5 ms·

Anyone living on planets orbiting those stars would not see many other stars at all, right?

by paul_f 12y ago

jessriedel 12y ago
Right. Almost all the stars we see in the night sky are within 1,000 light years. You could guess this from the fact that the Milky Way disk is about ~1,000 light years thick and ~100,000 light years in diameter. If we could easily see stars further than the thickness, then we'd see a lot more stars along the Galactic disk. But instead, the only sign of the disk is the diffuse glow of very distant, undifferentiated stars where the Milky Way gets it's name. Presumably stars outside of galaxies are at much lower density, so they wouldn't see hardly any points of light in the sky. If most such stars are still within the Galactic groups, then they would probably just see faint smudges of other galaxies like the very dim Magellanic Clouds we can see. (Note that these are still pretty big, though, in terms of angular size. Several moons wide.) On the other hand, planets around lone stars in voids away from Galaxy clusters would presumably have nothing visible to the naked eye from their surface. (Someone please correct me if I'm wrong. I'm not an astronomer.)
- dalke 12y ago
  To add to those excellent points, the Andromeda galaxy is visible with the naked eye, making it another type of smudge ("Little Cloud", to use an early name) that might be seen by a lone star in a galactic cluster.
  - deciplex 12y ago
    Andromeda is pretty close to us. In fact, we're going to collide with it in a few billion years. After that, what little remains of our solar system by then, might end up like one of these orphaned stars. So if Andromeda is already difficult to see by us, that gives some indication of what's going to be visible from a planet that is much further away from anything, than even that.
    - dalke 12y ago
      The premise in the g'parent comment is "If most such stars are still within the Galactic groups ...". This assumption is reasonable given the hypothesis expressed in the article: "Bock suspects that a lot of these renegade stars could have come from relatively lightweight galaxies, which can lose hold of their stars more easily than more massive galaxies." Our Local Group has a diameter of ~10 megalight-years . Andromeda is 2.54 MLy away. Thus, given the premise, and assuming that our own cluster is "typical", it's unlikely that a renegade star is all that much further from a galaxy like ours or Andromeda than we are from Andromeda.
- throwaway_yy2Di 12y ago
  I think that's right (also not an astronomer). I looked this up in the Hipparcos catalog [0][1] -- that's one that measures parallax distances to bright stars. It's supposed to be complete to at least +7.3 magnitude [0]. Cutting off at +6.5 magnitude [2] (the faintest visible stars according to [3]), Hipparcos reports * 7,943 visible stars * 1,078 (14%) beyond 1,000 light years; * 34 beyond 10,000 l.y. (these must be supergiants!) If I calculated right, a +6.5 magnitude star would demand an absolute magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years. Looking this up on the Hertzsprung–Russell chart [4], this would include only very bright giants, and supergiants, respectively. [0] https://en.wikipedia.org/wiki/Hipparcos https://en.wikipedia.org/wiki/Hipparcos [1] ftp://cdsarc.u-strasbg.fr/pub/cats/I/311/ (hip2.dat.gz for data; intro.pdf for field descriptions) [2] Hp magnitude is a 340-850 nm visible band, which diverges slightly from V band magnitude, http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html [3] https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_notable_celestial_objects https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_no... [4] https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_di...
