Bill Blair's

Vela Supernova Remnant File

Photo credit (left): Color optical image showing the filaments north of the pulsar (see below). Credit and copyright: David Malin and AAO.

Photo credit (right): Wide field X-ray image. Rosat Observatory, NASA

Go to the Hubble Heritage page about the Pencil Nebula (photo released June 5, 2003).

Welcome! This site provides some background about the Vela supernova remnant (SNR), a roughly 11,000 year old leftover from a supernova explosion in our Milky Way galaxy. This object is about 250 parsec (815 light years) away in the constellation of Vela (which represents the sail of the mythical Argonaut's ship). This distance makes the Vela SNR one of the closest SNRs to the solar system.

Our appreciation and understanding of this interesting object have changed dramatically over the last decade. The SNR has been recognized as such for decades, with a non-thermal radio source (characteristic of SNRs) and selected optical filaments showing indications of heating by fast shock waves from the supernova. A pulsar (spinning neutron star) has also been known since the early 1970's, and is thought to represent the collapsed core of the star that exploded. It is from the so-called "spin down rate" of this pulsar that the estimate of 11,000 years since the supernova has been derived, although most astronomers would not claim this number to be more accurate than a factor of two (either way).

Caption: The region right around the Vela pulsar, as viewed in X-rays by the Chandra X-ray Observatory. The region shown here is so small that it would be barely visible in the wide angle X-ray image shown in the next figure. The pulsar is moving toward the upper right in this picture. See THIS SITE for more information.

The full extent of the expanding remnant was not known until the mid-1990's when a sensitive X-ray image of the region was obtained with the Rosat satellite. This image is reproduced below.

Caption: This Rosat All-Sky Survey image was the first to show the full extent of the Vela SNR (from Aschenbach et al. 1995, Nature, 373, 587). Before this, only the inner bright, roughly circular region had been ascribed to the SNR. The bright clump of emission near 8h20m, -43 is a separate background SNR known as Puppis A.

You should note the angular scale on this figure: the full extent of the SNR on the sky is about 8 degrees! For reference, the full moon is about a half degree across, so this is some 16 times the angular size of the moon! Prior to this, only the inner bright ring (about 4 degrees across) had been identified in radio and X-ray observations. An interesting sidelight of this discovery was that it solved a long standing mystery. The Vela pulsar should have been close to the center of the SNR, but was off to one side of the bright 4-degree section. Now that the full extent is seen, it turns out that the pulsar is indeed close to the center!

In the optical, it is more difficult to separate out the emission due to the Vela SNR from other emitting regions along the line of sight. Here is as very wide field image in the light of the red hydrogen alpha emission line:

Caption: This very wide field Hydrogen-alpha image is from the Royal Observatory's SuperCOSMOS Halpha Survey project, centered on the Vela SNR. Only the filaments slightly above and right of center are obviously associated with the SNR itself. Nearly everything else is foreground or background emission.

Here is another nice image of the optical emission, provided to me by astrophotographer Bert Van Donkelaar:

Caption: In this "image", Bert has actually stitched together multiple images in multiple filters to make a single, color-composite image. He used a Canon 300mm f/2.8 lens mounted on the side of a guide telescope. Red shows the light of the hydrogen Balmer-alpha line and the blue picks up the [O III] 5007 Angstrom (ionized gas) line. More on Bert's photography can be found HERE! (Click image to see larger version. For a high-quality JPG of this image, GO HERE. For a wider FOV of the region, also by Bert, GO HERE.)

To put these images in context relative to each other, here is a figure that overlays the H-alpha and X-ray wavebands. (This has been done only in an approximate manner, for illustrative purposes only!) The X-ray is shown in blue/green with the optical remaining in red.

This is a bit messy, but hopefully you can see how the bright X-ray region north of the pulsar is bounded by the optical filaments. The bright X-ray blob to the NW (upper right) is actually a separate supernova remnant, Puppis A, which is much farther away (about 2000 parsecs, or 6500 light years) in the next spiral arm out in this direction).

The other region marked on the figure above is called "Vela D" or sometimes "Knot D." In the Rosat X-ray picture above, you can see a number of protruding regions outside the main shell of X-ray emission. In the discovery paper, Aschenbach et al. (1995, Nature, 373, 587) labelled these protrusions with letters, and the naming convention took hold. Here is a figure that shows this naming convention.

The Vela Knot D region is interesting, and my colleagues Ravi Sankrit, John Raymond and I have recently published a paper on the ultraviolet emissions from this region (links to a pdf file). The X-ray Knot D is bounded by a beautiful "fan" of optical filaments know commonly as the Pencil Nebula (NGC 2736), the subject of a June 2003 Hubble Heritage photo release. This image is centered near the region observed in the ultraviolet spectral region in our paper. See below for some additional notes about the Field of view images by the Hubble Telescope.

