Milky Way Versus Andromeda As Seen from Earth

Astronomers have used the Hubble Space Telescope
to forecast a future cosmic pile up: the titanic collision of the Milky Way and the Andromeda
galaxy in about four billion years time. The Andromeda Galaxy, some 2.2 million light-years
away, is the closest spiral galaxy to our home, the Milky Way. For around a century,
astronomers have known it is moving towards us, but whether or not the two galaxies would
actually collide, or simply fly past each other, remained unclear. Now, a team of astronomers
has used the Hubble Space Telescope to shed light on this question, by looking at the
motion stars in the Andromeda Galaxy. We wanted to figure out how Andromeda was
moving through space. So in order to do that we measured the location of the Andromeda
stars relative to the background galaxies. In 2002 they were in one place, and in 2010
they were in a slightly different place. And that allowed us to measure the motion over
a period of eight years. The motion is actually incredibly subtle,
and not obvious to the human eye, even when looking at Hubble’s sharp images. However,
sophisticated image analysis revealed tiny movements that the scientists were able to
project into the future. Based on these findings, it is finally possible
to show what will happen to the Milky Way over the next eight billion years, as the
galaxies drift closer, then collide and gradually merge into a single, larger, elliptical galaxy
with reddish stars. And yet the Solar System should in fact survive this huge crash. The reason we think that our Solar System
will not be much affected by this collision between the Milky Way and Andromeda is that
galaxies are mostly empty space. Even though our galaxy, as well as the Andromeda
Galaxy, has a hundred billion stars in it, they are very far apart. So if two galaxies
actually collide with each other, the stars basically pass right between each other and
the chance of two stars directly hitting each other is really, really small. So the likelihood
that our Solar System will be directly impacted by another star, for example, in Andromeda
as we collide with it is really, really small. Well, if life is still present on Earth when
this happens, the changes in the sky will be quite spectacular. Now they will be very
very slow because the timescales on the scales of galaxies in the Universe are very very
long. So you have to think, millions of years but even then over these timescales over millions
of years, we will see big changes. If we wait a few billion years, Andromeda will be huge
on the sky. It will be as big as our Milky Way because we’ll be very close to it. And then later, when the galaxies merge, the
merged remnant of the Milky Way Galaxy and Andromeda will look more like an elliptical
galaxy and we’ll be sitting right in it. So the view of the Milky Way on the night
sky will be completely gone and this band of light will be replaced by a more spheroidal
distribution of light. And so, the Sun, born in the Milky Way almost
5 billion years ago will end its life in a new orbit, as part of a new galaxy.

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