Veil Nebula (NGC 6960): Hubble's Close-up of a Supernova Shock Front

About 8,000 years ago a star some twenty times the mass of the Sun exploded in what is now the constellation Cygnus. The debris has been spreading ever since, and today it forms the Veil Nebula, also cataloged as the Cygnus Loop, a ring of glowing filaments about 110 light-years across and roughly 2,100 light-years from Earth. In April 2015 Hubble photographed a tiny piece of its edge, in the section listed as NGC 6960, and the picture was released on September 24 of that year.

The whole remnant is enormous on the sky. It spans about six full Moons, so a single Hubble camera field covers only a speck of it. The released image is a mosaic of six Wide Field Camera 3 pointings that together cover a strip about two light-years wide, a fraction of one percent of the full structure.

Where the shock meets the wall

What the mosaic records is a collision. Before it exploded, the star blew a strong wind for a long time and cleared a large bubble of thin, low-density gas around itself. When the supernova went off, its blast wave crossed that bubble quickly and then hit the denser, cooler interstellar gas at its rim. The Veil glows where that happens: gas at the wall is heated and excited by the shock and gives off light.

A helpful way to read the picture is to imagine a crumpled bed sheet seen from the side. Where the sheet folds over and is seen edge-on, a line of sight passes through a great deal of glowing gas and the filament looks bright. Where the sheet faces the viewer, the same gas is spread thin and looks faint. The brightest ridges also mark places where the shock has run into denser clumps.

How the 2015 colors were assigned

The UVIS channel of Wide Field Camera 3 recorded the region through three narrowband filters, each passing light from one kind of excited gas, plus two broad filters that mainly capture the stars.

Filters and assigned colors

Filter	What it records	Shown as
F502N	Doubly ionized oxygen [O III]	Blue
F673N	Ionized sulfur [S II]	Green
F657N	Hydrogen-alpha plus ionized nitrogen [N II]	Red
F555W	Broad visible light (V band), mostly stars	Blue
F814W	Broad near-infrared light (I band), mostly stars	Red

A narrowband filter lets through a slice of the spectrum only a few nanometers wide, centered on a line emitted by one element in one state of ionization. Each such exposure is therefore a map of a single gas. The broad filters pass hundreds of nanometers at once and record everything that shines in that range, which in a field like this mostly means stars.

This palette does not follow the strict order of wavelength. Sulfur, at 673 nanometers, has a longer wavelength than hydrogen-alpha at 657, yet it is shown green while hydrogen is red. Giving sulfur a color of its own keeps the two red-glowing gases apart in the filaments. The broad V and I filters mostly record starlight, so the stars keep a range of color. Why processors sometimes depart from the wavelength rule is explained in Are the Colors Real?

With that key in hand, the image sorts itself into layers. The blue features, oxygen emission, run along the outer side of the cavity wall and look smooth and gently arched. Behind them the green and red structures look fluffy and tangled. The red glow there comes from cooler gas that the shock passed through some time ago; it has since spread out into a chaotic shape. Here and there a few thin, sharp red threads stand out. These mark gas that has just been swept up into the shock, moving at close to one million miles per hour, fast enough to cover the distance from Earth to the Moon in about 15 minutes.

Watching the Veil move

Few objects in the sky change visibly within a human lifetime, but the Veil is close enough for its shock fronts to shift measurably. Part of the 2015 field had already been photographed in 1997 with the older Wide Field and Planetary Camera 2, giving a baseline of a little more than 18 years. Comparing the two epochs lets astronomers measure how far the filaments have moved and how the shock is advancing into the surrounding gas.

The stereo pair above uses that motion in an unusual way. Each panel shows one epoch. When the two are merged by crossing the eyes, the brain interprets the small shifts between them as depth, and the filaments seem to stand out in three dimensions. The effect is an illusion produced by motion over time, yet it gives a fair idea of how much the shock front changed in under two decades.

The release also included observations taken at the same time with the Advanced Camera for Surveys, which pointed at a neighboring patch of sky while Wide Field Camera 3 imaged the main field. Combining main and parallel data extends the coverage slightly without extra telescope time.

Third in the anniversary series

The data came from Hubble Heritage proposal 14056, with contributions from W. Blair, of Johns Hopkins University and STScI, and S. Porter, also of Johns Hopkins. Hubble was deployed from the space shuttle in April 1990, and the 25th anniversary of that deployment was marked with a series of special images released through 2015. The Veil was the third of them. The first was a new look at the Eagle Nebula in January 2015, and the second was the star cluster Westerlund 2 in April 2015. The remnant can be compared with a much younger one, the Crab Nebula, whose explosion was recorded in 1054.

Questions readers ask

How old is the Veil Nebula?

The star that produced it exploded about 8,000 years ago. Its blast wave has been expanding ever since, and it is still running into the gas at the edge of the cavity the star cleared before it died. The remnant now spans about 110 light-years.

How big is the area in the Hubble image?

The mosaic covers a strip roughly two light-years across, put together from six Wide Field Camera 3 pointings. The complete Veil Nebula is about 110 light-years wide, so the image shows only a tiny part of the edge near NGC 6960.

Why is the sulfur shown green instead of red?

Sulfur and hydrogen both glow red, at 673 and 657 nanometers. In this image hydrogen is kept red and sulfur is shown green so the two gases stay distinct. It breaks the usual wavelength order, but it makes the layered structure of the shock easier to read.

How massive was the star that exploded?

It was about twenty times as massive as the Sun. Before the explosion its strong stellar wind carved out a large bubble of thin gas. The Veil filaments seen today sit along the wall of that bubble, where the supernova shock is hitting cooler and denser interstellar gas.

What did the comparison with 1997 show?

Over the roughly 18 years between the Wide Field and Planetary Camera 2 view and the 2015 data, several shock fronts moved by measurable amounts. That motion lets astronomers study how the blast wave is still pushing into the surrounding gas, thousands of years after the explosion.