I was looking at the Astronomy picture of the day (here’s the real home page, but it changes daily, so I linked to the target picture), and I noticed that the bright blue stars appear to be discs.
Now I have always understood that it was virtually impossible to resolve a star into a disc due to the vast distances and infintessimal angles of arc involved. Based on this, I assume that the apparent discs in that photo are artifacts of the apparatus.
Questions:
Am I correct in assuming that those stars in the photo are really only points of light?
Is it still the case that we cannot see any star’s disc (excepting the Sun :))
AFAIK, Betelgeuse is the only one, and we’ve only gotten that with Hubble. I was told that Betelgeuse is actually only five pixels across, so I imagine that the images shown there are some kind of interpolated composite of multiple exposures. I could be wrong, though.
As for today’s APOD, yes, those stars are all actually smaller than one pixel, but CCD imaging always spreads the light out. Since that was taken with a ground-based telescope, I think the biggest thing, though, is atmospheric distortion (seeing).
I think diffraction is another reason why stars lookk like round discs in telescope images. Sometimes stars look, well, star-shaped, with rays of light coming out of the center. This is because some telescopes have a secondary mirror supported right in the middle of the aperture; the support structure (“spider”) causes diffraction.
By the way, we do have techniques for detecting features on stars. The simplest way is to observe the light curve (brightness variation over time); if the star is rotating and has a big sunspot, the star will be slightly darker when it is on our side. You can also use Doppler shift to resolve more features: because of rotation, one side of a star is moving towards us and the other side away from us. So if the blueshifted part of an emission line is slightly darker than the redshifted part, you might guess that there is a sunpot on the side of the star moving towards us. The diagrams on this page might help:
From what I’m gathering here, it is possible to resolve a few of the “nearby” supergiants into miniscule discs with the highest-resolution telescopes today. As of a couple of decades ago, when the 200-inch Hale telescope was the highest resolution available, the answer would have been a flat-out No.
However, there is a technique called “speckle interferometry” which I do not understand well. Interferometry is itself simple to explain – take a multiplicity of pictures taken at the same instant of the same object by telescopes well separated by physical distance, and digitally compare the different images for which are “in phase” and “out of phase” with each other. The net result is an image with the effective resolving power of a telescope of a diameter equivalent to the distances between the telescopes. Speckle interferometry is a variation on this.
The net result is an image of a star that is somewhat “grainy” but gives a reasonably accurate representation of the star’s appearance. Betelgeuse’s photosphere (visible disc), for example, seems to be heavily influenced by giant convection cells each occupying a large percentage (~16%) of the star’s distended body.
