Where does the Sun end?

The Sun is a giant blob of superheated plasma. There’s no obvious surface – the plasma just slowly thins out the farther out you go. So how exactly do we tell where the Sun ends? It’s not an easy question. Astrophysicists actually have a few different definitions for where the Sun ends, and different ones make sense for different areas of research. By the most generous definition, you’re actually in the Sun right now. If you go outside and look at the Sun (with proper safety equipment), the big glowing part you’ll see is called the Sun’s photosphere. It’s a layer a few hundred kilometers1 thick where the Sun’s plasma transitions from being opaque to transparent. We know the photosphere so broad because we can partially see through its thinner and cooler upper layer. That’s why the Sun looks dimmer and redder around the edges in photographs: Notice the dark red edge The standard way to define the Sun’s radius is to use the layer of the photosphere where the temperature equals the…

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