Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne With Dust, Why Do We See Stars
Name: Edward
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
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.


Replies:
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.

Sincerely

David H. Levy


Edward,

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 flashlight.

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.

Matt Voss


Hi Edward,

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)
Canisius College


Ed

Your observations testify more to the absence of material in space.

Twinkling is called Scintillation:

http://en.wikipedia.org/wiki/Scintillation_%28astronomy%29

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.

http://en.wikipedia.org/wiki/Occultation

It happens all the time, astronomically.

Here is a list of the galaxies closest to Earth in ascending order:

http://en.wikipedia.org/wiki/List_of_nearest_galaxies

So, yes, given the astronomical distances, we do have a largely Unobstructed view of (most) everything in space.

Sincere regards,

Mike Stewart


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 astonishingly empty.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming



Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory