NGC 2736.The expansion revealed

Please, click on the image to watch the animation that shows (repeated five times) the expansion movement of NGC 2736 between 1979 and 2016 (37 years). I recommend to enlarge it to full screen mode for a better view. The nebula was to the East (right in this image) in 1979 and it moved Westward (to the left) in the 2016 image.

NGC 2736, the Pencil Nebula, is located at the outskirts of the Vela Supernova Remnant, about 4.5 degrees West of its very core, the Vela Pulsar. The Type II Vela Supernova exploded about 11,000 years ago, and left a neutron star (the Vela Pulsar) and expanding debris that, now, rank among the beautiful objects in the sky. The supernova remnant, and NGC 2736 with it, is expanding at a rate in the range of 500,000 up to 650,000 km/h (according to different sources). The most reliable sources lean to the values in the lower part of the range (500,000-585,000 km/h). With this in mind, and taking its distance, around 800 light years, it had to be possible to detect its expansion by comparing old and recent pictures. Having taken an image of NGC 2736 recently, February 2016, the search started for older images. This has been a joint project with the renowned astrophotographer Don Goldman.

A thorough search provided some of the images and the contacts provided by Don Goldman and  Alan Strauss (University of Arizona) the rest of the nine main images that were collected for the comparison, they were images taken in: 1979 (DSS3), 1993 (David Malin), 1998 (DSS5), 2003 (HST), 2004, 2009 (Ken Crawford), 2012 (ESO), 2014 (Don Goldman) and 2016 (Josep Drudis). Out of these, the selected images were: 1979 (the oldest), 2016 (the most recent), 1993 and 2009 for being at intermediate dates. In order to get reliable measures, the image taken in 2016 (JD) was “deconstructed” by separating the stars from the nebula, and for all images, the stars were, separately, registered as well as the nebula (bright, very small knots were selected, in order to reduce the error). All shifts (in half pixels) needed to register the nebula, compared with the 2016 image (shift=0), were recorded and transformed into arc seconds (0.54 arc second/px). These movements were then converted, based on the tabulated distance to the nebula (800 light years) and the time elapsed between both images, into expansion rate in km/h. The values obtained were:

Year    Age(yr) Shift (arcsec)        Expansion rate (km/h)       Calc.Error

1979      37           4.6                               554,800                                        6%

1993       23          2.9                               566,978                                        9%

2009       7            0.9                               551,979                                      31%

All values are totally consistent with the expansion rates found in the literature, and the 1979 value is considered to be the most reliable due to its smaller error. Due to this, the final animation that shows the expansion of the nebula, has been prepared with the image of 2016 (basic, shift=0) and the “reconstructed” 1979, registering the 2016 stars with the 1979 stars and the 2016 nebula with the 1979 nebula. This has been made this way, in order to avoid an animation with big differences in color (1979 being B&W negative) and image density between both images. We are confident that it accurately represents the real position of NGC 2736 in 1979 and that the expansion shown is the real one.

Don Goldman and myself are very grateful to Dr. David Malin and Ken Crawford for promptly providing their highest resolution images (1993, DMalin and 2009, KCrawford), as well as Alan Strauss’s help for this project.

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