Ur home galaxy, the Milky Way, and its nearest neighbor

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ur home galaxy, the Milky Way, and its nearest neighbor, 

the Andromeda Galaxy, are on a collision course. Billions of 

years from now, the merger will drastically alter the struc-

ture of both galaxies and spawn a new city of stars we have 

dubbed Milkomeda (“milk-AHM-mee-da”). The event will 

also radically transform the night sky. But into what? 

Currently, the Milky Way’s thin disk of stars, dust, and gas appears as 

a nebulous strip arching across the sky. As Andromeda grazes the Milky 

Way disk, we will see a second strip of stars looming across the night sky. 

After the final merger between these galaxies, the stars will no longer be 

confined to two narrow strips, but instead get scattered across the entire 


In our research, we have explored the Milky Way’s fate by simulating 

the birth of Milkomeda in a supercomputer. The simulations are at a suf-

ficient level of detail, or “resolution,” to learn much about the coming 

merger and how it will change our perspective on the universe. Although 

we will not be alive to witness the event — nor to take responsibility for 

whether our forecast proves accurate — this is the first research in our 

The Milky Way is on a collision 

course with its neighbor, the 

Andromeda Galaxy. What will 

the night sky look like after the 


  ⁄ ⁄ ⁄


BY aBraham loeB and t.j. cox

5 billion years A.D.

24  Astronomy

⁄ ⁄ ⁄

June 08

Our galaxy’s

    date with


The Milky Way’s

date with


www.Astronomy.com 25

BIllIonS oF YearS From noW, the night 

sky will blaze with stars, dust, and gas from 

two galaxies: the Milky Way, in which we 

live, and the encroaching Andromeda  

Galaxy (M31). 

LyneTTe Cook for AsTronoMy

Our galaxy’s

    date with


26  Astronomy

⁄ ⁄ ⁄

June 08

careers that has a chance of being cited in 5 

billion years.

The Local group

The vastness of the night sky might suggest 

the Milky Way resides in a relatively remote 

part of the Universe. However, astronomers 

know the Milky Way to be the second larg-

est member of the Local Group of galaxies. 

The largest in the Local Group is Androm-

eda. It contains somewhat more mass than 

the Milky Way, resides nearly 2.5 million 

light years away, and is visible in the north-

ern sky with the naked eye. The remaining 

members of the Local Group — more than 

30 galaxies —  are a bevy of much smaller 

satellite galaxies. The satellites cluster near 

the Milky Way or Andromeda like celebrity 

entourages. Thus, the Milky Way and 

Andromeda are the celebrity couple of the 

Local Group.

In astronomical jargon, a galaxy group 

comprises two or more massive galaxies in 

relatively close proximity. As the headlights 

on a dark country road indicate the exis-

tence of an entire car, the luminous stars of 

a galaxy indicate the existence of an 

extended halo of “dark matter.”  The close 

proximity of galaxies in groups suggests 

that their dark halos are gravitationally 

bound and dynamically coupled to each 

other. “Dynamically coupled” simply means 

the haloes attract each other via their gravi-

tational fields, and a change in one galaxy 

affects the fate of the other. 

Evidence of the dynamical connection 

between the Milky Way and Andromeda 

comes from their relative motion. The gal-

axies are barreling toward each other at 

nearly 270,000 mph (120 kilometers per 

second). We know this because the spectral 

lines of Andromeda’s light appear to be 

blueshifted — displaced toward the blue 

end of the spectrum — by the Doppler 

effect. This is in sharp contrast to most gal-

axies in the universe, which are flying away 

from the Milky Way. This spreading 

motion induces a redshift in the light from 

distant galaxies, a fact used to establish the 

expanding universe since the time of the 

American astronomer Edwin Hubble (1889 

– 1953).

Timing is everything

Nearly 50 years ago, Franz Kahn and 

Lodewijk Woltjer pioneered the “timing 

argument.” This hypothesis held that the 

Milky Way and Andromeda formed close 

to each other, during the dense, early stages 

of the universe. Subsequently, they were 

pulled apart by the general cosmological 

expansion. Later, the Milky Way and 

Andromeda reversed their outward paths 

owing to mutual gravitational attraction. 

Since then, they have now traced out nearly 

a full orbit of each other. 

abraham loeb is professor of astronomy at Harvard University, a visiting professor at the 

Weizmann Institute of Science, and director of the Institute for Theory and Computation at the 

Harvard-Smithsonian Center for Astrophysics. t.j. cox is a postdoctoral fellow at the Institute 

for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics.

the andromeda GalaxY (m31)

 is a typical spiral of stars, dust, and gas — the type of 

galaxy that dominates the night sky in the Local Universe. Fourteen small satellite galaxies 

accompany Andromeda, including the two visible in this image: M32 (above Andromeda) 

and NGC 205 (below). 

