Interstellar dust

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zuz
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We know interstellar space and intergalatic space is not empty. Couldn't small dust particals block the light of distant galaxys?
 

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  • #2
Bandersnatch
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They can. But, before you ask, reddening by extinction is not the same as redshift. The former removes part of the spectrum, the latter shifts the entire spectrum.
 
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  • #3
Janus
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We know interstellar space and intergalatic space is not empty. Couldn't small dust particals block the light of distant galaxys?
To answer that, you have to consider just how close to empty intergalactic space is. It is estimated that that it contains roughly one particle per cubic meter, So if you had a one meter square column of intergalactic space 10 billion light years long, it would contain about 9.5 x 1025 particles, and this would be what would be between you and a galaxy 10 billion light years away.
This seems like a a lot. However, one cubic meter of air at sea level contains 5.2 x 1025 particles, and so a one meter square, two meter long column of air has more particles in it than that 10 billion light year long column of intergalactic space. Two meters of air doesn't block much light.

If the Earth's atmosphere were of constant density, it would be 8.5 km thick, which is how much air we are looking through when we look out at the night sky. This means that our atmosphere blocks many, many, many times more light that is lost over the 10 billion year distance of intergalactic space.
 
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  • #4
zuz
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What I'm saying is that one grain of dust, between us and a distant light sourse is enough to block that light sourse. There are probably particals much larger than that floating out there. I don't see why planet sized chunks of material can't be out there. They have been found in the galaxy. Couldn't one cover a whole slew of galaxys?
 
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Janus
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What I'm saying is that one grain of dust, between us and a distant light sourse is enough to block that light sourse.
Only if it were much, much, much closer to us than that galaxy. Grains of dust don't stand still and are in constant motion. The closer the particle is to us, the greater its apparent motion. For example, a dust mote 1 meter away moving sideways at cm/s has an apparent angular motion of 0.57 degrees per sec. It would cross more than the width of the full Moon in a sec. At 2 meters away its angular motion drops to 0.285 degrees/sec and it would take twice as long to cross the width of the Moon. However, its angular size would also decrease in half and it would block a much smaller part of the Moon as it passed.
There are probably particals much larger than that floating out there. I don't see why planet sized chunks of material can't be out there. They have been found in the galaxy. Couldn't one cover a whole slew of galaxys?
A planet-sized chunk close enough to us to have an angular size large enough to block a whole slew of galaxies would also have a large enough angular velocity to not stay in place to block those galaxies for very long. The images we have of distant galaxies require us to collect light from them for a long time, days in fact. An planet sized object close enough to block a significant section of these galaxies, would only be there for a fraction of the viewing time. A planet-sized object far enough away from us to have a small enough angular motion to stay in place over the whole observation period would also have too small an angular size to block much. And something both big enough and with a small enough angular motion to block our view of a number of galaxies, would reveal itself as blank spot surrounding by visible galaxies.
 
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  • #6
Vanadium 50
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. I don't see why planet sized chunks of material can't be out there.
Janus gave you the exact density. If that doesn't concvince you, what will?
 
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  • #7
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To answer that, you have to consider just how close to empty intergalactic space is. It is estimated that that it contains roughly one particle per cubic meter, So if you had a one meter square column of intergalactic space 10 billion light years long, it would contain about 9.5 x 1025 particles, and this would be what would be between you and a galaxy 10 billion light years away.
This seems like a a lot. However, one cubic meter of air at sea level contains 5.2 x 1025 particles, and so a one meter square, two meter long column of air has more particles in it than that 10 billion light year long column of intergalactic space. Two meters of air doesn't block much light.

If the Earth's atmosphere were of constant density, it would be 8.5 km thick, which is how much air we are looking through when we look out at the night sky. This means that our atmosphere blocks many, many, many times more light that is lost over the 10 billion year distance of intergalactic space.
The whole atmosphere weighs as much as 10 m of water, or 76 cm of quicksilver, or about 5 m of soil. Look under your feet. You cannot see through 76 cm quicksilver or 5 m soil, and only see anything through 10 m water if it is unusually clear lake or sea.
Weighs as much as just 2 m air? But so does 1 mm aluminum foil, or 5 mm of wood. Neither of which you can see through.
Of the intergalactic matter, how much is transparent gas, and how much is solid and opaque rock, coal and iron?
 
