The biggest stars are not the most luminous ones
Betelgeuse is about 1,000 times wider than the Sun and shines around 100,000 times as brightly. Yet R136a1, only 39 times the Sun's width, pours out more than 6.1 million times the Sun's power. The difference is heat: a star's output climbs with the fourth power of its temperature, so small and scorching beats vast and cool.
Luminosity is the total energy an object radiates each second, measured in watts or, more handily, in multiples of the Sun's output. It is a fixed property of the star. Brightness is what we see, and it depends on distance and on any dust dimming the light along the way. Light spreading from a star covers an ever larger sphere, so its intensity falls with the square of distance. Even the Sun's total includes a hidden share: about 2 percent of its energy leaves as neutrinos.
Measuring luminosity directly is harder than it sounds. A bolometer, which gauges radiant energy by how much it heats up, works on Earth but is not sensitive enough across the spectrum for stars. Astronomers instead sample light at chosen wavelengths and fit a model of the whole spectrum. Sometimes that means bold extrapolation, as with a hot Wolf-Rayet star seen only in infrared, where less than 1 percent of its output is observed.
Two routes lead to an answer. One combines a star's size and temperature, though size requires knowing both its apparent width in the sky and its distance, which is rarely possible. The other combines apparent brightness, distance and the dimming caused by interstellar dust. Getting each piece right is one of astronomy's central challenges. Helpfully, the widths of certain absorption lines in a star's spectrum can reveal its luminosity class without any distance at all.
Temperature spans a huge range, from Class O stars above 30,000 kelvins to Class M stars below 3,500, so luminosity varies even more wildly. On the main sequence, output rises roughly with mass to the power 3.5, which is why heavyweights burn out fast; the most luminous stars are always young, a few million years at most. Deneb, a white supergiant, shows the middle ground: about 200,000 solar luminosities from a star around 203 times the Sun's width.
Source: Luminosity
