The Sun. Our nearest star, and the only one we can really study closely. It's an imaging challenge because it's so bright. We all remember burning leaves with magnifying glasses as kids. Imagine the heat that an unfiltered Sun would create at the focus of a large aperture telescope! It would melt a camera sensor in a fraction of a second. So images of the Sun require that the aperture of the telescope be covered with a filter that only lets a fraction of a percent of the light through.
The Sun is composed mainly of hydrogen, with hydrogen-to-helium fusing core that is around 16 million degrees C, but is only about 5,700°C at its surface. The convective and rotational motion of the plasma that makes it up creates a strong magnetic field, and this field changes over time, following a 22 year cycle, which is most apparent in the 11 year sunspot cycle. During solar maximum, sunspot activity is high and large amounts of material may be ejected from the Sun's surface, causing auroras on Earth (and other planets, as well). Auroras are rare as far south as Colorado, but a several impressive events occurred around the 2001 solar maximum and again two years after that.
It is a remarkable coincidence that the angular diameter of the Moon in the sky is exactly that of the Sun. Because of this, when the positions of the Earth and Moon are just right, the Moon can perfectly block the disk of the Sun, allowing us to see the otherwise invisible solar corona. This is an sight of unbelievable beauty; once seen it is never forgotten. I have been fortunate to observe five total solar eclipses. I hope to see many more.
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