 So far, we've identified Wolf 359 as one of the least luminous stars in our neighborhood, and Vega as one of the most luminous, but we haven't been explicit as to what we mean by luminous. Stars have a wide range of apparent brightness as measured here on Earth. The variation in apparent star brightness is caused by two things. One, stars have different intrinsic luminosity, and two, stars are located at different distances from us. An intrinsically faint nearby star can appear to be just as bright to us on Earth as an intrinsically luminous star further away. Luminosity is what we use to put precise measurements on the idea of brightness. It measures the total amount of electromagnetic energy emitted by a star in watts, just like a light bulb. Bright brightness is measured in watts per square meter. Because light from stars spreads out over the surface area of a sphere, we can use the inverse square law to categorize luminosity for all the stars that have parallax distance information. Take the Sun, for example. The apparent luminosity the Sun has measured in my backyard is 1400 watts per square meter. If my backyard solar cells were 100% efficient, that's how much electricity each panel would create. Unfortunately, current technology is only 15% efficient, so I'm only getting around 200 watts per panel. Plugging our distance of the Sun into the inverse square law, we calculate its total luminosity. Here you can see that the answer is a very big number. Using Einstein's famous E equals mc squared, or energy equals mass times the speed of light squared, we calculate that the Sun is fusing 600 million tons of hydrogen into helium every second. And in the process, converting 4 million tons of matter into energy every second. To put this into perspective, this number is equivalent to around 4 billion hydrogen bombs exploding every second. Astronomers use a more complex set of classifications for calculating brightness called magnitudes and absolute magnitudes at 10 parsecs. But for our purposes, we'll stick with luminosity.