Resolving stars?

[astoc]

This is going to sound REALLY stupid...

I always thought stars discs are unresolvable at this distance, but in every image I see the brighter stars form circular discs with a definite width. Is this simply an imaging phenomenon?

John

[barkis]

Stars are at an Infinite distance, and unfortunately, the telescope, and in particular the reflecting telescope, cannot form a point source due to the laws of diffraction, and will always return a small disc called the diffraction spot. The larger the objective of the telescope, the smaller this disc will be, but still a disc.

Using interferometry, using large telescopes together as one, astronomers have been able to resolve the Red Giant star Betelguese into a small disc.

Ron.

[brianb]

in every image I see the brighter stars form circular discs with a definite width. Is this simply an imaging phenomenon?

Yes. Some of the light striking the sensor diffuses out & affects neighbouring pixels. The brighter the point, the further the diffusion goes. Same happens with film.

If you take a series of intensity readings across a star image, you'll find they all have the same shape (until overexposed enough to saturate the sensor!), it's just that the brighter ones have a taller curve, and obviously come clear of the background further from the centre ... which is why the spot appears bigger.

Barkis, you're perfectly right about stars not imaging to an exact point, but few people doing deep sky imaging use a plate scale big enough to resolve the diffration disc. In any case the size of the diffraction disc of stars - being created in the optics rather than a feature of the star - is independent of brightness of the star. And the 0.04 arc secs apparent diameter of even the "largest" stars (Betelgeuse) is not sufficient to have any effect on an image taken with a scope of less than a couple of metres aperture, even with perfect seeing or adaptive optics, even if the plate scale is adequate to show it.

[FraserClarke]

You can resolve Betelgeuse using aperture masking on a 4m telescope. I believe the VLT (8m) has also just about managed to resolve it with adaptive optics, but it's right on the limit. With the 30m class telescopes coming on line in ~10 years, you should be able to start directly mapping the surface of the largest stars, and get 5--6 resolution elements across the surface. Very cool

But, as the others say, with anything less than a 4m, all stars can be treated as perfect point sources (which the telescope produces a diffraction limited image of).

[Shibby]

See my post here about interferometry: http://stargazerslounge.com/astro-lounge/94491-betelgeuse-imaged.html

You'll find a few links to images of resolved stars...