A reader asked: “Heat kills a vacuum so is the sun in a vacuum? What surrounds it?”
The questioner most likely heard the phrase, ‘heat kills a vacuum’ in reference to high-vacuum systems in laboratories on Earth.
Heating a laboratory high-vacuum system can release atoms from the walls of the system enclosing the vacuum. The vacuum is then not as good, so the heat essentially killed the vacuum.
This loss of vacuum does not apply to the Sun, which is located in the vacuum of space. The quick answer is therefore that the Sun is surrounded by the vacuum of space.
As for most things, however, delving deeper reveals more complexity.
The Sun’s Outer Layers
Like the Earth’s atmosphere, the Sun does not have a sharp boundary between the top of the atmosphere and space. Rather they both gradually decrease in density as measured by the number of atoms in a cubic centimeter of volume.
The Sun’s visible surface is the photosphere. Above the photosphere, the Sun has the chromosphere and corona, which are only visible when a solar eclipse blocks the bright light from the photosphere.
As the distance from the Sun’s surface increases, the corona gradually thins into the vacuum of interplanetary space. There is however no sharp dividing line between the corona and the space surrounding the Sun.
The Vacuum of Space
We think of interplanetary and interstellar space as being a vacuum, but in a very strict sense, it isn’t. In the strictest definition a vacuum does not contain even a single atom. Such a vacuum does not occur anywhere on Earth – and probably not anywhere in the universe.
Interstellar space, far from any stars or nebulas, contains, on the average, about one hydrogen atom for every cubic centimeter of volume. This density is a much much better vacuum than scientists can make in any laboratory on Earth, but still isn’t a perfect vacuum.
Interplanetary space within the solar system is a bit more crowded than interstellar space. In addition to planets, moons, asteroids, comets, and meteoroids, interplanetary space contains gas atoms, protons, electrons, and dust grains. Like interstellar space, however, interplanetary space would qualify as a very good vacuum in any laboratory on Earth.
Space, Vacuums, and Our Solar System
Given the caveats that the vacuum of space is not actually completely empty, and that there is no sharp boundary between the Sun and space, space is still an excellent vacuum. To answer the original question: The Sun, and other stars, are surrounded by the vacuum of space.