Practical astronomy

Observing the closest star, during the day

"What is the nearest star?" Perhaps it is a bit of a lame question. Yet it is often my first question during a lay talk on astronomy. I am always very impressed by the names of stars I receive. The star mentioned most often is Proxima Centauri. This star is indeed located very nearby, at just about a distance of 38 000 000 000 000 kilometres from Earth. A trip through space with destination Proxima Centauri would only take a mere 75 000 000 years.

Now, when you think, "But wait a minute, this is not the nearest star", you are absolutely right. As a matter of fact, there is a star that is even closer to us. This star is so often forgotten because of its name, the way we draw it in cartoons and because we cannot see it at night but instead only during the day. She stands on what we in astronomy call exactly one 'astronomical unit'. Any lightbulb moment? Exactly, you are correct, it is our own sun!

Ok I have said enough about distances. Now, I really would like to move on to the question: how can we actually observe our sun in the best way possible? For telescopes we use during the night, the aim is to capture as many photons emitted or reflected by the object we want to look at. Well, one thing is for sure, there is no shortage of photons while trying to observe the sun. On the contrary: never look directly at the sun when you are not using a solar filter. Be it with a telescope or eclipse glasses, without a solar filter your eyes will be blinded within a tiny fraction of a second.

Instead of collecting as many photons as possible, a dedicated solar telescope has another important goal: to obtain the highest possible spatial resolution. Because of this, solar telescopes usually have a smaller mirror than night telescopes and their optical setups offer larger focal points to approach the so-called diffraction limit as closely as possible. This makes solar telescopes easily distinguishable from night telescopes. Instead of a dome, you typically see a tower-like structure that facilitates the large focal length. There is another feature of a solar telescope. The light paths of professional modern solar telescopes are located in either a vacuum or helium. Together with a temperature controller for the optics, this is used to limit the heating of the air (also known as turbulence) inside the telescope.

Since 2015, we have had a solar telescope at the Old Observatory in Leiden. It is composed of an ingenious mirror system that captures sunlight on the roof and reflects it down towards the visitor centre in the basement. That is where the telescope projects a live image of the sun with a diameter of about a metre onto the wall. This allows us to get a close-up view of the sun. This semester I taught undergraduate astronomy students how to use the telescope. By using the heliostat, we can introduce them to our nearest star, its sunspots and eruptions.