September 24, 2017

Variable Star Observing 101.

An artist’s conception of an accreting binary system. (Credit: NASA).

Bored and looking for something new to do in astronomy? Tired of hauling out that imaging rig you took out a 2nd mortgage for just to see “how M31 looks in my 10-inch SCT tonight?” Let me introduce you to the fun field of variable star observing, an exciting endeavor that you can actually contribute some real science to. But first, a little history;

Sky-watchers have noticed that certain stars vary in brightness since pre-telescopic times. Certain stars such as Algol (known as the “Demon Star” to the Arabs) gave themselves away to attentive observers of yore. Unfortunately, the concept of the variability of stars faced some stiff opposition in the West, due to the dogmatic factors that plagued scientific thinking right up until the Renaissance. Aristotle said that heavens simply weren’t supposed to change, and old thinking died hard. Opinions began to make a slow shift on August 3rd, 1596, when astronomer David Fabricius needed a reference star to contrast the planet Mercury with and used nearby Mira in the constellation Cetus. When Mira “disappeared” on successive nights, Fabricius assumed he had caught a spurious nova, but later observations by Johannes Holwarda in 1638 cinched the variability of this now famous star.

Beta Lyrae Animation (Credit: Public Domain graphic).

Other discoveries such as Chi Cygnus by G. Kirch in 1686 and R Hydrae by G.D. Maraldi in 1704 soon followed. By the year 1800, a dozen variable stars where known. But this class would go from an astronomical curiosity to an indispensable astrophysical tool in the coming century.

What good are variable stars? The utility of these stars became apparent with the discovery of a class of stars known as Cepheid variables via a brilliant piece of deduction by Henrietta Swan Leavitt in 1908. It was discovered that the luminosity and variation of these stars was intimately related. Once the first parallax to the nearest Cepheid variables was uncovered, a new rung could be added to the cosmic distance ladder, one that extended right out to galactic distances.

Eruption of T Pyx earlier in 2011. (Photo by Author).

The Basics: Daunted yet? Well here’s the good news…to start, all you need is clear skies and a good pair of eyes. In fact, I advise you to first do this before you move on to the fancy robotic rig that counts photons of thousands of stars simultaneously while you sleep. A simple approach will give you a hands-on feel that even modern professional astronomers fail to receive in their training. (This is because many astronomers come into the field via theoretical physics; that’s a separate post!)

This is the point at which we’ll evoke the AAVSO to the rescue: the American Association of Variable Star Observers is your friend in your variable star observing adventures. Founded in 1911, this outstanding organization serves as a clearing house for all things in the sky variable. The organization has evolved along with the technology, and features custom printable star maps, alert notices, ad-hoc observing campaigns, and a handy dandy variable star of the month featurette. Really, it’s THAT good. Dig around the site a bit, and you’ll get a feel for the community that is variable star observing. Fear not the wilderness… Astroguyz is here with you. I would advise starting into variable star observing the old fashioned way; by visual comparison. This can be done tonight with no equipment,  by monitoring naked eye stars such as Algol. A recent citizen sky campaign for the bizarre long term variable star Epsilon Aurigae trained folks to do just that. All you’re doing is comparing the brightness of a target star with the known magnitude of other stars in the field. This is a kind of “this is brighter/fainter than that,” eye-chart test; seasoned observers can pick out a variation of 0.1 of a magnitude, which is pretty darned good. Remember, a 1 magnitude change is equal 2.5x difference in apparent brightness. Get enough naked eye observations, and you can build a pretty smooth light curve over time. The AAVSO still takes and encourages observations done via naked eye; it’ll also give you a feel for how astronomers of yore operated. Sketching the field for later comparison is also a great way to get to know the sky intimately, and may just allow you nab that undiscovered nova that’s out of place…

Not all variable stars are created equal. Some vary over hours, some years. Some are extremely predicable, while some types are prone to fits of outburst activity after years of slumber. Some different classes include eclipsing binaries, Cepheid variables, cataclysmic variables, novae and recurrent novae.  Eclipsing binaries are just what their name suggests; binary stars that happen to be inclined to our line of sight. Cepheid and RR Lyrae-type variables have a predictable rhythmic pulse that makes them useful astrophysical tools for measuring astronomical distances. Recurrent novae and cataclysmic variables are more exotic and are often due to a companion star dumping material into an accretion disk which builds up and periodically ignites. The rarest types of variables that amateurs monitor are quasars, novae and extragalactic supernovae which often burst onto the scene.

