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Astronomers detect most distant Milky Way stars known

July 9, 2014
Courtesy of Haverford College
and World Science staff

The dis­tant 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.

A sim­u­lat­ed im­age show­ing how small the Milky Way would look from the lo­ca­tion of ULAS J0744+25, near­ly 775,000 light years away. (Cred­its: Vis­u­al­i­za­tion Soft­ware: Uni­view by SCISS Da­ta: SO­HO (ESA & NA­SA), John Bochan­ski (Hav­er­ford Col­lege) and Jack­ie Fa­herty (A­mer­i­can Mu­se­um of Nat­u­ral His­to­ry and Car­ne­gie In­sti­tute's De­part­ment of Ter­res­tri­al Mag­net­ism) )


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.”


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The distant 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. Astronomers led by John Bochanski, a visiting assistant professor at Haverford College, 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.”