With Dust, Why Do We See Stars
A solar eclipse is when the shadow of the moon passes
over and blocks our view of the sun. Applying that concept to the
rest of the sky, given the fact that "empty space" is actually
filled with debris, such as tiny pebbles, grains of sand etc,. Q:
Shouldn't distant objects, which appear to us as tiny pin-points of
light (or smaller and require help from telescopes to be seen) in
the night sky, be blinking (instead of merely twinkling) constantly
as tiny objects between here and there pass by and block the light?
Surely it cannot be true that, given such incomprehensible
distances, that we have a largely unobstructed view of everything
especially the most distant, ancient objects.
Thanks, good question. Actually we do not. Interstellar dust blocks
our view of many distant objects in the Milky way. Supernovae that
should be visible in daylight in our own galaxy are often blocked by
such dust. Dust particles do cause the blinking to which you refer
but the result is infinitesimal and we do not see it.
Such a brief "blink" from one part of the double quasar in Ursa
Major led Rudolf Schild in 1996 to observe what could be a planet
out there, so it can happen! I described the event in my book Deep
Sky Objects: The Best and Brightest four Decades of Comet Chasing.
(Prometheus, 2005), p. 262.
David H. Levy
There are two concepts that can explain why stars twinkle and do not
blink. While there is lots of dust in space, the vast majority of
space contains no solid matter. I think a rough estimate is that
there are an average of 9 molecules of hydrogen in a given cubic
meter. Given that hydrogen is close to 98% of the atoms in the
universe, there is a lot of empty space that is not blocking light
from reaching us. Another point is that the size of debris compared
to a given star is so much smaller that it would take a lot of
debris indeed to fully block all of the light from a star in order
to make it blink. It is the equivalent of chicken wire blocking a
The second point is that matter generates gravity and due to this,
the gravity has an effect on the wave of light. Light can actually
bend around object, can reflect off of objects and depending on the
material, shine right through it (i.e. transparency or
translucency). While debris generally wouldn't have a measurable
gravitational pull on light, planets, stars and other objects,
especially black holes, do. In order for debris to cause a star to
blink it would have to have a significant gravitational pull and it
would need to be traveling past the star so quickly as to divert
light completely away from Earth. If this were to ever occur,
chances are very good that we would miss a single blink.
There are two issues here: the issue of scale (sizes, dimensions)
and the propagation of light. In terms of scale: what appears to us
as tiny points of light are incredibly gigantic objects and dust
particles are incredibly small compared to stars. In terms of light
propagation: (a) while light may be said to travel in a straight
line, every point on a star's surface is a light source that
produces light and so an object in front of a star would be
receiving light from different points of the star and the object's
shadow will tend to get smaller (or less defined) with distance -
this is why a beam from a flashlight gets bigger as you shine it at
more distant objects, and why a shadow gets more diffuse as it gets
farther from its source; (b) light is affected by gravity - massive
objects like planets tend to bend light - so an object like the
planet Mercury creates a much smaller shadow (than its actual size) on Earth.
So, for these reasons we do not see much of the effect of small
debris in between here and other stars - the objects' shadows (which
are very tiny relative to a star) get diffused and ultimately become
imperceptible with the vast distances. It does not matter that there
might be a lot of debris in between here and a particular star, all
those debris do not form a coherent, combined shadow.
On the other hand, several exo-planets (planets around other stars)
have been discovered because they do cause a diminishing of the
light - but these planets are incredibly large compared to dust
particles or even asteroids.
Greg (Roberto Gregorius)
Your observations testify more to the absence of material in space.
Twinkling is called Scintillation:
It is caused by changes in density of the air column in Earth's atmosphere
through which you are observing the star.
Blinking is called occultation.
It occurs by a planet, dense gas cloud or "something else" moving in front
of the observed body.
It happens all the time, astronomically.
Here is a list of the galaxies closest to Earth in ascending order:
So, yes, given the astronomical distances, we do have a largely
Unobstructed view of (most) everything in space.
The eclipsing events to which you refer do occur, but they are rare.
One reason they are not routinely detected is that long exposure
times are used for most astronomical observations. A brief
"wink-off" would not be noticed any more than you notice the
flickering of a fluorescent light bulb. However, that is a very
small effect. Light received from very distant objects has not been
blocked, scattered, or absorbed by intervening material.
You hit upon the reason in your question, but rejected it as
implausible. Space is incomprehensibly big, and it really is
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012