(See description below)
Milky Way and Andromeda
Galaxies Merging
movie
The Milky Way and Andromeda galaxy are on a collision course! In about 3 billion years, the two galaxies will collide. After a very complex gravitational dance lasting about 1 billion years, they will merge to form an elliptical galaxy.
The Milky Way
Our home galaxy, the Milky Way, is a collection of about
400 billions stars spread out in a thin disk more than 100,000
light year across. Our sun is one of those stars sitting about
midway out in the disk moving around with the others on nearly
circular orbits. The Milky Way would look like an average looking
spiral galaxy if we could see it from
the outside.
Andromeda
The nearest big spiral galaxy to the Milky Way is the Andromeda galaxy. Appearing as a smudge
of light to the naked eye in the constellation Andromeda, this
galaxy is about four times as massive as the Milky Way but very
similar in many ways. At the moment it is about 2.2 million light
years away from us but the gap is closing at 500,000 km/hour.
Andromeda is the only big spiral galaxy galaxy moving towards the
Milky Way, and the best explanation is that the Milky Way and
Andromeda are in fact a bound pair of galaxies in orbit around
one another. Both galaxies are thought to have formed close to
each other shortly after the Big Bang, initially moving apart
with the overall expansion of the universe. But since they are
bound to one another, they are now falling back together, and one
very plausible scenario puts them on a collision course in 3
billion years.
Interacting Galaxies
Galaxies collide and interact occasionally and there are
several well-known examples in the vicinity of the Milky Way. We
see interacting pairs as snapshots in time and the results are
often very dramatic. Long streams of stars thrown off in
beautiful open spiral patterns are characteristic of these
collisions and are known as tidal tails and bridges because of
their origin in the strong mutual gravitational tides of the two
interacting galaxies. Colliding galaxies also tend to merge with
one and the final outcome after some violent convulsions lasting
a few hundred million years is another kind of galaxy called an
elliptical. During this period, the gas in these galaxies can be
ignited violently in a starburst creating stars at rates hundreds
of times greater than normal. Galaxy interactions are not that
common an event in the local neighbourhood (maybe one in a
hundred galaxies) but the rates of merging and interaction
increase dramatically at early times in the universe. Galaxy
merging is fundamental to building up structure in the universe
and explains many of the peculiar features of young galaxies seen by the Hubble Space
Telescope.
A Simulation of the Milky Way/Andromeda Collision
It is fun and instructive to see how the collision of
the Milky Way with Andromeda might play itself out. John Dubinski
of the University of Toronto generated a numerical simulation of
the collision using Blue Horizon, a 1152 processor IBM SP3 at the
San Diego
Supercomputing Centre. Each spiral galaxy is represented by
about 40M stars and is surrounded by a 10M particle dark matter
halo for a total of more than 100M particles for the galaxy pair.
Click on movie above to see a
graphical representation of the simulation. In the still image above, the
Milky Way galaxy is the smaller galaxy near the top of the image. In the
movie, the Milky Way galaxy starts out near the bottom.
These simulations reveal a tremendous amount of detail in the destruction and unravelling of the galaxies as they collide and merge to form an elliptical galaxy. The Milky Way is shown face-on and moves from the bottom up to the left of Andromeda and the to the upper right. Andromeda is tilted from this perspective. The images are 1 million light years across. After the initial collision, a open spiral pattern is excited in the both the Milky Way and Andromeda and long tidal tails and a connecting bridge of stars form are apparent. The galaxies move apart and then fall back together for a second collision and then after a few convulsions which throw off more stars in complex ripple patterns they settle into something looking like an elliptical galaxy.
A View from the Inside
An interesting fact on the timing is that the Sun will
still be burning brightly when this collision occurs and maybe
life of some sort will still be around on Earth at that time. So
what would people see in the night sky during this billion year
galactic dance? As Andromeda approaches, it will grow in size and
just before the collision the night sky will be filled by a giant
spiral galaxy. When the two galaxies intersect, our familiar
Milky Way arch over the sky will be joined by a second
intersecting arch of stars but this will only last for 100
million years or so and will be a very confusing state of affairs
for galactic astronomers. Finally, when the two galaxies merge
our view will depend on which direction the Sun is thrown. There
are two possible fates of the Sun which depend closely on the
details of where it is in its galactic orbit at the time of the
collision. In the first case the Sun may take a ride on a tidal
tail and be ejected into the darkness of intergalactic space. In
this case, our star would be all alone with few stellar
neighbours so the night sky would be very dark with few stars to
see—maybe like the view of the nightsky from downtown
Toronto. In the second case, the Sun is thrown right into the
centre of the merging pair where a great starburst will be
underway. The huge number of stars forming will result in
supernovae going off at a rate of a few per year in the new
merged galaxy. While these will not present a direct hazard to
the Earth, they will truly light up the sky letting you read at
night but probably frustrating the endeavours of backyard
astronomers!
Examples of actual galaxies in collision: Mice Galaxies, Tadpole Galaxy, Antennae Galaxies, Bird's-Head Galaxy