Lens­ing events be­tween stars

Sci­en­tists have done what Ein­stein said was im­pos­si­ble – used rel­a­tiv­ity to mea­sure a star’s mass.

The Star Malaysia - Star2 - - Science - By AMINA KHAN

AS­TRONOMERS scan­ning the skies with Nasa’s Hub­ble Space Tele­scope have dis­cov­ered the bend­ing of one star’s light by another star’s grav­ity – and have even used that dis­tor­tion to mea­sure a star’s mass.

The find­ings, pub­lished in the jour­nal Sci­ence, con­firm a key tenet of Albert Ein­stein’s the­o­ries even as they of­fer a new tool with which to ex­plore a fun­da­men­tal prop­erty of stars.

Ein­stein’s gen­eral the­ory of rel­a­tiv­ity, pre­sented in 1915, de­scribes how grav­ity can dis­tort the path of light, al­ter­ing its tra­jec­tory. In 1919, the the­o­rist was proved right when, dur­ing a so­lar eclipse, an ex­pe­di­tion by Sir Arthur Ed­ding­ton dis­cov­ered that stars near the edge of the blocked sun’s disc were not where they were sup­posed to be. Their ap­par­ent po­si­tion had moved be­cause the sun’s grav­ity had dis­torted the path of their starlight, just as Ein­stein had pre­dicted.

Since then, as­tronomers have used this as a pow­er­ful tool with which to ob­serve dis­tant phe­nom­ena. That’s be­cause, when lined up just right, a mas­sive ob­ject in the fore­ground can bend the light of a back­ground light source and mag­nify it the way a lens does. This phe­nom­e­non, known as grav­i­ta­tional lens­ing, has al­lowed as­tronomers to ob­serve dis­tant gal­ax­ies that, with­out this ef­fect, would be too faint to study.

But lens­ing events by large struc­tures such as gal­ax­ies have been fuzzy at best, said Terry D. Oswalt, an astronomer at Em­bry-Rid­dle Aero­nau­ti­cal Univer­sity’s Day­tona Beach cam­pus, who was not in­volved in the study.

“They are lousy lenses be­cause they’re not point sources,” Oswalt said. “They’re big and splotchy. They’ve got spi­ral arms and nu­clei and some­times com­pan­ion gal­ax­ies, and some­times there are clus­ters of gal­ax­ies.”

But stars are point sources, not large and lumpy like gal­ax­ies. If you could catch a lens­ing event be­tween two stars, it could of­fer a much more fo­cused ef­fect. You might even be able to cap­ture an Ein­stein ring – a phe­nom­e­non in which a lens­ing ob­ject eclipses a back­ground light source so per­fectly that the back­ground ob­ject is ren­dered as a lu­mi­nous ring.

(This has been doc­u­mented for gal­ax­ies, but not for in­di­vid­ual stars.)

For this pa­per, lead au­thor Kailash Sahu of the Space Tele­scope Sci­ence In­sti­tute in Bal­ti­more and his col­leagues set out to find a lens­ing event be­tween two stars. This was a much more dif­fi­cult feat, partly be­cause the ef­fect for sin­gle stars is so tiny com­pared with the size of gal­ax­ies. It’s also much rarer be­cause it’s less likely to catch two stars over­lap­ping than it is to find two gal­ax­ies do­ing so.

Sahu’s team used the Hub­ble Space Tele­scope to look for stars that were set to cross in front of back­ground stars in the hopes of catch­ing a stel­lar Ein­stein ring. They ze­roed in on a white dwarf star called Stein 2051 B, which was set to pass in front of a more dis­tant star. This was no easy task: The back­ground star was 400 times dim­mer than Stein 2051 B.

“It’s like mea­sur­ing the mo­tion of a fire­fly next to a light bulb from 2,400km away,” Sahu said.

Ein­stein ac­tu­ally de­scribed such rings in a pa­per in 1936 but said that be­cause of their rar­ity and be­cause our in­stru­ments were not pow­er­ful enough, they weren’t likely to ever be seen.

“Of course, there is no hope of ob­serv­ing this phe­nom­e­non di­rectly,” he wrote in that pa­per in Sci­ence.

But as sci­en­tists ob­served Stein 2051 B, the back­ground star seemed to jump, ap­pear­ing to do a tiny som­er­sault over the white dwarf passing in front of it.

Here’s what was hap­pen­ing: As Stein 2051 B be­gan to line up with the back­ground star, its grav­ity dis­torted the back­ground star’s light, cre­at­ing an Ein­stein ring. But be­cause the two stars’ align­ment was not per­fect rel­a­tive to Earth, that Ein­stein ring took the form of an el­lipse, with one side brighter than the other.

As Stein 2051 B moved in front of and across the dim­mer star, the el­lip­ti­cal Ein­stein ring shifted po­si­tions, with the brighter side ap­pear­ing as a point that traced a tiny arc across the sky.

While Hub­ble is not strong enough to re­solve that el­lipse, the tele­scope does see the back­ground star ap­pear to shift po­si­tions.

“Even though you can’t see the ring it­self, it’s lop­sided, and so the po­si­tion of the ob­ject ap­pears to move,” said Oswalt, the Em­bryRid­dle astronomer.

“It’s not ac­tu­ally mov­ing – it’s (an) ap­par­ent mo­tion caused by the bend­ing of the light.”

What’s more, the fact that this se­ries of Ein­stein rings is el­lip­ti­cal rather than a per­fect cir­cle ac­tu­ally al­lows sci­en­tists to cal­cu­late the mass of Stein 2051 B– a mea­sure­ment that has dogged the as­tro­nom­i­cal com­mu­nity for years.

Stein 2051 B is ac­tu­ally part of a bi­nary pair of stars that cir­cle one another, and re­searchers have used the mo­tion of the pair to cal­cu­late the white dwarf’s mass. Ac­cord­ing to this method, the star ap­par­ently was so heavy that it would have to have an iron core, which doesn’t make sense for a white dwarf. It also would mean this star was an­cient, about as old as the uni­verse it­self, which sci­en­tists were pretty sure could not be right.

But now, thanks to this grav­i­ta­tional lens­ing event, sci­en­tists have been able to di­rectly de­ter­mine Stein 2051 B’s mass. They found that the white dwarf weighs in at 0.675 suns – much more in line with our un­der­stand­ing of white dwarf evo­lu­tion.

“This is like putting the star on a scale and just see­ing how the scale changes,” Sahu said of the lens­ing method. “The de­flec­tion (of light) is the move­ment of the scale, and that tells you the mass. So it’s a very direct way to de­ter­mine its mass.”

White dwarfs are the rem­nants of dead stars; some 97% of the stars in our gal­axy are des­tined to be­come one. Sur­pris­ingly lit­tle is known about their masses – only a hand­ful have been mea­sured, typ­i­cally in­di­rectly by us­ing bi­nary star pairs. This lens­ing method could change that.

“This is the de­but of a new tool,” Oswalt said of the re­sults. Un­der­stand­ing the masses of stars is key to un­der­stand­ing their ori­gins and devel­op­ment, he added.

— TNS

In this Hub­ble Space Tele­scope im­age, the white dwarf star Stein 2051 B and the smaller star be­low it ap­pear to be close neigh­bours. The stars, how­ever, are far away from each other. Stein 2051 B is 17 light-years from Earth; the other star is about 5,000 light-years away.

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