system
Given the following post, respond as if you're commenting on Reddit:
human
How can we know the Sun's surface temperature when the Sun's corona around it is hundreds of times hotter?

I read today about the fact that the Sun's corona is hundreds of times hotter than the photosphere (surface) of the Sun. The photosphere measures around 6000 K, goes over 20,000 K in the chromosphere and way over 1 million K in the Sun's corona.

As I was glad to learn something new, my little science brain started thinking about **how we can possibly measure 'lower' temperatures that's surrounded by such extreme high temperatures?**

The problem I'm visualizing, is like measuring the heat of a warm coffeecup that is hidden behind a larger, hotter campfire (or something hotter). Maybe a bad analogy, but I'm curious about the temperature measurement techniques used **if it's explainable in ELI5 format** :)
gpt
The corona and chromosphere are very tenuous, so most of the light emitted by the photosphere passes right through them. By comparing the emission spectrum of the sun that's observed when looking directly at the central region and the spectrum that's observed when looking only at the outer layers (achieved by observing the outer edges of the sun), we can separate the two.

Note that we we can get a very good picture of the photosphere itself even without doing any of the above. The photosphere is tens of thousands of times denser than the chromosphere and about a trillion times denser than the corona, so without very sensitive equipment (or an eclipse blocking out the photosphere), the chromosphere and corona are invisible. What we see when we look at the sun *is* essentially just the photosphere.

Unlike the corona and chromosphere, the photosphere is opaque so we can't make similar direct observations of the layers below it. Understanding the inner layers of the Sun requires different techniques, including seismology, neutrino astronomy, nuclear and particle physics experiments and computer simulations.