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Brightness changes throughout the life of a star. The luminosity of a star is a measure of its brightness. That's it. Simple. Astronomers measure a star's brightness by examining the amount of energy emitted (given off) every second. The more energy emitted, the higher the brightness. Scientists use Watts to measure energy. Since our Sun is so close, scientists have measured about 390,000,000,000,000,000,000,000,000 Watts being given off all of the time. We need to remind you that not all of that energy is sent to Earth. Most of it is just sent out into space as heat and other EM radiation.

Luminosity Is Caused By...

There are some specific factors that affect luminosity. As the size of a star increases, luminosity increases. If you think about it, a larger star has more surface area. That increased surface area allows more light and energy to be given off. Temperature also affects a star's luminosity. You don't even need to look at a star for this idea. Just think about a stove, or a fire. When you see very little light, chances are the temperature is lower. When an electric stove is off, it is black. But when it is on high, the stove glows bright red. The same idea applies to stars. As a star gets hotter, the number of nuclear reactions increases. More reactions, more energy.

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> Luminosity
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- Development II
- Black Holes


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Keywords for Review

Kinetic Energy: The energy of an object related to the motion of the object. On a simple level, an object that is not moving has no kinetic energy. An object that is moving has some amount of kinetic energy. The more an object moves, the more kinetic energy it has. An object increases its kinetic energy if it accelerates and increases its velocity. For example, as you increase the temperature of a gas, the molecules become more energetic and the system has an overall increase in kinetic energy. Kinetic Energy=(0.5)* mass * (velocity)2

Exothermic: A chemical reaction that releases energy after the reaction is complete. The energy is usually released as heat, but it can also be released as light or sound. On a small scale, a burning candle releases light and heat because of exothermic reactions as the wax burns. On a large scale, an explosion might occur when blasting with sticks of dynamite (TNT).

Activation Energy: The least amount of energy needed for a chemical reaction to occur. Reactions often require some amount of energy to get moving. For example, placing hydrogen and oxygen gases in a container will not give you water. There is a certain amount of energy required to get the first reaction going. Catalysts are substances that help to lower activation energies so that reactions can proceed.

Viscosity: A term used to measure the fluidity of a liquid. As the attractions between the molecules increase, viscosity increases. Fluids with high viscosities don’t flow easily. Some substances such as honey or sap are very slow moving and have high viscosities. Other fluids such as water or mercury (Hg) have very low viscosities.

Volatile: Volatility is the likelihood that a substance will vaporize (become a gas). Volatility measurements are all about comparing two substances. Substances with a higher vapor pressure are more volatile. Alcohol is more volatile than water because it evaporates at a lower temperature.

Reference Materials

Encyclopædia Britannica:

- Cosmos4Kids: Star Classes
- Chem4Kids: Matter
- Chem4Kids: Astrochemistry
- Chem4Kids: Atoms
- Chem4Kids: Hydrogen
- Chem4Kids: Helium
- Geography4Kids: Earth Energy
- Geography4Kids: Solar Radiation
- Physics4Kids: Magnetic Fields
- Physics4Kids: Light

- NASA: Home Page
- NASA: Kids Home Page
- ESA: Home Page
- ESA: Kids Home Page

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