Light Years Away

buzzdiamond

OK Tim, thanks for the link. But, I've checked some images out and deduced that the Hubble space images are not real.

They show pictures of galaxies billions of light years away, outside of our galaxy, but there are no pictures that focus on a star in our galaxy. Why is that..? If Hubble can supposedly show us a beautiful color photo of some distant galaxy, bazillions of light years outside of our galaxy, why can't it produce a photo of a star in our galaxy to appear as close as the photo's we have of our sun?

Can someone please explain to me why the hubble pictures of the planets in our solar system http://hubblesite.org/gallery/album/solar_system/ don't show any stars in the background..? The pictures of the planets really look fake.

Here's a link http://www.nasa.gov/mission_pages/LR...llo-sites.html showing pictures of the moon, with so-called lunar rover tracks. Would someone please estimate the distance of those tracks for me..?

OK people, have at it...

Chronos

Buzz, you've deduced your way into the twilight zone of conspiracy theories. Planets are vastly brighter than background stars, which also explains the lack of stars in pictures taken on the moon by astronauts.

buzzdiamond

I'm sorry Chronos, but Mars does not give off light or is burning at extreme temperatures. Planets reflect light and are cold. Therefore, Mars and the other planets in our solar system are not brighter than stars..!!

Nice try. Do you wish to comment on my other two paragraphs..?

Looking for others input. Thanks.

Drakkith

Because the diameter of a star is so much smaller than a galaxy we run into the problem of not being able to get enough resolution to see the full extent of the star. When viewed in a picture a star is referred to as a "point source" of light. To understand what this means you need to learn a few things about optics.

First, when light is focused down to a point, called an airy disc, that point has a finite size that directly depends on the diameter of your optical aperture and the wavelength of the light. Small telescopes cannot focus the light onto as small a point as a larger telescope if the focal length is the same for both scopes. When we increase the focal length of a telescope we increase the magnification of the image. When we do this the airy disc itself also gets larger, which keeps us from just zooming in until we can see detail on the star. Our "angular resolution" simply isn't high enough to see almost all stars. When the size of the airy disk is much larger than the apparent diameter of the object, that object is referred to as a "point source" because you can treat it in almost all aspects as a point "infinitesimal in size" that emits light. (These don't really exist, but at a certain point the difference between a "true" point source and a real object is small enough that it simply doesn't matter)

Now, stars are VERY small compared to galaxies. In optical terms we refer to how "big" something looks as it's angular diameter. Closer objects look bigger than objects that are further away, such as watching as a car shrinks in apparent size as it moves away from you.

So a star, being so very far away compared to its physical size has a very small angular diameter. Stars further away look even smaller than closer ones do. They are generally so small that we actually don't have telescopes capable of seeing detail on any stars but a very select few, such a Betelgeuse. (Which Hubble has actually taken a picture of, because the star is close enough and large enough to be within the Hubble's capability. See here: http://en.wikipedia.org/wiki/File:Be...8Hubble%29.jpg)

Galaxies, especially closer ones, are so big that we can see plenty of detail in them even though we cannot see individual stars in our own galaxy. Realize that a galaxy is about 100,000 LIGHT YEARS in diameter, while a star is only around 200,000 - 1 billion kilometers across, which is FAR less than even 1% of one light year. (The sun is about 650,000 km in diameter for comparison. The largest stars are about 1,000 times larger)

Stars are very far away, so the intensity of the light has dropped off substantially by the time it gets here. Put simply, the amount of light reflecting off a planet into a camera is FAR more than any single star. Because of this the exposure time has to be short enough to keep the image of the planet from saturating the sensor, which means that there isn't enough light entering from the background stars in that time to register in the image.

If we take the planets and put them at the same distance that the stars are at then of course they aren't brighter than the star for the reasons you stated. However, stars are light-years away and the apparent brightness of an object falls off with the inverse square of the distance.

cepheid

LOL! Good on you for your powers of deduction! Too bad they have no basis whatsoever and demonstrate a clear misunderstanding of basic physics.

A telescope has a certain limit to its ability to resolve two distinct objects as being separate. Suppose two objects are very close together on the sky. This does not necessarily mean that they are close to each other physically in space, it just means that the direction of your line of sight to one object is not very different from the direction of your line of sight to the other object. In other words, there is a very small angle between these two lines of sight. This is how we measure the separation of objects on the sky: by the their angular separation. If two objects have 0 angular separation (i.e. the line of sight to one is the same as to the other), then they'll appear superimposed on top of each other. Now, as a fundamental limitation of how telescopes work (imposed by the laws of physics), every point of light in the scene being imaged is not mapped to a perfect point of light in the image. Instead, the light from that point is smeared out a little over a finite area. The light is spread out into a circular disc, because of a phenomenon called diffraction. Now, the size of this diffraction disc, in other words, the angle over which this light gets smeared out, depends on two things: 1. the diameter of your telescope, and 2. the wavelength of the light you're observing. The bigger the telescope diameter, the smaller the angular size of this diffraction disc. You want this disc to be as small as possible (i.e. the light from a point source should not get spread out too much). Think about it: this diffraction disc places a fundamental limit on your ability to resolve two objects as being separate. If the angular separation of those two objects is smaller than the angular size of the diffraction disc of each one, then the diffraction discs of those two sources will overlap. In other words, the light from one will overlap with the light from the other, in your image, and you'll not be able to tell that there are two separate objects there. So your ability to resolve fine detail is limited by this.

The diffraction-limited angular resolution of the Hubble space telescope is 0.05 arcseconds. There are 60 arcminutes in a degree (of angle), and 60 arcseconds in an arcminute. That means that 1 arcsecond = 1/3600 of a degree. So Hubble can see two distinct objects as being distinct, even if they are separated on the sky by an angle of less than 0.000014 degrees (I just converted the 0.05 arcseconds to degrees).