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  • #8
phinds
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We know interstellar space and intergalatic space is not empty. Couldn't small dust particals block the light of distant galaxys?
zuz, I don't understand the point of your question. As Janus has pointed out, it could only happen (if at all) under the most particular circumstance but even if it DID happen, my question to you is, so what? Why does it matter? What is it you are really trying to find out?
 
  • #9
zuz
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My point is maybe we're not seeing the whole picture. But Janus makes a solid argument. Now my question is, do we see any thing like that?
 
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Yes, a lot. We do not see most of Milky Way, nor distant galaxies beyond.
 
  • #11
zuz
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Density of air is no comparison. It's too close. I can block out the moon with my thumb. If you look at Mars on any given night, it is probably blocking the light of at least a few galaxies, if the Hubble Ultra Deep Field is a example of the entire sky. Yes, planets move. I'm not arguing that. What I would like to know is are there any voids in our line of sight that could be caused by dust, particals, planets...etc.
 
  • #12
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What I would like to know is are there any voids in our line of sight that could be caused by dust, particals, planets...etc.
Yes. Starting with Great Rift of Milky Way.
 
  • #13
davenn
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Density of air is no comparison. It's too close. I can block out the moon with my thumb. If you look at Mars on any given night, it is probably blocking the light of at least a few galaxies, if the Hubble Ultra Deep Field is a example of the entire sky. Yes, planets move. I'm not arguing that. What I would like to know is are there any voids in our line of sight that could be caused by dust, particals, planets...etc.
yes it is a comparison, for the exact reasons Janus gave you.
Blocking the moon with your thumb is a similar comparison to blocking a distant star, galaxy etc with the moon.
It's all a matter of scale and that is what you need to understand :smile:

thumb. If you look at Mars on any given night, it is probably blocking the light of at least a few galaxies, if the Hubble Ultra Deep Field is a example of the entire sky. Yes, planets move. I'm not arguing that. What I would like to know is are there any voids in our line of sight that could be caused by dust, particals, planets...etc.
You seem not to have understood what Janus said.
Everything is moving, and any light blockage from a planet/ moon etc will last hours at most for a given spot of the sky.

You originally said dust particles, which are NOT the size of the moon or planets. And again Janus related your dust
particles to the same in our atmosphere. It's only when the collection particles of dust or gas are VERY dense as in a
nebula etc that the light objects behind gets blocked out.
Hence why we cannot see the other side of our own Milky Way galaxy or what is beyond it in that direction

Take this random example .....

upload_2018-12-23_9-17-9.png


I could almost 100% guarantee that all the stars you see within the main darkest parts of the nebula will be foreground stars
or stars embedded in the nebula. The only stars that would shine through the nebula would be ones that are quite bright
and immediately behind the nebula. Almost every other star behind the nebula would be blocked out

Reread the two posts from @Janus again and understand the particle size and density scales that are involved.


cheers
Dave
 

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  • #14
phinds
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My point is maybe we're not seeing the whole picture.
OK, well, I'd say you are WAY WAY off base on that thought, as Janus has already explained.
 
  • #15
stefan r
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An object that is closer to you can block a larger object. A 10 cm diameter ball held one meter from your face can block a 1 meter diameter object that is 10 meters away. The Milky Way goes out to around 100,000 light years. Jupiter has a radius of around 70,000 km. If you had a Jupiter size rogue planet outside of our galaxy it could block an object 7 x 109 km wide if that object was at least 1010 light years away. 1.3 x 1010 light years is the extreme limit of anything that we could see because that goes back to the big bang. So a Jupiter radius rogue planet could block an object with the width of Neptune's orbit. All rogue planets will be about equal or smaller and all galaxies will be closer. So at most you lose one star system in a galaxy with billions of stars.
 
  • #16
DaveC426913
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So at most you lose one star system in a galaxy with billions of stars.
And, as Janus alludes to, when your shutter speed minutes, hours or even days, even that problem goes away.
 

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