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GOLDILOCKS IN THE DUMBBELL NEBULA

ROBERT J. VANDERBEI

The Dumbbell Nebula (M27) is one of my favoritenebulae. I’ve taken many pictures of it over theyears. I like to joke to my friends that it is namedafter me—the dumbbell. One night back inSeptember, I invited my freshman seminarastronomy class out to my house to show themhow to do some astrophotography. Eightstudents came. We took some snapshots ofstars and stuff and then I suggested that we takea sequence of pictures of M27. So, for the nextfew hours, that’s what my telescope and cameradid while my students and I had some tea andbrownies in the house and talked aboutastronomy.

The resulting picture came out prettygood—almost as nice as one I took two yearsearlier with the same telescope and camera. So,I thought it would be fun to combine the twopictures into one thereby achieving a picture witha longer total exposure time. Of course, the firststep in this process is to load both pictures into astacking program and then align the images. Inthe alignment process, I flipped back and forthbetween the two images a few times and Inoticed that a fairly prominent star in the 2016image was totally missing from the 2018 image.This caught me completely by surprise. I’d neverseen a star totally disappear like that before.

I uploaded the star-showing picture toastrometry.net to get an astrometricallyannotated fits-file version of the picture that I wasthen able to load into my computer planetariumprogram, Cartes du Ciel (aka Star Charts), andthen determine the right ascension (RA) anddeclination (Dec) of the star of interest. Next, Ivisited the Simbad website and entered thesecoordinates. Simbad told me that the star is aknown Mira-type variable star and provided linksto further information about this star.

Interestingly, this variable star in M27 wasdiscovered only about thirty years ago byanother amateur astronomer, Leos Ondra.

Prior to my “(re)discovery”, I was completelyunfamiliar with Mira type variable stars. I haveover the years had an interest in globular clustersand the RR-Lyrae variable stars containedtherein. These variable stars are noticeably blue,they have a period that’s typically shorter than aday, and they vary in brightness by only amodest amount. Mira variables, on the otherhand, are red giants with long periods (on theorder of a year) and dramatic dips in brightness,generally a few orders of magnitude. It is this lastproperty that surprised me the most. It meansthat a Mira variable star can seemingly disappearand then return. Its brightness can vary thatmuch! And, as shown below, this is exactly whatwe see in the pictures I took.

A few days after my discovery, I blinked the twopictures for my class. The students were amazedat how this star appeared and disappeared.

One final note... In blinking the two pictures, Ialso noticed that there’s a second Mira variablestar in this field of view. It’s over toward the upperleft side. This star is dim, but not absent, in the2016 picture and it is significantly brighter in the2018 picture.

PS. I’m a mathematician by training and so I likevariables of all kinds, including variable stars.

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Picture of the Dumbbell Nebula taken August 4, 2016. Image taken by the author.

Picture of the Dumbbell Nebula taken October 1, 2018. Image taken by the author.

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Cropped close up showing Goldilocks.

Cropped close up of a second Mira variable located near upper left part of original picture.