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GOLDILOCKS IN THE DUMBBELL NEBULA ROBERT J. VANDERBEI The Dumbbell Nebula (M27) is one of my favorite nebulae. I’ve taken many pictures of it over the years. I like to joke to my friends that it is named after me—the dumbbell. One night back in September, I invited my freshman seminar astronomy class out to my house to show them how to do some astrophotography. Eight students came. We took some snapshots of stars and stuff and then I suggested that we take a sequence of pictures of M27. So, for the next few hours, that’s what my telescope and camera did while my students and I had some tea and brownies in the house and talked about astronomy. The resulting picture came out pretty good—almost as nice as one I took two years earlier with the same telescope and camera. So, I thought it would be fun to combine the two pictures into one thereby achieving a picture with a longer total exposure time. Of course, the first step in this process is to load both pictures into a stacking program and then align the images. In the alignment process, I flipped back and forth between the two images a few times and I noticed that a fairly prominent star in the 2016 image was totally missing from the 2018 image. This caught me completely by surprise. I’d never seen a star totally disappear like that before. I uploaded the star-showing picture to astrometry.net to get an astrometrically annotated fits-file version of the picture that I was then able to load into my computer planetarium program, Cartes du Ciel (aka Star Charts), and then determine the right ascension (RA) and declination (Dec) of the star of interest. Next, I visited the Simbad website and entered these coordinates. Simbad told me that the star is a known Mira-type variable star and provided links to further information about this star. Interestingly, this variable star in M27 was discovered only about thirty years ago by another amateur astronomer, Leos Ondra. Prior to my “(re)discovery”, I was completely unfamiliar with Mira type variable stars. I have over the years had an interest in globular clusters and the RR-Lyrae variable stars contained therein. These variable stars are noticeably blue, they have a period that’s typically shorter than a day, and they vary in brightness by only a modest amount. Mira variables, on the other hand, are red giants with long periods (on the order of a year) and dramatic dips in brightness, generally a few orders of magnitude. It is this last property that surprised me the most. It means that a Mira variable star can seemingly disappear and then return. Its brightness can vary that much! And, as shown below, this is exactly what we see in the pictures I took. A few days after my discovery, I blinked the two pictures for my class. The students were amazed at how this star appeared and disappeared. One final note... In blinking the two pictures, I also noticed that there’s a second Mira variable star in this field of view. It’s over toward the upper left side. This star is dim, but not absent, in the 2016 picture and it is significantly brighter in the 2018 picture. PS. I’m a mathematician by training and so I like variables of all kinds, including variable stars. 1

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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.