All About Space
How big is Antares?
The local red supergiant star has been mapped in extraordinary detail by radio telescopes
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.