B How Can Astronomy Even Occur? (Oort Cloud)

[Carpe Physicum]

I watched a show on TV about the Oort cloud, that featured a visual. I think it mentioned something like several trillions with a t objects surrounding the Sun outside the solar system. And the image showed just that, a cloud so dense and thick you couldn't see the sun. Combine that with all the millions of pieces of space junk circling the earth, how in the world can any serious astronomy occur? How do we know the light (of any type) isn't being affected by all these objects, especially the ones in the oort cloud, even indirectly? When we're talking about such minute things as shifts in a sun's orbit indicating existence of planets, I don't know, it just seems to go beyond belief.

 

[Orodruin]

a show on TV about the Oort cloud, that featured a visual

Was that actual footage or an artist's impression?

 

[Carpe Physicum]

Oy, if you're implying that because it's an artist's impression, and artists aren't astronomers, it can't possibly be accurate and is just for the amusement of us rube laymen, then can I get a real answer please. I see that too much in this forum..."ya that's just popular claptrap ha ha what dunces you laymen are for even watching that crap".

It was from a reputable producer of scientific shows and I'm assuming a real astronomer verified the image was realistic.

 

[Arman777]

I watched a show on TV about the Oort cloud, that featured a visual. I think it mentioned something like several trillions with a t objects surrounding the Sun outside the solar system. And the image showed just that, a cloud so dense and thick you couldn't see the sun. Combine that with all the millions of pieces of space junk circling the earth, how in the world can any serious astronomy occur? How do we know the light (of any type) isn't being affected by all these objects, especially the ones in the oort cloud, even indirectly? When we're talking about such minute things as shifts in a sun's orbit indicating existence of planets, I don't know, it just seems to go beyond belief.

Well, for the space junk, it simply does not reflect light (because enable to see things light should reflect from an object and come to Earth or to the telescope, which that's most likely impossible for such small objects) in that sense it should not affect our observation.

In general we use Hubble telescope which its already in space.

For Oort cloud, I think we can apply the same principle. Also as you know when the objects are farther they take up little space in the sky. Like Jupiter much more larger than the moon but since its far away it looks much smaller.

Also, it says " The Oort cloud was "discovered" by Dutch astronomer Jan Hendrik Oort in 1950, not through telescopic observations, but through a theoretical study of the orbits of long-period comets (comets with periods greater than 200 years)."

http://www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/astronomy-general/oort-cloud

In this sense, we may conclude that it doesn't affect our observations.

 

[Carpe Physicum]

Interesting. Aren't there other ways in which light can interact with objects, for example getting blocked entirely by them? Or slight shifts in their paths due to gravity (with some of the larger objects). So we know for a fact that there are no objects that would have this effect? Or does it boil down to the old 'the universe is just so big" that even a cloud of trillions of objects is still basically see through?

 

[Bystander]

Or does it boil down to the old 'the universe is just so big" that even a cloud of trillions of objects is still basically see through?

Yes.

 

[Arman777]

I didn't quite understand your question actually.

Aren't there other ways in which light can interact with objects, for example getting blocked entirely by them?

Like Solar eclipse ? I don't know any kind of other example in general but maybe someone can give other examples on that one.

Like also, If it's interesting for you, you can check this
https://www.universetoday.com/120006/why-cant-we-see-the-center-of-the-milky-way/

Or does it boil down to the old 'the universe is just so big" that even a cloud of trillions of objects is still basically see through?

Dust is a big problem sometimes, but we can use infrared telescopes to see through the dust. But again, yes. As you said, the universe is huge so I don't think it would be a problem for observation.

 

[snorkack]

By contrast, we CAN see zodiacal light.
There are 25 asteroids in main belt with mean diametre over 200 km. I suppose all of them are individually discovered.
What is the prevalent size of grains that form the zodiacal light?

 

[Orodruin]

ya that's just popular claptrap ha ha what dunces you laymen are for even watching that crap

I never said that. I would urge you to stop putting words into people's mouths. What I said was that you have to take all such images with a pinch of salt, just because it is a respectable show does not mean that they will not take artistic freedoms. An actual image of the Oort cloud would be rather uninteresting. Even if there are a trillion objects, you have to consider the solid angle that each of those objects take up when seen from Earth. For example, the Milky Way contains something like 200 billion stars and that is clearly not enough to fill the sky because each star takes up a very small solid angle. Also, the fact that you can see those stars essentially undistorted should already tell you that the Oort cloud cannot be affecting the light coming from outside the solar system to any significant extent.

