. . o gahma-ray bursts hold secrets of the infant .

~~How,,astrono.mers? -.$ . .. : are finding out.

g:& , g:& a&'" - .

' Bob Berman Master the How to create your on finding art of wide- own astronomy another Earth field imaging '..weather forecast p. 13 p. 66 - ,p. 64 > : I Ii. . . . BILLIONS OF YEARS FROM bJW, ttp night sky will glow with stars, dust,.md psfrom two galaxies: the Milky Way, in ~Mchwe' live, and the *roaching Anqromeda

Galaxy (M31). LYNE~TECOOKFOR ASTRONOMY / THE ANDROMEDA GALAXY (M31) is a typical spiral of stars, dust, and gas. Spiral galaxies the Sun's mass. It also suggests the Milky I dominate the night sky in the local universe. Fourteen satellite galaxies accompany / Andromeda, including the two visible in this image: M32 (above Andromeda) and NGC 205 Way and Andromeda \c.ill nialie a close pass : (below). Andromeda is the largest in the Local Group of galaxies. TONYANUDAPHNEHALLAS in about 4 billion years. Kahn and Woltjer inspired a generation and is visible in the northern sky with the effect. In contrast, most galaxies in the uni- of studies that further constrained the mass naked eye. The remaining members of the verse are flying away from the Milky Way. of the Local Group and revealed important Local Group - several dozen - are a bevy characteristics of Andromeda's orbit, such of much smaller satellite galaxies. Timing is everything as its total energy of motion. A galaxy group con~prisestwo or more Nearly 50 years ago, Franz Kahn and But the timing argument does not have relatively close, massive galaxies. The com- Lodewijk Woltjer pioneered the "timing the ability to follow the coniplex dynamics pactness of galaxies that form groups sug- argument." This hypothesis held that the that accompany the merger of extended gests that they are gravitationally bound Milky Way and Andromeda formed close galaxies. Therefore, it cannot predict the and dynamically coupled to each other. to each other, during the dense, early stages future arrangement of the Local Group. For This simply means the galaxies attract each of the universe. processes as conlplex as galaxy mergers, other gravitationally, so a change in one Subsequently, the general expansion of astronomers need more powerfill tools. affects the fate of the other. the universe pulled the two galactic neigh- Evidence of the dynamic connection bors apart. Later, the Milky Way and Simulating the Local Group between the Milky Way and Andromeda Andromeda reversed their out\vard trajec- Numerical simulations are indispensable comes from their relative motions. The gal- tories owing to mutual gravitational attrac- for understanding processes too complex to axies are barreling toward each other at tion. Since then, they have completed solve with pen and paper. In galactic merg- nearly 270,000 mph (190,000 kilometers nearly a full orbit around each other. ers, for example, simple gravity shapes the per hour). We know this because the spec- The timing argument, combined with merged galaxy. But the sheer number of tral lines of Andromeda's light appear to be estimates of the galaxies' relative velocities atoms interacting over time makes it dif- blueshifted - displaced toward the blue and other factors, indicates the Local ficult or impossible to simulate the merger end of the spectrum - by the Doppler Group's total mass is about 3 trillion times without massive computer power...... ------...... ~....~~ To simulate the evolution of the Local Abraham Loeb is a professor of astronomy at Harvard University, a visiting professor at the Group, first we create a mathematical model Weizmann lnstitute of Science, and the director of the lnstitute for Theory and Computation describing its present state. This is straight- at the Harvard-Smithsonian Center for Astrophysics. T. J. Cox is a postdoctoral fellow at the forward for the Milky Way and Andromeda. lnstitute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. Several decades of observations enable us to

