Earth Science Essay

– Discuss stellar evolution (describing each stage in brief). What forces are opposing one another throughout the life of a star and how do they influence the various stages in the life cycle of a star Stellar evolution stars exist because of gravity. The two opposing forces in a star are gravity (contracts) and thermal nuclear energy (expands). Stage 1 Birth is where gravity contracts the cloud and the temperature rises, becoming a protostar. Protostars are a hypothetical cloud of dust and atoms in space which are believed to develop into a star. Astronomers are fairly certain of their existence. Protostars are formed about a million years after a gas clump from an interstellar gas cloud has started to rotate and from a disk. The protostar is simply the core of the disk that formed from the clump of gas that was compressed inside the gas cloud. The star becomes a stable main-sequence star, which are characterized by the source of their energy. They are all undergoing fusion of hydrogen into helium within their cores. The rate at which they do this and the amount of fuel available depends upon the mass of the star. Mass is the key factor in determining the lifespan of a main sequence star, its size and its luminosity. Stars on the main sequence also appear to be unchanging for long periods of time. Any model of such stars must be able to account for their stability. Ninety percent of a stars life is in the main-sequence. A red giant is a luminous giant star of low intermediate mass that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The burnout and death final stage of a star depends on its mass. After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is no longer any source of heat to support the core against gravity. Hydrogen burning continues in a shell around the core and the star evolves into a red giant. When the Sun becomes a red giant, its atmosphere will envelope the Earth and our planet may be consumed in a fiery death. Meanwhile, the core of the star collapses under gravity’s pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and  the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking â€Å"planetary nebula†, much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star. Reference Lutgens, F. K. & Tarbuck, E. J. (2011). Foundations of earth science (6th ed.). Upper Saddle River, NJ: Prentice Hall ES 1010, Unit 8, Question 12 – How do we calculate or determine the distances to stars? What units do we use and what are the limitations (if any) of the method used for such calculations? Measuring distance to stars has been considered a very difficult task. Stellar parallax is a method used to determine distance, the extremely back and forth shifting in a nearby star’s apparent position due to the orbiting motion of earth. The farther away a star is, the less its parallax. The light year is a unit used to express stellar distance, which is the distance light travels in a year, which is approximately 9.5 trillion kilometers (5.8 trillion miles). The parallax angles are very small. Proxima Centauri is the parallax angle nearest to the star. It is less than one second or arc, which equals 1/3600 of a degree. A human finger is roughly 1 degree wide. The distances to stars are so large that conventional units such as kilometers or astronomical units are often too cumbersome to use. Some limitations are that parallax angles of less than 0.001 arcsec are very difficult to measure from Earth because of the effects on the Earth’s atmosphere. This limits Earth based telescopes to measuring the distances to stars about 10.01 or 100 parsecs away. Spaced based telescopes can get accuracy to 0.001, which has increased the number of stars whose distance could be measured with this method. However, most stars even in our own galaxy are much further away than 1000 parsecs, since  the Milky Way is about 30,000 parsecs across. Reference Lutgens, F. K. & Tarbuck, E. J. (2011). Foundations of earth science (6th ed.). Upper Saddle River, NJ: Prentice Hall