Emission and Absorption Spectra

Ever wondered how we know what stars are made of without actually visiting them? The answer lies in emission and absorption spectra - the cosmic fingerprints that reveal stellar secrets.

Stars naturally emit a continuous spectrum of electromagnetic radiation from their surfaces, much like a rainbow containing all colours. However, when this light passes through the star's atmosphere, something fascinating happens - specific atoms absorb particular wavelengths, creating dark lines in what we call a line absorption spectrum.

Think of it like this: gas clouds act as selective filters, absorbing one specific wavelength and then re-emitting it as an emission line. This process creates unique patterns that help astronomers identify which elements are present in distant stars.

Key Insight: Each element has its own unique absorption pattern, like a barcode that tells us exactly what a star contains!

Black Body Radiation and Wien's Law

Here's where stellar physics gets really clever - we can actually measure a star's temperature just by looking at the colour of light it emits most intensely. Stars behave like black bodies, theoretical objects that absorb and emit all incident radiation.

The black body spectrum shows us that hotter objects emit shorter wavelengths more intensely. As temperature increases, the peak wavelength shifts towards the blue end of the spectrum and becomes much brighter overall.

Wien's displacement law gives us the mathematical relationship: λp = W/T, where W = 2.9 × 10⁻³ m·K. This means peak wavelength is inversely proportional to absolute temperature. Remember to convert Celsius to Kelvin by adding 273.15!

Stefan's law takes this further, stating that power emitted per unit area is proportional to T⁴. For a complete star: P = σAT⁴ = 4πr²σT⁴, where σ = 5.67 × 10⁻⁸ Wm⁻²K⁻⁴.

Pro Tip: The inverse square law explains why distant stars appear dimmer - intensity decreases with the square of distance as radiation spreads over increasingly larger spherical areas.