Double Dipping: Spring 2010

Variable Stars Near Deep Sky Treats

March is typically the time of year observers take on the challenge of the Messier Marathon. The Messier Marathon is an attempt to view all 110 Messier objects in one night, and it takes the entirety of a night to achieve it. You have to start as early as you can, to get the first few objects, as Cetus and Pisces are setting, and the last few objects in Aquarius and Capricorn are a challenge, even if you’ve managed to say fresh after a whole night. I personally have never completed the marathon, and I can’t say I know anyone who has actually done it.

The important thing for this piece is that as warmer spring temperatures arrive, there is no shortage of deep sky treats to keep your friends and relatives entertained. Meanwhile, you and I will figure out how to squeeze in some variable star observing in between galaxies, nebulae and star clusters. It will be a bit of a challenge at this time of year because there just aren’t that many stars in the sky. As fate would have it, in spring the plane of the Milky Way lines up pretty closely with the horizon in early evening. So we don’t see the fullness of the galaxy in the sky. Instead, everywhere we look we are looking out into deep space, away from the center or arms of the Milky Way.

We ended our winter tour in Orion, so we’ll head east and start with the brightest star in the sky, Sirius. Glowing at magnitude -1.08, alpha Canis Majoris is a blinding object in a telescope. 8.5 arc minutes southeast, embedded in the glare from Sirius, is the UGZ type variable, HL CMa. Ranging from magnitude 10-14.5, it is an active, unpredictable star that is visible most of the time in 10” or bigger scopes. Four degrees south of Sirius is the beautiful star cluster, M41. It contains about 100 stars, including several red giants, the brightest glowing at magnitude 6.9, and situated near the cluster's center.

Heading north into Monoceros, we are going to luck out and observe two nebulae that have variable stars embedded inside them. It doesn’t get any easier than that. Two for one, two times in one constellation!

NGC 2346 Credit: NASA and STScI 

First on the tour is NGC 2346 also known as Butterfly NGC 2346. Discovered by William Herschel in 1785, NGC 2346 is fairly conspicuous and has been extensively studied. Among its most remarkable characteristics are, its unusual butterfly shape, and its unusually cool central star, V651 Mon, which varies irregularly from 11.3 – 15.3.

The variable is a spectroscopic binary and has a period of about 16 days. Variability is probably due to clouds of dust in orbit around the binary. As this dust is heated by the central star it glows brightly in the infrared.

The evolutionary track taken to create this dusty, double variable system is pretty interesting. Millions of years ago, the more massive star expanded to become a red giant, swallowing up its companion and setting in motion a process that created this butterfly effect. As the less massive companion spiraled inward towards its massive partner, rings of gas were expelled. As the hot core of the red giant was gradually exposed, powerful stellar winds inflated two huge bubbles of gas, producing the butterfly effect.

Continuing north through Monoceros, we come to NGC 2261, ‘Hubble's Variable Nebula’. It’s named after Edwin P. Hubble, who carried out some of the early studies on this object. The nebula is a fan-shaped cloud of gas and dust that is illuminated by R Mon, the bright star at the apex of the nebula. The variability comes from dense condensations of dust near the star casting shadows out into the nebula. As they move the illumination changes, giving rise to the variations first noted by Hubble. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.

NGC 2261 Credit: HST, NASA, JPL

The star, R Mon, can't be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to be a massive young star, with a mass of about 10 times the Sun, and may be only 300,000 years old. It varies irregularly from about magnitude 11- 13.2.

By this time, someone will have asked you, “Hey, what is that bright red star up there?” You can make their day as you tell them it’s the planet Mars. Unfortunately, it will have shrunken to about 10.4 arc seconds by mid-March and will be a pretty underwhelming site in most telescopes on an average night. But everyone will want to look anyway, just so they can say they saw it.

Lucky for us, about 35 arc seconds away to the southwest lies RR Cnc, a pretty interesting Mira with a range of 9.8-15. Mars never moves far from RT Cnc in March, so you can the red planet as a guidepost to one of my favorite red stars in Cnc.

Continuing east-southeast you’ll run smack into the enormous star cluster M44, the Beehive Cluster, or Praesepe (Latin for manger). This cluster is easily visible to the naked eye from suburban skies and contains several variable stars within it, including the eclipsing binary, TX Cnc and several Delta Scuti variables. But I’m going to point you to U Cnc, about 1 degree southwest of the center of the cluster. U Cnc is a Mira that varies from 8.5- 15.5. The charts and sequence have been extensively revised to cover the entire range adequately, making this star a joy to observe nowadays.

