PAUL ABEL

PRO AS­TRONOMER

Sky at Night Magazine - - LETTER FROM THE EDITOR -

Paul’s Vis­ual Ob­serv­ing Guide con­tin­ues with a look at how you can track the wax­ing and wan­ing of vari­able stars.

At first glance, the stars in the night sky all ap­pear alike – cold points of light ly­ing far away. How­ever, af­ter a few min­utes sub­tle dif­fer­ences be­come clear: some are dif­fer­ent colours, some ap­pear fainter than oth­ers, and so on. But there is one prop­erty of some stars that can take longer to pick up: not all stars stay at the same bright­ness all the time.

Th­ese won­der­ful ob­jects are called vari­able stars. Many of them ‘vary’ – that is, com­plete a cy­cle where they dim and then brighten again– over the course of a night, but some take over a year. Study­ing th­ese stars has pro­vided some fas­ci­nat­ing in­sights into mod­ern as­tro­physics and the pri­vate lives of stars.

It took a sur­pris­ingly long time for vari­able stars to be iden­ti­fied. In the mod­ern era the first one was dis­cov­ered by Jo­hannes Hol­wards in 1638 when he no­ticed that Omi­cron Ceti var­ied over the course of 11 months. Since then, many thou­sands of vari­able stars have been iden­ti­fied. While the as­tro­physics of vari­able stars is of great in­ter­est to pro­fes­sional as­tronomers, most ob­ser­va­tions of th­ese stars come from am­a­teurs.

The main fo­cus of vari­able star ob­ser­va­tion is mak­ing es­ti­mates of mag­ni­tude. Th­ese es­ti­mates can be plot­ted against time to gen­er­ate a light curve like the one above, which re­veals the pe­riod of time in which the star varies, its bright­est and faintest mag­ni­tudes, and any long-term fluc­tu­a­tions hid­den

in its con­stant changes. There are many dif­fer­ent types of vari­able star, and or­gan­i­sa­tions such as the Bri­tish As­tro­nom­i­cal As­so­ci­a­tion (BAA) and the Amer­i­can As­so­ci­a­tion of Vari­able Star Ob­servers (AAVSO) run a vast num­ber of vari­able star cam­paigns, in which stars are mon­i­tored by many am­a­teur as­tronomers,

and their ob­ser­va­tions are used to gen­er­ate the much-needed light curves.

Get­ting started

Be­com­ing part of the move­ment that mon­i­tors th­ese stars is sim­ple and there are many vari­ables you can make a start with. Gary Poyner of the BAA Vari­able Star Sec­tion has pro­vided some good tar­gets for begin­ners on the op­po­site page; as a rule it is best to start with a cou­ple of stars then build up to a man­age­able list.

Ob­serv­ing vari­able stars doesn’t re­quire ex­pen­sive kit: you can ob­serve with a small or large tele­scope – there are plenty of stars to go around! It’s a good idea to have a se­lec­tion of eye­pieces as it’s eas­ier to lo­cate stars at low mag­ni­fi­ca­tion, then switch to high mag­ni­fi­ca­tion to home in on the ones close to the lim­it­ing mag­ni­tude of your tele­scope. A sturdy note­book to act as your log, a re­li­able pen and a red light are also help­ful.

A cor­rect star chart is also es­sen­tial. It must show the lo­ca­tion of the vari­able star in the sky, and in­di­cate com­par­i­son stars to use for mak­ing mag­ni­tude es­ti­mates. This is im­por­tant since we can’t use other vari­able stars for mak­ing com­par­isons. The BAA Vari­able Star Sec­tion and the AAVSO are the two most re­li­able sources for charts – just go to their web­sites, find the star you want a chart for and print it out. Re­mem­ber to record which chart you are us­ing in your log­book.

Make sure you’re fully dark adapted be­fore you start ob­serv­ing. The hard­est part is track­ing down the star in ques­tion, and for this I al­ways use a lower power eye­piece (100x or so). If your tele­scope has

set­ting cir­cles, use them to get your scope aimed at the right lo­ca­tion, oth­er­wise use binoc­u­lars to track down the right area of sky. Don’t be dis­heart­ened if it takes a num­ber of at­tempts to find your cho­sen star– you’ll get there in the end.

Con­tribut­ing to science

The main task is to es­ti­mate the mag­ni­tude of the vari­able (see ‘Es­ti­mat­ing star mag­ni­tudes’, above, to find out how). Record th­ese in your log­book at the tele­scope along with the date, time (in UT) and tele­scope de­tails. You will need to do this for each ob­ser­va­tion, and I find it best to keep a sep­a­rate book for each vari­able star.

It’s es­sen­tial that you re­port your ob­ser­va­tions so that they can be used by the whole com­mu­nity. Both the BAA ( w w w.britas­tro.org/vss ) and the AAVSO ( w w w.aavso.org ) will be grate­ful for them. Th­ese or­gan­i­sa­tions have knowl­edgable as­tronomers who can help you with any prob­lems you may ex­pe­ri­ence. You can upload your ob­ser­va­tions us­ing their web­sites, or al­ter­na­tively you can post a pa­per ver­sion. Both sites also al­low you to gen­er­ate light curves for your data so you can com­pare them with other ob­servers.

Vari­able star ob­serv­ing is very re­ward­ing. There is some­thing spe­cial about study­ing the life of a star that may be hun­dreds of lightyears away from us on planet Earth. Your cho­sen star will need ob­serv­ing in­def­i­nitely, so a vari­able star is a life­long friend.

Light curves can re­veal pat­terns: this one shows the fluc­tu­a­tions in the bright­ness of cat­a­clysmic vari­able star SS Cygni over a pe­riod of 10 years

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