Time for me to add a few more comments.
I just love Lorenzo Comolli's portrait of Comet Garradd. The colors and structural details are fantastic. Note the two tails pointing in opposite directions. Note the color of the tails, too. One tail is yellow-white, and it is made of dust from the comet. Its color reflects the color of the Sun, just possibly with some reddening. The dust tail often points in a direction that is affected both by the motion of the comet and the solar wind and it forms a curved tail.
The other tail, the blue ion tail, always points directly away from the Sun. Check out
this page to find out which ways the two comet tails point in relation to the motion of the comet and the direction to the Sun.
The ion tail glows blue because of ionized gases. Unfortunately, Wikipedia is sufficiently impolite not to explain what gases cause the blue color, but I'm pretty sure we are talking about ionized versions of carbon-oxygen combinations and hydrocarbons. I believe the ionization of the gases happens because of interactions with charged particles in the solar wind.
The blue-green coma, the head of the comet, glows the way it does because of other ionized gases. Again I don't know which ones. Wish I knew why the gases in the coma and the ion tail are always different!
But Lorenzo Comolli's picture really underscores that a comet is made of a nucleus surrounded by a coma, a dust tail and an ion tail. His picture underscores the fact that the dust tail and the ion tail and differently colored and point in different directions, and that the nucleus is yet another color.
Finally, check out the color of the star. It is a K-type star, redder than Arcturus, but not as red as Aldebaran and significantly less red than Betelgeuse. So what a color symphony this picture is!
Another very interesting image is Alistair Symon's portrait of several emission nebulae. Two are round, smallish, rather bright and concentrated. One is
very small. One is large, round and spread out. I get the impression that the small concentrated ones are regions where star formation may still be in progress, or else it has only just come to an end, and there is still a lot of gas near the hot young stars (or star? Maybe just one really hot massive star?). The large one reminds me of the tenuous gas around a hot massive star that has blown away most of the gas cloud it was born from. In other words, the small concentrated nebulae remind me of the
Cocoon Nebula, whereas the large tenuous nebula reminds me of the
Lambda Orionis Nebula, seen at top in the image I linked to which I believe is taken by Bill and Sally Fletcher.
Ann
