Sam Cleary, age 16, of Hutchinson, Kan., for his question!

WHAT'S ASTROPHYSICS?

Astrophysics is the branch of astronomy that deals with the physical nature of the heavenly bodies. Tt seeks to understand the birth, evolution and end states of celestial ohjects and systems in terms of the physical laws that govern them.

A scientist in the field is called an astrophysicist.

Astrophysicists, for each object or system under study, observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time. This information is then interpreted with the aid of theoretical models. Tt is the task of such a model to explain the mechanisms by which radiation is generated within or near the object and how the radiation then escapes.

Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms making up the object.

Stars are among the best understood celestial objects, thanks to the work of the astrophysicist.

If the light of a star is dispersed into its wavelength spectrum, the relative intensities at various wavelengths yield considerable information about the star. The surface temperature can be estimated, using the law of thermal radiation.

If the distance of the star is known, its luminosity can be found by adding the observed intensities over all wavelengths. Tts radius can then be found by using the fact that the luminosity is the product of the energy emitted per unit area (which depends only on the surface temperature) and the total surface area.

If the spectrum of a star is studied under high resolution, many dark lines are seen at specific wavelengths. These lines are due to the absorption of light from deeper layers by atoms to the cooler layers above, astrophysicists tell us.

Astrophysicists analyze the light that the telescope collects. One of the important tools that he uses Is a spectrograph, a device that forms a spectrum. By analyzing a spectrum, the astrophysicist can determine the chemical composition and the temperature on the surface of a star.

When the conditions at the surface are known, astrophysicists can calculate the pressure and density increase toward the center by applying the known principles and methods of theoretical physics.

As a result. of these calculations, astrophysicists know that the main source of energy of all stars comes from nuclear reactions converting hydrogen nuclei to helium nuclei.

Another theoretical investigation is to calculate the history and future development of the stars.  Other problems in astrophysics include the study of the planets.