Mercury and its comet-like tail

A 24 million km long plume of gas is ejected from Mercury's thin atmosphere due to the sun, very much like a comet.
This is only visible using a narrowband filter that captures the bright yellow sodium light at 589nm.

A tail was predicted in the 1980s, and first discovered in 2001.
Multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015 revealed more details of this tail.

Telescope: Borg 90FL with Borg #7872 0.72X suer-reducer, which makes the scope f/4
Camera: ZWO ASI 294MM Pro, cooled to -10C
Filter: Edmund Optics 589nm +/5nm narrow band filter, installed in a custom 3-D printed enclosure to fit in a standard Starizona filter slider
mount: SkyWatcher EQ6R-Pro equatorial mount running with EQMOD ASCOM

30x30 seconds processed in PixInsight