Size Matters - The Universe, Our Place in it, and the Star of Bethlehem

Jan. 02, 2003

by Seth Shostak - Senior Astronomer

A recent (but fortunately, hard-to-find) survey has confirmed the obvious: if you visit a planetarium in December, you have a one-in-three chance of seeing yet another show on the Star of Bethlehem. There is enough speculative literature on this seasonal topic to fill an average, two-car garage.

Given that eyewitnesses to this stellar event are long gone, you might wonder how anyone could presume to guess what inspired the Magi to speed their steeds to the Holy Land. The answer is as follows: From the accounts of the Gospels, we know that Jesus was born during King Herods reign (in case you missed it in Latin class, Herod was appointed King of Judaea in 37 BC by the Romans. He wasnt popular.) The important point is that Herod died in 6 BC, a fact that sets a "latest date" for the Star of Bethelehems appearance. Todays sophisticated astronomy software has permitted hordes of the curious to use their home computers to "run the clock backwards" to 6 BC or so, and look for unusual things that appeared in the night sky.

What have they found? Among the various possibilities, most experts favor peculiarities in the apparent positions of the naked eye planets. Planetary conjunctions, retrograde motions, and even the occultation of Jupiter by the Moon have all been suggested as the "Star," particularly as these types of geometric events would have been interpreted by Roman-era cognescenti as having important astrological significance.

Now its worth stepping back a bit to note that celestial alignments and such are remarkable only because the sky, in general, doesnt seem to change much (a circumstance of value to star chart publishers and planetarium projector manufacturers.) If you think about it, youll realize that this is simply a consequence of the fact that the universe is large. Everything in the universe moves, and at speeds that would impress the folks in Indianapolis: most stars have random motions that careen them through the galaxy at 10 20 miles a second. The entire Milky Way rotates at 150 miles a second, and galaxies shuffle relative to their nearby neighbors at a similar clip.

But because the universe is large, you dont notice.

What if it werent? What if the galaxy were a hundred times smaller than it is, so that the stars (not the planets) would be a hundred times closer? (Note that this is still large: our chemical rockets would require a millennium to bridge the distance to Alpha Centauri.) In this smaller universe a universe that doesnt exist, but might have would planetary alignments still be something noticeable? Or is the Star of Bethlehem only something that could occur in our particular kind of universe?

In fact, if the stars were only one percent as far away as they are, they would be much less "fixed" on the sky. Nearby ones would shift their apparent positions in a few months time by amounts noticeable to the unaided eye! While this would be an obvious annoyance to anyone with a heavy investment in star charts, youd expect that special planetary arrangements would still be visually seductive to wise men of the ancient Middle East. After all, even with a shrunken galaxy, the planets would still dance faster than most stars. And just as important, the naked eye planets would catch their attention because theyre bright.

Wrong. In this smaller galaxy, the sky would shine with many thousandsofstars brighter than the naked-eye planets. Sirius would glow with 500 times the intensity of Venus, and the light from this star alone would be enough to read the stock listings at night.

In such an imposing, star-speckled sky, the sorts of planetary alignments that apparently caught the attention of the Magi might not be obvious.

But, you coolly reason, this is merely imaginative hypothesis. After all, the galaxy is the size it is a consequence of the laws of physics and the particulars of the Big Bang. However, the impressive stellar spectacles weve described actually do exist in our galaxy in globular clusters and in the central regions of the Milky Way. In these regions, stellar densities are as high or higher than in our fictional example. For creatures on planets that might inhabit these galactic realms, worldly alignments might not be a siren song of sufficient volume. Announcements of transcendent import would have to be made using a different approach; a nova, for example. But, of course, this would alert the entire galactic neighborhood, and its unclear whether thats theologically justified.

Its a peculiar thought, but the Star of Bethlehem as a signaling device only worked because the part of the universe we happen to live in is relatively empty.