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How large an area is that brightly illuminted patch from stars covering?

  1. Jun 11, 2015 #1
    If the sun is at least 90million miles away and illuminates earth, and stars are the same as our sun, then am i correct in assuming that each star we see in the sky is illuminating an area of at least 90million miles!!!??

    We see a little white dot from this far away but, does that little dot on each star actually cover 90 million miles? Is that little area honestly that large and if not, then how large is the illuminated area?

    Surely my assumption is wrong!

  2. jcsd
  3. Jun 11, 2015 #2


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    The stars vary greatly in size.

    Last edited: Jun 11, 2015
  4. Jun 11, 2015 #3
    I know that they vary in size but my questions are:

    Am i right in believing the illuminated area we see from the average star on earth must cover at least 90million miles? (if the star is the size of our sun)

    If the our sunlight reaches earth and we are 90million miles away from our sun, then surely the little dots in the sky made from stars the size of our sun must be illuminating at least 90million miles in area. Thse little white dots are actually 90million miles accross. If not how?
  5. Jun 11, 2015 #4


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    I don't get your connection between the earth-sun distance and the brightness of other stars. The photons emitted by a star start off with a given density per area. As the distance from the star increases, the density of photons falls off because they are spread over a larger area.
  6. Jun 11, 2015 #5


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    To put it another way, any star that you can see, is illuminating the earth - just not very brightly.
  7. Jun 11, 2015 #6
    The light from stars reaches us from the star but the width of the star covers an area of the sky about a millimter accross when looking at it from the earth. I am not asking about brightness.

    My question is, how large an area on average are little dots we see from earth actually covering? Are those little pinpoints millions of miles accross?
  8. Jun 11, 2015 #7
    So how big on average is the are we see illuminated in the sky? i understand it is illuminating the earth but it is aso illuminating a very bright area in the sky wich from earth looks like only a mm in diameter, but how large an area is the highly illuminated patch of space covering?
  9. Jun 11, 2015 #8


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    The 'area' that you're thinking of is affected by many things like the size of the star, it's distance from earth and scattering of the light as it passes through the earth's atmosphere. Take a look at the Wiki article on Angular diameter in Astronomy.
  10. Jun 11, 2015 #9
    i just want a very approx number for an average star. Is it thousands of miles, millions or just hundreds?
  11. Jun 11, 2015 #10


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    The sun has a radius of 696,342±65 km which means its diameter is just under 1.7 million km. There are larger stars and there are smaller ones. If I remember correctly, most are smaller. Of course those are harder to see.
  12. Jun 11, 2015 #11
    Could anyone please give me a very approximate answer. Im sure it is known by someone without me having to study the formulas for the angular diameter. Angular diameter is new to me, this is just a hobby to me i have not even purchased a telescope yet. Its the incomprehensible numbers of the universe i find amazing and this is a number i cant even get an approximation for.

    How large is the area of space is brightly illuminated from an average star?

    From earth it looks to be about a mm, but how large do you think that bright area actually is? I am not asking how far starlight shines, as we all know it shines forever.

    Still, that very bright area we see from earth must have a size. So is it just a few hundred miles or is it thousands, millions or lightyears?

    Sorry to ask but i am unsure if anyone understands what it is i am asking.
    Thanks for you patientce.
  13. Jun 11, 2015 #12


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    I think the problem is that we are not sure that YOU understand what you are asking.

    If a star has only very small planets then what we are seeing is just the star. We are not seeing anything millions of miles away from the star.

    If a star were in the middle of a gas cloud 500 million miles in diameter, then what we are seeing is an illuminated area 500 million miles in diameter.

    In other words, you are asking for a definitive answer to something that doesn't HAVE a definitive answer.
  14. Jun 11, 2015 #13
    Yes the sun is about 900,000 miles in diameter, so is it fair to say the area of a star that we seeon earth must be much larger.

    If the light reaches us from that star then the light must of traveled at least that far in all other directions too. We only see it as a pinpoint because the star is so far away but that still does not explain say how large that pinpoint actually is.

    If the star is the size of our sun aprrox 900,000 miles then the pinpoint must be at least 900,00 miles across but that doesnt take into account the amount of area that has been illuminated areound the cirumfernce of that star.

    Do we just see the actuall size of the star from earth (900,000miles) and if so how come we dont see any light illuminating any other direction except straight towards us?
  15. Jun 11, 2015 #14


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    You just asked questions that I answered in the post right before yours.
  16. Jun 11, 2015 #15
    Okay, now thanks this makes sense i need sleep!

    What we see from earth is just the stars diameter, the light comes towards us and hits my eyes, so the light has traveled millions of miles to earth. We dont see the light going in all the other directions for millions of miles because the light has nothing to reflect off of (unless there was a nearby object). IS this correct? Thanks by the way.
  17. Jun 11, 2015 #16


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    Yes, it is correct. When we see stars its only their surface.
  18. Jun 11, 2015 #17


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    You are a master of understatement.
  19. Jun 11, 2015 #18


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    You're correct. You CANNOT determine the actual size of a star just by looking at how large it appears with the naked eye (or even with telescopes). The reason for this is that the angular diameter of the star is MUCH smaller than your eye (or even a telescope) can resolve.* The incoming light from the star is focused onto your retina, but diffraction spreads the light out into a 'spot', called an airy disk. The brighter the star, the larger the spot appears to be. (The spot doesn't actually change size, its just that the light at the outer areas of the spot becomes great enough for your eye to detect when you look at brighter stars) That's why bright stars appear larger when there's actually no connection between the stars real size and how big they appear when looking at them.

    See the following links:

    *The only exceptions are a few very massive, very close stars, like Betelgeuse. The Hubble Space Telescope was actually able to resolve the star into a disk instead of just a pinpoint. The image of the star fit on just 10 pixels of the camera. See here: http://iopscience.iop.org/1538-4357/463/1/L29/pdf/5023.pdf

    The latter parts are correct, but the part about seeing the diameter is not, as I explained just above. Also note that the nearest star is almost 4 light-years away. One light-year is is 5.8786×1012 miles, or 5.8 trillion miles. That puts the nearest star at over 20 trillion miles away, and most stars are hundreds or thousands of light-years from us.
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