Angular size of stars

eggsaladinspace · Jan 30, 2006

Is it possible, with the largest telescope in the world and the largest angular-sized star, to see the body of a star? Or would glare be too much of a factor? How does the angular size of stars compare to, say, that of Pluto's? I've always been curious about this..

CalliArcale · Jan 30, 2006

The glare isn't a problem; the angular size is, as you've correctly divined. A very few stars have been imaged in this way; Betelgeuse is a notable example, as it's actually big enough for Hubble to image it and show a disk. It's truly huge. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

CalliArcale · Jan 30, 2006

For comparison, here's Hubble pic of Pluto and Charon. Don't read too much into the relative sizes of Betelgeuse and Pluto; I have no idea how much cropping and enhancing was done to these pictures. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

eggsaladinspace · Jan 30, 2006

I think I've seen this picture before, but I still can't make out a disc. I assume it's the brightest yellow, toward the bottom of that blob. Thanks

nexium · Aug 29, 2006

The pixtal boundries are not noticeable for Charion, Pluto nor Betelgeuse. Surely this does not mean these objects are 100 pixtals wide? Neil

doubletruncation · Aug 29, 2006

Both of these images were taken with the FOC. The pixel scale of the FOC is ~0.014" per pixel. The image of Betelgeuse is in the UV and has an angular diameter of ~0.125" so it's about 10 pixels across on the camera. (In the optical it would be ~0.044", that it's larger in the UV indicates an extended chromosphere, so this is really an image of the chromosphere of the star rather than of its photosphere). However, in making this picture of Betelegeuse they resampled the image on 5x finer resolution grid (using bicubic spline interpolation) and then deconvolved the image using the known PSF for the FOC, so the image that you see is more like 45 pixels across for the diameter. I don't know about the pluto images. The source for this info is from the paper where this Betelgeuse image was first published: Gilliland & Dupree, 1996, ApJ, 463, L29. To directly measure the angular diameter of a star you don't necessarily have to make an image of it. You can use interferometry to measure the angular size of an object - this was actually first done for Betelegeuse back in 1921 (by Michelson and Pease). And you can also measure angular diameters using lunar occultations (time how long it takes the star to disappear as the moon passes in front of it). Using optical interferometry you can get angular resolutions down to 1 mas (0.001"), so main sequence stars are actually measureable. People have used interferometry to do a number of very neat things like measuring the actual diameter variations of variable stars like Cepheids, imaging spots on the surfaces of very large stars, and measuring the limb darkening/oblateness of other stars (Altair is a prime example of an oblate star). <div class="Discussion_UserSignature"> </div>

qso1 · Aug 30, 2006

eggsaladinspace: Is it possible, with the largest telescope in the world and the largest angular-sized star, to see the body of a star? Me: In the case of Betelgeuse, I imagine it would be difficult to see a discernable disc because the star is so large and its atmospheric gasses so diffused and spread out, it tends to blur the visible disc edge of the star. But I'm not a professional astronomer so I don't know for sure. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

qso1 · Aug 30, 2006

I have never seen any other star that it was claimed was imaged as a disc. I'd like to see an image of a star showing starspots, are there any links to such images or images of other stars imaged as discs? <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

doubletruncation · Aug 30, 2006

Besides Betelgeuse (for which there are interferometric maps of the optical photosphere that show two spots besides the HST UV images of the chromosphere), Antares, Rasalgethi and Mira have also been imaged interferometrically. Perhaps some others have as well, I'm not aware of the results though. These are all supergiant stars and except for Mira they all show evidence for bright spots (they're not thought to be the same phenomenon as sunspots though, for one thing they show up as hot spots instead of cool spots - instead large-scale photospheric convection is the theoretical explanation). The photospheres for these supergiants are very dynamic and diffuse - so they don't look like round objects (see some simulations at: http://www.astro.uu.se/~bf/movie/movie.html - look at the numerical simulations of a red supergiant mpegs). Some papers with actual observations (which show the maps) are: Buscher et al., 1990, MNRAS, 245, 7 - no preprint, presents maps of the photosphere of Betelgeuse (with spot features) Wilson et al., 1992, MNRAS, 257, 369 - no preprint, presents maps of Betelgeuse and Mira (shows Mira is asymmetric) Gilliland & Dupree, 1996, ApJ, 463, L29 - paper presenting the above images of Betelgeuse Burns et al., 1997, MNRAS, 290, L11 - again maps of Betelgeuse Tuthill et al., 1997, MNRAS, 285, 529 - imaged Betelgeuse, Antares and Rasalgethi finds hot spots on each (they only give maps of Betelgeuse and Rasalgethi though) Unfortunately I don't have links for these - you can search for them on NASA ADS ( http://adsabs.harvard.edu/bib_abs.html ). Rapidly rotating stars can also have images reconstructed via doppler imaging, and maps for the surfaces of smaller stars (including some G-dwarf stars) have been generated in this way for a number of stars. These images often sho <div class="Discussion_UserSignature"> </div>

