spectral line broadening
If atoms emitted and absorbed at one perfectly sharp colour, every spectral line would be infinitely thin. They are not. Each line is smeared out over a small range of wavelengths — it has a width and a shape. That blurring is called line broadening, and far from being a nuisance, it is one of the richest sources of information in a spectrum.
Several effects widen a line, and each measures something different. Thermal broadening comes from the random thermal jiggling of atoms: hotter gas means faster jiggling, which Doppler-shifts each atom's contribution a little to the red or blue and smears the line wider, so the width is a thermometer. Pressure (or collisional) broadening comes from atoms being jostled by frequent collisions in dense gas, so a broad line hints at high pressure — and thus at a small, dense, high-gravity star rather than a puffy giant. Rotational broadening comes from a whole spinning star: one limb rushes toward us and one away, blurring every line by an amount that reveals how fast the star turns.
Reading these widths turns a spectrum into a physics laboratory. From the shape of the lines astronomers extract the temperature of the gas, the pressure and hence the surface gravity (which separates dwarfs from giants), the rotation speed of a star, and even the turbulent motions churning in its atmosphere. A line's width, in short, is a story about the conditions where it was born.
A rapidly spinning star can have its lines smeared so wide and shallow they almost vanish, while a slowly rotating star of the same type shows the same lines sharp and deep.
A line's width is a measurement — of heat, of pressure, of spin — not a flaw in the data.
Don't confuse broadening with a Doppler shift. Broadening makes a line wider but keeps it centred at the same place; a bulk Doppler shift slides the whole line sideways. The two effects are measured separately and tell different things.