11.   Astronomers have known for millenia that the sun was not unchanging, instead following a predictable cycle of gaining and losing sunspots.

12.   Astronomers know so little about it and its orbit that they cannot predict its exact location with the precision normally desired for such an encounter.

13.   Astronomers now know that there are no copies of our Galaxy nearby.

14.   Do astronomers know of any objects that might be wandering Trojans?

15.   It will take many more studies, using more refined measurements, before astronomers know whether the crisscross pattern is merely a statistical illusion.

16.   Previously, astronomers knew little more about Eros than that it had a highly reflective surface.

17.   Supernova explosions, produced by certain types of dying stars, are the most violent blasts astronomers knew until now.

18.   Thanks to pictures from the Hubble Space Telescope, astronomers now know that it has the most varied surface of any planet.

19.   Astronomers do not know their origin or precise cause.

20.   But NASA must wait several weeks before astronomers will know if the two major additions to Hubble -- a spectrometer and a spectrograph -- are functioning properly.