Briefly, when first discovered the Knot D region (as well as the other protrusions seen in X-rays) was thought to represent a place where "ejecta" (chunks of the precursor star) were being shot out at high velocity from the explosion site. However, even with extensive observations of the region, it has been difficult to confirm (or refute!) this claim. The ultraviolet and optical observations are consistent with the region being a "breakout" from the main shell of the SNR. That is, a weaker, lower density portion of the shell may have allowed the main shock wave to "run ahead" in this region, and the shock wave is now encountering a denser region again, causing the X-ray and optical/UV emission we see. We do not find evidence for peculiar abundances or physical conditions that would indicate stellar ejecta are involved in creating this structure Future observations, including some with the Chandra X-ray Observatory, are being pursued to reconcile which of these models might be correct.

One final surprise discovered fairly recently is that yet another supernova remnant is present along this line of sight! When the Rosat X-ray data shown above were processed in a way that showed only the higher energy X-rays, a separate, smaller diameter shell popped out:

Caption: These two images show low (left) and high (right) energy X-ray images of the same piece of sky (from Aschenbach, B. 1998, Nature, 396, 141). In the right panel, a separate supernova remnant, about 2 degrees across, is revealed.

The distance to this new supernova remnant (known formally as RX J0852.0-4622, but known affectionately as "Vela Jr.") is a matter of hot debate. It could be a background object, similar to the Puppis A SNR mentioned above. A more tantalizing possibility, though, is that it is physically associated with the Vela SNR, and represents an even younger supernova from within the same generation of stars! The discovery image above even raises the possibility that the Vela D/Pencil Nebula region could be associated with THIS OBJECT instead of the Vela SNR itself.

It's amazing sometimes how surprises pop out of new observations, and how difficult is can be to discern the true situation!

Further Notes about the Hubble Heritage Image Released June 5, 2003

The Hubble Space Telescope image of the Pencil Nebula region is an excellent demonstration of Hubble's ability to resolve fine scale structure. But by comparing to the images above, you should also realize that the field imaged with Hubble is a tiny portion of a MUCH larger object!

It is important to remember that what we see as a flat image is really a 3-dimensional structure in space, in particular extending into the screen (or page) as well as across it. (Astronomers say it extends "along the line of sight".) On the large scale, the Pencil nebula appears to be an example of where the supernova blast wave (or shock front), moving from right to left in the picture, is encountering a region of enhanced density in the interstellar medium. Think of the blast wave as a sheet in the wind (perhaps a sail?), rippling along the line of sight. Where ripples in the sheet become tangent to our sight line, we see sharper edges or brighter regions, and where the sheet is less edge-on, we see more diffuse or clumpy emission (and generally fainter). The HST picture shows us this process in glorious detail, showing that the spatial scales of these ripples extend to finer scales than can be seen from the ground.

Now what about the differences in the red and blue emission in the picture? The blue comes from [O III] 5007, and the red from (primarily) H-alpha (with perhaps some [N II] 6584 mixing in. Never mind the differences in "elements", in this case. What we are really seeing is changes in ionization of the gas. Just think of the blue stuff as being "hotter" than the red stuff.

Think of a SN shock wave as an "impulsive" heating mechanism: the shock wave hits a clump of gas and heats it up, but then the clump is free to cool down (by emitting the light that we see). This is different than heating by starlight (photoionization), where the star keeping pumping in energy over a period of time. In the HST picture, the regions in blue have cooled to the point where the hotter [O III] emission line is strong, but they have not cooled to where they can emit in H-alpha (and/or [N II]). The red regions have cooled beyond the point where the hotter [O III] emission is dominant, and the other lines pick up and become bright. Then, of course, because of the aforementioned line of sight effects ("projection effects"), there are places where the situation appears more confused.

One of the neat things in the HST picture is the variety of scales over which we see this phenomenon occurring. On the large scale (and remembering that the shock wave's motion is directed toward the left), fluffy wisps of blue filaments lead the entire structure. But then in the regions "behind" the brightest filaments (again in projection anyway, toward the right in the picture), one sees many lovely examples of where this process is occurring on much smaller spatial scales, that is, where a blue filament lies directly on the leading edge (left side) of a group of red filaments.

One last thing I'll mention is that there are some small, brighter "clumps" of emission scattered about the picture. Brightness comes from one of two things: either higher local density of gas or more gas projected locally along a given line of sight (or a combination of the two). It is not always possible to distinguish between these two possibilities (at least without a spectrum), although given the general patchiness or clumpiness observed, one can imagine that the interstellar gas being encountered by the SN shock wave is itself somewhat clumpy.

Go to Hubble Heritage Page on the Pencil Nebula (June 2003 release).

			-- Bill Blair

Last updated: Apr. 5, 2009

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