlikely that from one intergalactic star planet you could see other intergalactic stars with naked eye? – Pere Sep 17 '16 at 11:58

Since Andromeda is already visible to the naked eye, to a civilization located at half the distance from the Milky Way, Andromeda would be still be visible. Its total brightness would be four times higher, but since its area would grow by the same factor, its surface brightness would stay constant. The Milky is less bright by a factor of ~2.5, but also smaller, so the surface brightness and hence visibility would be similar.

Individual stars would not be visible except if 1) one exploded as a supernova or 2) the civilization has evolved into beings with extremely large pupils, so that more light is being recorded by their retinas. In fact, we can calculate how large:

According to this list, the brightest star known in the Milky Way is called WR 25, and has an absolute magnitude of $M = -12.25$. Half of the distance from the MW to Andromeda is $d \sim 390,000$ pc, so the distance modulus is $$\mu = 5\log d - 5 = 22.95,$$ and the apparent magnitude of WR 25 would be $$m = M + \mu = 10.7.$$

Under the darkest conditions, humans are able to see objects down to an apparent magnitude of ~6, i.e. a factor of $$f = 2.5^{10.7 - 6} = 75$$ smaller. Thus, to be able to see WR 25, the diameter of the aliens' pupils would have to be a factor of $\sqrt{75} \sim 9$ larger. Since human pupils are roughly 6 mm wide, their pupils would be 5 cm wide, and the eyeballs somewhat larger, incidentally roughly the size of a teacup.

Thus, it seems that my compatriot H. C. Andersen predicted the existence of these beings in his fairytale The Tinderbox.

• Your relationship between flux and magnitude is incorrect. The correct factor is a somewhat less dramatic 75, not 50,000. – ProfRob Feb 12 '15 at 22:18
• Dang, you're right! @RobJeffries Aw, there goes my point with the dog with eyes as big as millstones. Lemme edit that… – pela Feb 13 '15 at 8:45
• Ha! Saved by the smaller dog. – pela Feb 13 '15 at 8:46
• "Andromeda would be even more visible, in fact four times brighter". I think it has 4 times the apparent area in the sky and the brightness stays the same. This is the infamous N. D. Tyson error in the first new Cosmos episode, in which he said when the Moon was 1/10 its current distance, it was 100 times brighter. That would mean the Apollo missions would have been melted as they approached the Moon and been turned to plasma as brightness went to infinity as they went to the surface. – C. Towne Springer Feb 13 '15 at 23:15
• Yes, that's true, +1 for that! The surface brightness would stay the same. The total brightness would increase, though. – pela Feb 14 '15 at 0:12

Planets, if any in that star system, would be visible to the naked eye, the way you can see Mercury, Venus, Mars, Jupiter, Saturn and Uranus in our system. On a virtually empty sky, they would draw that much more attention.

Of course, if you had a Moon (or several moons), that would be visible too.

Very rarely and briefly, asteroids passing by very close to the planet might also be visible. In an empty sky, that would be a big event - and difficult to explain with an under-developed astronomy.

Comets would be visible as usual, if the system has an Oort cloud and enough perturbations within it.

Other than planets, moons, asteroids and comets:

From a desert, or a farm way out in the boondocks, you will see the nearest galaxies as faint "clouds" of luminescence in the sky, the way you can see Andromeda now from Earth. Any light pollution from cities nearby would kill it.

There would be no individual stars visible to the naked eye, since all the stars you can see that way must be very, very close (within your own galaxy, if you're located in one). It goes without saying that naked-eye star clusters that you can see from Earth (like the Pleiades) would not be visible there.

There are nebulae visible to the naked eye from a place far away from cities on Earth, like the Great Orion Nebula, but there would be no such thing visible from your star system.

So most people (city-dwellers) in a modern civilization in a place like that would not see anything in the night sky except planets (if any).

Keep in mind that the Milky Way and Andromeda are fairly close, as galaxies go. If your system was outside a cluster, and had no planets, then the sky would be completely empty to the naked eye no matter where you're looking from. Only telescopes would be able to see anything.

So there could be a civilization that exists in which the night is completely void of light?

unlikely but yes (unless they make telescopes)

that would suck

yes

It may also retard the development of astronomy, cosmology and fundamental physics - especially if they had no other planets and moons.

You can see the Andromeda galaxy with a naked eye, even with some level of light pollution. So if the star you are asking about would be even closer to Andromeda, you would see at least that galaxy. There are some other galaxies that can be seen from Earth with a naked eye or with binoculars, so sky of such a lonely planet would still have some night lights on it (and of course its own "sun" that it would orbit). You would not see any stars, as all the stars we see on our sky are from our galaxy. You might be able to see some Active Galactic Nuclea, which are far away galaxies that look like stars (point sources), and are extremely luminous, so we are able to see them even though they are really far away. But if you consider the probability of a stellar system that would be far away from any other galaxies, such probability is defnitely nonzero, as the universe is very inhomogenous. The issue of any planets having civilizations on them is a separate topic, so I will leave that to your personal opinion.

• Which AGN can be seen with the naked eye? – ProfRob Feb 12 '15 at 22:19
• This site claims that Centaurus A, which has an AGN, is visible under exceptional conditions. Seems a bit unserious, though. – pela Feb 13 '15 at 9:23
• The question was about planetary system outside of any galaxy, so in a different location in space. Generally speaking it might be possible then. – Luise Feb 14 '15 at 3:03