Can you measure a star's diameter directly?

[Chemicalsuperfreak]

I can understand that given a star's mass, composition, age, temperature, etc. it could be relatively straightforward to calculate it's size. But do the big telescopes have enough resolution to directly measure a star's size? Or does it have to be calculated from distance, spectrum, and intensity? I was thinking about dwarfs and giants the other day and was wondering if the size could actually be observed.

 

[chroot]

Yup, it can be done (and has been done). Only a handful of stars have had their angular diameters directly measured. One of the first, I believe, was Betelgeuse. Its diameter was measured not by an interferometer, but by the Hubble Space Telescope. Interferometric techniques, of course, are far superior to HST photos.

- Warren

 

[Phobos]

Originally posted by chroot
One of the first, I believe, was Betelgeuse.

Sounds right. The HST captured the first image of the surface of a star other than our sun...
http://antwrp.gsfc.nasa.gov/apod/ap990605.html

 

[Labguy]

Originally posted by Chemicalsuperfreak
I can understand that given a star's mass, composition, age, temperature, etc. it could be relatively straightforward to calculate it's size. But do the big telescopes have enough resolution to directly measure a star's size? Or does it have to be calculated from distance, spectrum, and intensity? I was thinking about dwarfs and giants the other day and was wondering if the size could actually be observed.

Your question was about measuring the size of a star, not specifically imaging it. Chroot was right and Betelgeuse was first, but it was not an image and it was done with a telescope. It was sometime in the 1960's I believe (didn't look it up) and it was done with speckle Interferometry. Until the HST, no telescope had the resolving power to directly measure a star's angular diameter.

Labguy

 

[Nereid]

Cursed atmosphere!

At a mere 2m, the HST is tiny compared with Keck, Yepun, the Geminis, ... even venerable Palomar. The pesky atmosphere gets in the way - direct images in optical wavelengths are smeared by 'seeing' (as astronomers quaintly call it); the best seeing I recall being reported is ~0.2", yet the largest stellar disc is ~0.05" (amateur astronomers do well if their images have a FWHM of 2", though there are some really cool low-light videocams, with nice software to integrate only the best sub-second images, that produce stunning planetary pictures).

Hanbry-Brown (sp?) in Australia developed a really neat (optical) interferometer - not your usual phase interference - and measured the diameters of a good many bright stars ... as long ago as the 1970s!

Of course, there are several other ways to measure a star's diameter - good observations of eclipsing binaries, lunar occultations, analysis of the radio pulses from pulsars, gravitational lensing, ...

 

[Labguy]

Originally posted by Nereid
<snip>
Of course, there are several other ways to measure a star's diameter - good observations of eclipsing binaries, lunar occultations, analysis of the radio pulses from pulsars, gravitational lensing, ...

Just to pick a nit, how is a star's diameter measured by either lunar occultations or gravitational lensing? (just those two). I know thay can measure the gravity of a large (massive) object by the star's light displacement, but that doesn't measure the star's diameter.

Lunar occultations use "pinpoint" stellar light sources to measure the precise diameter of the moon, but that also doesn't measure the star's diameter. The problem there is that most any lunar surface feature (valleys, mountains, rilles, crater walls, etc.) have a larger angular distension as seen from Earth than even the largest observable star's diameter. If this method was accurate for stellar diameters, I would think it would have been done in the 1890 to 1910 timeframe since the technology was accurate enough at that time. (?)

Labguy

 

[Nereid]

Labguy wrote: Just to pick a nit, how is a star's diameter measured by either lunar occultations or gravitational lensing? (just those two)

Lunar occultations give the angular diameters of stars through the diffraction patterns at time of insertion (absolute diameters follow once you know the distance).

Some resources:
http://www.eso.org/~arichich/publications/1997iau158...71.pdf
http://www.ifjungo.ch/041_ggn00.htm
http://aanda.u-strasbg.fr:2002/articles/aa/abs/2001/42/aa1477/aa1477.html

To quote from the first: "the lunar limb acts as a straight edge that diffracts the light from a background star: the high angular resolution (HAR) information is embedded in the fringe pattern, which is rapidly scanned by the telescope as the Moon moves across the sky. Two important facts should be recognized: firstly, the diffraction phenomenon takes place well outside the Earth's atmosphere; and secondly, the telescope aperture plays no role in it."

Gravitational lensing? Oops! I thought I'd read a paper on its use in double star systems, but I can't find it again.

 

[Labguy]

Originally posted by Nereid
Lunar occultations give the angular diameters of stars through the diffraction patterns at time of insertion (absolute diameters follow once you know the distance).

Some resources:
http://www.eso.org/~arichich/publications/1997iau158...71.pdf
http://www.ifjungo.ch/041_ggn00.htm
http://aanda.u-strasbg.fr:2002/articles/aa/abs/2001/42/aa1477/aa1477.html

To quote from the first: "the lunar limb acts as a straight edge that diffracts the light from a background star: the high angular resolution (HAR) information is embedded in the fringe pattern, which is rapidly scanned by the telescope as the Moon moves across the sky. Two important facts should be recognized: firstly, the diffraction phenomenon takes place well outside the Earth's atmosphere; and secondly, the telescope aperture plays no role in it."

Gravitational lensing? Oops! I thought I'd read a paper on its use in double star systems, but I can't find it again.

The second link was the most useful and direct. I didn't know they were doing the "diffraction thing" these days, with large scopes. The first link wouldn't fully load, for some reason (my computer?). Either way, you are correct about the use of the diffraction pattern being used for high angular resolution, that's news to me, and cool.

I think the gravitational lensing you mentioned has been used for double star separation measurements, like you just mentioned, but I haven't heard anything about it determining stellar diameters. (?) If you find a link on that part, I'd like to read it.

Labguy