 

[snorkack]

Hell, not just Oort cloud.
How much would a short scale trillion raindrops block or even reflect light on Earth?
Earth is 510 million square km.
A trillion raindrops, short scale, is just 2000 raindrops per square km. A single drop per 500 square m.
Would it be even noticeable?

 

[russ_watters]

Oy, if you're implying that because it's an artist's impression, and artists aren't astronomers, it can't possibly be accurate and is just for the amusement of us rube laymen, then can I get a real answer please.

Sure. The oort cloud starts at a distance of about 1.5 quadrillion km from the sun. Using the assumption that all trillion objects are arrayed on a circle of that radius, what is the average spacing between the objects?

It was from a reputable producer of scientific shows and I'm assuming a real astronomer verified the image was realistic.

Oh, no, you should never assume that. Regardless of if an astronomner is consulted, the purpose of a tv show is to attract viewers to watch advertisements. So artists impressions need to be pretty and interesting, not accurate. And an image of the reality - a totally black picture of nothing - just isn't interesting enough.

[edit] I should say this is not entirely the tv show producer's fault. The vast differences in scale make it impossible to show the objects and the cloud in the same image, so choices have to be made about how to represent it.

 

[phyzguy]

I watched a show on TV about the Oort cloud, that featured a visual. I think it mentioned something like several trillions with a t objects surrounding the Sun outside the solar system. And the image showed just that, a cloud so dense and thick you couldn't see the sun. Combine that with all the millions of pieces of space junk circling the earth, how in the world can any serious astronomy occur? How do we know the light (of any type) isn't being affected by all these objects, especially the ones in the oort cloud, even indirectly? When we're talking about such minute things as shifts in a sun's orbit indicating existence of planets, I don't know, it just seems to go beyond belief.

Do a simple calculation. Suppose there are 10^12 objects in the Oort cloud and each one is 10km in diameter (most are probably smaller, but if you don't like this number try 100km or 1000km). Then the total surface area of all of those objects is about 10^12 * 10^2 = 10^14 km^2. The Oort cloud is about 5000 AU or 10^13 km in radius so it has a surface area of at least 10^27 km^2. Actually many of the Oort cloud objects are even further away, but let's try these numbers. So those trillion objects only block 10^(-13) of the light coming through. So we don't even notice them. The reason the image you saw exaggerated the size of the objects was that if it didn't there would be nothing to see.

 

[Orodruin]

Do a simple calculation. Suppose there are 10^12 objects in the Oort cloud and each one is 10km in diameter (most are probably smaller, but if you don't like this number try 100km or 1000km). Then the total surface area of all of those objects is about 10^12 * 10^2 = 10^14 km^2. The Oort cloud is about 5000 AU or 10^13 km in radius so it has a surface area of at least 10^27 km^2. Actually many of the Oort cloud objects are even further away, but let's try these numbers. So those trillion objects only block 10^(-13) of the light coming through. So we don't even notice them. The reason the image you saw exaggerated the size of the objects was that if it didn't there would be nothing to see.

I have to say: Excellent post.

 

[lekh2003]

To give another analogy following what @phyzguy said. Imagine somebody making a documentary about dust particles and skin cells in the air. In a normal day you can't see any of it, even though there are hundreds of particles floating in the air. If somebody, even a reputable science researcher, was making a documentary, they would have close to zero chance of taking a nice picture with dust particles actually being shown. Artistic representations might make it more clear to any viewer (not just laymen) that there are actually several dust particles in the air; which does not necessarily mean that's how it literally looks.

@Orodruin is not saying that you are a "dunce", he is explaining that documentaries and films usually have artistic representations of things which would otherwise not be apparent. No need to be stingy about it.

 

[davenn]

What is the prevalent size of grains that form the zodiacal light?

from Wiki

The dust in the Solar System forms a thick, pancake-shaped cloud collectively known as the zodiacal cloud, which straddles the ecliptic plane. The dust particles are between 10 and 300 micrometres in diameter, most with a mass around 150 micrograms.[3]

 

[Carpe Physicum]

Do a simple calculation. Suppose there are 10^12 objects in the Oort cloud and each one is 10km in diameter (most are probably smaller, but if you don't like this number try 100km or 1000km). Then the total surface area of all of those objects is about 10^12 * 10^2 = 10^14 km^2. The Oort cloud is about 5000 AU or 10^13 km in radius so it has a surface area of at least 10^27 km^2. Actually many of the Oort cloud objects are even further away, but let's try these numbers. So those trillion objects only block 10^(-13) of the light coming through. So we don't even notice them. The reason the image you saw exaggerated the size of the objects was that if it didn't there would be nothing to see.

