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 “effective temperature” of the Sun’s light. The color of the Sun’s light matches that of a glowing object with a temperature of 5,772 Kelvin, so by this definition the “edge” of the Sun is the layer of the photosphere that’s also 5,772 Kelvin. This essentially represents the average layer that a photon of sunlight came from. By this definition, the Sun’s radius is 695,700 km or 432,300 mi – about 109 times that of Earth.

The photosphere is a decent edge if you’re taking a photo of the Sun, or looking at it through eclipse glasses. But a total eclipse reveals the main flaw:

The wispy white corona, with bits of the red chromosphere peeking through

The Sun has more layers beyond the photosphere. There’s the chromosphere, so named because it appears brightly colored during the beginning and end of a total eclipse, and beyond that the wispy corona, where the Sun’s magnetic field bends plasma into long streamers that extend much farther out than the photopshere.

The photosphere, chromosphere and the corona are often together described as the Sun’s atmosphere,² even though they’re all still made primarily of plasma just like the rest of the Sun. So how far out does this so-called atmosphere extend?

One definition is the “Alfvén surface”, which is the point of no return for matter streaming out from the Sun. Within the Alfvén surface, the Sun’s gravity and magnetism can pull plasma back in. Any plasma that passes it is fated to escape the Sun and become part of the Solar wind that pervades the entire solar system. It’s an irregular shaped surface that is constantly shifting with the Sun’s magnetic field, and extends about 10 to 20 times farther than the photosphere (13 million km or 8 million miles – about a quarter of the way to Mercury). We know where it is because we’ve sent a robotic probe through it: Parker Solar Probe, the fastest object humanity has yet created.³

Artist’s visualization of the Alfvén surface

The Solar wind is arguably still part of the Sun’s atmosphere, though – after all it’s still plasma from the Sun. It’s constantly blowing past Earth, creating the aurorae when it hits our atmosphere and magnetic field. In fact, the tenuous solar wind blows all the way out past Neptune and Pluto until it reaches the heliopause, where it hits the even more tenuous matter coming from interstellar space. Our most distant space probes, Voyager 1 and Voyager 2, reached the heliopause in the 2010s, after more than 30 years of travel from Earth – hitting it at about 18 billion km or 11 billion miles, 4 times the distance from the Sun to Neptune.⁴

The heliosphere, including the heliopause

There are Solar System objects that lie far beyond the heliopause, since the Sun’s gravity reaches even further than the solar wind. The most distant comets in our Oort cloud are hundreds of times further away than even the heliopause. Still, it’s pretty incredible that humanity’s machines have now both touched the Sun’s outer layers, and finally left its physical reach.