Astropedia

Intended for all audiences, this textbook is an introduction to the nature of the universe. Use it to research or review our solar system, stars, galaxies, and the history of the universe. Each chapter has a set of corresponding homework questions.

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Articles related to: Stefan-Boltzmann Law  Read article

1. The Hertzsprung-Russell Diagram  Read article

When astronomers realized the variety of stars that exist, their first step was to devise a sensible way to arrange and study the data to find relationships among the various forms of stars. This could have been done in numerous ways, but one method has become traditional. Building on the work of...

2. Stars of Different Sizes  Read article

As astronomers studied the properties of large samples of stars, they found stars with many different combinations of luminosity and temperature. Most of these stars had properties that placed them at different positions on the main sequence of the H-R diagram, slanting from high luminosity and h...

3. Star Properties  Read article

Stars are luminous balls of gas like the Sun, but hundreds of thousands of times more distant that the Sun. Star distances are measured in units of light-year or parsecs. Astronomers can rarely measure the size of a star directly; the images of all stars are blurred to about the same angular size...

4. Understanding the Main Sequence  Read article

Why are distinctive types of stars, such as the main sequence stars, the giants, and the white dwarfs found at different positions on the H-R diagram? The simple answer is: stars have different masses. High-mass main sequence stars are hotter and brighter and bigger than low-mass main sequence st...

5. The Size of Stars  Read article

Size is a fundamental property of a star. It is extremely difficult to measure the size of a star by direct observation. We can use the small angle equation to see why. Imagine a star like the Sun at a distance of a parsec. The angle subtended by a solar-type star at that distance is 206,265 (d /...

6. Stellar Luminosity  Read article

One fundamental property of a star is the total amount of energy it radiates each second. This energy output is called the luminosity or absolute brightness. Unlike apparent brightness, which depends on how far from us the star is, luminosity is intrinsic to a star, similar to the way that wattag...

7. Volume and Brightness Selected Samples  Read article

The dramatic difference between what we can easily see and what really lies out in space is an important issue in astronomy. To use Hertzsprung's analogy, the whales are much easier to see than the fishes. We can see whales out to large distances, but in any volume of water, there are far fewer w...

8. Temperature and Spectral Class  Read article

Spectroscopy is vital to understanding the temperatures of stars. We can study stellar spectra and measure which color is the most strongly radiated by a star. We can then use Wien’s law to calculate the temperature. There are other clear patterns in the spectral features of stars — t...

9. Absolute Brightness  Read article

The amount of light is that reaches the Earth from a distant object depends on a variety of factors; the distance to the object, the amount of light the object is giving off in all directions, and the amount of intervening material that is blocking some of the light from reaching us here on earth...

10. Wien's Law  Read article

The concept of thermal radiation is related to the ideas of energy and temperature. The key principles were discovered in 1898 by the German physicist Wilhelm Wien (pronounced Veen). Wien discovered that all bodies constantly emit thermal radiation. Thermal radiation is always concentrated at cer...