Excellent post. Thanks.

 

[Carpe Physicum]

Sure. The oort cloud starts at a distance of about 1.5 quadrillion km from the sun. Using the assumption that all trillion objects are arrayed on a circle of that radius, what is the average spacing between the objects?

Oh, no, you should never assume that. Regardless of if an astronomner is consulted, the purpose of a tv show is to attract viewers to watch advertisements. So artists impressions need to be pretty and interesting, not accurate. And an image of the reality - a totally black picture of nothing - just isn't interesting enough.

[edit] I should say this is not entirely the tv show producer's fault. The vast differences in scale make it impossible to show the objects and the cloud in the same image, so choices have to be made about how to represent it.

I don't think all tv producers are so low. Maybe some crap reality shows. But not necessarily or I should say primarily stations like Nat Geo (I think that's where I saw it). Regardless I think your edit was more to the point. It would be interesting to have a show just on that...comparing renderings made for laymen, illustrative purposes only, compared to some kind of actual representation. Carl Sagan was always good at that.

 

[Carpe Physicum]

So let me ask a follow on question. I have a rough understanding of things like the Doppler shift, etc. So when light is received at a ground based telescope, do astronomers really take into account all the motions of the earth? There's rotation on its axis, angle of it's axis relative to..??, effects caused by the moon, motion around the sun, motion of the sun in the galaxy, and all the angles involved in those things as well. I know astronomers are smart people, compared to whom I'm literally a three-legged frog, but that just seems outrageously complex to the point of being impossible.

 

[Orodruin]

The effects you mention are too small to significantly affect most observations. For example, the motion of the Sun relative to the galactic center has a speed of around 200 km/s. Compare this to the speed of light, which is 300000 km/s.

 

[Carpe Physicum]

What about for example how planets are detected? I was under the impression the slight "wobble" of the host star due to the orbitting planet was being detected. That seems like a pretty fine tuned thing, which would also then have to take into consideration their movements as well. Seems like something that miniscule (I'm guessing of course) would be affected.

 

[Orodruin]

Yes, those are very small effects, smaller than the effects of the motion of the Earth for example. However, it is a relative effect. Let me make up some numbers to show the general idea:

Imagine that the motion of the star you observe relative to the observer is 2000 km/s. The star's motion due to a planet changes by ±1 m/s. Assuming that you have sufficient precision, it does not matter that the star is moving at 2000 km/s because you measure it to be 2000001 m/s at one instant and 1999999 m/s at another so you know that the motion changed by 2 m/s. If the star was instead moving at 3000 km/s you would still have enough precision to know that the motion changed by 2 m/s.

Of course, there may be situations where motion could be relevant. However, those are usually pretty well known and can be easily accounted for by having a computer analyse the data. It is standard procedure in science to analyse your experiment and identify the possible sources of backgrounds and account for them appropriately.

 

[phyzguy]

What about for example how planets are detected? I was under the impression the slight "wobble" of the host star due to the orbitting planet was being detected. That seems like a pretty fine tuned thing, which would also then have to take into consideration their movements as well. Seems like something that miniscule (I'm guessing of course) would be affected.

For these types of precision measurements, astronomers do indeed need to take all of the things you mention into account. This is done by defining an "International Celestial Reference System", which is centered on the center of mass of the solar system, and has a fixed orientation in space. Distant quasars, which are basically motionless because they are so far away, are used to calibrate the motions involved, and you can look up tables of how your location on Earth is moving relative to that coordinate system at any given time. Luckily we have computers to keep track of all of this stuff.

 

[Carpe Physicum]

Wow, great answers! It's so interesting to get some of the details that obviously go way beyond the TV show level of things, especially when you guys explain them so nicely without having to go into the deep mathematical aspects. Thanks.

 

[stefan r]

So let me ask a follow on question. I have a rough understanding of things like the Doppler shift, etc. So when light is received at a ground based telescope, do astronomers really take into account all the motions of the earth? There's rotation on its axis, angle of it's axis relative to..??, effects caused by the moon, motion around the sun, motion of the sun in the galaxy, and all the angles involved in those things as well. I know astronomers are smart people, compared to whom I'm literally a three-legged frog, but that just seems outrageously complex to the point of being impossible.

It is much easier. Far away objects like galaxies and bright far away stars all move the same.

You can add things like people jumping around on the observatory foundation, wind, or temperature distortions of the apparatus. Everything moves the far away objects the same way.