30 Astronomy/// June 08 estimate the quantity gas, stars, and other matter involved. We can determine a plau- sible mass estimate for the Milky Way and Andromeda to well beyond the visible inner portion of each galaxy. However, the combined mass of the Milky Way and Andromeda is still less than nearly every number the timing argument yields. This implies there is additional mass in the Local Group. The missing mass turns out to be the diffuse "intergalactic medium" of atoms, gas, and dust between the galaxies. Galaxies ...... ,, ,. . ". . . _. . '...: L - . . ,' . . . :. ,...... :I .. - , , . , . are simply the visible peaks of massive ice- .... :. ..\ .,:.. - .... :.. ,...... '? I...... , . .< :..,,..,. ... '- . .. , <."...... , .- . .... , ,: . ' . .... :, , ...... : .. ... bergs of matter. Much of the mass is not ...... , ...... readily apparent, just as most of an iceberg's bulk lies underneath the water's surface. j FROM EARTH, we see the Milky Way from an insider's perspective. Depending on the time 1 of year, an Earth-bound observer can see 3 or 4 different arms of the spiral. JOHNCHUMACK When galaxies collide Full-scale simulations typically require 2 nucleii (quasars), and transform pinwheel- dynamic impact upon each other owing to weeks of number crunching. This task shaped spiral galaxies into smooth spheroi- their mutual gravitation. As a result, it's requires the power of the equivalent of 16 dal or elliptical galaxies. possible the Sun - and Earth and the other fully loaded desktop computers. One of the distinguishing characteristics planets - will be dragged into a tidal tail. Since the early days of astronomy, merg- of galaxy interactions is the appearance of During this period, an observer would have ing galaxies have remained curiosities long streams of stars and gas that stretch one of the most unique vantage points owing to their complex and irregular from one or both of the participant galax- imaginable. Torn shreds of the Milky Way shapes. But astronomers now appreciate ies. We commonly call these features tidal will fill a large fraction of the night sky as that mergers significantly drive galaxy evo- tails. They result from the powerful gravita- our galaxy experiences its gravitational lution. Galaxy mergers touch off bursts of tional forces at work between merging gal- dance with Andromeda. star formation, give birth to bright galactic axies. As the tails form,.they rip stars and Because only a small fraction of a gal- gas from the host galaxy and hurl it into axy's mass ends up in tidal tails, it is more intergalactic space. likely the Sun will go for a much less dra- As the Local Group evolves, the Milky matic ride. Most of the stars in merging Sextans A Sextans T Way and Andromeda will begin to have a galaxies remain relatively close to their host The IN THE LOCAL GROUP OF GALAXIES, the Milky Way, Andromeda Local Group (M31), and the Pinwheel Galaxy (M33) are the'largest in the group. of galaxies Dozens of smaller satellite galaxies accompany them. The group's members are all bound by mutual gravitational attraction. Its total 'LeOAlfilled space spans 6 million light-years. A~TRONOMZROENKELLY

Galaxies larger than 4,000 light-years

Satellite galaxies smaller than 4,000 light-years

WLM 1 .I. . -- L. I GALAXY MERGERS IN CYBERSPACE Astronomers don't simulate galaxy mergers just to create pretty pictures. The images below, sampled from a merger simulation by Harvard astrono- The simulations are serious and time-consuming scientific experiments. mersT. J. Cox and Avi Loeb, depict the merger of the Milky Way and Simulations enable astronomers to test new ideas about the merger pro- Andromeda galaxies.These frames highlight important milestones and .g *- cess and the role of mergers in the evolution of galaxies and the universe. events in the merger process. UNLESS~iRWSENOTEQMERGERIMACESBYTJJCOX~C~A~

2 billio~ 2.5 billion 4.5 billion years 5.5 billion yc from nc :iom now from no\-. frem now

2 BILLION YEARS from the pres- j IN 2.5 BILLION YEARS, the i IN 4.5 BILLION YEARS, the galax- IN 5.5 BILLION YEARS, Milkon ent, the galaxies swing around galaxies are still moving ! ies loop around another time and / eda is born. Tidal swirls, tails, 5 each other in a close pass. Mutual 1 apart. A ghostly bridge of then finally coalesce into a single and eddies left over from the attraction draws tenuous tidal j gas and stars still connects i mass. Their dense cores, each har- 1 violent merger slowly relax anc tails of stars and gas.Tidal tails [ them. Stars in the bridge, / boring a black hole, gradually j dissipate. Individual stars are hallmarks of mergers in the 1 perhaps some with plan- \ combine into a single nucleus.The ! spread out, forming a more real universe (see image below), j ets, could end up literally j merging galaxies experience a / homogenous elliptical galaxy lost in space as the galaxy j brief pulse of star formation as 1 similar to M32 (right), a satellit bridge dissipates. ! the black holes merge. 5 i1 L___ a*. --.--

i

I. ! ' -P -1 S z E 2

II rnZ

I 0 Z 0, 1 - _.' "NGC 2207 (lower galaxy) as it / THE BEAUTY of merging galaxies stands out in this simulation of another possible model of the Milky -c, , merges with smaller IC 2163. j Way-Andromeda collision by astronomer John Dubinski.The simulation reveals more detailed structure , , NASNESAMJBBLE HERITAGE TEAM iSTScl1 ,1 . / than Cox and Loeb's images because it includes more than 300 million particles of interacting matter.

