Is there a hole in the sun?

How a strange find­ing has left us ques­tion­ing our star

All About Space - - Contents - Re­ported by Kul­vin­der Singh Chadha

It may be the brightest ob­ject in our sky, but the Sun is a dark horse. The more we study it, the more quirks it re­veals and re­cently we have found quite a fun­da­men­tal one. A new way of modelling the Sun's at­mos­phere has fur­ther widened a gap in our knowl­edge of its core. This could have im­pli­ca­tions for the whole of as­tro­physics, not just the Sun, and it may mean hav­ing to re­think a whole cen­tury of so­lar physics. The age, mass and lu­mi­nos­ity of bil­lions of stars would have to be re­cal­cu­lated.

Sci­en­tists from the Max Planck In­sti­tute of As­tro­physics (Aldo Serenelli and Martin As­plund), Yale Univer­sity (Sar­bani Basu) and Wi­chita State Univer­sity (Ja­son Fer­gu­son) for­mu­lated a new

model of the Sun's sur­face, its pho­to­sphere, that was dy­namic and three di­men­sional. Pre­vi­ous mod­els had been static and one di­men­sional. In stars, el­e­ments heav­ier than he­lium are called 'met­als' by as­tro­physi­cists whether they're me­tal­lic or not. The 1D mod­els had a higher pro­por­tion of th­ese than the 3D one found was re­quired. Some have put for­ward the idea that this means that a cer­tain amount of mass has 'gone miss­ing' from the Sun; up to 1,500 Earths' worth.

How­ever, one of the sci­en­tists in­volved in the work, Dr Ti­ago Pereira of Oslo Univer­sity has poured cold wa­ter on this no­tion. “I find the idea of miss­ing mass in the Sun to be ridicu­lous. Some peo­ple have made mod­els of the so­lar in­te­rior. Those mod­els are in­com­plete and lead to more mass than ob­served. This doesn't mean any­thing is miss­ing, only our knowl­edge is in­com­plete.“

Un­til the early 20th cen­tury no one re­ally knew what pow­ered the Sun's in­te­rior. Bri­tish sci­en­tist Lord Kelvin and the Ger­man sci­en­tist Her­mann von Helmholtz both pro­posed that a ball of gas of the Sun’s size could ef­fi­ciently gen­er­ate heat by con­tract­ing un­der its own grav­ity. The fig­ure cal­cu­lated by Kelvin in 1862 came to 100 mil­lion years; far shorter than the 300 mil­lion year-es­ti­mate of Earth's age by ge­ol­o­gists of the time. But Kelvin also knew the lim­its of knowl­edge of the Sun's pro­cesses at that time, say­ing in March's Pop­u­lar Lec­tures and Ad­dresses, vol­ume 1, 2nd edi­tion: “Mu­tual grav­i­ta­tion be­tween the dif­fer­ent parts of the Sun’s con­tract­ing mass must do an amount of work, which can­not be cal­cu­lated with cer­tainty, only be­cause the law of the Sun’s in­te­rior den­sity is not known.“So this couldn't be the way the Sun pow­ered it­self.

It wasn't un­til Ein­stein pub­lished his work on spe­cial rel­a­tiv­ity in 1905 that the true mech­a­nism was dis­cov­ered. De­riv­ing the equa­tion E=mc2 from his work, Ein­stein showed that mass and en­ergy were two sides of the same coin, and that a small amount of mass could be con­verted into enor­mous amounts of en­ergy. This was the foun­da­tion on which sci­en­tists from the 1920s to the 1950s for­mu­lated stel­lar nu­clear fu­sion equa­tions. They showed the Sun is pow­ered by hy­dro­gen nu­clei

(74.9 per cent of its mass) fus­ing into a heav­ier el­e­ment, he­lium (23.8 per cent of its mass), and re­leas­ing en­ergy in the process. The other 1.3 per cent is heav­ier el­e­ments – the 'met­als' – such as oxy­gen, car­bon and mag­ne­sium. 4.25 mil­lion tonnes of mass are con­verted di­rectly into en­ergy ev­ery sec­ond in the core, mak­ing the Sun shine. That may sound like a lot, but it only amounts to 0.15 per cent of Earth's mass since di­nosaurs died out around 66 mil­lion years ago.

But that isn't the end of the story. The 3D model was tested against the lat­est spec­tro­scopic ob­ser­va­tions from the Swedish So­lar Tele­scope in La Palma. The re­sults were good, and also in agree­ment with other spec­tro­scopic data sug­gest­ing the sci­en­tists were on the right track. But when

“The prob­lem is most likely with our in­com­plete knowl­edge of pro­cesses in the core of the Sun”

Ti­ago Pereira

Charged par­ti­cles from the Sun trapped in the mag­ne­to­sphere re­sult in au­ro­rae

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