The Sun is a star. Not many people know that. It’s a star like all the other stars in the night sky. Or, put differently: all the stars in the night sky are suns like our own. So why are the stars tiny pinpricks of light, while the Sun is so large, bright and hot? It’s just because the Sun is so nearby. Our planet Earth circles around the Sun, at a distance of 150 million kilometers. The nearest star, in contrast, is 40 billion kilometers away.
Stars are giant balls of hot gas. Their energy is produced in the star’s core, by nuclear reactions. The Sun is 1.4 million kilometers across – over a hundred times the diameter of the Earth. The temperature in the core of the Sun is some 15 million degrees Centigrade.
Of all the stars in the Universe, the Sun is of course the best known, because it is so close to us. Still, there are many mysteries left. One mystery concerns the sunspots – dark splotches on the bright surface of the Sun that are cooler than their surroundings. Every eleven years or so, the number of sunspots is larger than average. The same is true for the number of flares and explosions on the Sun. No one knows what produces and maintains this ‘solar cycle’.
Another mystery is the ‘atmosphere’ of the Sun, called the corona. The corona is very tenuous, but it is incredibly hot: a few million degrees. That’s much hotter than the surface of the Sun, which is a ‘mere’ 5,600 degrees Centigrade. No one knows how the Sun’s corona gets so hot.
These questions are important. The Sun is the source of all light and warmth for living things here on Earth. If the solar cycle would stop, Earth might experience a new ice age. So we need to understand how this cycle works. And giant explosions in the hot corona of the Sun blow dangerous particles into space. When these particles arrive on Earth, they can damage satellites. Or they can cause the breakdown of computer networks and power supplies. So we need to understand how the corona gets so hot and how solar explosions are generated.
The Sun through different eyes. This video shows what the Sun looks like when viewed through different types of receivers, ranging from optical light, similar to what our eyes see, to X-ray wavelengths, like those used to take x-ray images of bones. ALMA can observe at larger wavelengths, which are known as radio waves. Credit: NASA Solar Dynamics Observatory
ALMA can help by observing the millimeter waves emitted by the solar flares and explosions. Astronomers can then study them and they also hope that ALMA’s observations will help to solve the mystery of the corona heating.
The Sun is extremely bright. If you look straight into the Sun, you could get blind! So it seems very stupid to aim a telescope at the Sun. But ALMA can look into the Sun without any problem. The surfaces of the ALMA antennas reflect the light and heat of the Sun in all possible directions, so it is not concentrated on the sensitive receivers. Only the millimeter waves from the Sun are focused on the receivers, for further study.