Tony AnD DAPHne HALLAs  


Milky Way




Leo A

Sextans Dwarf

Sextans A

Sextans B 

NGC 3109 



IC 1613

NGC 185

NGC 147

NGC 6822

Fornax Dwarf

Andromeda (M31) 


NGC 205

IC 10

1 million



www.Astronomy.com 27

The timing argument, along with 

knowledge of the current separation, rela-

tive velocity, the age of the universe, yields 

an estimated total mass for the Local Group 

of more than 3 trillion times the Sun’s mass 

(solar-masses). In addition, it suggests the 

Milky Way and Andromeda will make a 

close pass in about 4 billion years. The mass 

estimate, in particular, generated significant 

amount of interest at the time, because it is 

more than 10 times the mass of all visible 

matter. This result became one of the first 

pieces of evidence for the existence of dark 


Kahn and Woltjer inspired a generation 

of studies that further constrained the mass 

of the local group and revealed important 

characteristics of Andromeda’s orbit, such 

as its total energy of motion, or “angular 

momentum.” But the timing argument does 

not have the ability to follow the complex 

dynamics that accompany the merger of 

extended galaxies. Therefore, it cannot pre-

dict the future arrangement of the Local 

Group. For processes as complex as galaxy 

mergers, astronomers need more powerful 

tools: supercomputers.

simulating the Local Group 

Numerical simulations are an indispensable 

tool to understand astronomical processes 

too complex to solve with pen and paper. A 

perfect example is the merger of two galax-

ies. Simple gravitational forces govern the 

structure of the galaxies, but the sheer 

number of atoms of matter interacting over 

time makes it difficult or impossible to 

solve without massive computer power.

To simulate the evolution of the Local 

Group, first we construct a physical model 

describing its present state. This task is 

straightforward for the Milky Way and 

Andromeda, since several decades of obser-

vations enable us to estimate the amount of 

gas and stars involved and the contribution 

from dark matter. Combined with cosmo-

logical simulations, a plausible mass model 

for the Milky Way and Andromeda can be 

determined 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 

turned out to be the diffuse “intergalactic 

medium” of atoms, gas, and dust between 

the galaxies. Galaxies are simply the visible 

peaks of massive icebergs of matter. Much 

of the mass is not readily apparent, just as 

most of an iceberg’s bulk lies beneath the 


When galaxies collide

After we construct a model that includes all 

the stars, gas, and dark matter in the Local 

Group, we evolve the system over time in a 

computer and see what happens.   Full-

scale simulations typically require 2 weeks 

of number crunching, using the equivalent 

of 16 fully loaded desktop computers.

Galaxy interactions are spectacular 

From earth, 

we see the Milky Way from an insider’s perspective. Only one of the galaxy’s 

spiral arms is visible. 


In the local GroUP oF GalaxIeS, 

the Milky Way, Andromeda 

(M31), and the Pinwheel Galaxy (M33) are the largest in the group. 

Dozens of smaller satellite galaxies accompany them. The group’s 

members are all bound by mutual gravitational attraction. It’s total 

filled space spans 6 million light-years. 



Local Group 

of galaxies

28  Astronomy

⁄ ⁄ ⁄

June 08

Astronomer astronomers don’t simulate galaxy mergers just to create 

pretty pictures. The simulations are experiments to test hypotheses about 

how mergers work whether they are a significant process in the formation 

and evolution of galaxies. These images, taken for an animated film on T.J 

Cox’ web site, depict the complex merger of the Milky Way and Androm-

eda. These frames highlight important features of the galaxies and the 

merger process. 

UnLess oTHerWIse noTeD, MerGer IMAGes by T.J. Cox (HArVArD-sMITHsonIAn sfA)


events. Since the early days of astronomy

merging galaxies have remained curiosities 

owing to their complex and irregular 

shapes. But astronomers now appreciate 

that mergers are one of the most significant 

drivers of galaxy evolution. For example, 

galaxy mergers touch off huge bursts of star 

formation (starbursts), trigger the birth of 

quasars, and transform rotating spiral gal-

axies, such as the Milky Way and Androm-

eda, into smooth spheroidal or “elliptical” 

galaxies. (see “What happens when galaxies 

collide?,” March 2008.)

You can view numerous spectacular 

images of interacting galaxies captured by 

the Hubble Space Telescope and other great 

observatories. These images are snapshots 

of the dynamic merger process and paint an 

amazing story. One of the distinguishing 

characteristics of galaxy interactions is the 

appearance of long streams of stars and gas 

that stretch from one or both of the partici-

pant galaxies.  These features are typically 

referred to as “tidal tails,” and result from 

the powerful gravitational forces at work 

between merging galaxies. As the tails 

form, they rip material from the host galaxy 

nGc 2207

 (LEFT) merging with 

smaller IC 2163. 

nAsA/esA/HUbbLe HerITAGe TeAMs (sTsCI)

2 BIllIon YearS

 from the pres-

ent, the galaxies loop around 

each other in a close pass. Mutual 

attraction draws tenuous “tidal 

tails” of stars and gas. Tidal tails 

are hallmarks of merger in the 

real universe (see image at right).

