ALMA looks at the Sun
New discoveries

ALMA looks at the Sun

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ALMA has done something that you should never do yourself. The huge observatory in northern Chile has stared directly into the Sun. It’s something that no living person ever should try. The bright light of the Sun would damage your eyes. In the past, people have even become blind by looking into the Sun too long.

ALMA has no real eyes. However, it has very sensitive and expensive detectors. They, too, could be damaged by the bright light of the Sun. So ALMA technicians took good care to protect the detectors from heat and visible light. Only then did they dare to point the ALMA dishes in the direction of the Sun, to observe the Sun’s radiation at millimeter wavelengths.

Since 2014, ALMA has looked at the Sun every now and then for two-and-a-half years. Sometimes just with one single dish; at other times with the full array of 66 dishes. The main goal: to test ALMA’s capability as a solar observatory. In the future, astronomers may want to use the observatory more often to study the Sun in detail.

The Sun’s visible light is emitted by its bright, gaseous ‘surface’. Instead, the millimeter radiation observed by ALMA is emitted by a hot layer of gas just above the surface, known

as the chromosphere. In fact, the ALMA images reveal temperature differences in the chromosphere. By observing at different millimeter wavelengths, ALMA can look to different depths into the chromosphere.

Some of the most striking ALMA images of the Sun show a large sunspot. Sunspots are regions at the Sun’s surface that are a bit cooler than their surroundings. As a result, they look dark. The lower temperatures are caused by strong magnetic fields. The sunspot that was imaged by ALMA on 18 December 2015 is more than twice as large as the Earth.

Astronomers hope that ALMA observations of the Sun will provide them with more information on how the Sun behaves. It’s very important to really understand the Sun. After all, it’s our main source of heat and light – without the Sun, there could be no life on Earth.


The Sun is the star at the center of our Solar System. The Earth and the other planets orbit the Sun and bask in the Sun’s heat and life. The Sun’s diameter is more than 100 times as large as the diameter of the Earth. If the Sun were hollow, it could contain 1.3 million Earths! Like other stars, the Sun is a huge ball of extremely hot gas, mainly hydrogen and helium. The surface temperature of the Sun is some 5,500 degrees Celsius, but the temperature in the Sun’s core is more like 15 million degrees.


The ALMA observations of the Sun have been carried out over a period of 30 months by a large collaboration of astronomers from all over the world, including Tim Bastian of the National Radio Astronomy Observatory in the United States. According to Tim, ‘to fully understand the Sun, we need to study it across the entire electromagnetic spectrum, including the millimeter and submillimeter portion that ALMA can observe.’