Colliding Galaxies

The year 1924 was a turning point in the study of Andromeda.  For hundreds of years astronomers believed that Andromeda was a constellation in the Milky Way Galaxy and that the Milky Way Galaxy was what we knew as “our universe”.  From the work of Edwin Hubble, we now know that this is not true.

Andromeda Galaxy

Andromeda is a large galaxy ~ 260,000 light years in diameter and holding ~ 1 trillion stars.

Edwin Hubble discovered Andromeda as an independent spiral galaxy in 1924.  Prior thinking
that Andromeda was a constellation in the Milky Way Galaxy was equally wrong, but the most astounding finding was that our universe consisted of additional galaxies, a discovery that has grown into current thinking that the universe includes billions of galaxies.   

Astronomers place Andromeda in what they term the local group,a group of 30 galaxies including Andromeda and our galaxy, the Milky Way.  Within the local group Andromeda and the Milky Way are ~ 2.5 million light years apart and approaching each other at ~ 100-140 kilos/sec.

Milky Way Galaxy

 The Milky Way is about half the size of Andromeda, being ~115,000 light years in diameter and containing ~ 100-400 billion stars.  It had been anticipated for many years that the two galaxies would collide in ~ 4.5 billion years based on present courses and star growth information detected in Andromeda which suggests that the galaxy had suffered through an earlier collision with another unknown galaxy some millions of years earlier.  The collision of any galaxies cannot be compared to the collision of two or more vehicles on a highway.  In the case of the vehicles there is immediate physical contact at the moment of collision.  When speaking of galaxies colliding, consideration must be made for the immense distances between adjacent astro bodies making up the individual galaxy.  It is possible for the majority of the material in each galaxy to pass each other except for tidal forces which may occur during close passage.  Yet, it is still possible that stars and planets may be stripped from one galaxy and transferred to the other.  In some cases stars or planets may be torn from a galaxy not to be captured, but to be thrown into space to die in the cold and blackness.  The latter is less likely to happen because of the many interacting gravitational actions of the many disturbed stars which would be likely to initiate a capture.

Having said all this, the question of the consequences of the collision on our Solar system must be considered and answered.

Our star Sol is ~ 4.6 billion years old and middle aged.  The average radius of Sol is 695.5 km of which 20-25% is in the core.  The Sun burns at a surface temperature of ~ 5,500 degrees C using hydrogen as a fuel.  The Sun is almost 3/4 hydrogen with most of the remaining mass, helium.  It's energy is generated at the core by nuclear fusion with hydrogen being converted to helium.  Huge gravitational forces keep it from exploding.

When all the hydrogen has been consumed the Sun will continue to burn for another ~ 130 million years using helium as fuel.  During the latter period internal pressures and a reduction in gravity from the huge loss of of hydrogen and helium will allow the Sun to expand until it reaches the orbit of Earth, engulfing the planets Mercury, Venus and Earth in the process.  At this stage in the life of Sol, it would have become a Red Giant.  

After it's Red Giant phase and the continued burning of helium, the Sun will collapse, retaining its enormous mass while assuming the approximate volume of the former Earth, and become a White Dwarf. 

By the time the two galaxies collide, Sol would be a White Dwarf and Earth would, at best, be a burned-out cinder remaining in its original orbit.

Our Solar System is roughly 27,000 light years from the galactic center, in a remote backwater of the Orion Arm, one of its spirals.  Since the Milky Way Galaxy is a spiral galaxy, the location of Sol at the time of collision will have much to do with the effect of the collision on our Solar System -- not that it would matter.  As our galaxy rotates , since Sol is closer to the edge than the core, the position of Sol will vary greatly.  If the spiral rotation places Sol in position closest to the on-coming Andromeda, the risk to Sol would be much greater than when Sol was on the opposite side of the spiral.