... So those trillion objects only block 10^(-13) of the light coming through. So we don't even notice them.

Even if they did block 90% of the light Earth is moving so all far away objects could be imaged. Earth is moving at nearly 30km/second so a one second exposure would show a star that was eclipsed by a 10km object (exception near dawn/dusk near the ecliptic plane). You can wave your hand in front of your face and still read this. The hand decreases the brightness a little but you can still get a lot of information.

What about for example how planets are detected? I was under the impression the slight "wobble" of the host star due to the orbitting planet was being detected. That seems like a pretty fine tuned thing, which would also then have to take into consideration their movements as well. Seems like something that miniscule (I'm guessing of course) would be affected.

The wobble is in frequency not in intensity. A 100 watt bulb has higher intensity than a 60 watt bulb. Frequency is more like a color. Yes, it can be very fine tuned.

A specific peak frequency will Doppler shift because of both our motion and the star's motion. Galaxies will shift because of only Earth's motion. You can subtract one from the other and get the shift that is only due to the star's motion.

 

[russ_watters]

I don't think all tv producers are so low. Maybe some crap reality shows. But not necessarily or I should say primarily stations like Nat Geo (I think that's where I saw it).

I think Nat Geo may be the best, but be careful even with such non-fiction based tv stations because some produce very problematic content: History has "Ancient Aliens" and Discovery has "Moonshiners" and "Amish Mafia", both of which look and are marketed as "reality" tv shows, but are in fact completely fake; the people in the shows are actors and the depicted characters do not exist in real life.

Regardless I think your edit was more to the point. It would be interesting to have a show just on that...comparing renderings made for laymen, illustrative purposes only, compared to some kind of actual representation. Carl Sagan was always good at that.

We can do that here. This image is from NASA:

The distance scale is logarithmic, which is fine, but none of the objects are to scale; not even to each other. And the Oort cloud looks like a dense cloud of individual objects. If the objects in the picture we're shown to the scale of the Sun, you might also see the gas giants in the image, but the entire rest of the image would be black.

This is from NASA(!), and even from them if it isn't explicitly stated it is tough to know what is real and what is just drawn to look pretty.

 

[russ_watters]

What about for example how planets are detected? I was under the impression the slight "wobble" of the host star due to the orbitting planet was being detected. That seems like a pretty fine tuned thing, which would also then have to take into consideration their movements as well. Seems like something that miniscule (I'm guessing of course) would be affected.

Well consider this; just to track a certain star across the sky, you have to account for most of those movements. But there are only half a dozen or so and the the telescope in my avatar photo came with a computer as powerful as a decent pocket calculator to do the calculations and run the tracking.

More sophisticated, even free home astronomy software does thousands of such calculations.

...some more detail:

To my telescope's computer, the celestial sphere is a fixed surface, all objects are points and nothing outside the solar system has relative motion. So the only motions I can think of it takes into account are:
1. Earth's rotation.
2. Earth's revolution (and orientation change with it).
3. Your motion on Earth's surface (manual input, then included in the calcs).
4. The moon and planets' orbits (and I think a few major comets and asteroids).

Home astronomy software additionally takes into account/includes:
5. Precession of the polar axis.
6. Proper motion of stars.
7. Rotation of planets and the sun.
8. Orbits of dozens of moons around their planets.
9. Orbits of thousands of comets and asteroids.
10. Orbits of thousands of artificial satellites and space probes.
11. Rotation of satellites (for flare/magnitude prediction).
12. Object size.
13. Object reflectivity.
14. Lunar libration.
15. Atmospheric scattered and absorbed light (twilight, light pollution).

Probably more I couldn't think of.

 

[PAllen]

Oy, if you're implying that because it's an artist's impression, and artists aren't astronomers, it can't possibly be accurate and is just for the amusement of us rube laymen, then can I get a real answer please. I see that too much in this forum..."ya that's just popular claptrap ha ha what dunces you laymen are for even watching that crap".

It was from a reputable producer of scientific shows and I'm assuming a real astronomer verified the image was realistic.

OK, when you look at the night sky what do you see? Loads of stars, meteors (on a clear night), etc. or something else. This alone suggests either your interpretation of the show you saw, or the show, are grossly inaccurate. I have no idea which is the case, but it is one or the other.

 

[stefan r]

OK, when you look at the night sky what do you see? Loads of stars, meteors (on a clear night), etc. or something else. This alone suggests either your interpretation of the show you saw, or the show, are grossly inaccurate. I have no idea which is the case, but it is one or the other.

