The dis�tant out�skirts of our gal�axy har�bor val�u�a�ble clues for un�der�stand�ing its forma�t�ion and ev�o�lu�tion. But the stars out there are few, far be�tween, and far, far away. Now, as�tro�no�mers are re�port�ing the dis�cov�ery of two stars in this dis�tant "ha�lo" that are the fur�thest ev�er dis�cov�ered in our gal�axy, the ...

The dis�tant out­skirts of our gal­axy har­bor val­u­a­ble clues for un­der­stand­ing its forma­t­ion and ev­o­lu­tion. But the stars out there are few, far be­tween, and far, far away.

Now, as­tro­no­mers are re­port­ing the dis­cov­ery of two stars in this dis­tant “ha­lo” that are the fur­thest ev­er dis­cov­ered in our gal­axy, the Milky Way, and are be­ing de­scribed as pos­si­ble ghosts of ga­lax­ies past.

They're so far away that if you could stand on them and look up to the sky, you'd see the entire Milky Way spi­ral out there, not just a small part of it, as we see from Earth.

As­tro­no­mers led by John Bo­chan­ski, a vis­it­ing as­sis­tant pro­fes­sor at Hav­er­ford Col­lege in Penn­syl­vania, were tar­get­ing stars in this “ha­lo,” a sparse shroud of stars that sur­rounds the flat­ter, spir­al disk of our gal­axy. The ha­lo stretches to at least half a mil­lion light years away. A light-year is the dis­tance light trav­els in a year.

Pre­vi­ously, only sev­en stars were known be­yond 400,000 light years.

Bochan­ski and col­leagues pub­lished a let­ter July 3 in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters de­tail­ing the dis­cov­ery of the two stars, clas­si­fied as “cool red gi­ants” cat­a­logued as ULAS J0744+25 and ULAS J0015+01.

They lie an es­ti­mat­ed 775,000 and 900,000 light years away, re­spec­tive­ly. To put it in per­spec­tive, when light from the fur­ther star left the ob­ject, “our early hu­man an­ces­tors were just start­ing to make fires here on Earth,” Bo­chan­ski said.

Red gi­ant stars are rare com­pared to near­by “cool red dwarf” stars, but are nearly 10,000 times brighter, and thus more vis­i­ble. Us­ing a com­bina­t­ion of fil­ters high­light­ing dif­fer­ent com­po­nents of the light from these gi­ants, the team iden­ti­fied cool red gi­ant can­di­dates from sur­veys, then con­firmed the find­ings us­ing the 6.5m tel­e­scope at the MMT Ob­serv­a­to­ry on Mt. Hop­kins in Ar­i­zo­na.

“It is re­mark­a­ble to find stars this far out in the Milky Way gal­axy,” said Dan­iel Ev­ans, lead for In­di­vid­ual In­ves­ti­ga­tor Pro­grams at the U.S. Na­t­ional Sci­ence Founda­t­ion's Di­vi­sion of As­tronomical Sci­ences, which funded the re­search. “These re­sults will un­doubtedly shed new light on the forma­t­ion and ev­o­lu­tion of our ga­lac­tic home.”

The stars' sig­nif­i­cance goes be­yond their record-holding dis­tances be­cause they in­hab­it the ha­lo, the as­tro­no­mers said. Some as­tro­no­mers think that the ha­lo is like a cloud of ga­lac­tic crumbs, the re­sult of the Milky Way's merg­er with smaller ga­lax­ies over our gal­ax­y's life­time, said Hav­er­ford Col­lege as­tron­o­mer Beth Will­man, a co-author of the stu­dy.

“The­ory pre­dicts the pres­ence of such an ex­tend­ed stel­lar ha­lo, formed by the de­stroyed re­mains of small dwarf ga­lax­ies that merged over the cos­mic ages to form the Milky Way it­self,” Will­man said. “The prop­er­ties of cool red gi­ants in the ha­lo thus pre­serve the forma­t­ion his­to­ry of our Milky Way. These stars are truly ghosts of ga­lax­ies past.”

Source : world-science.net

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worldscience, "Astronomers detect the farthest known star of Milky Way", MachPrinciple, July 11, 2014, https://machprinciple.com/post/astronomers-detect-the-farthest-known-star-of-milky-way