qso1 · Aug 30, 2006

Basically, no actual image that does not have to be reconstructed or inferred by data? <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

doubletruncation · Aug 30, 2006

I would consider the interferometric images to be actual images that are effectively as bonafide as any single telescope dish image. An intereferometric image is made by combining observations from two telescopes (or a telescope with a mask that has two holes over it). If you take every possible combination of positions between those two telescopes that would cover the surface of a bigger telescope you can combine all those separate observations and effectively obtain a single observation with the bigger telescope (to some extent the Earth's rotation usually helps fill in some of the positions). Interferometry lets you make giant telescopes with less glass. The observations obtained this way are just as legitimate as ones obtained with a single pointing of a big telescope. The only assumption that you have to make about the source that you're imaging is that it isn't changing over a timescale shorter than that used to build up the image - you have to make this same assumption when you expose for several minutes with a single telescope anyway. That means that you can get better time resolution for varying objects with a single dish than you could interferometrically. On the other hand, I would consider the doppler images to be inferred from data since you have to use physics and some assumptions about how the star's atmosphere works to back out the image. <div class="Discussion_UserSignature"> </div>

Saiph · Aug 30, 2006

the image of betelguese above is a single image observation, and photometry would show that it's got a distributed light curve across the width, not the typical diffraction limited light curve you get with point sources. I.e. it's a real disk, not just a blurred point source. As for "actual images" inteferometry is the same thing as if you were to cover up a band bisecting a large mirror in a telescope. The images are dimmer, but the mirror still acts as the same size. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

erioladastra · Aug 30, 2006

"Basically, no actual image that does not have to be reconstructed or inferred by data? " Depends on how you define images. If you don't like interferometry you could use Doppler Imaging. This is where the assyemtries in spectral line profiles as a function of rotation phase can be used to provide a fairly accurate 2-D or even 3-D image of a star. In this way star spots, plages and other active regions have been imaged on dozens of stars. In fact many have been imaged over cycles! And you don't need the worldest biggest scope!

qso1 · Aug 31, 2006

doubletruncation: I would consider the interferometric images... Me: Why havn't any been published, or if they have, where are they being published? <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

qso1 · Aug 31, 2006

Saiph: the image of betelguese above is a single image observation, Me: Thanks for the info. On Betelguese however, I basically knew it was an actual image of the disc because it was widely publicised when it came out years ago. I also knew it was one of the few stars large enough to be imaged as a disc. I have not seen any other actual images since the Betelgeuse image was released. Especially of stars much closer than Betelgeuse. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

qso1 · Aug 31, 2006

erioladastra: "Basically, no actual image that does not have to be reconstructed or inferred by data? " Me: Thanks. Thats kind of what I was thinking since I hadn't seen any other star images announced as first image of such and such star seen as a disc with spots or whatever. When interferometry is used to infer starspots, would an image reconstruction for visual purposes be able to accurately represent the spots distribution and density? <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

doubletruncation · Aug 31, 2006

Why havn't any been published, or if they have, where are they being published? Buscher et al., 1990, MNRAS, 245, 7 - presents maps of the photosphere of Betelgeuse (with spot features) Wilson et al., 1992, MNRAS, 257, 369 - presents maps of Betelgeuse and Mira (shows Mira is asymmetric) Gilliland & Dupree, 1996, ApJ, 463, L29 - paper presenting the above images of Betelgeuse Burns et al., 1997, MNRAS, 290, L11 - again maps of Betelgeuse Tuthill et al., 1997, MNRAS, 285, 529 - imaged Betelgeuse, Antares and Rasalgethi finds hot spots on each (they only give maps of Betelgeuse and Rasalgethi though) Young et al., 2000, MNRAS, 315, 615 - images of Betelgeuse + detailed modelling. Unfortunately I couldn't find pre-prints for these so if you want to see the papers you have to go look at the journals (MNRAS = Monthly Notices of the Royal Astronomical Society, ApJ = Astrophysical Journal) which requires either going through a university library or paying to view the article on the journal website. Note that usually people plot contour maps of the surface brightness rather than an image since its easier to see the features in a contour map. <div class="Discussion_UserSignature"> </div>