What you see on a TV is a monitor. Suppose it has 1920 x 1080 pixels. Each of the things you are show need at least one pixel or you are not showing it. 2 million items should be a full screen. Talking about the Ooort cloud and then showing a normal photo of the night sky would be rather confusing. All Ooort cloud objects are smaller than one pixel. If they zoom into accurately show the sized of one object at 1/1080 then overall image does not look like the night sky or a cloud.

 

[sophiecentaur]

How do we know the light (of any type) isn't being affected by all these objects,

The point has been made, already, that there are many more gaps than objects to get in the way of things. Say an object actually found itself right between the astronomer and a distant star. Firstly, that sort of occlusion will not last long because the sort of objects that occur in the Oort Cloud will be subtending a minuscule angle (small fractions of an arc second) and the Earth is constantly on the move, taking the astronomer out of the suggested shadow and into a massive e gap until the next Oort object gets in the way. But, even if someone were actually chasing the Earth's motion to track its position in line with the star, Diffraction is at work. Light spills round the edges of the object and it only needs to be diffracted by a tiny amount (that small fraction of an arc second we were considering) in order to fill in the shadow. Even for relatively large objects, relatively close, there is a phenomenon called the Poisson Spot, which is a bright spot, formed dead in the middle of a shadow. A small ball bearing at a distance of a metre or so can have a Poisson Spot. So, basically, a large lump of rock at the distance of the Oort Cloud has no chance of producing an actual shadow. It may reduce the brightness of the star for a short while but that is what we find with the atmosphere in any case. And the dip would not, I suggest, even be detectable by Hubble because it would just be a one off miniscule blip, which would just be interpreted as a typical event in Electronic Noise.
So, @Carpe Physicum, astronomers can carry on with no worries about missing major (or even minor) events and objects out there.

 

[Jenab2]

The inner Oort cloud is a torus having an inner radius of about 2000 AU, or 3e+11 kilometers.
The outer Oort cloud is a spherical shell having an outer radius of about 60000 AU, or 9e+12 kilometers.

 

[sophiecentaur]

It just struck me that we are as likely to be aware of Oort objects between us and stars behind them due to shadowing as we are to see them due to them being illuminated by stars that are behind us - or even the Sun (the most likely candidate). We do not see Oort objects, as far as I know and their presence has not been inferred by any shadowing effect either. We only infer the presence of the clouds because of objects that appear occasionally, much nearer (comets).

 

[Jenab2]

When an incoming comet has an elliptical orbit, it is possible to find the orbit's semimajor axis from the comet's observed speed and heliocentric distance, using the Vis Viva equation, which is essentially an expression of the conservation of energy in a gravity-dominated physical system. Then with the polar equation of the ellipse, pole at the sun, you can get the eccentricity of the comet's orbit, though this is most easily done when the comet passes through its perihelion. With the semimajor axis and eccentricity both known, you can calculate the aphelion distance.

 

[sophiecentaur]

When an incoming comet has an elliptical orbit, it is possible to find the orbit's semimajor axis from the comet's observed speed and heliocentric distance, using the Vis Viva equation, which is essentially an expression of the conservation of energy in a gravity-dominated physical system. Then with the polar equation of the ellipse, pole at the sun, you can get the eccentricity of the comet's orbit, though this is most easily done when the comet passes through its perihelion. With the semimajor axis and eccentricity both known, you can calculate the aphelion distance.

I'm not sure what that has to do with the visibility of Oort objects?

 

[Jenab2]

I'm not sure what that has to do with the visibility of Oort objects?

You don't need to see Oort Cloud objects to know that they're there. You only need to compile statistics on the original distances (now aphelion distances) from which comets come. Then you know how far away the Oort Cloud is. This doesn't work with old comets, because old comets might have brushed by objects in the inner solar system and had their aphelions reduced. But it ought to work for young comets just fine.

 

[rootone]

The Oort cloud is not packed full of objects and for that matter the much nearer asteroid belt of the inner solar system is not,
There are enough objects to make a distinct feature, but nothing like the worst cases of pop-sci presentations with hundreds of kilometer sized objects constantly colliding.

 

[sophiecentaur]

You don't need to see Oort Cloud objects to know that they're there. You only need to compile statistics on the original distances (now aphelion distances) from which comets come. Then you know how far away the Oort Cloud is. This doesn't work with old comets, because old comets might have brushed by objects in the inner solar system and had their aphelions reduced. But it ought to work for young comets just fine.

Oh yes. I realize that we can deduce the presence of the Oort Cloud but we can't actually 'see it'. That's pretty a pretty normal situation in Science.