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Professor Questions Essay 1. Discuss how astronomers compare the stars. There may be several terms astronomists use to compare stars, including luminosity, flux, energy flux, apparent magnitude and absolute magnitude. Luminosity is the total amount of energy given from a body every second, measured in Watts Energy flux is the flow of energy out from a particular surface area and is measured in Watts per meter square. Observed flux – As the radius of the circle placed around the radiating object increases, the flow of energy per meter square would decrease.    The observed flux would be higher if the radius of the circle is less (inverse square law) Apparent Magnitude is the amount of light that is received from a particular star.   It is determined using the luminosity of the star and the distance from us.   The apparent magnitude would be low for a brighter object. Absolute magnitude is the amount of light that is received from a particular object when the object is placed at a distance of 10 par secs (AB’s Astronomy Lab, University of Columbia, 2002). 2. From nebulae, stars are formed. Discuss the factors that affect the gestation time for the birth of protostars. The nebulae from which clouds form are nothing but clouds of dust and gas.   They slowly begin to contract to begin the process of star formation.   The mass is known as the ‘protostars’ and it would continue to condense and slowly heat up.   Once it reaches a critical mass phase, the nuclear fusion reaction takes place hydrogen atoms fuse together to form helium, two protons and tremendous amounts of energy.   Once this phase begins, the star is born. Factors that affect gestation time for the birth of protostars †¢ Gravity of the passing star (external energy to help star formation) †¢ Shockwave from a nearby supernova (external energy to help star formation) †¢ Heating by contraction (Giant molecular clouds need to contract to heat up and begin the thermonuclear processes) †¢ Magnetic fields available (helps in contraction) †¢ Rotation or angular momentum of the protostars †¢ Size of the star (smaller stars live longer than larger stars as they take longer to burn their fuel) (Sea Sky, 2008, UNT, 2008) 3. Discuss how a stars life affected by its `fuel`? Once the protostar gets heated up, nuclear fusion reaction begins (thermonuclear processes), and the star now enters its stable phase.   The star would be combating gravitational collapse by fusing atoms together and forming energy.   The star’s life span depends on how much of matter it actually contains.   The process of nuclear fusion in the stars would involve conversion of hydrogen to helium, helium to carbon, until iron is formed.   Once iron is formed, the energy required to fuse iron is much higher and hence the reaction stops.   Larger stars tend to use up larger amounts of fuel compared to smaller stars, and hence would live much shorter. Several large stars in the universe live for about a few hundreds of thousands of years.   As smaller stars would be spending their fuel much more slowly, they would live for billions of years.   Once a star’s fuel gets exhausted, it would transform into a red supergiant (expanding in size), and would be present for sometime.   The star would then collapse, as the force of the nuclear reactions is not effective to combat the force of gravity.   Some of the stars blow away, to form planetary nebulae.   The core of the star would contain some amount of fuel which would burn as white dwarf.   Once they completely burn away, they would remain as a dark ball known as a ‘black dwarf’.   Some massive stars may trigger a violent explosion, known as ‘supernova’ (Airty Nothing, 2008 Sea Sky, 2008). 4. Discuss basic ideas of the main sequence chart (Hertzsprung-Russell (HR) Diagram). The HR diagram is basically a graph in which the luminosity of the stars (absolute magnitude) is charted against the surface temperature (color).   This chart was discovered separately by Ejnar Hertzsprung and Henry Norris Russell in the 1910’s.   This was following several scientists query whether the temperature of the object could be determined from the color of radiation it emitted.