doubletruncation · Aug 31, 2006

Here is a link which shows some images of Betelgeuse from the COAST interferometer, you can see three hotspots on the surface of the star in the 700 nm image. http://www.mrao.cam.ac.uk/telescopes/coast/betel.html <div class="Discussion_UserSignature"> </div>

qso1 · Aug 31, 2006

Thanks doubletruncation. Least now I know some have been published but I'd hoped to see a few images in magazines like Discover. I'll have to go to a University library. BTW, great link. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

erioladastra · Aug 31, 2006

For some DI imaging: http://www.aip.de/groups/activity/DI/results.html http://www.noao.edu/noao/noaonews/dec99/node2.html

qso1 · Sep 1, 2006

Thanks for those links. I checked out the animated gif files. HD 12545 is especially interesting due to its enormous star spot. <div class="Discussion_UserSignature"> My borrowed quote for the time being: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. </div>

serak_the_preparer · Jun 1, 2007

Another one - but not nearly as big: Gazing up at the Man in the Star? by Josh Chamot (NSF) May 31, 2007 Using a suite of four telescopes, astronomers have captured an image of Altair, one of the closest stars to our own and a fixture in the summer sky. While astronomers have recently imaged a few of the enormous, dying, red-giant stars, this is the first time anyone has seen the surface of a relatively tiny hydrogen-burning star like our own sun.... The CHARA telescopes were able to make the breakthrough observation because they were outfitted with a novel system to clean up some of the distortions from Earth's atmosphere, a technology called the Michigan Infrared Combiner, developed with NSF support at the University of Michigan in Ann Arbor. Recent advances in fiber optic telecommunication technology made this new combiner possible.... Altair spins so quickly, about 300 kilometers per second at its equator, that it's shape is distorted: the star is a full 22 percent wider than it is tall. The new telescope measurements confirmed the oblong shape, yet showed slightly different surface temperature patterns than what models predicted. Altair is one of the closest stars in our neighborhood, only about 15 light years away, and the researchers hope to image Vega as well as more distant stars in the future....

serak_the_preparer · Jun 3, 2007

Probably my last post to this thread on the subject. Shortly after putting up my previous post on Altair above, this discussion got going in the Space Science & Astronomy forum. Recommend checking that thread for further information. Interferometry lets you make giant telescopes with less glass. A little more re CHARA (Center for High Angular Resolution Astronomy) and Altair. First up, the AAAS article: A Distant Sun, Revealed by Phil Berardelli (ScienceNOW) 1 June 2007 Although still a bit blobby to the untrained eye, astronomers have captured the most detailed images yet of a star outside our solar system. The breakthrough could pave the way to observing solar systems in the process of forming--and even visualizing extrasolar planets--years ahead of the launch of spacecraft specifically designed for such missions.... Hoping to snap a better picture of a distant sun, an international team of astronomers focused its attention on Altair, a star only about twice as large as our sun and a mere 17 light-years away. Even so, the challenge is like taking a photo of a child's plastic swimming pool on the moon. Yet that's exactly what the researchers accomplished employing the four-telescope CHARA array on Mount Wilson in California.... The resulting images show a whirling star, shaped more like a pumpkin than a basketball. It sports a large, dark bulge at its equator, something existing computer models did not predict. This puzzling feature could be connected to Altair's spin rate, which the images reveal is 90% of the velocity required to break the star apart. The bulge is about 20% farther from the star's core than the rest of its surface.... BBC's article:

MeteorWayne · Jun 3, 2007

Nice, thanks <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

3488 · Jun 4, 2007

Yes I agree. Fascinating links. Will be great to Chara image other well known stars like Sirius, Procyon, Arcturus, Capella, etc. Like Altair, Vega & Regulus have been seen to be severly oblate too, due to their rapid rotations (12 hrs for Vega, 15 hours for Regulus). Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

Angular size of stars

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