CHAPTER 12: Observing Deep-Sky Objects

 
The following section describes in more detail an aspect of deep-sky observing that many amateurs find rewarding and, unlike most deep-sky viewing, actually has a scientific purpose. 

Variable Stars — Stars That Do Something

“I feel it my duty to warn any others who may show signs of star susceptibility that they approach the observing of variable stars with the utmost caution. It is easy to become an addict and, as usual, the longer the indulgence is continued the more difficult it becomes to make a clean break and go back to a normal life.” Writing in his 1965 autobiography Starlight Nights, Leslie Peltier, one of the greatest amateur astronomers of the 20th century, provides the warning based on his decades of watching variable stars.
Only a small subset of the amateur community gets hooked on variable stars, but those that do gather immense satisfaction from watching something actually happening in the heavens—and reporting on it. While today’s amateurs can apply the latest digital technology to record these stars, all the aspiring variable star observer really needs is a 3- (80mm) or 4-inch (100mm) telescope, keen eyes, and the patience to “stay with the program.” Simple or sophisticated, amateurs are the prime sources of data on the antics of variable stars. Few professional observatories have the staff or telescopes to devote to routine monitoring programs. They turn their telescopes to variables only when the word goes out from the amateur network that something strange is going on.
Stars that fluctuate in brightness, either predictably, or in random pulses, do so for one of several reasons. The cause provides the prime means of classifying variables. 

Eclipsing Binaries
Some stars are binaries that orbit so close to each other we can’t resolve them. Yet we can tell they are binaries because they eclipse each other as they dance around together in space. As one star hides the other the total light output of the system drops. If one star is dimmer than the other (the usual case) we see two repeating dips in brightness—a major dip when the dim star hides the bright one, and a lesser drop when the dim star becomes eclipsed. The two best examples are the naked eye stars Algol (Beta Persei) and Beta (β) Lyrae. Every 2.8 days Algol drops from magnitude 2.1 to 3.3, a major decline that lasts 10 hours and is easy to follow with just unaided eyes. Beta Lyrae, a system made of two stars so close they are in contact, drops from magnitude 3.4 to 4.3 every 12.9 days. 

Pulsating Variables
Most variables belong to one of several sub-classes of stars that pulsate in size and brightness. Typically, these “intrinsic variables” are giant stars passing through an unstable phase near the end of their lives. The best known are the Cepheids, yellow giant stars that pulse with periods of 1 to 70 days. The prototype example is Delta (δ) Cephei, a colorful double star whose yellow component drops smoothly from magnitude 3.5 to 4.4 then back up again over a predictable cycle of 5.36 days. RR Lyrae stars, named for their prototype, are hot white stars that cycle up and down by one to two magnitudes in mere hours.
At the other end of the time scale are “long-period variables” that take months or years to vary through a complete range in brightness. Like the archetype Mira, or Omicron (ο) Ceti, all are red giant stars suffering from some deep-seated instability that can make these stars vary widely over several magnitudes. Many are predictable but some of these unstable stars, the so-called irregular variables, pulsate with random fluctuations that defy explanation. Perhaps the star is expelling clouds of bright starstuff from a rapidly rotating surface. A good example is Herschel’s Garnet Star, Mu (μ) Cephei. You can find it shining anywhere between magnitude 3.6 and 5.1 depending on the star’s health that night.
A variation are the R Corona Borealis stars, sometimes called reverse novae. R Cor Bor, the prototype, usually shines at magnitude 5.8, just visible to the naked eye. Without warning it can drop suddenly to as dim as 14.4, out of range of a small telescope. The likely cause is a short-lived shell of dark sooty material cast off the star that temporarily hides it from view.

Naming of Variable Stars
The brightest variable stars are popularly known by their Greek letters, the so-called Bayer designations devised in 1603, such as Beta (β) Persei and Delta (δ) Cephei. In the mid-19th century Friedrich Argelander devised a method of labeling the increasing number of variables that he and other astronomers were finding. The first new variable found in a constellation was labeled “R.” Many of the R stars—R Leonis, R Leporis, R Scuti—are long period variables.
After R, the scheme works up the alphabet to Z, then restarts at RR, then RS, and so on. After RZ, the lettering restarts at SS, then ST, to SZ, back to TT, TU, etc. until it reaches ZZ. When astronomers realized they needed more letter combinations, they restarted at AA, AB, to AZ, then BB, BC, to BZ until QZ was reached. (Combinations starting with J are not used.) This arcane system gave 334 letter combinations, still not enough for a census of crowded constellations. More variable stars within a constellation are simply called V335, V336, etc. 


Eruptive Variables: Stars That Go Boom in the Night
The “high-profile” variable stars are the explosive kind. After years, centuries, even millions of years of quiet life, a star suddenly explodes in brilliance. The best of these give us a brief “new star” in the heavens. 

Flare Stars
Some red dwarfs let loose giant flares on their surfaces that can brighten the entire star by two or more magnitudes. While always telescopic targets, the attraction of flare stars such as EV Lacertae is that the fluctuations last mere minutes, making these superb objects for rapid-fire CCD imaging. 

Dwarf Novas
White dwarfs orbiting around more normal Sun-like stars often draw off material from their normal companions. A sudden infall of new material causes the dwarf to flare up in brilliance. A classic example is SS Cygni, which flares four magnitudes from 12.1 to 8.1 with some regularity every 50 days. 