  - jessriedel 12y ago
    OK, but most of those 1,077 are quite dim. WIkipedia's list of the 93 brightest stars (magnitude <2.5) lists only 6 that are further than 1,000 ly. http://en.wikipedia.org/wiki/List_of_brightest_stars http://en.wikipedia.org/wiki/List_of_brightest_stars
Florin_Andrei 12y ago
All stars you see at night with naked eye are within this galaxy. All of them. Yes, Andromeda (M31) is visible to the naked eye, but it's pretty huge and quite close to us. Orbiting a lone star, the naked eye at night would see something between absolutely nothing, and maybe a few faint patches of light. And that's in perfect conditions - middle of desert or ocean, zero light pollution.
- JoeAltmaier 12y ago
  I don't believe it. I can see many galaxies from where I stand. They would appear even brighter without a local galaxy to wash them out. The sky would appear totally covered in fuzzy dots - galaxies.
  - Florin_Andrei 12y ago
    > I can see many galaxies from where I stand. If half a dozen is "many", then sure. But you probably live far from any cities then. http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_galaxies http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_gala... (The list includes objects that are very difficult to see, and some only "reportedly" seen naked-eye.) > They would appear even brighter without a local galaxy to wash them out. There's no reduction in contrast in the direction perpendicular to the galactic plane. And yet the naked eye doesn't perceive the sky there "totally covered in fuzzy dots". The naked eye has a lot of trouble seeing anything beyond the Local Group - and these are a handful of close-knit galaxies. You need telescopes for that, with plenty of aperture. Once the aperture is comparable to the diameter of your car's wheels, or bigger, then yes, you get the "my god, it's full of galaxies" effect.
    - JoeAltmaier 12y ago
      I'm an idiot. I confused nebulae etc for galaxies. Not an astronomer. So most of what we see are constructions inside our own galaxy? I see what you mean - the sky would be a very lonely place for the intergalactic skygazer.
cmsmith 12y ago
Making it fun to imagine the invention of the telescope on those planets. It would be like the Hubble deep field image, but MUCH more unexpected. On the other hand, who knows how much science and telescope-making would have been set back without there being anything to look up at. Without other stars (and our rather extraordinary moon), it would take a lot longer than 1500 years to break out of the Ptolemaic mindset.
- IndianAstronaut 12y ago
  I would imagine other galaxies being visible like faint blobs in the sky, not to mention that you would still see the planets around the given star.
  - kijin 12y ago
    Other planets would definitely help stimulate the mind. But depending on how isolated the system is, galaxies in the night sky might be indistinguishable from clouds. Especially if the planet has lots of clouds.

