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Messier Monday: The Little Dumbbell Nebula, M76

It’s the faintest and hardest object to see in the entire catalogue, but the rewards — and knowledge you gain — are priceless!

“If there is nothing new under the sun, at least the sun itself is always new, always re-creating itself out of its own inexhaustible fire.” –Michael Sims

The night sky is a dazzling array of wonders, with thousands of stars visible to the naked eye alone on a clear, dark night. But none of these points of light are going to live forever; none of these cosmic fires — despite the billions or even trillions of years some of them have left to live — will burn forever. And when they do, their ends will be spectacular. Among the 110 deep-sky objects in the Messier catalogue, which consists of many of the brightest and most prominent night sky wonders visible from Earth, only five of them are remnants of dead or dying stars.

Image credit: Mike Keith, of http://cosmicneighbors.net/PeriodicMessier.htm.

One of them — the Crab Nebula — is an ultra-massive star that died in a supernova, while four others are Sun-like stars that have recently blown off their outer layers while their inner layers contract down to a white dwarf. Today’s object, Messier 76, is known as the Little Dumbbell Nebula due to its resemblance to the brighter Dumbbell Nebula, Messier 27, but it itself happens to be the faintest of all Messier objects! In order to find it, make sure the Moon hasn’t yet risen, and look for the big “W” in Cassiopeia to start.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Underneath the “W” are four bright stars in a row: Mirphak, Almaak, Mirach and Alpheratz. The bottom of the first “V” in the “W” is the star Ruchbah, while the relevant star in the line we just took a look at is the second one, Almaak. What you want to do, if you’re searching for this elusive stellar remnant, is to look midway between these two stars, at the imaginary line connecting them.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Messier 76 is roughly halfway between them, but it’s incredibly difficult to find without a guide. Lucky, the Universe has given us one in the form of other bright stars easily identifiable in the sky! Almost midway between Ruchbah and Almaak — slightly closer to Almaak — are a pair of prominent stars: the blue φ Andromedae and the orange (and slightly brighter) 51 Andromedae.

If you point your telescope — and don’t even mess around with binoculars unless you’ve got a pair like this — at the blue one, you’re almost there.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Just a little bit east of φ Andromedae is a star right at the limit of human vision under ideal conditions: HIP 8063, an orange giant. And right next to it, just a tiny fraction of a degree north, lies Messier 76, the Little Dumbbell Nebula.

Image credit: Fred Espenak of Astropixels, via http://astropixels.com/planetarynebulae/M76-01.html.

It was barely visible to Messier with the equipment at his disposal, and in fact it was his assistant — Pierre Méchain, who by many accounts was the superior observer — who first discovered it. Here’s Messier’s account:

Nebula at the right foot of Andromeda, seen by M. Méchain on September 5, 1780, & he reports: “This nebula contains no star; it is small and faint”. On the following October 21, M. Messier looked for it with his achromatic telescope, & it seemed to him that it was composed of nothing but small stars, containing nebulosity, & that the least light employed to illuminate the micrometer wires causes it disappear: its position was determined from the star Phi Andromedae, of fourth magnitude.

What’s really amazing about this is the difference you’ll find if you look with simply a monochrome (e.g., unfiltered) eyepiece…

Image credit: forum user Starf of UK Astro Imaging, via http://ukastroimaging.co.uk/forums/index.php?topic=50603.0.

where you can see all the wavelengths of light equally, versus what you can see if you look for light in very particular wavelengths: if you filter by various colors and then stack them together!

Image credit: Jeff Woods of flickr, cropped by me, via https://www.flickr.com/photos/41622748@N05/8111671153/.

What you’ll find, instead, is an incredibly intricate tapestry of color. Why that’s the case is an even more remarkable story: due to the fact that this was once a Sun-like star — a star that burned hydrogen into helium and then helium into carbon, undergoing a little bit of further fusion into nitrogen, oxygen and neon — that ran out of fuel and then blew off its outer layers, what we’re seeing is a concentration of different elements the farther away we look!

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona, via http://www.caelumobservatory.com/gallery/m76.shtml.

The outer layers, in red, highlight the hydrogen gas from what was once a Sun-like star, while the inner layers in green and blue showcase the regions rich in nitrogen and oxygen.

