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New map reveals just how enormous the supergiant star Antares really is

An artist's impression of the atmosphere of the supergiant star Antares. (Image credit: NRAO/AUI/NSF, S. Dagnello)

Astronomers have revealed the gigantic atmosphere of the red supergiant star Antares in stunning, unprecedented detail. 

Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the National Science Foundation's Karl G. Jansky Very Large Array (VLA) in New Mexico, an international team of researchers has created the most detailed radio map yet of Antares' atmosphere. In fact, this is the most detailed radio map ever created of any star other than our sun. 

While Antares' diameter is about 700 times larger than the sun in visible light, this map revealed that, as seen in radio light, the star's atmosphere stretches even farther and is even more enormous. With this detailed map, the team found that Antares' chromosphere, a gaseous layer that creates a star's outer atmosphere along with its corona, stretches to 2.5 times the star's radius. For context, our sun's chromosphere only extends to 0.5% of our star's radius. 

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"The size of a star can vary dramatically depending on what wavelength of light it is observed with," Eamon O'Gorman, a researcher at the Dublin Institute for Advanced Studies in Ireland and lead author of this study, said in a statement. "The longer wavelengths of the VLA revealed the supergiant's atmosphere out to nearly 12 times the star's radius."

To create this map, ALMA observed Antares' photosphere, or the layer that the majority of the stars' visible photons (light particles) stream from, in shorter wavelengths. The VLA observed the longer wavelengths in the star's atmosphere further out. The radio telescopes also observed and measured the temperatures of gas and plasma in the star's atmosphere. They were able to, for the first time, detect the chromosphere using radio waves and, not only did they find that it stretches 2.5 times the star's radius, they also found its temperature.

The team found that Antares' chromosphere is cooler than previous optical and ultraviolet observations suggested, peaking at 6,400 degrees Fahrenheit (3,500 degrees Celsius). This is significantly cooler than our sun's chromosphere, which is almost 36,032 degrees F (20,000 degrees C).

Antares, a red supergiant star in the constellation Scorpius, shines as the bright-red sparkle at the center of the image. (Image credit: Babak Tafreshi/TWAN)

"We found that the chromosphere is 'lukewarm' rather than hot, in stellar temperatures," O'Gorman said in the same statement. "The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star's atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma."

"Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard," Chris Carilli of the National Radio Astronomy Observatory, who was not involved in this study but who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998, added in the same statement. 

This study was published June 16 in the journal Astronomy & Astrophysics.

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  • rod
    For some who enjoy geeky stats, this report is interesting, 'ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse', https://www.aanda.org/articles/aa/abs/2020/06/aa37756-20/aa37756-20.html
    "Abstract We first present spatially resolved ALMA and VLA continuum observations of the early-M red supergiant Antares to search for the presence of a chromosphere at radio wavelengths. We resolve the free-free emission of the Antares atmosphere at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA S band). The projected angular diameter is found to continually increase with increasing wavelength, from a low of 50.7 mas at 0.7 mm up to a diameter of 431 mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular diameter, respectively..."

    My note, Wikipedia reports Antares is 619.7 LY distance from earth or 190 pc. The mass listed is 11 to 14 solar masses. Using 12 solar masses and 700 solar radii, mean density is 4.927300E-08 g cm^-3. At 190 pc, 35 mas = 714.67 solar diameters for Antares, close to the values reported or 6.65 AU diameter. 50.7 mas = 1035.26 solar diameters or 9.633 AU seen in the abstract using ALMA.
    Reply
  • rod
    FYI, some may find this interesting. My note, the size calculations obtained for some stars using the heliocentric solar system in the mid and later 1600s, was used as an argument against the heliocentric solar system. From the link and arxiv report, "Robert Hooke in his 1674 'An Attempt to Prove the Motion of the Earth' called attention to the ideas of one of the people he identified as being among the “great anti-Copernicans”: André Tacquet. Hooke was describing— ...a grand objection alleged by divers of the great Anti-copernicans with great vehemency and insulting; amongst which we may reckon Ricciolus and Tacquet, who would fain make the apparent Diameters of the Stars so big, as that the body of the Star should contain the great Orb many times, which would indeed swell the Stars to a magnitude vastly bigger then the Sun, thereby hoping to make it seem so improbable, as to be rejected by all parties.1", ref - https://ui.adsabs.harvard.edu/abs/2019arXiv190912074G/abstract
    Reply
  • Helio
    I wonder if the outer atmosphere for Antares extends much farther than our own Heliosphere? The Sun's higher wind speeds -- I assume they are -- makes this a fair question, I think. It's interesting to me that Earth and the other planets are orbiting within the outer solar atmosphere.

    rod said:
    FYI, some may find this interesting. My note, the size calculations obtained for some stars using the heliocentric solar system in the mid and later 1600s, was used as an argument against the heliocentric solar system. From the link and arxiv report, "Robert Hooke in his 1674 'An Attempt to Prove the Motion of the Earth' called attention to the ideas of one of the people he identified as being among the “great anti-Copernicans”: André Tacquet. Hooke was describing— ...a grand objection alleged by divers of the great Anti-copernicans with great vehemency and insulting; amongst which we may reckon Ricciolus and Tacquet, who would fain make the apparent Diameters of the Stars so big, as that the body of the Star should contain the great Orb many times, which would indeed swell the Stars to a magnitude vastly bigger then the Sun, thereby hoping to make it seem so improbable, as to be rejected by all parties.1", ref - https://ui.adsabs.harvard.edu/abs/2019arXiv190912074G/abstract
    I think Galileo was also intimidated with this argument but he early-on favored Copernicus and set aside the need for extreme distances for stars to address their lack of parallax, which was about the only argument against Cop's theory.

    It's odd to me that they never seemed to take the time to determine if the eye itself was the issue. Given how lousy optics were back then, and the eye is also an optical device. You'd think they would simply light a flame, step-off beyond about 80 meters and recognize that the flame (now a spot) doesn't diminish in size but only intensity. The ~ 1 arcminute resolution of the eye would have been very useful information in those highly scientific debates. Experiments weren't that popular and some of that may have been because it was the college philosophers that were the top of the totem pole. Galileo, when starting as a math prof., was about the least paid.
    Reply
  • mike215
    I'm not so much impressed by the luminosity "size" of a star is as by its mass.
    Antares is only 15 times more massive but 680 times as large as the sun. Most of that is nebulous. An interesting comparison would be density as a function of radius.
    Reply
  • Helio
    mike215 said:
    I'm not so much impressed by the luminosity "size" of a star is as by its mass.
    Antares is only 15 times more massive but 680 times as large as the sun. Most of that is nebulous. An interesting comparison would be density as a function of radius.
    That is pretty amazing. The Sun will in about 4 billion years become perhaps 200 times greater in radius than it is today.
    Reply