ASTRONOMY N('YY

The swirling hydrogen gas on the surface of the star Rho Gassiopeia is turbu at best; a seething, frothing mass, normally radiating at a temperature of Z,OOO Kelvin. Keith Cooper explains what happens when, periodically, Rho Gassiopeia's turbulence gets the better of it.

he onset of an eruption is sig- naled when gas begins to swirl and fall in towards the centre of the star.The pressure rises, compressingand heating the gas,causing the starto brightenbriefly.Essentially, the star is coiling up like a spring on a trampo- line and.within months.it bouncesback. In the summer of the year 2000hydro- gen gas measuringten thousandEarth masseswas blastedout from the surface ofRho Cas,the largestsurface eruption ever observed on a star. A shock wave of material now encircles the star that could form aplanetarynebulaearound Rho Cas similar to the Homunculus Nebula around the star Eta Carinae.During the eruption Rho Cas'surface temperature dropped three thousand degreesto a relatively cool4,000It andalreadythe star's surface is recoiling again,perhaps preparing for The constellation of Cassiopeia, home of the hypergiant Rho Cas. lmage: Nik Szymanek. an evenbigger eruption. Welcometo the explosivelife of ahypergiant. ity of these stars,the individual stepson of a solar mass,and in the spaceof ten Rho Casbelongs to a rare breed of stars the path to this void haveyet to be seen. thousandyears it will undergo two hun- known as'hypergiants', of which only Very little is known about the stellar dred eruptions. In total, if each eruption twelve are known to exist in the Galaxy, mechanics involved, or even whether is similar in size,this will accumulateto 'yellow and only sevenare hypergiants' hypergiants can actually reach the Void the equivalent of twenty solar masses like Rho Cas.These stars are extremely before they blow themselvesapart in a ejected from the star. luminous. Indeed,Rho Casis five hundred supernovaevent, all of which makes the However, Rho Cas is a huge star, and thousandtimes more luminous than the recentmass-ejectionof Rho Cascrucialin will be rapidly using up all the hydrogen Sunand, even at a distanceoften thousand studiesof this type of star. within its core.The helium flashwill come lightyears,itcan be detectedbythe naked Hypergiant stars must lose sufficient quickly, as will death through supernova eyein the constellationofCassiopeia. masstocrosstheVoid boundary, which is explosion,which may indeed already what Rho Cashas been doing,not only in havehappened. Therefore, the probabil- Headingjforthe void the year 2000but alsoback in 1946,when ity of Rho Cas evolving into the Yellow Placing yellow hypergiants on the astronomersobserved a similar erup- EvolutionaryVoid is low, if that was ever Hertzsprung-Russelldiagram is difficult tion. Then an eruption causedthe star to possible in the first place.As such, Rho asthey do noteasilyfitinto the previously alter from an F-Class to an M-Class, Casis now the number one suspectto be acceptedstellar models. Sometheorists dropping in temperature by 2,000 the next supernovain the Galaxy. speculatethatthey could be somekind of degrees.The eruption in 2000was even The Yellow Evolutionary Void is aptly evolvedstar, between the red supergiant bigger. Becausewe seethe star as it was named,for at presentit doesseem to be a phaseand the blue supergiantphase. All ten thousandyears ago, ifwe cantake the void, absentof any stars. One star that has the known yellow hypergiants seem to last two eruptions as being typical of the comeclose to it is the star HR 8752,which heading for what is known as the Yellow star,Rho Cas could have alreadyejected is teeteringupon the edgeof the Void.At Evolutionary Void', where the atmos- so much material that the star has all but this particular stage in a hlpergianfs life, pheresofthe hypergiantsreach a com- dissipatedinto the glowing relic of a cool, starsapproaching theVoid showincreased paratively more settled state.The prob- billowing cloud of hydrogengas. On each activity,with regular variationsin tempera- lem for astronomers is that due to the rar- eruption the star is jettisoning one tenth ture and densityinversions which may

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The Wlliam Herschel Telescope on La Palma was used to study Rho Gassiopeia. lmage: Nik Szymanek

accountfor some of the instability. HR away from the star in a shell moving at 8752is alsoshedding mass, though not fourtimes the speedof sound. quite as spectacularlyas Rho Cas. It is thereforeno surprisethat there are As astronomersbecome more awareof no starswith luminosities of one million these stars,spectroscopic analysis of solar luminosities or more becausethey them over the coming years could begin simply don't hang around long enough to clear up the muddy waters that sur- for astronomersto detect them. Higher round their existence.More crucially, it massand higher luminosity starshave may assistin solving one of the enduring short lifetimes as it is, and if they keep problemsin stellar astrophysics- why erupting like Rho Cashas, they'Il be there areno starswith luminositiesover a aroundfor an evenshorterperiod. We are million times that of the Sun. fortunatethat there are severalgood can- The astronomerswho observed the didatesfor observation- Rho Cas,Eta Rho Cas eruption, led by Alex Lobel of Carinae,HR 8752and P Cygni amongst the Harvard-SmithsonianCentre for others - just at a time when we have Astrophysics,calculated how much mass developedequipment sensitive enough had been ejectedby using spectroscopy. to examinethese stars in great detail. The eruption had altered Rho Cas'spec- If astronomers do finally see one of tra markedly,and a computermodel that them explode,itwill be the closestsuper- matchedthe new spectragave the aston- nova to the Earth since the star that ishing figure of ten thousandEarth explodedto create the Crab Nebula in masses ejected from the star. 1054,and itwillyield anwnazingamount Furthermore, the spectroscopicanalysis of informationregarding supernovae in showeddark bandsin the opticalspectra, general.It may also clear up other rid- indicating the formation of moleculesof dles, such as whether gammaray bursts titanium-oxide in Rho Cas' expanding really do originate from extremely mas- atmosphere that did not usually exist sive starsgoing nova. there. The samebands were also seenin In that case,perhaps it is fortunatethat A science team led by Alex Lobel watched 1946,and are a symptomof the dimming we are not too closeto any of these stars Rho Cas develop dark bands in its optical spectrum. The last time this happened was and cooling ofthe star during its erup- when they get hyper. during 1946 when the star's atmosphere tion. The shocloravethat blastedout the 'chilled' to 3OOOK. lmage sequence cour- ejectedhydrogen was pushing the gas KeithCooper is a freelancescience writer. tesy: Alex Lobel and the Harvard, Smithsonian Center for Astrophysics.

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