Now, we should ask ourselves, what's the apparent (angular) size of even the closest star? In other words, by what angle does the line of sight to one end of the object differ from the line of sight to the other end of it? What angle does it span, or how much of my field of view does it take up? To measure the angular size of something, you just take its physical size, and divide that by the distance to it. (This gives you the angle in radians, which you can then convert to degrees).

The closest star is Alpha Centauri at a distance of 4.366 light years, and having a radius of 1.227 times the radius of the sun, or about 853,000 km. When I divide the second number by the first, I get an angular size for this star of about 0.00000119195 degrees or about 0.004 arcseconds. (EDIT: multiply these numbers by 2 since I used the radius of the star and not the diameter) That is smaller than the size of Hubble's diffraction disc, by a factor of 10 (EDIT: a factor of 5, actually). So the image of this star (and any other star) just looks like whatever the shape of the diffraction disc is. All of the light from this star gets smeared out into an area much larger than the actual size of the star's disc itself. So it is not possible to resolve any of the details of the structure of the star itself.

Now let's compare that to trying to image a galaxy. I think that an image of this type:

http://hubblesite.org/newscenter/arc...mat/large_web/

is the kind that is troubling you so much. It's an image of the Pinwheel galaxy, also known as M101. It has a diameter of something like 170,000 light years, and its distance is given as 21 million light years away. When I divide the physical size by the distance to get the angular size, I get a result of 0.46 degrees or 1670 arcseconds. The thing is 1670 arcseconds across, and the Hubble can resolve details even if they are as closely spaced as 0.05 arcseconds. So, Hubble has more than enough angular resolution to make out fine details of the structure of this galaxy.

As you can see, it's the angular size of an object that determines whether a telescope will be able to produce a resolved and detailed image of it, and since stars are all so small (relative to how far away they are), their angular sizes are all so small that they will just appear as points of light (i.e. their images will be diffraction disks or worse) in even the best telescopes. In contrast, galaxies are much larger (relative to how far away they are) causing them to have larger angular sizes, large enough that we can resolve them as being extended objects.



Chronos's explanation is correct, that a planet, (which is really close by, and therefore reflects a lot of sunlight towards us), appears much much brighter than a star, which although it is much more luminous, appears much fainter due to its extreme distance away from us. In fact, all you have to do is go outside and look up at the night sky to confirm for yourself that most of the planets in our solar system appear much brighter than the brightest stars in the sky. I could run the numbers in as much detail as I did above to show you why this is, but I'm too tired now.

If you understand anything about photography, then Chronos's explanation should make sense to you. Hubble, in order to take a nice photograph of Mars, needs to take an exposure for a certain amount of time. Since Mars is so much brighter than the background stars, an exposure time that exposes Mars nicely is not long enough of an exposure for the faint stars to show up in it. Conversely, if Hubble took a long enough exposure that the background stars became visible, Mars would then be horribly overexposed.

The key take home message of my post is this: just because something seems "obvious" to you doesn't mean that it is correct. You have to support the claims that you make quantitatively, you can't just make a bunch of assertions with no evidence to back them up other than "it's obviously true." The things you said in your previous post are neither obvious nor true. If you're here to learn, that's great, but if you're just here to spout off a bunch of conspiratorial nonsense, then I'd advise you not to waste everyone's time, and your threads will get locked.

buzzdiamond

I'm not here to spout off, I'm here to learn and sorry that I have some different thoughts. The knowledge everyone here has (besides me) is really amazing and I appreciate the interaction. I've learned that with computers, even though numbers are the same, you can have two distinct differences in the outcome, and/or reactions, so please excuse my "conspiratorial nonsense", so to speak. =:^D

Maybe I can open a new window of thought for people, which in turn, will lead people to a new conclusion of our universe. Having said that, I'm gonig to start a new thread, called My Theory of the Solar System. Please feel free to analyze it with your thoughts and comments.

Ryan_m_b

If you keep with an attitude better suited to learning you will find it far more rewarding. Asking what scientists have discovered and dismissing what doesn't match your personal feelings is no way to learn or develop.
My advice would be to get rid of any ideas regarding your own theory. If you haven't learnt what scientists have so far discovered then the chances that your theory will hold anything of merit are virtually zero. Instead start from the ground up by learning what is currently known.

buzzdiamond

I understand your point Ryan. Sometimes you come up with an idea first, then the physics and numbers put it to the test afterwards. I see things in the future, then try to explain it coming backwards. It's a different way of thinking, and with all the knowledge here, I thought this was the place to be to get some answers to my thoughts. I guess you can't have an open mind and/or your own thoughts here. Sorry to intrude and please accept my sincere apology.

I don't want to offend anyone, so if anyone cares to read and analyze my theory outside this forum, feel free to pm me. Thanks.

Ryan_m_b

Making something up then trying to prove it is not science and it is not open minded. Scientific enquiry starts with observation, from that we form a hypothesis that explains the observation, we then design experiments to test the hypothesis and finally conclude based on the data we recieve. You cannot do it the other way round. I believe that you believe you are right and think that you have formulated great explanatory theories but trust me, you haven't. You've done what so many others have done and constructed a descriptive analogy riddled with factual errors and logical fallacies (I know, I read it before I deleted it). I implore you to use this site to learn. There is no shame in being wrong and learning why, we all do it. But delluding yourself into thinking you've done something grand and dismissing anything that doesn't fit into your ideas is detrimental to yourself and society.

This site exists to teach and discuss what scientific enquiry has so far discovered, it is not the place for theory development. That doesn't mean you can't have your own thoughts but personal theories are not something that fits with what we do here.

Given that your questions on this topic have been answered this thread is locked.