Ready to go hunting? Here is a quick sampler of 12 classic variables for beginners to whet the visual appetite;

Name

Type

Right Ascension

Declination

Range

Period

Algol

Eclipsing Binary

03:04:54

+40 46’ 00”

2.1-3.4

2.9 days

Notes: The “Demon Star”

Epsilon Aurigae

Eclipsing Binary

05:01:58

+43 49’ 24”

2.9-3.8

27 Years

Notes: Long term eclipsing binary: dims 640-730 days every 27 years.

R Leonis

Mira-type

09:47:33”

+11 25’ 44”

4.3-11.7

312 days

Notes: Red-hypergiant star.

U Geminorum

Dwarf Nova

07:55:05

+22 00’ 05”

15-9.0

100 days

Notes: White dwarf with a red dwarf companion star.

Delta Cephei

Cepheid Var.

22:29:10

+58 24’ 55”

3.6-4.3

5.4 days

Notes: Classic Cepheid variable and the first identified.

Beta Lyrae

Eclipsing Binary

18:50:05

+33 21’ 46”

3.4-4.6

12.9 days

Notes: Semi-detached binary star system.

Chi Cygnus

Mira-Type

19:50:34

+32 54’ 51”

5-13.0

407 days

Notes: Visible to the naked eye at maximum.

V Hydrae

Carbon Star

10:51:37

-21 15’ 00”

7.0-11.5

530 days

Notes: B-V Color index +5.5 and one of the reddest stars in the sky.

R Leporis

Carbon Star

04:59:36

-14 48’ 23”

5.5-11.7

430 days

Notes: Hind’s Crimson Star.

Mira

Mira-prototype

02:19:21

-02 58’ 39”

2.0-10.1

1,000days

Notes: Variability can range from 80-1,000 days.

RR Lyrae

RR-Prototype

19:25:28

+42 47’ 04”

7-8

13 hours

Notes: Another prototypical standard candle.

These stars listed above range from naked eye classics (Algol) to the historically significant (Delta Cephei) to those that may unlock key secrets of astrophysics (Epsilon Aurigae)… if your favorite isn’t listed, let us know!

Hind’s Crimson Star. (Photos by Author).

Ready for more? Advanced techniques available to backyard observers always run a neck-and-neck arms race with the big boys; some observers now have such technology at their disposal that would have been unimaginable even a decade ago. Each technique could (and hopefully will) be a blog post onto its own; the scope here is to merely mention what’s out there. Many amateurs now have access to photometers, and are routinely making high quality photometric observations of variable stars. Some have even coupled this with wide field robotic surveys, which is similar to the modern technique that professional observatories now employ. The realm of exoplanet hunting not even out of the question for advanced amateurs! One of the most intriguing projects we’ve heard of allows amateurs to simply use wide field DSLR photography and some basic processing techniques to do photometry; this solely involves tech that many average mortals may have lying around. Finally, in the “Bizzaro Astronomy” category, we’ve built a cardboard interferometer to separate binaries with a separation of less than 1” that could also be employed for spectroscopic variable star observing, but that’s another post…

Backyard Interferometry! (Photo by Author).

Of course, no 101 post would be complete without the exhaustive list of resources;

-AAVSO: The gathering point of all things variable.

-And the AAVSO on Twitter, which is designed just for this sort of fast breaking event(s).

-An exhaustive list of variable stars.

-SLOOH: A robotic telescope that YOU can use!

-Cloudy? Try amateur light-curve sleuthing at Planet Hunters.

Also, keep an eye on this space as we periodically feature a variable star favorite of the season when time permits. Happy hunting, and let us know of your exploits!

Trackbacks

  1. [...] we all go there; you may have even read a recent “how-to” post of our not-so-secret addiction: variable star observing. Unlike much of deep sky astronomy, variable stars actually change and show activity on time scales [...]

  2. [...] will scarcely notice this over decades. One spectacular exception, however, is the realm of variable stars. One of the most famous and easy to observe is the “demon star” known as Algol in the constellation [...]

  3. [...] to the fascinating world of eclipsing binary stars. This week, we’d like to turn our attention towards a special star in [...]

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