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The galaxy next door—our destined home?

May 10, 2007
Special to World Science  
Link to online paper added May 11

As­tro­no­mers have run new sim­u­la­tions to see what could hap­pen when an ex­pected col­lision takes place be­tween our gal­axy and an­other big one—pos­si­bly with­in our de­scen­dants’ life­times.

The sur­pris­ing re­sults: lit­tle of the ce­les­tial fire­works that were wide­ly ex­pected to oc­cur as great gas clouds crunch to­geth­er to form new stars. In­stead, a more out­land­ish pos­si­bil­i­ty arose.

The An­drom­e­da gal­axy, al­so called M31. (© R. Gend­ler)
The com­put­er sim­u­la­tions in­di­cated there is a one in 37 chance we’ll end up liv­ing in that oth­er gal­ax­y—majestic An­drom­e­da, said the re­search­ers, T. J. Cox and Abra­ham Loeb of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­b­ridge, Mass.

“Fu­ture as­tro­no­mers in the so­lar sys­tem might see the Milky Way,” our pre­s­ent gal­axy, “as an ex­ter­nal gal­axy in the night sky,” Cox and Loeb wrote in a pa­per on their find­ings. In oth­er words, the Milky Way would be no more the fa­mil­iar sil­very strip across the heav­ens, but a dis­tant smudge of light.

Par­a­dox­i­cal­ly, this might give us a much bet­ter view of our old ga­lac­tic home than we had when we lived there. The Milky Way vis­i­ble in our night sky is but a small part of our galaxy, and it blocks our view of the rest.

The Milky Way, Andromeda and about 40 smaller com­pan­ion ga­lax­ies make up our ga­lac­tic neigh­bor­hood, wrote the as­tro­no­mers. As such, this is “the near­est lab­o­r­a­to­ry, and there­fore the most pow­er­ful tool, to study the for­ma­tion and ev­o­lu­tion of ga­lac­tic struc­ture.”

Be­cause these “lo­cal group” ga­lax­ies are linked by grav­i­ty, they break a gen­er­al rule, that ga­lax­ies through­out the uni­verse are re­ced­ing from one anoth­er. In par­tic­u­lar, An­drom­e­da is ap­proach­ing us at an es­ti­mat­ed 120 km (75 miles) per sec­ond—though it re­mains 2.5 mil­lion light years away, so the fam­i­ly re­un­ion is still a ways off. A light year is the dis­tance light trav­els in a year.

An ear­li­er sim­u­la­tion of the Milky Way and An­drom­e­da col­lid­ing, per­formed by John Du­bin­ski at the Uni­ver­si­ty of To­ron­to. The new sim­u­la­tion in­cor­po­rat­ed more re­cent da­ta and al­so sought to pre­dict our So­lar Sys­tem's fate specif­i­cally. 
As­tro­no­mers gen­er­ally don’t ex­pect a head-on col­li­sion. Rath­er, the two star­ry be­he­moths would cir­cle each oth­er clos­er and clos­er, drawn in by each oth­er’s grav­i­ty. This would lead to a few close brushes be­tween the two, dis­tort­ing their shapes, fol­lowed by a fi­nal merg­er in­to a big, in­el­e­gant blob.

The two ga­lax­ies are like­ly to merge with­in five bil­lion years, thus “with­in the Sun’s life­time,” Cox and Loeb wrote in their pa­pe­r, posted on­line and sub­mit­ted to the re­search jour­nal Month­ly No­tices of the Roy­al As­t­ro­no­m­i­cal So­ciety.

By the end of the sec­ond close en­coun­ter, there is a 50 per­cent chance our Sun will be dragged along with its plan­ets and oth­er stars in­to a long “ti­dal tail” ex­tend­ing out from our gal­axy, and caused by An­drom­e­da’s grav­i­ta­tion­al pull, the re­search­ers wrote.

But there is al­so a 2.7 per­cent chance, they added, that our So­lar Sys­tem won’t even stay in the gal­axy. With its grav­i­ty, An­drom­e­da could steal our So­lar Sys­tem al­to­ge­ther: “it could be more tight­ly bound to An­drom­e­da than to the Milky Way.” Of course, af­ter the merg­er is eventually over, we would re­turn to our old, bashed-in and some­what un­rec­og­niz­able ga­lac­tic home. We would like­ly find our­selves at the fringes of the com­bined gal­axy, added Cox and Loeb, who dubbed this fu­ture con­glom­er­a­tion “Milk­o­me­da.”

Gal­axy col­li­sions aren’t ex­pected to pose much dan­ger to their in­hab­i­tants. “Plan­e­tary sys­tems ride out col­li­sions,” said Uni­ver­si­ty of Ha­waii as­tron­o­mer Josh­ua Barnes in a 2005 pre­s­entation at Hi­lo, Ha­waii. “Their cen­tral stars may be launched in­to tid­al tails or scat­tered in ran­dom di­rec­tions, but grav­i­ty acts so grad­u­al­ly that plan­e­tary or­bits are not dis­turbed.”

Cox and Loeb al­so played down the pos­si­bil­i­ty of any great burst in star for­ma­tion, which some ex­pected to re­sult from col­li­sions of gas clouds in the two ga­lax­ies. Stars form from re­gions of such clouds where the gas be­comes more com­pact, and starts to fall to­geth­er un­der its own grav­i­ty. Al­though such bursts of star for­ma­tion are com­mon with gal­axy col­li­sions, both Milky Way and An­drom­e­da are too “gas-poor” for much of this, Cox and Loeb wrote. 

