CHAPTER 13

BEYOND THE SOLAR SYSTEM

I. Stars

A. Properties of stars.

• Stellar parallax the slight back and forth shifting in the apparent position of a nearby star due to the orbital motion of the Earth. Parallax is determined by photographing a nearby star against the background of distant stars. Then, six months later when the Earth has moved halfway around its orbit, a second photograph is taken. When the photographs are compared, the position of the nearby star appears to have shifted with respect to background stars.

o       Stellar brightness is controlled by how big the star is, how hot the star is , and how far away it is.

• Maginitud is the measure of the star's brightness.
• Apparent magnitude is the brightness of a star as it is viewed from Earth. Stars that appear the brightest are of the first magnitude, while the faintest stars visible to the unaided eye are of the sixth magnitude.
• Because some stars are brighter than first magnitude stars, zero and negative magnitudes have been introduced.
• Absolute magnitude is the true brightness of stars.

B. Stellar color and temperature.

• Very hot stars with surface temperatures emit most of their energy in the form of short-wavelength light and therefore appear blue.
• Red stars are much cooler and emit longer-wavelength red light.
• Stars with middle temperatures appear yellow.

C. Binary stars are used to determine the star property most difficult to calculate, its mass. The mass of a body can be established if it is gravitationally attached to a partner, which in the case with any binary star system.

D. Hertzaprung-Russell diagram exhibits stellar properties that can provide information about the sizes, colors, and temperatures of stars. 90 percent of all stars fall along a band that runs from the upper left corner to the lower right corner of an H-R diagram. These stars are called main-sequence stars. The luminosity of the main-sequence stars is also related to their mass.

• Red giants are very luminous stars, large, and are above to the right of the main sequence in the H-R diagram. Supergiants are very large stars.
• In the lower central portion of the H-R diagram the stars are much fainter than the main-sequence stars. This group of stars are called white dwarfs.

II. Interstellar Matter

A. Nebula is the term applied to interstellar matter, such as dust and gas accumulations. If the interstellar matter is close to a very hot (blue) star it sill glow. This is called a bright nebula.

B. There are two types of bright nebulae: emission and reflection nebulae. Emission nebulae are gaseous massed that consists of hydrogen, and they absorb ultraviolet radiation. Reflection nebulae reflect the light of nearby stars. They are composed of dense clouds of large particles called interstellar dust. When a cloud of interstellar material is not close enough to a giant star to be illuminated, it is called a dark nebulae.

III. Stellar Evolution

A. Stars exist because of gravity. The gravity attraction of particles in a thin, gaseous squeezed to unimaginable pressures, its temperature rises, igniting its nuclear furnace, and a star is born.

B. Stages of stellar evolution.

• Dust and gases exist as a cloud.
• A protostar is created.
• From a protostar, a star becomes part of the main-sequence.
• From the main-sequence, a star goes to the giant stage.
• As the star continues its demise, it becomes a white dwarf stage star.
• Eventually, it becomes a black dwarf stage object.

IV. Stellar Remnants

A. White dwarfs are extremely small stars with densities greater than any known terrestrial material. They are thought to be low mass stars whose internal heat was able to keep these gaseous bodies from collapsing under their own gravitational force.

B. Neutron stars are smaller than white dwarfs and are remnants of a supernova event. During a supernova event, the implosion produces the core of very hot neutron star.

C. During a supernova event, remnants of stars greater than 3 solar masses apparently collapse into objects even smaller and denser than neutron stars. Their surface gravity is so immense that even light can not escape the surface. Thus, these objects are called blackholes.