# Diameter of stars

#### Zman

Is it possible with our current technology to measure the diameter of stars in galaxies other than our own?

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#### mgb_phys

Homework Helper
Not directly.
You can infer their diameter from the color which gives you the surface temperature and the light output which tells you the surface area required at that temperature.

#### Zman

Thanks mgb_phys
Following on, can we directly measure the diameter of all the stars in the Milky Way or is there a distance beyond which we have to use the surface temperature method.

In other words what is the approximate maximum distance from the Earth that we can directly measure the diameter of stars, beyond which we have to use indirect methods?

#### D H

Staff Emeritus
The highest resolution camera is the Hubble's Advanced Camera for Surveys' High-Resolution Channel instrument, with a pixel resolution of 0.025 arcseconds. Suppose a giant star with a diameter 2000 times that of the Sun was a mere 120 light years away or so. The HRC would see that star as 20 pixel diameter circle. This is adequate to measure the star's diameter to within 10% accuracy. The HRC could resolve such a star as being more than a spot (larger than one pixel in diameter) at ten times that distance. For a star the size of the Sun, the star would have to be less than 0.61 light years away for the HRC to resolve it as being more than a spot.

Astronomers can do better with arrays of telescopes. Milliarcsecond resolutions are now feasible. At the milliarcsecond level, a sun sized star remains larger than a spot even at 15 light years distance.

#### mgb_phys

Homework Helper
The largest ground based visible light interferometers are around 650m baseline so have an angle resolution of 1:10^9 or 0.2mas.
You can resolve a nearby large star like Betelgeuse with a 4m telescope it has a diameter of around 55mas and is 600lyr away so an interferometer like SUSI would be able to resolve it at a distance 275x as great or about to the edge of the galaxy.

#### Zman

I looked up the SUSI website which seemed to be up-to-date and it says that although there is a baseline capacity of 650m, they currently only go up to 80m.
I guess that in the next few years they will probably upgrade and be able to measure large bodies at the edge of our galaxy as mentioned.
If SUSI currently has a maximum baseline of 80m then (after browsing the web) the CHARA possibly has the greatest resolution (0.5mas) with a baseline of 330m?

#### mgb_phys

Homework Helper
Most interferometers COAST,VLTi,Keck and Chara are aimed at imaging - so you have more than 2 telescopes.
SUSI was the only large 2 dish system I could think of.
These can take actual pictures rather than just measure diameters although they need more signal and so aim at nearer/brighter/larger stars and currently have smaller baselines. There is no real theoretical limit to the baseline it's just a question of engineering and cost.

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