But there is enough gas, they added, to possibly make the black hole or holes at the cen­ter of the merged gal­axy light up more brightly. Black holes are ex­tra­or­di­nar­i­ly com­pact ob­jects whose in­tense grav­i­ty sucks in ev­erything near­by. New gas mov­ing into the area would thus fall in­side. The gas would heat up in the pro­cess and start emit­ting pow­er­ful ra­di­a­tion, which could af­fect life forms, de­pend­ing on their dis­tance. Cox and Loeb’s pa­pe­r did­n’t ex­am­ine these pos­si­bil­i­ties in de­tail. 

Un­cer­tain­ties in where the Sun ends up, they added, stem large­ly from un­cer­tain­ties in where it will be when An­dro­meda hits. The Sun’s dis­tance from the Milky Way cen­ter is pre­dict­able—it’s always about the same—but the rest is hard to pre­dict. So Cox and Loeb looked at all stars at that dist­ance from the cen­ter to gauge the odds of what will hap­pen to ours. An­dro­me­da’s dir­ec­tion, too, is known only roughly.

Wherever we wind up, Cox and Loeb added, stars out­side the Lo­cal Group may lat­er re­cede from view en­tire­ly, be­cause the uni­verse has been ex­pand­ing at an ev­er-growing rate. Thus with­in 100 bil­lion years—if an­yone is left to watch—Milk­o­meda, and the Lo­cal Group, “will con­sti­tute the en­tire vis­i­ble Uni­verse.”

* * *

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Astronomers have run new computer simulations to learn what could happen when our galaxy makes an expected run-in with a big neighboring galaxy, perhaps within our descendants’ lifetimes. The surprising result: little of the celestial fireworks that were widely expected to occur as great gas clouds crunch together to form new stars. Instead, a more outlandish possibility arose. The simulation suggested there is a one in 37 chance we’ll end up living in the other galaxy—majestic Andromeda, said the researchers, T. J. Cox and Abraham Loeb of the Harvard-Smithsonian Center for Astro physics in Cambridge, Mass. “Future astronomers in the solar system might see the Milky Way,” our present galaxy, “as an external galaxy in the night sky,” Cox and Loeb wrote in a paper detailing the findings. In other words, the Milky Way would be no more the familiar silvery strip across the heavens, but a distant smudge of light. Paradoxically, this might give us a much better view of our old galactic home than we ever had when we lived there. The Milky Way visible in our night sky is but one strip of hazy light from stars in our galaxy; the dust among these blocks our view of the rest. The two spiral galaxies and about 40 smaller companion galaxies “comprise our galactic neighborhood,” wrote the astronomers. As such, this is “the nearest labora tory, and therefore the most powerful tool, to study the formation and evolution of galactic structure.” Because these “local group” galaxies are linked by gravity, they break a general rule that galaxies throughout the universe are receding from one another. In particular, Andromeda is approaching us at an estimated 120 km (75 miles) per second—though it remains 2.5 million light years away, so the family reunion is still a ways off. A light year is the distance light travels in a year. Astronomers generally don’t expect a head-on collision. Rather, the two starry behemoths would circle each other closer and closer, drawn in by each other’s gravity. This would lead to a few close brushes between the two, distorting their shapes, followed by a final merger into a big, inelegant blob. The two galaxies are likely to merge within five billion years, thus “within the Sun’s lifetime,” Cox and Loeb wrote in their paper, submitted to the research journal MNRAS. By the end of the second close encounter, there is a 50% chance our Sun will be dragged along with its planets and other stars into a long “tidal tail” extending out from our galaxy, caused by Andromeda’s gravitational pull, the researchers wrote. But there is also a 2.7% chance, they added, that our Solar System won’t even stay in the galaxy. Thanks again to gravity, “It could be more tightly bound to Andromeda than to the Milky Way.” Of course, after the merger is eventually over, we would return to our old, bashed-in and somewhat unrecognizable galactic home. We would likely ourselves at the very fringes of this new galaxy, added Cox and Loeb, who dubbed this future conglomeration “Milkomeda.” Galaxy collisions aren’t expected to pose much danger to their inhabitants. “Planetary systems ride out collisions,” said University of Hawaii astronomer Joshua Barnes in a 2005 presentation at Hilo, Hawaii. “Their central stars may be launched into tidal tails or scattered in random directions, but gravity acts so gradually that planetary orbits are not disturbed.” Cox and Loeb also played down the possibility of any great burst in star formation, which some expected to result from collisions of gas clouds in the two galaxies. Stars form from regions of such clouds where the gas becomes more compact, and starts to fall together under its own gravity. Although such bursts of star formation are common with galaxy collisions, both Milky Way and Andromeda are too “gas-poor” for much of this, Cox and Loeb wrote. But there is enough gas, they added, to fuel a possible rebirth in gas-consumption activity by the black hole or holes at the center of the merged galaxy. Black holes are extra ordinarily compact objects whose intense gravity sucks in everything nearby. According to astronomers, the infalling gas would heat up and start to emit powerful radiation, which could pose problems for life forms, depending on their distance. Cox and Loeb’s paper didn’t examine these possibility in detail, but left it to future studies. Uncertainties in the Sun’s fate, they added, stem from uncertainties in how precisely the Sun will be positioned when the crash occurs. Either way, they added, stars outside the local group may later recede from view entirely, because the the universe is found to be expanding at an ever-growing rate. Thus within 100 billion years—if anyone is left to watch—Milkomeda, and the local group, “will constitute the entire visible Universe.”