All About Space

How big is Antares?

The local red supergiant star has been mapped in extraordin­ary detail by radio telescopes

- Reported by Lee Cavendish

The night sky, especially when viewed from an area with low to no light pollution, can reveal a beautiful array of celestial gems – twinkling stars that generation­s before have marvelled at. The difference between then and now is that our generation has the capability to map these stars with precision.

New research has taken this to a whole new level by mapping the famous star Antares, learning more about its atmosphere and how this can be applied to other red supergiant stars.

Throughout the years people have gazed upon ruddy Antares, the brightest star in the constellat­ion of Scorpius and one of the brightest stars in the night sky, which lies 554 light years from Earth. As telescopes, spectrosco­py and multi-wavelength observatio­ns have become more advanced, we have been able to learn more about the stars. Scientists have been able to categorise Antares as a red supergiant star that has swollen as it approaches the end of its life, despite it being only 11 million years old. By comparison the Sun is 4.6 billion years old. This is because Antares weighs 12 solar masses – 12 times the mass of our Sun – and with stars the biggest and brightest burn out the quickest. Astronomer­s have deduced over many years of visible-light observatio­ns that Antares is approximat­ely 700-times wider than the Sun, and previous research into its climate has suggested that the temperatur­e of the red supergiant star’s chromosphe­re should be between 5,700 and

12,700 degrees Celsius (10,000 and 22,900 degrees Fahrenheit).

Now the plot has thickened as researcher­s have been able to map the atmosphere of Antares with unpreceden­ted levels of precision, revealing unusually cool regions residing in localised bubbles in addition to highlighti­ng the true extent of the star’s impressive stellar wind. This research was only made possible by utilising two of the

most powerful tools in radio astronomy: the Atacama Large Millimeter/submillime­ter Array (ALMA) located in the Atacama Desert, Chile, and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA), located in New Mexico. Dr Eamon O’Gorman of the Dublin Institute for Advanced Studies in Ireland and his team have been able to measure the temperatur­e and size of Antares’ stellar atmosphere, publishing their results in a new scientific paper.

“We had looked at Betelgeuse a lot already at radio wavelength­s, albeit with the old VLA, and at one wavelength with ALMA,” O’Gorman tells

All About Space. “Antares is the only other red supergiant that can currently be resolved at radio wavelength­s, so we decided to focus on it. We could then compare our results to what we already knew about Betelgeuse.”

However, something unexpected arrived in the results. “[The results] have revealed a new ‘lukewarm’ chromosphe­re component,” explains paper co-author Dr Keiichi Ohnaka, associate professor at the Universida­d Católica del Norte in Chile, to All About Space. “It is not known why the temperatur­e goes up in the chromosphe­re instead of falling off, so this new lukewarm component is important for decipherin­g the heating mechanism of the chromosphe­re.”

“[This discovery] is important firstly because we have managed to resolve a star’s atmosphere at many different radio wavelength­s, a feat which is very, very difficult to do,” says O’Gorman. “These observatio­ns allowed us to find out a lot about the size and temperatur­e of the chromosphe­res of red supergiant stars.”

But what exactly is the chromosphe­re? “A chromosphe­re is an atmospheri­c layer just above a star’s surface that is heated to high temperatur­es by mechanisms due to the motion of the plasma taking place beneath or on the star’s surface. This had never been detected at radio wavelength­s before for an evolved star,” explains O’Gorman. “The inhomogene­ous nature of the chromosphe­re is the result of the complex environmen­t that exists in the atmosphere­s of these stars – Antares and Betelgeuse – and stars in general,” he continues.

Astronomer­s had already deduced that Antares is roughly 700-times wider than the Sun; it would extend to somewhere around the asteroid belt if it was placed into the Sun’s spot at the centre of our Solar System. Now the star’s chromosphe­re through to the region containing its stellar wind have been mapped in unrivalled detail using

ALMA for shorter wavelength­s and the VLA for longer wavelength­s. These new results have shown that the chromosphe­re is ‘lukewarm’ at best. The temperatur­es peak around 3,500 degrees Celsius (6,400 degrees Fahrenheit) and then gradually decrease the further out it gets from the energy source. This is minuscule compared to the Sun’s chromosphe­ric temperatur­e of around 20,000 degrees Celsius (36,000 degrees Fahrenheit).

“[This result] does not mean that the temperatur­es of 5,700 to 12,700 degrees Celsius (10,000 to 22,900 degrees Fahrenheit) from the previous works are wrong. Such hot gas does exist. If we take into account the infrared images of Antares, there should also be cool gas at temperatur­es lower than 1,700 degrees Celsius (1,400 degrees Fahrenheit),” says Ohnaka. “Therefore we conjecture that both ‘hot’ and ‘lukewarm’ chromosphe­res, as well as cool gas, may coexist in distinct cells or pockets. As we think that Antares and Betelgeuse are representa­tive of red supergiant­s, the inhomogene­ous atmosphere is probably a usual trait in this class of star.”

The inhomogene­ous nature of Antares is an extremely intriguing aspect in itself. It appears that because of the evolved nature of the star the temperatur­e decreases in random pockets of the star’s atmosphere, and the mysterious magnetic fields and shock waves emanating from the star’s surface are the reason for this.

Something else that the study revealed is the true size of Antares, as the chromosphe­re stretches out to 2.5 times the star’s radius – extraordin­ary considerin­g the Sun’s chromosphe­re is only 0.5 per cent of its radius. However, astronomer­s’ current understand­ing of heliophysi­cs is limited, and they are still unable to understand these different features, factors and mechanisms for our own Sun, let alone a star over 550 light years away.

When O’Gorman and his team observed Antares in longer wavelength­s, courtesy of the VLA, they were able to make a clear distinctio­n between the chromosphe­re and the region where Antares’ stellar wind begins to form. This is the first time

this distinctio­n has been detected, and it is shown in amazing clarity. The images that the VLA took show a huge gust of stellar wind that is illuminate­d by Antares B, Antares’ smaller but hotter companion star. This wind is carrying stellar material out into the cosmos. At the start of Antares’ life it had a mass 15 times that of the Sun, but has lost three solar masses worth of material during its lifetime This all originates from the interior of the star.

In the interior of a red supergiant there is more than just hydrogen and helium and the occasional bit of lithium burning, as in most stars. Instead there is an iron core with layers of different elements being burned to keep the star shining for just a little bit longer. These layers contain carbon, oxygen, nitrogen and so on, which can all find themselves in the stellar wind and sent out into starformin­g clouds throughout the cosmos. This is similar to the elements that were once in the Solar System’s original cloud of dust and gas. This is the carbon that is the basis for our DNA, the oxygen we breathe and the nitrogen that makes up most of Earth’s atmosphere.

There are so many stars in the night sky, each with its own story, and only by investigat­ing them in all sorts of wavelength­s and using different methods can we begin to build a firm understand­ing of how all stars differ, how they evolve and their overall influence on the cosmos.

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 ??  ?? Below: ALMA currently holds the title of the world’s largest radio telescope
Below: ALMA currently holds the title of the world’s largest radio telescope
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 ??  ?? This is the first time another star’s chromosphe­re has been detected and measured using radio waves.
This is the first time another star’s chromosphe­re has been detected and measured using radio waves.
 ??  ?? Left: The VLA is comprised of 28 25-metre (82-foot) radio telescopes
Right: Red supergiant stars eventually phase into an extravagan­t supernova explosion
Left: The VLA is comprised of 28 25-metre (82-foot) radio telescopes Right: Red supergiant stars eventually phase into an extravagan­t supernova explosion
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