galaxies. The chance of the Sun being ban- chaotic owing to the rapid fluctuations in ing life span. Currently, our Sun is about ished to the tidal-tail boondocks is rela- gravity induced by the merger. What would halfway through its lifetime and eventually tively small, based on our simulations. this mean for Earth and its residents? will begin to expand. As it does so, it will Our research suggests the Milky Way consume all its available hydrogen and Change of fortune and Andromeda will begin to interact evolve toward a red-giant phase within 5 The Sun's peaceful orbit around the center strongly 2 billion years from now, and then billion years. In short, the Sun will be in its of the Milky Way - which it has traversed complete the merger in about 5 billion death throes on Milkomeda's birthday. nearly 20 times since its birth - will for- years. The latter date is especially notable The Sun's red-giant stage will make life ever change. Its new path will be far more because it coincides with the Sun's remain- on Earth rather uncomfortable. Indeed, it

32 Astronomy/// June 08 THE MERGER OF SPIRALS often produces a single, new sphere-shaped type of galaxy called an elliptical.The elliptical galaxy above, M32, is, one of the 14 known satellite galaxies of Andromeda. Most galaxies in the Local Group are small satellites. WOLFGANGPROMPER ;,A4 will spell the end of life as we know it. contrast, Milkomeda will be nearly spheri- our final resting place. A hundred billion However, it does not preclude the possibil- cal in shape and much smoother in appear- years later, Milkomeda also will represent ity for colonization of habitable planets ance than any spiral galaxy. Stars within our entire visible universe. around nearby stars. Thus, it is possible . Milkomeda will follow more complex In the next tens of billions of years, the future astronomers will be able to witness orbits. The stars will spend brief periods accelerating expansion of space itself will some, if not all, of the Local Group evolu- near the dense galactic center, but orbit pull all distant galaxies farther and farther tion we have simulated. much farther away most of the time. away from us. Once the recession rate of Although the Mdky Way and Androm- Milkomeda's spheroidal shape is not any distant galaxy exceeds the speed of light eda will merge, stars within the two galax- unusual, as it characterizes a major class of relative to us, its light will be unable to tra- ies, such as our Sun, will not physically objects called elliptical galaxies. Such galax- verse the inflating gap. collide. The reason is the extremely large ies typically host relatively old stars. Pre- At that point, we will no longer be able distances between individual stars in galax- sumably, many of these galaxies in the to see those galaxies. They will gradually ies. For example, if the Sun were the size of present-day universe formed by mergers wink out of reach of the most powerful a ping-pong ball, the nearest star (Proxima between galactic disks, in which the stars telescopes. No longer will astronomers be Centauri) would be another ping-pong ball had formed at earlier cosmic times. able to gaze into the sky and study distant nearly 1,000 miles (1,600 km) away.. The Sun's likely fate will be to spend galaxies in order to learn about our own. much of its time in the galactic outskirts. However, the prelude to this final Our final resting place The merger will redistribute the orbital gloomy fate would be full of fun. Over the The Sun's orbit will follow a chaotic path momentum (energy) of the Milky Way and next 5 billion years, astronomers will wit- until the merger concludes. At that point, Andromeda among individual stars in ness the stellar fireworks in one of the the system will relax and expand. And the Milkomeda. The stars will end up with greatest shows of all time: the transforma- Sun will reside inside a new galaxy: more momentum after the merger, and tion of the Milky Way and the Andromeda Milkomeda. It will look very different from thus will encircle Milkomeda's center at a Galaxy into Milkomeda. So sit back and either the Milky Way or Andromeda. larger average distance. wait for the show to begin. rn The Milky Way and Andromeda are spiral galaxies, with most stars concen- The Local Group's far future fate WEB Watch animation of the Milky Way trated into a disk and moving in nearly cir- Because of the universe's accelerating EXTRA and Andromeda galaxies merging at cular orbits around the galactic center. In expansion, Milkomeda will not merely be