Kirchhoff's laws of spectroscopy
/ KEER-khofs laws /
In the 1860s Gustav Kirchhoff and Robert Bunsen worked out three simple rules that say which of the three spectrum types you will see, and why. The rules connect the look of a spectrum to the physical state of whatever made the light — a wonderfully practical map from appearance to cause.
The three laws, in plain words: first, a hot dense object (a solid, a liquid, or a thick gas) glows with a smooth continuous spectrum, an unbroken rainbow. Second, a hot thin gas glows with an emission-line spectrum, just a few bright lines of its own colours. Third, a cool thin gas sitting in front of a hotter continuous source produces an absorption-line spectrum, dark lines carved at exactly the colours that the same gas, if heated, would emit brightly. The bright and dark lines of a given element always land at the very same wavelengths.
These three laws are the foundation of stellar spectroscopy. A typical star shows the third case: a hot dense interior making a continuum, wrapped in a cooler thin atmosphere that prints absorption lines onto it. Knowing the laws, an astronomer glancing at a spectrum can immediately read the geometry — what is hot, what is cool, what is dense, what is thin — long before doing any detailed calculation.
The Sun is Kirchhoff's third law in action: a dense interior makes a continuum, and the cooler photosphere above it carves thousands of dark absorption lines into the sunlight.
Three rules linking what a spectrum looks like to what kind of gas, hot or cool, dense or thin, made it.
The laws are an excellent rule of thumb, not exact physics: real sources can be partly transparent and layered, so a single object may show emission and absorption together. The modern, exact statement is the theory of radiative transfer.