What is Interstellar Space?

We humans love to categorize things.  The physical space between objects in the universe is no exception!  Let’s start with the smallest of spacely categories – interplanetary space.  Interplanetary space is the area between planets in the solar system.  Moving up on the scale is interstellar space or the area between stars within the same galaxy.  Finally, the largest physical space is the area between galaxies themselves which is called intergalactic space!

I think that it’spretty easy for us to grasp the concept of interplanetary space.  This is the space between planets in our solar system.  It is something that we can easily see and measure.  Just go outside tonight and you won’t have any trouble seeing any number of planets that share the sun’s orbit with us.  In fact, Venus and Mercury are putting on a delightful show for us this month just after sunset.  While the majority of this space is almost an absolute vacuum, you can still find cosmic rays, some subatomic particles, gasses of various kinds, plasma, dust and the occasional organic molecule.  We generally measure the space between objects in the solar system by using Astronomical Units (AUs) – which is the distance of the Earth to the Sun – approximately 93 million miles.  Earth would be 1.0 AU from the sun and Mars is 1.52 AU from the Sun.

The Space Between the Stars

Interstellar space or the physical space between stars or planetary systems  within the same galaxy is enormous.   This space is also near absolute vacuum  with matter being very diluted.  Most of this matter is gas.  It is difficult to grasp the vast amount of distances between stars.  In fact, they are so large that Astronomical Units don’t really work.  Instead, distance is measured in terms of light years – which is the distance that light can travel in one year or about six trillion miles (10 trillion kilometers).  As a point of reference, the closest star to us, Proxima Centauri, is about 4.22 light years away.

Finally, intergalactic space is the physical space between galaxies in our universe.  This space is generally free of dust and debris and not somewhere you want to be!  These distances are so vast that even a light year is not enough!  Out here, distances are measured by parsecs – which is about 3.26 light years. Of course, the parsec is too small to measure some distances, which is why megaparsecs (1 million parsecs) and gigaparsecs (1 billion parsecs) were created.  The Andromeda Galaxy is .77 megaparsecs from Earth.

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