In 2.5 BIllIon YearS,


galaxies are still moving 

apart. A ghostly bridge of 

gas and stars connects the 

galaxies. Stars in the 

bridge, perhaps some with 

planets, could end up liter-

ally lost of intergalactic 

space bridge dissipates.

In 4.5 BIllIon YearS,

 the galax-

ies loop around again and come 

back together to finally merge. 

Their dense cores, each harboring 

a supermassive black hole, gradu-

ally combine. The merging galax-

ies experience a brief pulse of star 

formation as the two black holes 


In 5.5 BIllIon YearS,


eda is born. Tidal swirls,tails, 

and eddies left over from the 

violent merger slowly relax and 

dissipate. Individual stars 

spread out, forming a more 

smooth, internally homogenous 

elliptical galaxy similalr to the 

barrEd elliptical galaxy IN CEN-

TAURUS NGC 2207 at right.

montreal aStronomY 

John Dubinski created this image of the merger of the Milky Way with 

Andromeda. It highlights to fine, elegant contours of Milkomeda in the making, although at a cost of less 

detailed scientific contting in the images. The Dubinsky image reflects the skeletal structure of the margin 

2 billion years 

from now

2.5 billion years 

from now

4.5 billion years 

from now

5.5 billion years 

from now


Is this available 

in a hi-res?

www.Astronomy.com 29

and hurl it into intergalactic space.

As the Local Group evolves, the Milky 

Way and Andromeda will begin to have a 

dynamical impact upon each other owing 

to their mutual gravitation. As a result, it’s 

possible the Sun — and Earth and the other 

planets — will be dragged into a tidal tail. 

During this period, an observer will have 

one of the most unique vantage points ever 

imagined. Torn shreds of the Milky Way 

will fill a large fraction of the night sky as 

our galaxy experiences its gravitational 

dance with Andromeda.

Because only a small fraction of a gal-

axy’s mass ends up in tidal tails, it is more 

likely the Sun will go for a much less dra-

matic ride. Most of the stars in merging 

galaxies remain relatively close to their host 

galaxies. The chance of the Sun being ban-

ished to the tidal-tail boondocks is rela-

tively small, based on our simulations. 

There is also a 3 percent change the Sun 

will end up in Andromeda after its second 

close encounter with the Milky Way. In that 

case, earthly observers could gaze across 

space at their own former home, seeing it 

truly for the first time.

Change of fortune

However, the Sun’s peaceful orbit around 

the center of the Milky Way — which it has 

traversed nearly 20 times since its birth — 

will forever change. Its new path will be far 

more chaotic owing to the rapid fluctua-

tions in gravity induced by the merger. 

What would this mean for the Earth and its 


Our computer studies suggest the Milky 

Way and Andromeda will begin to strongly 

interact 2 billion years from now, and then 

complete the merger in about 5 billion 

years. The latter date is notable, because it 

coincides with the Sun’s remaining lifespan. 

Currently, our Sun is about halfway 

through its lifetime and will soon begin to 

expand as it slowly consumes all available 

hydrogen and evolves toward a red giant 

phase within 5 billion years.  In short, the 

Sun will be in its death throes on Milkome-

da’s birthday.

The Sun’s red-giant stage will make life 

on Earth rather uncomfortable. Indeed, it 

will spell the end of life (as we know it). 

However, it does not preclude the possibil-

ity for colonization of habitable planets 

around nearby stars, and thus it is possible 

that future astronomers will be able to wit-

ness some, if not all, of the Local Group 

evolution we have simulated.

Although the Milky Way and Androm-

eda will merge, stars within the two galax-

ies, such as our Sun, will not physically 

collide. The reason is the extremely large 

distances between individual stars in galax-

ies. For example, if the Sun was the size of a 

ping pong ball, the nearest star (Proxima 

Centauri) would be another ping pong ball 

nearly 1,000 miles (1,600 km) away.

our final resting place


he Sun’s orbit will follow a chaotic path 

until the merger concludes and the system 

relaxes and expands. At this point, the Sun 

will reside inside a new galaxy, Milkomeda. 

It will look very different from either the 

Milky Way or Andromeda.  

The Milky Way and Andromeda are 

spiral galaxies, with most stars concen-

trated into a disk and moving in nearly cir-

cular orbits around the galactic center. In 

contrast, Milkomeda will be nearly spheri-

cal in shape and much smoother in appear-

ance than any spiral galaxy. Stars within 

Milkomeda will follow more complex 

the merGer oF SPIralS

 often produces a single, new sphere-shaped type of galaxy called an elliptical. The galaxy above, Centaurus A 

(NGC 5128), is a “peculiar elliptical” visible in the Southern Hemisphere. 


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