(This post is a collection of some ideas I had in a conversation with Dan Korman about six months ago, at the APA Pacific in Pasadena.)
Are there such things as constellations? I’ll presume that there are such things, which then of course raises the question of what they are. The natural thought is that a constellation is a collection of stars, which means that it’s a bunch of balls of hydrogen and such, each glowing from the heat of its fusion, scattered across large expanses of space.
But this seems to give the wrong persistence conditions. Consider the constellation of <A href=http://en.wikipedia.org/wiki/Orion_(constellation)>Orion</a>, which is probably the most easily recognizable and visible constellation in the northern hemisphere (unfortunately, at this time of year in the southern hemisphere it doesn’t rise until about 1 am, and whenever it does appear it’s upside-down). If <A href=http://en.wikipedia.org/wiki/Epsilon_Orionis>the middle star in his belt</a> were to suddently cease to exist, it seems that we wouldn’t say that Orion has ceased to exist (as would be the case if Orion were just a set of stars), but rather that Orion’s belt is now missing a buckle. Similarly, if a new, extremely bright star were to appear in the vicinity of <A href=http://en.wikipedia.org/wiki/Lambda_Orionis>Orion’s head</a>, we would say that Orion’s head is now brighter, and not that we’re looking at a new constellation. So we might suggest that a constellation is not a set of stars, but is rather an object composed of them.
Now, when I say “in the vicinity of”, it doesn’t actually matter how close this new star is to the star that already exists there – the star that currently serves as Orion’s head is about 1000 light years away, while <A href=http://en.wikipedia.org/wiki/Gamma_Orionis>Orion’s right shoulder</a> (the less bright one) is only about 240 light years away. A new bright star could count as an addition to Orion if it was over 1000 light years away (like the head and middle of the belt), or if it was closer to the shoulder, or if it was only 1 or 2 light years away, in which case it would be much closer to the stars in <A href=http://en.wikipedia.org/wiki/Crux>the Southern Cross</a> than it would be to most of the other stars in Orion. The important thing is just what angle you’d have to look at to see it from Earth.
Thus, I suggest that rather than being composed of stars (as in the actual glowing balls of gas), a constellation is composed of beams of light reaching Earth. [UPDATE: see comments for a modification of the “beam of light” view.] To be part of Orion, it doesn’t matter where the ball of gas is in relation to the other balls of gas (after all, a few of those balls of gas are closer to stars in constellations that can’t even be seen from most of the northern hemisphere than they are to most of the other balls of gas in Orion), but it does matter what angle the beam of light reaches Earth. As further confirmation of this view, note that if the middle star in Orion’s belt were to explode right this instant and stop shining, we wouldn’t actually say that Orion has lost his belt buckle yet – that wouldn’t happen for another 1300 years. Although the ball of gas would no longer exist, the beam of light reaching the Earth still would for quite a while.
This suggestion then raises another question – if constellations are composed of beams of light rather than of balls of gas, then are constellations really made of stars? I think the natural answer here is that the word “star” is actually ambiguous between a glowing ball of gas and a beam of light reaching the Earth, and that constellations are composed of the latter but not the former. As it turns out, a few “stars” aren’t really beams of light from individual glowing balls of gas at all. Some of them are <A href=http://en.wikipedia.org/wiki/Binary_star>binary star systems</a> (for instance, <A href=http://en.wikipedia.org/wiki/Sirius>Sirius</a>, which is the brightest “star” in the sky, and which you can find conveniently by following the line of Orion’s belt down and to the left (reverse the directions in the Southern hemisphere of course). And the bright middle “star” in Orion’s
belt [sword] is actually <A href=http://en.wikipedia.org/wiki/Orion_Nebula>the Orion Nebula</a>, which is a cloud of gas that is giving birth to many stars in the other sense.
Of course, not just any beam of light coming down to Earth counts as a star – some are planets, some faint ones are asteroids or moons of planets, and some are man-made satellites. (For instance, a few weeks ago I was able to spot the International Space Station using a guide <A href=http://heavens-above.com/PassSummary.aspx?satid=25544&lat=0&lng=0&loc=Unspecified&alt=0&tz=CET>here</a> – you’ll need to input your own location and time zone for that to be helpful.) Presumably, for a beam of light to count as a star in this sense, it must be bright enough to be visible to the naked eye, but also stable enough that it doesn’t noticeably move from year to year, and must come from far enough away that it doesn’t noticeably move as the observer moves from point to point on Earth. But constellations in the ordinary sense I would say are composed of these sorts of stars, and not of balls of gas in space.
Thus, I think it’s incorrect to say (as we standardly do) that stars are glowing balls of gas in space, and that constellations are made of stars. This involves an equivocation on the word “star”. Stars in one sense tend to be created by stars in the other sense, but the examples pointed out above show that one can exist without the other.
(Astronomers do have terms for certain natural collections of balls of gas, like <A href=http://en.wikipedia.org/wiki/Star_cluster>star clusters</a>, which are balls of gas of the relevant type that are gravitationally bound to one another. They also have a<A href=http://en.wikipedia.org/wiki/Constellation>technical use</a> of the term “constellation” to refer to one of 88 specific regions of the sky and all the stars in them – thus for instance, the astronomical constellation of <A href=http://en.wikipedia.org/wiki/Crux>Crux</a> consists not just of the five stars of the Southern Cross seen on <A href=http://en.wikipedia.org/wiki/Flag_of_Australia>the Australian flag</a> (for some reason the <A href=http://en.wikipedia.org/wiki/Flag_of_New_Zealand>New Zealand flag</a> has only four stars) but actually has <A href=http://en.wikipedia.org/wiki/List_of_stars_in_Crux>at least a dozen</a> stars. However, they use the technical term <A href=http://en.wikipedia.org/wiki/Asterism_(astronomy)>“asterism”</a> for something very much like the ordinary term “constellation”.)