When a star ends its life in a supernova, huge amounts of material are ejected in all directions.  This material continues to travel outward for thousands of years, gradually producing a structure that expands and becomes more diffuse.  The Vela Nebula is one example of such a supernova remnant (SNR).  This image is just a small portion of the Vela Nebula.  If the star on the left side of this image were moved close to the center, the image would closely resemble another SNR portion, the Western Veil Nebula, also know as the Witch’s Broom:

But Vela is in a southern hemisphere constellation of the same name, while the Veil Nebula is in the northern hemisphere constellation Cygnus.

The SN that produced Vela happened about 12,000 years ago at a distance of about 800 light-years from Earth, while the Veil Nebula was born about 8,000 years ago at a distance of about 1,500 light years.  Thus, in terms of both absolute and apparent size, Vela is much larger.  It’s apparent size is about 8 degrees, while the Veil is about 3 degrees.

The diffuse nature of these nebulae makes them quite dim, and this is evidenced by the numerous stars visible in these exposures.  In a brighter nebula, the narrowband filters used would suppress many of the stars, but the long exposures (and heavy stretching) needed to show this faint nebulosity also reveals a lot of stars.  In contrast, the Crab Nebula is a SNR that was born less than 1,000 years ago, in the year 1054:

As you can see, the structure is much more dense and intricate.  Less obvious is that this nebula is much smaller and brighter because it has not expanded as much.

The Vela image above is, of course, just the hydrogen-alpha band.  I’m still waiting for better weather in Australia to capture the other bands.