The other open cluster in Cancer is of course, M67. M67 is one of the oldest known open clusters, believed to be about 3.5 billion years old. Open clusters usually self-destruct before reaching this age. Being such a mature cluster, it has many evolved red giants within its population of 500 or so stars. To me, M67 is merely a star hop jumping off point to get to AK Cancri, a nice little cataclysmic variable in Cancer. For such a small constellation, Cancer is endowed with lots of cataclysmic variables, namely DW, AK, SY, GY, GZ, DE, CC, AT, EG, YZ, HH Cnc and SDSS0808.

AK Cnc is dangerously close to the ecliptic, so it’s not unusual to find an asteroid or two in the field when observing. You need to be sure you are actually witnessing an outburst or you may find yourself embarrassed by a pesky asteroid.

Regulus is another bright star in the spring sky, and makes a perfect place to begin your star hop to one of the all time favorite Miras in the AAVSO program, R Leo. From Regulus, alpha Leonis, you pan west to nu Leonis and then about the same distance again to 18 and 19 Leonis. R Leonis is just south of 19 Leonis. Once you’ve found the triangle of 18 and 19 Leonis and the star due east of this pair, you will never forget it. R Leo varies from 4.4 to 11.3, so it is always visible in a small telescope, and often only binoculars are needed.

Leslie Peltier recounts in ‘Starlight Nights’ how, after several unsuccessful attempts at locating it he finally found and observed his first variable, R Leonis, on March 1, 1918.  Like many of us, after that first success, we feel foolish when we realize our own mistakes that prevented our initial success. It was the same for Peltier. He wrote, “every March first since that night, whenever the skies were clear, R Leonis and I have recalled our first meeting by making a mutual estimate of our brightness”. Even though there isn’t a deep sky wonder of any consequence nearby, I think a spring night out with the telescope warrants a visit to R Leonis. As it happens, the active cataclysmic variable, X Leonis resides just northeast of the anonymous member of the trio with 18 and 19 Leonis, making this location a frequent stop on my patrols.

10.5 degrees north of R Leo is one of the finest galaxies in the sky that is not a Messier object, NGC 2903. How Messier ended up missing this bright galaxy has got to be purely bad luck. Three of Messier’s comet discoveries came very close to this galaxy and one, the second comet of 1760 actually passed within 1.6 degrees of it on the night of march 11-12, 1760. William Herschel eventually discovered it, in November of 1784.

NGC 2903 Credit: NOAO

On and two thirds of a degrees east-southeast is AB Leo, a semiregular variable with a range of 10.7-13.2. This one is easy to find, there are plenty of stars and asterisms to help identify the field, and the current AAVSO charts cover the range nicely.

Also in Leo is M105, a bright elliptical galaxy that actually looks like a comet in the eyepiece. It is flanked closely by NGCs 3384 and 3389, forming a nice trio of deep sky delights. 1.5 degrees to the northeast is W Leo, a Mira with a period of 392 days, varying between 8.9 and 14.8. There are so many galaxies in this region of the sky you’ll be forgiven if you end up off the beaten track and miss W Leo. But she’s a fine Mira worthy of your attention once you’ve taken in an eyeful of galaxies.

Heading due east through the lion’s tail, beta Leonis, and then another 10.5 degrees, lays M88. This is an impressive spiral galaxy. The infrequently outbursting cataclysmic variable, AL Com lies in the same field of view, and is not likely to be in outburst when you visit, so go ahead and take some time to wander in the ‘Realm of Galaxies’ here. There are a lot of Messier galaxies around here- M49, 58, 59, 60, 64, 84, 85, 86, 87, 88, 89, 91, 98, 99, 100, you get the idea. You may spend more time looking at galaxies than variables tonight, but we’ll let it go this time.

For this season’s ‘piesta resistance’ we have a trio made in heaven. The constellation Virgo features the giant spiral galaxy M61, host of at least six supernovae, the planet Saturn with its rings opening up from edge on and the excellent Mira variable SS Virginis. Together they form a nice triangle with something to please everyone at each apex. I would start with M61, to get people looking for faint fuzziness and to sharpen their eyepiece skills, then move to the variable before ruining your night vision staring at brilliant Saturn and his impressive rings.

We’ll continue this series with the final installment in summer. Until then, happy hunting.


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