Star of the Month

R Coronae Bo­re­alis – the sooty star that dims with no pre­dictable pat­tern

Sky at Night Magazine - - CONTENTS -

Corona Bo­re­alis, the North­ern Crown, is a semi­cir­cu­lar pat­tern of stars lo­cated off the north­east shoul­der of Boötes. It is prom­i­nent dur­ing spring but can be seen all year round if you are pre­pared to put in some early morn­ings. The shape en­closes an area of sky in which you may, or may not, see a faint star on the thresh­old of naked eye vis­i­bil­ity. This is R Coronae Bo­re­alis, a re­mark­able vari­able star with an ir­reg­u­lar pe­riod.

At its bright­est R Coronae Bo­re­alis is on the edge of naked-eye vis­i­bil­ity at mag. +5.9, but it is known for its un­pre­dictable drops in bright­ness, tak­ing typ­i­cally a year to re­cover. Back in 2007 it be­gan to fade to an un­prece­dented min­i­mum, reach­ing 15th mag­ni­tude by the sum­mer of 2009. It re­mained dim be­fore even­tu­ally ex­hibit­ing a slow bright­en­ing to mag. +12.0 by sum­mer

2011. Ap­prox­i­mately two years af­ter it started to climb, R Coronae Bo­re­alis had dimmed again. This col­lapse took it back to 14th mag­ni­tude. It re­cov­ered to 12th-mag­ni­tude by spring 2012, peak­ing at mag. +10.5 in spring 2013, then plunged back to mag. +14.5 that au­tumn. An­other brief peak oc­curred be­fore it fell back to mag. +15.0 by the end of 2013. By the start of 2015, it had al­most man­aged to

re­cover, reach­ing 7th-mag­ni­tude. This again was short-lived, but by the start of 2017 it was on the in­crease again.

The rea­sons for the vari­a­tions are re­lated to R Coronae Bo­re­alis be­ing a car­bon star. It sheds car­bon rich clouds into space which cool to form soot. The soot blocks the star’s light caus­ing it to dim but when the clouds dis­perse, it ap­pears to brighten once more.

You can check the cur­rent bright­ness of R Coronae Bo­re­alis by com­par­ing it to its neigh­bours

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