Imagine a group of 3 stars that appear to be equally luminous ,star A, B, and C in the shape of an equilateral triangle. A,B and C look like they form an equilateral triangle right now on the Earth but when the light from star B reaches the Earth, the light from star C still has say, 10 years of traveling to go. So when B is observed in its present position, the apparent position of C is from 10 years in the past. Where C is now relative to the present position of B might be sufficiently different so as to not to appear to form an equilateral triangle. The 3 stars 'right now ' could form a 'scalene triangle' pattern if star C's position ,where it is 'right now', were visable . If you wanted to 'imagine' the 'smallest' sphere 'covering these 3 stars 'right now' , what would be it's radius? ( given star A is 100 light years away, star B is 110 light years away and star C is 120 light years away.)

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    $\begingroup$ Can you clarify what you're asking. Constellations may be important for astrology but play little role in physics. $\endgroup$ Jul 13, 2014 at 7:59
  • $\begingroup$ @JohnRennie This is just a guess - I am not a native speaker of English, and something in my mind tells me that the OP also isn't, and that the question is about whether all stars in any given constellation are spatially "close" to each other. $\endgroup$ Jul 13, 2014 at 8:30
  • $\begingroup$ No and there is no need to do so either. Light is emitted continuously from the stars, so path lengths do not matter, only their relatively fixed position on our night sky. $\endgroup$
    – Kyle Kanos
    Jul 14, 2014 at 0:03
  • $\begingroup$ If you saw the 3 stars I mentioned above that seem to be in the shape of an equilateral triangle from your point of view ; and say you wondered what would be the smallest sphere 'covering' all 3 stars what might its radius be? $\endgroup$
    – user128932
    Oct 5, 2014 at 6:06
  • $\begingroup$ That's quite a heavy edit of your original question. $\endgroup$
    – Dr Chuck
    Oct 10, 2014 at 6:59

1 Answer 1


The shapes of constellations (and there are several different depictions of any particular constellation) only depend on how they look at any specific time. Stars in any constellation are not necessarily close to each other in space. For example, the main stars in Ursa Major vary in distance from 58 to 124 light years.

The boundaries of constellations are determined by the International Astronomical Union, and are not set by reference to the positions of the stars.

  • $\begingroup$ Can one known constellation have at least two very different 'shapes' that its stars form and are apparent in the night sky at two different times? $\endgroup$
    – user128932
    Oct 18, 2014 at 2:50
  • $\begingroup$ Imagine there are two stars A and B that in there present positions point to a star in Orion's belt. Imagine A is 100 light years away and B is 200 light years away. When the image from A is 'just leaving' towards Earth the image from B has been 'travelling for about 100 years. As the images from A and B then 'travel' to Earth a lot could have happened to star B. It might have blown up or been otherwise destroyed , or it could have moved to a position were it is now not aligned with star A and Orion. So when the observer sees them aligned now it could be a problem of the distances involved. $\endgroup$
    – user128932
    Nov 27, 2014 at 6:11
  • $\begingroup$ If a constellation was the shape of an approximate equilateral triangle and it was observed to 'keep' this shape over the next year of observation would this be possible if it was known one of the stars was 100 light years more distance from the Earth than the other two? $\endgroup$
    – 201044
    May 13, 2015 at 3:10

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