Relative vs Absolute sizes of stars

[ajna]

When we see stars, how much bigger is the apparent size compared to its actual size? Looking at Sirius for example, would the diameter we see (naked eye) be say as large as its Oort cloud (assuming it has one)?

 

[doubletruncation]

Hi Ajna,<br /><br />We actually can't see the diameter of the stars with your naked eye. The eye has a resolution of about 1 arcminute (1/60th of a degree), whereas the star with the largest apparent diameter, R Dor, is only 0.057 arcseconds (about 1/60000th of a degree). If Sirius looks bigger to you than another star, it is only because it is brighter. If you look at pictures of stars, in all but a few cases, the light from the stars is spread out over many pixels due to turbulence in the atmosphere (which causes the stars to twinkle) or due the resolution limit of the camera - brighter stars look bigger simply because it's easier to see the small fraction of their light that gets spread over pixels far away from the center pixel of the star. If you could actually resolve the surfaces of the stars they would look much much smaller than they do in the images. The Oort clouds of stars are basically invisible to us, though you can possibly infer their existence, for some stars, as a slight excess in the infrared light emitted from the star compared to what you would expect based on a model for the star's spectrum. If you look at a star with your eyes, you don't see its Oort cloud. Even our own Oort cloud has never actually been seen (its existence is hypothesized to explain the fact that the inner solar system is constantly visited by comets coming from all directions on orbits with aphelia of about 50000 AU). The sun's Oort cloud is thought to have a diameter that is about 10 million times larger than the sun itself. Sirius A, for example, has an angular diameter of about 0.007 arcseconds (and is about twice the radius of the Sun), so its Oort cloud, if it has one that is the same linear diameter as the one hypothesized for our solar system, would have an angular diameter of about 10 degrees (the moon has an angular diameter of about half a degree). If this seems surprisingly large to you, keep in mind that the Oort cloud is hypothesized to have a radius of almost <div class="Discussion_UserSignature"> </div>

 

[qso1]

Another comparison would be Sirius and the sun. Sirius is roughly twice the size of the sun. For more info, try the link below.<br /><br />http://solstation.com/stars/sirius2.htm <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[ajna]

yeah i wonder how Sol looks from Sirius, it may look as big as the Kiuper belt. I notice from your link that Sirius B is 20AU from Sirius A which would put it within the apparent disc of A as view with the naked eye from Earth. Given this I think Sol's disk would appear to be about 30AU from Sirius. Very rough estimate! The Kiuper belt objects would surely create some kind of halo or limit???

 

[qso1]

I would think the KB would be noticeable from Sirius. The dust disc around Beta Pictoris is probably that systems Kuiper Belt. Sirius is so bright that its probably next to impossible to see its KB.<br /><br />From just light output alone, the sun would probably be roughly as visible from Sirius as Tau Ceti is from us. Tau Ceti is not nearly as bright as Sirius despite being only 3 or 4 light years further.<br /><br />Future higher resolution scopes may yet spot a KB around Sirius if it has a significant one. Come to think of it, IIRC, looking at dust discs is usually done in the infrared or near infrared wavelengths but I don't recall ever seeing Sirius imaged in this way. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>