Answer to Question 1
Hot stars emit a great deal of ultraviolet radiation that you can't see, and cool stars emit plenty of infrared radiation. To add in the energy you can't see, astronomers make a mathematical correction that depends on the temperature of the star. With that correction, astronomers can find the total electromagnetic energy output of a star, which they refer to as its luminosity (L).Astronomers know the luminosity of the Sun because they can send satellites above Earth's atmosphere and measure the amount of energy arriving from the Sun, adding up radiation of every wavelength, including the types blocked by the atmosphere. Of course, they also know the distance from Earth to the Sun very accurately, which is necessary to calculate luminosity. The luminosity of the Sun is about 4 x 1026watts (joules per second).
Answer to Question 2
In an eclipsing binary system, if the plane of the orbits is nearly edge-on to Earth, then the stars can cross in front of each other as seen from Earth. When one star moves in front of the other, it blocks some of the light, and the star is eclipsed. Such a system is called an eclipsing binary system. Seen from Earth, the two stars are not visible separately. The system looks like a single point of light. But, when one star moves in front of the other star, part of the light is blocked, and the total brightness of the point of light decreases. A light curve is a graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries.The light curves of eclipsing binary systems contain plenty of information about the stars, but the curves can be difficult to analyze. A light curve shows how long it takes for the stars to cross in front of each other, and multiplying these time intervals by the orbital speeds gives the diameters of the stars. There are complications due to the inclination and eccentricity of orbits, but often these effects can be taken into account, so observations of an eclipsing binary system can directly tell you not only the masses of its stars but also their diameters.
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