Andromeda is Coming!
| The most spectacular and
violent cosmic fireworks that astronomers can foresee will be happening
right here on July 4th, in the year 3900001776..(just kidding on
Our galaxy, the
Milky Way, is the second largest of the 50+ galaxies in
a cluster called the Local Group, which spans 10 million light-years
(60 million trillion miles).
Andromeda is our
big sister— the largest galaxy in the Local Group.
Andromeda and the Milky Way dominate the cluster, with all the other
galaxies being much smaller. Each of these two great galaxies has over
a dozen satellite galaxies orbiting around them. Andromeda and the
Milky Way have kept their distance from each other for billions of
years—at least so far.
time for an intimate family gathering of colossal
Space Telescope observations reveal that Andromeda will
hit our galaxy head on…in 3.9 billion years. You may want to reconsider
your long-term investment strategy.
collisions are common in our universe. That’s how the biggest
galaxies grew so large. We’ve observed many distant galaxies in various
stages of colliding, and we have seen what results.
Andromeda is now
2.5 million light-years (15 million trillion miles)
away and is moving toward the Milky Way at 246,000 mph. On Earth, we
see Andromeda approaching at 677,000 mph, but that’s because our Solar
System orbits the Milky Way at 500,000 mph and is now moving more or
less toward Andromeda. About 110 million years from now, the Solar
System will be on the opposite side of its orbit and Earth will be
moving away from Andromeda. Our changing perspective clearly doesn’t
change the galaxies’ motion.
As the two great
galaxies come closer, their mutual gravitational
attraction will increase and their closing velocity will accelerate.
The Milky Way’s 300 to 400 billion stars will be flung at Andromeda’s
trillion stars at 1.3 million mph. Just imagine how exciting that might
In the galactic
suburbs, there’s typically a vast distance between
individual stars—galaxies are mostly empty space. If the collision
occurs in the spiral arms, it’s likely there will be a lot of
gravitational pushing and shoving but no star-on-star collisions.
However, the latest estimate says it’s 41% likely that the galaxy
centers will collide head on, which would be much more dramatic.
After the initial
impact, astronomers predict the two galaxies will
pass through one another, move apart, and reach a maximum separation of
560,000 light-years, 4.8 billion years from now. They will then start
falling directly toward one another for a final climatic merger 6.3
billion years from now. The resulting system, dubbed Milkomeda, will be
an immense elliptical galaxy, devoid of spiral arms.
The benign, nearly
circular orbits now enjoyed by stars in both
galaxies will be severely disrupted and may become chaotic. Billions of
stars will be flung into supermassive black holes and billions more
will be flung outward by “tidal” forces. Tidal tails may span over 5
times the Milky Way’s current size. The latest measurements are still
not precise enough to determine where our Solar System will be during
the collision, but it unlikely that our Sun will be separated from its
spiral galaxies will exist no more, replaced by a vast
elliptical blob. The neighborhood will never be the same again.
I’m often asked:
How can Andromeda be coming toward us when the
universe is expanding and everything is supposed to be moving away from
us? The answer is that things move due to a variety of factors, which
can be complicated.
After teaching at
UCLA, I drive home at 15 mph on the 405 Freeway. But
I’m also moving east at 800 mph due to Earth’s rotation, and moving
70,000 mph due to Earth orbiting the Sun, and 500,000 mph as the Sun
orbits the Milky Way, and…
The universe is
expanding at 50,000 mph per million light-years—the
Hubble expansion rate. This means a distance that is 1 million
light-years today is expanding 50,000 miles per hour, and a distance
that is 10 million light-years is expanding 500,000 miles per hour.
Andromeda is now
2.5 million light-years away, so the distance between
us is expanding at a rate of 125,000 mph. But at the same time,
Andromeda and the Milky Way are falling toward one another at 371,000
mph because of their mutual gravitational attraction. The net
motion is the galaxies are approaching at 246,000 mph.
If Andromeda were
10 times farther, 25 million light-years, the space
between us would be expanding at 1,250,000 mph. At that distance, the
gravitational force would be 100 times less, too weak to overcome the
expansion. As the numbers work out, galactic motions within clusters
are dominated by mutual gravity, while the clusters themselves are
moving apart with universal expansion.
Below is NASA’s
concept of what this collision will look like from
Starting in the upper
left, the broad band of the Milky Way is seen cutting across our sky at
a slight diagonal, with Andromeda being the modest oblong object left
of the Milky Way. In the upper right image and then the
second-highest image on the left, we see Andromeda approaching and
growing larger. The fourth and fifth images show the first collision,
with the sixth showing the galaxies' maximum separation before the
final merger in the last image.
Have a Safe and
Wonderful 4th of July!
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Author of "Everyone's
Guide to Atoms, Einstein, and the Universe",
Can Life Be
Merely An Accident?"
World Without Einstein"