Arthur C. Clarke noticed another curious feature of the constellation Aquila.
There have been an inordinate number of novae detected in that
Cosmic Search Vol. 1, No. 1
“According to Norton’s Star Atlas, there have been twenty fairly bright novae between 1899 and 1936. No less than five of them have been in one small area of the sky, in the constellation Aquila. There were two in a single year (1936), and the 1918 Nova Aquila was one of the brightest ever recorded.
“What’s going on in this constellation? Why did 25 percent of the novae in a forty-year period appear in only 0.25 percent of the sky? Is the front line moving in our direction?”
On the space bbs Habitablezone.com this has been attributed to only a selection effect:
Trouble in Aquila – a mystery solved?
Posted by ER on 2/19/2007 10:23:22 AM
Unfortunately, the link to that 6 year old post (has it been that long?) no longer works, but I do recall it. The apparent flurry of novae in Aquila is an artifact resulting from a combination of several observational constraints, a selection effect, not a true physical phenomenon.
1) Aquila is in the Milky Way, and novae are more common in the Milky Way because there are more stars there. Almost all the novae in our galaxy have been seen in the Milky Way.
2) The center of the Milky Way is in Sagittarius, well south of the celestial equator. The closer you look towards the galactic nucleus the more novae you see because the more stars you see, and the more likely you are to see a nova.
3) Aquila straddles the equator, so it is an area nearer the galactic nucleus than constellations further north. However, it is still easily visible from the northern hemisphere, where (until recently) most astronomers worked, and most nova photographic patrol programs were based (Mostly N America and Europe).
These observational selection effects combine so that Aquila appears to have more nova actitivity, especially since Norton’s Star Atlas is based on maps and data acquired prior to 1950.
If it were possible to plot all novae (not just those observed by astronomers), they would cluster on the Milky Way, increasing in number as we approached Sagittarius, and be most numerous near the galactic nucleus where we are looking through a volume containing the maximum number of stars.
If we subtract the ones that couldn’t be seen from the N hemisphere, the distribution noted by Mr Clarke is what remains. Until fairly recently, the onlly major observatories in the S hemisphere were in Argentina and S Africa.