NGC 7293 is the PN with the highest integral-magnitude of the whole sky, which takes it at the fist place among the brightests of its class.
Its large size, thanks to its vicinity to the earth and also to its real dimension of one light-year circa, makes it an easy target for astroimaging. On the other side, the low surface brightness and the low altitude from the horizon, make the observation difficult, so that this nebula is visible only in dark sites. For these reasons OIII filter is quite mandatory for this nebula, and it will enhance our observation. It will be visible with its typical anular structure and its "helix" (that's the cause of it's nickname "Helix Nebula") drawn by two symmetrical hooks in the border af the nebula.
Only in the last years its structure has been resolved by recent cinematic techniques. They have revaled that NGC 7293 is a bipolar-toroidal nebula with its axis looking at us, so that we can see the toroid frontally, but it prevents us to see the development of polar lobes. Outside the nebulosity there are two couples of highly deformated lobes because of the interaction with interstellar matter; these lobes' axis doesn't seem to coincide with the interior lobes'. Thanks to the Space Telescope it's been possible to deeply investigate on this nebula, and it revealed a disomogenity matter distribution, infact, there are several comet-like knots of denser matter caused by the hard stellar wind. Because of this denser knots of matter, moving to the boundary slower than normal gas, forming litle shadow-cones, the nebula is less energically photoionized.
With normal common-use astro-imaging techniques we can also evidence this huge stratification of the ionization. This happens because the central star is on its white-dwarf finale sequence, so that its energy is not enough to ionize the outer parts of the nebula.
Interesting level: * |