Novas
These are the unpredictable variables. From out of nowhere, a previously anonymous dim star flares up as much as 15 magnitudes. A few reach first or second magnitude, changing the shape of a constellation in a way obvious to even casual stargazers. Novas as bright as that are once-a-decade events. Most are barely unaided eye or binocular objects at best, often as not first sighted by an amateur astronomer patrolling for stars “that don’t belong.” A nova is likely the result of another hungry white dwarf devouring an infrequent but massive infall of starstuff ripped from its companion star. Though dramatic the explosion is not enough to blow apart the dwarf star. It remains, perhaps to explode another day.

Supernovas
The most violent stellar explosions are also the most rare. When a massive star destroys itself and explodes as a supernova it briefly outshines all other stars in a galaxy put together. The last supernova visible to the naked eye flared up on the night of February 23, 1987 when a blue supergiant star exploded 160,000 light years away in the Large Magellanic Cloud, a companion galaxy to the Milky Way. No supernova has been sighted in our Galaxy for 400 years.
Nevertheless amateur and professional astronomers hunt for supernovas, and find them. They scan other galaxies, looking for single bright stars shining amid the haze of a dim galaxy. Backyard astronomers with CCD-equipped telescopes can record supernovas in galaxies as far away as 300 million light years. But as with watching more sedate variables, fancy equipment isn’t essential. In Australia, Reverend Robert Evans has discovered 42 supernovas (as of 2007) using no more than a 10- or 16-inch telescope wheeled into his driveway. His only other equipment is his eyes, aided by charts to confirm if a suspect star doesn’t belong, and a remarkable knowledge of the normal appearance of hundreds of galaxies.
Like searching for comets, hunting for “new stars” requires patience and perseverance. Even at that the “big ones” can get away. For a quarter of a century, Leslie Peltier watched the recurrent nova star T Coronae Borealis. It had last burst forth in 1866 and no one knew when it might explode again. Then on a cold night in February 1946 it shot up in brilliance. “And where was I, its self-appointed guardian on that once-in-a-lifetime night when it awoke? I was asleep!” Peltier has set his alarm for a 2:30 a.m. wakeup call to check on his flock of variables, including T Cor Bor. “Self-pity comes easy at 2:30 on a cold February morning so I went back to my warm bed...And thus I missed the night of nights in the life of T Coronae.” (from Starlight Nights)

The Pursuit of Knowledge: the AAVSO
In a more innocent time stargazing guidebooks had such charming names: The Friendly Stars by Martha Evans Martin (1907); Half-Hours with the Telescope by Richard Proctor (1868); and In Starland with a Three-Inch Telescope by William Tyler Olcott. Published in 1909, a year before Comet Halley’s return, Olcott’s little volume guided stargazers to a legion of double stars and a smattering of deep-sky objects. 
Inspired by a 1909 lecture on the subject given by Harvard College Observatory’s Edward Pickering, Olcott decided he should also observe variable stars. In March 1911 Olcott’s article “Variable Star Work for the Amateur with Small Telescopes” appeared in the American magazine Popular Astronomy. For the first time an amateur astronomer encouraged other amateurs to take up the cause of monitoring this class of stars. “Only by the observation of variable stars,” wrote Olcott, “can the amateur turn his modest equipment to practical use, and further to any great extent the pursuit of knowledge in its application to the noblest of the sciences.”
Amateurs heeded Olcott’s call and later in 1911 founded an organization to coordinate and collect observations from around America. In 1912 there were 19 observers. Today, the American Association of Variable Star Observers based in Cambridge, Massachusetts has hundreds of members in 40 countries who submit 300,000 observations a year. While many members keep track of variables using classic “eyeball” techniques Olcott would know well, an increasing number now employ CCD cameras to follow a star’s fluctuating brightness with fraction-of-a-magnitude accuracy. With robotic telescopes amateurs now patrol for supernovas in dozens of galaxies a night. With hobby-grade CCD cameras amateurs now seek out the short-lived optical afterglows of gamma ray bursts, the most powerful explosions in the universe. 
Today’s technologies give amateurs the recording power only professionals enjoyed a few years ago, and no astronomers could conceive of in Olcott’s time. Though the equipment may be far less modest than the 3-inch refractor Olcott employed, pursuing knowledge for “the noblest of sciences” remains the goal of the AAVSO’s network of members. 

For more information on observing variable stars, visit the American Association of Variable Star Observers, at ...
www.aavso.org
...the prime source for variable star information and coordinating amateur observations.http://en.wikipedia.org/wiki/Robert_Evans_(astronomer)http://www.aavso.orgshapeimage_2_link_0shapeimage_2_link_1

Belgian amateur astronomer Tonny Vanmunster used his private observatory to detect the exoplanet TrES-1 as it transited its host star. Amateur astronomers like Tonny, with well-equipped private observatories, are now capable of finding planets orbiting other stars, an amazing accomplishment made possible by CCD cameras and software.


Courtesy AAVSO and Tonny Vanmunster, CBA Belgium Observatory.

Click on the image above to go to the AAVSO webpage to download their introductory manual for aspiring variable star observers who wish to make visual estimates.


Courtesy AAVSO

Click on the image above to go to the AAVSO webpage to download their manual for variable star observers who intend to use CCD cameras.


Courtesy AAVSO

Click on the image above to go to the AAVSO webpage to download their manual for Supernova searching.


Courtesy AAVSO