Why doesn’t the sun melt?

This week, it seems, I’ll be writing about the sun. Why? Well, when we were walking home from preschool yesterday, I asked my son if he knew what day it was. “It’s the first day of summer!” he declared. And then he asked me a whole bunch of questions, one right after the other:

To be honest, I felt like I’d been hit by an avalanche of curiosity. But they’re all great questions.

What is melting?

Let’s start with the Dictionary.com, and a definition of melt that states:

  1. to become liquefied by warmth or heat, as ice, snow, butter, or metal.
  2. to become liquid; dissolve
  3. to pass, dwindle, or fade gradually (often followed by away)
  4. to pass, change, or blend gradually (often followed by into)
  5. to become softened in feeling by pity, sympathy, love, or the like
  6. Obsolete. to be subdued or overwhelmed by sorrow, dismay, etc.

From a more technical perspective, ‘melting’ is a first-order phase transition in which a material’s latent heat increases and it’s density or volume decreases sufficiently that it moves from the solid phase to the liquid phase.

Wait. Phases? Phase transitions? What now?

You’ve probably seen a diagram like this before:

Phase_diagram_of_water.svg

That is a phase diagram for water, showing the different states (solid, liquid, or vapor) that water can be in based on temperature and pressure. Under one atmosphere of pressure (1 bar or 100 kPa), it is a solid at or below 0 degrees Celsius, a liquid between 0 and 100 degrees Celsius, and a vapor above 100 degrees Celsius. Phase is just the technical term for these states, and a phase transition is simply where the material changes (or makes a transition) from one phase to another.

While researching these phase changes, I ran into the terms “first-order phase transitions” and “second-order phase transitions” a lot. I’ll be honest and say I don’t fully understand them, because the best definitions all seem to involve a whole lot more physics than I understand. But here’s my best attempt at an explanation, after reading several articles and staring hard at Wikipedia:

  • A first-order phase transition is driven by heat, and the material transitions from one phase to another at a set rate based on the energy added to the system. Think of melting ice, or boiling water.
  • A second-order transition is also called a continuous phase transition, and appears to a uniform change across the material – imagine a block of ice instantly becoming water, for example.

340px-Phase_change_-_en.svg
The first-order phase changes

The first-order phase changes are the ones we’re all familiar with. There are a whole lot of other types of phase change, so have fun reading up on them some time.

Right. So what does all of this have to do with a star?

All of this is a long walk to the answer I originally gave my son, when he asked me why the sun didn’t melt. “The sun can’t melt,” I told him. “Melting is when a solid turns into a liquid, and the sun is made of gas and plasma. It’s way hotter than melting.” Which is more or less true, and also led to the next question he asked me. But we’ll handle that tomorrow.

What is a star?

I’ll be honest here. I thought the International Astronomical Union would have a formal definition of a star, much like they do for planet and dwarf planet and the like. But, if they do, I couldn’t find it. So, in brief, a star is “a luminous sphere of plasma held together by its own gravity”. It can’t melt, because “melt” is not a phase transition available to plasma. Anything in it that could have melted has already melted, then vaporized, then ionized around 4.6 billion years ago.

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