Anyone living on planets orbiting those stars would not see many other stars at all, right?

by paul_f 12y ago

Anyone living on planets orbiting those stars would not see many other stars at all, right?

Right. Almost all the stars we see in the night sky are within 1,000 light years. You could guess this from the fact that the Milky Way disk is about ~1,000 light years thick and ~100,000 light years in diameter. If we could easily see stars further than the thickness, then we'd see a lot more stars along the Galactic disk. But instead, the only sign of the disk is the diffuse glow of very distant, undifferentiated stars where the Milky Way gets it's name. Presumably stars outside of galaxies are at much lower density, so they wouldn't see hardly any points of light in the sky. If most such stars are still within the Galactic groups, then they would probably just see faint smudges of other galaxies like the very dim Magellanic Clouds we can see. (Note that these are still pretty big, though, in terms of angular size. Several moons wide.) On the other hand, planets around lone stars in voids away from Galaxy clusters would presumably have nothing visible to the naked eye from their surface. (Someone please correct me if I'm wrong. I'm not an astronomer.)

dalke 12y ago
To add to those excellent points, the Andromeda galaxy is visible with the naked eye, making it another type of smudge ("Little Cloud", to use an early name) that might be seen by a lone star in a galactic cluster.
- deciplex 12y ago
  Andromeda is pretty close to us. In fact, we're going to collide with it in a few billion years. After that, what little remains of our solar system by then, might end up like one of these orphaned stars. So if Andromeda is already difficult to see by us, that gives some indication of what's going to be visible from a planet that is much further away from anything, than even that.
  - dalke 12y ago
    The premise in the g'parent comment is "If most such stars are still within the Galactic groups ...". This assumption is reasonable given the hypothesis expressed in the article: "Bock suspects that a lot of these renegade stars could have come from relatively lightweight galaxies, which can lose hold of their stars more easily than more massive galaxies." Our Local Group has a diameter of ~10 megalight-years . Andromeda is 2.54 MLy away. Thus, given the premise, and assuming that our own cluster is "typical", it's unlikely that a renegade star is all that much further from a galaxy like ours or Andromeda than we are from Andromeda.
throwaway_yy2Di 12y ago
I think that's right (also not an astronomer). I looked this up in the Hipparcos catalog [0][1] -- that's one that measures parallax distances to bright stars. It's supposed to be complete to at least +7.3 magnitude [0]. Cutting off at +6.5 magnitude [2] (the faintest visible stars according to [3]), Hipparcos reports * 7,943 visible stars * 1,078 (14%) beyond 1,000 light years; * 34 beyond 10,000 l.y. (these must be supergiants!) If I calculated right, a +6.5 magnitude star would demand an absolute magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years. Looking this up on the Hertzsprung–Russell chart [4], this would include only very bright giants, and supergiants, respectively. [0] https://en.wikipedia.org/wiki/Hipparcos https://en.wikipedia.org/wiki/Hipparcos [1] ftp://cdsarc.u-strasbg.fr/pub/cats/I/311/ (hip2.dat.gz for data; intro.pdf for field descriptions) [2] Hp magnitude is a 340-850 nm visible band, which diverges slightly from V band magnitude, http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html [3] https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_notable_celestial_objects https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_no... [4] https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_di...
- jessriedel 12y ago
  OK, but most of those 1,077 are quite dim. WIkipedia's list of the 93 brightest stars (magnitude <2.5) lists only 6 that are further than 1,000 ly. http://en.wikipedia.org/wiki/List_of_brightest_stars http://en.wikipedia.org/wiki/List_of_brightest_stars

Florin_Andrei 12y ago

All stars you see at night with naked eye are within this galaxy. All of them. Yes, Andromeda (M31) is visible to the naked eye, but it's pretty huge and quite close to us. Orbiting a lone star, the naked eye at night would see something between absolutely nothing, and maybe a few faint patches of light. And that's in perfect conditions - middle of desert or ocean, zero light pollution.

JoeAltmaier 12y ago
I don't believe it. I can see many galaxies from where I stand. They would appear even brighter without a local galaxy to wash them out. The sky would appear totally covered in fuzzy dots - galaxies.
- Florin_Andrei 12y ago
  > I can see many galaxies from where I stand. If half a dozen is "many", then sure. But you probably live far from any cities then. http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_galaxies http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_gala... (The list includes objects that are very difficult to see, and some only "reportedly" seen naked-eye.) > They would appear even brighter without a local galaxy to wash them out. There's no reduction in contrast in the direction perpendicular to the galactic plane. And yet the naked eye doesn't perceive the sky there "totally covered in fuzzy dots". The naked eye has a lot of trouble seeing anything beyond the Local Group - and these are a handful of close-knit galaxies. You need telescopes for that, with plenty of aperture. Once the aperture is comparable to the diameter of your car's wheels, or bigger, then yes, you get the "my god, it's full of galaxies" effect.
  - JoeAltmaier 12y ago
    I'm an idiot. I confused nebulae etc for galaxies. Not an astronomer. So most of what we see are constructions inside our own galaxy? I see what you mean - the sky would be a very lonely place for the intergalactic skygazer.

cmsmith 12y ago

Making it fun to imagine the invention of the telescope on those planets. It would be like the Hubble deep field image, but MUCH more unexpected. On the other hand, who knows how much science and telescope-making would have been set back without there being anything to look up at. Without other stars (and our rather extraordinary moon), it would take a lot longer than 1500 years to break out of the Ptolemaic mindset.

IndianAstronaut 12y ago
I would imagine other galaxies being visible like faint blobs in the sky, not to mention that you would still see the planets around the given star.
- kijin 12y ago
  Other planets would definitely help stimulate the mind. But depending on how isolated the system is, galaxies in the night sky might be indistinguishable from clouds. Especially if the planet has lots of clouds.