
Is the need for definition just human
natures quest for solid answers, a more refined interpretation of the world
around us, or is it always a physical boundary?
Astrophysics is one of those subjects
plagued by hand-wavy definitions of ‘the edge’. Stars, atmospheres, nebulae,
solar systems, even the Universe itself have only vague definitions of the
edge.
The dictionary defines the ‘edge’ as - The
outside limit of an object, area, or surface; a place or part farthest away
from the center of something.
So with this in mind there is the
possibility of multiple ‘edges’ for any one thing.
The Sun and stars for example have a
visible surface called the photosphere a layer in the solar atmosphere where
light in the visible wavelengths emerges. So this is the inner edge from which
light emerges from a star, but where does it stop, does it stop? As you go
farther and farther away from a star the amount of light you see will reduce
with distance. But there will be no point at which the light will get weaker
and stop. This gives the stars an inner edge but not an outer one.
“Ahhh, but…!” I hear you cry. Electromagnetic
radiation is not the only defining physical parameter associated with stars.
What about their gravity, how do we define the edge of a stars gravitational
influence?

So if light has no outer edge and neither
does gravity what is it that separates us from interstellar space?
This leads us to another edge. Formed by the
separation of one magnetic region from another, producing a physical boundary
to an otherwise handwavy realm.
The sun’s magnetic field.
While planetary magnetic fields like the
Earth’s are shaped by the solar wind, causing a tadpole like tail in the magnetic
field to drag behind the dark side of the planet, the Sun’s is relatively
spherical forming something called the heliopause. This is the edge that defines
the boundary between our Sun and interstellar space.
It is this giant magnetic bubble, this
defined edge, which Voyager 1 has been attempting to breach, trying to reach
out a human hand into interstellar space.
This is the edge. Voyager 1 marks the edge.
The edge of humanity as it endeavors to explore the
unknown. Voyager 1 is the
farthest from the Sun any man-made object has ever been and it will continue on
its journey into the reaches of the solar system long after we are able to
communicate with it. Maybe one day we will go pick it up but until then it will
continue to test the boundaries of our Suns influence on its profound voyage
from the Earth.
Next step the edge of the universe.
Whatever that may be.
What’s Next?
There have been some great articles about
Voyager 1’s journey but I think these two really speak to the facts the most.
NASA JPL – How do we know when Voyager
reaches interstellar space?
and
IEEE’s – Voyager hasen’t really left the
solar system, but that’s OK
I hope you had fun let me know what you
think by leaving a comment.
Thanks.
PICTURE CREDIT: Main image - http://www.psu.com/Cliff-Edge--wallpaper-847.php
middle and final image - NASA/JPL