There’s also an incredibly asymmetric structure to this nebula: something that appears to be two main lobes with a rectangular structure at the center. As it turns out, this is an optical illusion caused by this nebula’s orientation to us. But the right type of view can help us see what’s really going on.

Image credit: Terry Evans of http://bte999.jalbum.net/Terry’s%20Astrophotos/Messier%20Catalogue/slides/M76.html.

Like many dying stars, this has created a bi-polar planetary nebula, where you get two “lobes” of gas on either side, with a donut-shaped region at the center. Viewed right down the center of one of the lobes, this would look like a ring, but since this is viewed from the side, we see mainly the lighter, rapidly expanding gas farther away with the heavier nitrogen and oxygen closer to the center.

Image credit: Robert J. Vanderbei of Princeton, via http://www.princeton.edu/~rvdb/images/NJP/m76.html.

Almost all of the visible light emitted from this nebula comes from a single emission line: the green doubly-ionized oxygen line (O[III]-line) at 500.7 nanometers, with the central star very likely a binary star system. It’s only an orbital system like this — where there are likely two large masses — that are thought to cause a non-spherical nebula like this.

Image credit: Don Goldman, via APOD at http://apod.nasa.gov/apod/ap081121.html.

In fact, it’s theorized (and argued) that non-binary systems, like our Sun, might not produce bright planetary nebulae like this, since they won’t have a strong gravitational source to help throw that expanding gas far out into space!

As it stands, the binary star at the heart of the Little Dumbbell Nebula is doing an outstanding job: despite the nebula being only a few arcminutes in size, it’s estimated to be 2,500 light years distant, meaning the visible gas from this star has been flung to a distance greater than a light year in diameter, or more accurately, some 2,000 times the distance that Pluto is from the Sun!

Image credit: Paul Tankersley, via http://ptank.blogspot.com/2012/04/m76.html.

You don’t have to worry about the fact that there might only be one star at the center; there are images out there that clearly show that not only are there two stars at the center of this nebula, but that they’re two different colors! The blue one, by the way, is the one contracting down to become a white dwarf; the color/temperature is a dead giveaway.

Image credit: Stefan Seip, magnification by me, via http://apod.nasa.gov/apod/ap061102.html.

As you might expect from the hot, diffuse nebula, its nucleus is at a much higher temperature than any known star would be, with an average gas temperature of around 88,000 K, or around 15 times the surface temperature of our Sun.

Back in the 18th and 19th centuries, it was thought by most that this was two nebulae, but the advent of astrophotography showed that it was, in fact, a single nebula, and (correctly) identified as a planetary nebula approximately only a century ago.

Image credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA), from Spitzer (in the IR), via http://www.spitzer.caltech.edu/images/5686-sig13-015-Little-Dumbbell-Nebula.

If you look closely for other elements, like sulphur, you can find them as well, as J-P Metsavainio did, below (in the reddish hues) and along the central “donut” part of this nebula.

Image credit: J-P Metsavainio, via http://astroanarchy.blogspot.com/2010/11/m-76-little-dumbbell-nebula.html.

If we were going to have one nebula that the Hubble Space Telescope could shed some remarkable light on, I would have expected it to be this one. And yet, it never took a wide-field image of this nebula at all. What it did manage to accomplish was to view the very center of this nebula, and find that the “binary star” we identified was actually at least a trinary system!

Image credit: NASA / ESA and the Hubble Legacy Archive, via http://hla.stsci.edu/hlaview.

But there are — fortunately — a great many “amateur” views that are spectacular. I’ve shown you a great many, but here are two more:

Image credit: contributor Skyman1138 of Meade4m forums, via http://www.meade4m.com/index.php?/gallery/image/224-m76-little-dumbbell-nebula/.

and finally, one of my favorites and perhaps the most recent spectacular image of the Little Dumbbell Nebula, Fred Herrmann’s composition for Astronomy Magazine, cropped and enlarged by me, below.

Image credit: Fred Herrmann, 2014, via http://cs.astronomy.com/asy/m/nebulae/489616.aspx.

And with that, we come to the end of our final Planetary Nebula for our Messier Monday series, and find ourselves with only two objects left! In the order in which they were added to the catalogue, here are all the Messier objects we’ve covered so far:

So come back next week for our penultimate entry, where a late-rising Moon means a spectacular star cluster — one of the best in the sky — will be your Messier treat!

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