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Writing this answer got me thinking.

As far as I understand it, stars themselves radiate most of their energy at optical wavelengths (near-IR through near-UV).

Various types of stellar objects may flare in various ways when in-falling matter from an accretion disk or companion is heated in magnetic fields or undergoes transient nucleosynthesis, but I'm excluding these types of processes and asking only about radiation from the stellar object itself, in steady-state.

Question: How far have individual stars been seen by radio telescopes?

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Some types of red giant stars are susceptible to intense water and silicon oxide MASER activity in their outer envelopes. This results in intense radio emission. Examples of such maser activity have been observed in the Small Magellanic Cloud,at a distance of $\sim 60$ kpc (e.g. van Loon 2013).

IRAS 05298−6957 water maser van Loon 2013 Click for full size.

IRAS 05298−6957 (van Loon 2013): Note the logarithmic wavelength axis; the water maser peak lies at about 18 cm / 1670 MHz.

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  • $\begingroup$ Thanks, this is just what I was looking for. $\endgroup$
    – uhoh
    Commented Jul 25, 2019 at 21:01
  • $\begingroup$ Wow. I guess this means we have no actual proof that other Galaxies are made of stars! $\endgroup$
    – SurpriseDog
    Commented Jul 26, 2019 at 12:06
  • $\begingroup$ @Benjamin Other stars can be observed in other galaxies, just not at radio wavelengths. $\endgroup$
    – ProfRob
    Commented Jul 28, 2019 at 14:56
  • $\begingroup$ @RobJeffries I was under the impression that Radio Telescopes had the highest resolution due to interferometry working for them. That's why the telescope that imaged the black hole in another galaxy (the EHT) was as radio interferometry telescope. $\endgroup$
    – SurpriseDog
    Commented Jul 28, 2019 at 15:01
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    $\begingroup$ Stars are much less luminous at radio wavelengths. Cepheid variables were identified in other galaxies in 1908. en.wikipedia.org/wiki/Henrietta_Swan_Leavitt $\endgroup$
    – ProfRob
    Commented Jul 29, 2019 at 9:13
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From Vega stars types, observations studies, include star HD 23362 (Table I.) with a distance 1004 light years

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    $\begingroup$ When I recommended you write it up as an answer I was thinking of a proper answer post. This is what we call a "link-only" answer, without the link there is no answer. Can you add some information here explaining more about the observation, and what was the source of the radio emission? Is it the star, or a dust cloud around it? I had to read through the paper myself to find HD23362 in Table 1, each reader shouldn't have to do the same. Thanks! $\endgroup$
    – uhoh
    Commented Jul 24, 2019 at 16:18
  • $\begingroup$ As far as Vega is concerned, there are two radio observations listed in your link. 1 and 2. These are helpful because they show the actual radio images of Vega, so a screen shot or two might make the answer more interesting as well. $\endgroup$
    – uhoh
    Commented Jul 24, 2019 at 16:21
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How far have individual stars been seen by radio telescopes?

The disks of stars have been imaged by radio telescopes at least out to 700 light-years!

From https://www.star-facts.com/betelgeuse/

ALMA image of Betelgeuse click for full size

note: The link says that the bright spot is "a convective hot spot on the photosphere". To read further about the size of convection cells on other stars see this answer.

This image, made with the Atacama Large Millimeter/submillimeter Array (ALMA), shows the red supergiant Betelgeuse — one of the largest stars known. In the millimeter continuum the star is around 1400 times larger than our Sun. The overlaid annotation shows how large the star is compared to the Solar System. Betelgeuse would engulf all four terrestrial planets — Mercury, Venus, Earth and Mars — and even the gas giant Jupiter. Only Saturn would be beyond its surface. Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

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