Stars 2 Essay Research Paper Although they

Stars 2 Essay, Research Paper

Although they are tiny in size, white dwarfs have played a great role in astronomy. These compact stars are very different from familiar objects like our sun. They also pose a seemingly endless series of puzzles, whose solutions provide new insights into many areas of physics and astronomy. To unravel a wide variety of phenomena such as cataclysmic variables (novae, dwarf novae), planetary nebulae, and some types of supernova, we have to understand white dwarfs. These stars may even hold clues to one of the most fundamental questions of all- how old is the universe.

White dwarfs come in a variety of types ranging from hot to luminous to cool and dim. Some are among the faintest of all stars, but paradoxically their study began with that of Sirius, the brightest star in our night sky. The Dog star is a slightly scaled-up version of the Sun, with about twice the mass and 25 times the luminosity. It is an unremarkable object comfortably fusing enough hydrogen into helium in its core to supply the energy radiated from the surface and to provide the thermal pressure needed to balance the inward force of gravity. Sirius appears bright to us because it is so close, less than 9 light-years away.

In the last century Sirius attracted the attention of Friedrich W. Bessel. By carefully measuring the Dog star, position from 1834-1844, he found irregularities in its motion across the sky. He attributed this behavior to the influence of an unseen companion.

Despite many efforts the proposed attendant escaped visual detection until 1862. In that year, Alvan Graham Clark, the 3 telescope maker, was testing the objective lens of a new 18.5-inch refractor when he unexpectedly spotted the elusive companion, now known as Sirius B, or the Pup.

Today, it is clear why the detection was so long delayed. In 18844, when the search began, Sirius B was only 3 arc seconds from the primary star, but by 1862 the separation between the pair had reached 9.5 arc seconds. This made the discovery much easier. Also hindering the detection was the tremendous brightness difference between the two companions, the Pup being 10,000 times fainter than Sirius itself.

The white color of Sirius B shows a high surface temperature of about 30,000 Kelvin. Although the name “white dwarf” applies well to a star like the Pup, some members of the class are much cooler and have very different colors.

ORIGINS

The origins of white dwarf stars are examined in order to understand their surface compositions and evolution. A star runs out of fuel at its center at the end of its main-sequence phase. While the outer part of the star expands enormously, hydrogen continues to burn in a narrow shell surrounding the now pure helium core. As it becomes a red giant, its luminosity increases and its temperature falls. The core helium eventually gets hot enough to fuse into oxygen and carbon. If the mass of the star is between 2 and 8 Suns, helium ignition occurs quietly and evolution continues with a helium-burning shell. This star lies on the horizontal branch on the H-R diagram.

Helium and hydrogen continue to burn in relatively thin shells surrounding a now degenerate core of oxygen and carbon, when the new fuel in the core is in turn exhausted. While this is happening, the outer envelope swells even more, until it extends out to several times the Earth-Sun distance. The star is now on the asymptotic giant branch.

The mass of the core increases and the star gets brighter as the hydrogen and helium burning shells eat their outward from the center. The star moves upward on the H-R diagram and becomes a red super-giant. During this phase, two significant processes occur. First, the rarified outer envelope starts to evaporate. Second, the3 hydrogen-burning shell consumes material faster than the helium burning one does; resulting in a configuration that is unstable. This instability leads to rapid increases in luminosity-”flashes”-each time the mass of a newly formed helium deposited on the inner shell exceeds some critical value.

In some way, the steady mass loss plus the possible influence of the flashing helium shell remove much of the extended hydrogen envelope in only a few tens of thousands of years. The star, reduced to some 20% of its initial mass, evolves quickly and moves rapidly to the left across the top of the H-R diagram. Ultraviolet light emitted from the increasingly hot star causes the ejected material to glow, forming a planetary nebula, the birth pang of many white dwarfs. Mass loss may continue until helium-rich material is exposed to the surface, so whether the remnant becomes a DA or DB white dwarf depends a lot on what happens at this stage.

What remains of the expose core of the asymptotic giant branch star is called a planetary nebula nucleus, or PNN. Typically, one mass of 0.6 Sun evolves across the H-R diagram in only 10,000 years. As the shell-burning energy sources die out, the star’s luminosity drops, and it rounds the “knee” of its evolutionary track. As the planetary nebula disperses, the hot, degenerate remnant emerges from its cocoon and settles down as a cooling white dwarf.

While the nuclei of planetary nebulae are thought to be major contributors to the white-dwarf population of the galaxy, other evolutionary paths are also important for single stars. One example is the