EDUCATIONAL ACTIVITIES - EPOCH 3

A Spectroscopic Eclipsing Binary System (PDF file format—requires Adobe Acrobat Reader)

Sometimes two stars are closely associated with each other in binary systems. In some cases, the separation between stars in a binary system may be so small that the stars cannot be seen as separate entities; rather we see the combined light from both stars. On the other hand, however, since the stars are relatively close together, their relative velocities will be rather high. If the orbit of such a system is more or less edge-on to the Earth, radial velocity variations may be noticed as first one star and then the other moves towards and away from us. These orbital motions contribute to the Doppler effect. At times, one star may be moving away from the Earth faster than the other star. This produces a greater redshift and results in a splitting of the spectral lines into two slightly displaced components, one from each star. When one star is either in front of or in back of the other star, both stars have the same recession velocity and the Doppler shift is the same for both. So, as the stars orbit, the double spectrum collapses to a single spectrum - the mark of a spectroscopic binary system. The following activity uses the Doppler shift of two stars in such a system to determine their separation and mass.

Stellar Evolution (PDF file format—requires Adobe Acrobat Reader)

The evolution of a star depends upon its mass at birth. The mass determines where on the main sequence of the H-R diagram a star is located - and its mass and position tell the story of its evolutionary history once its nuclear fires sputter and die and the star leaves the main sequence. Some stars die a spectacular death - literally blowing themselves apart in spectacular supernovae explosions, others throw off planetary nebulae, and some show no evidence of their final demise. The mass of a star at birth pre-determines the type of death and final product after leaving the main sequence. This activity presents you with the opportunity to arrange two groups of images; one set representing the stages in the life and death of sun-sized star, and the other a massive star, in a sequence from birth to final end product.


 

Questions for discussion or short essays:

Students will find these exercises akin to a pre-test, largely evaluating one's understanding either based on prior knowledge or on the brief presentation in the Prologue. They are therefore meant to provoke critical thinking. Much more information regarding each of these questions will be developed in greater detail in the principal epochs of this Web site.

 

1. Name some of the agents at work against the inward pull of gravity when a star initially forms from an interstellar cloud.

 

 

 

2. How many atoms must come together for gravity to hold a star together? (a) 57 (b) 570,000 (c) 1057 (d) 10570

 

 

 

3. At what point does a protostar become a star? That is, what is the defining characteristic of a star, as opposed to a protostar or a planet?

 

 

 

4. Using an HR diagram, describe what happens during the Kelvin-Helmholtz and Hayashi contraction phases. Why do they run diagonally opposite on the HR diagram?

 

 

 

5. Explain why Jupiter is almost, but not quite, a star. Could Jupiter be plotted on an HR diagram?

 

 

 

6. Explain the process of triggered, or sequential, star formation. What types of phenomenon can cause such a scenario?

 

 

 

7. How hot does the core of a Sun-like star have to be in order to initiate hydrogen, then helium, burning? Why does it have to be so hot?

 

 

 

8. Describe the contraction-heating-fusing cycle that gives rise to the creation of the heavy elements in stars.

 

 

 

9. What happens to stars with masses (a) much less than the Sun, (b) about the same as the Sun, and (c) much greater than the Sun once they leave the main sequence?

 

 

 

10. Identify and briefly explain the stage of stellar evolution associated with each of the following observations: (a) optical emission from a bright, hot star and its surrounding three-dimensional shell of glowing gas; (b) x-ray emission from a region near an ordinary star not normally hot enough to emit x-rays on its own; (c) infrared emission from a dark, lukewarm blob embedded within an interstellar cloud of gas and dust; (d) bursts of radio and optical emission from a compact region at the center of an expanding diffuse cloud of gas; (e) ultraviolet spectral features showing evidence for strong "winds" emanating from a young star.

 

 

 

11. Compare and contrast the physical characteristics of a white dwarf and a neutron star: What are they made of? How big are they? How dense, hot? What process might make each of them?

 

 

 

12. Draw to scale the typical sizes of a red-giant star, a normal main-sequence star, a white-dwarf star, and a neutron star. How would the size of the Earth compare?

 

 

 

13. You are a space cadet aboard a galactic exploratory ship. While you were on a space walk, you mistakenly become trapped by the gravitational pull of a black hole. Being the good space explorer that you are, you decide to utilize this rare, if unfortunate, opportunity for the benefit of science—you radio back to the ship and describe what you see and experience on your way down. What do you tell them? What do they hear? Does anything special happen at the event horizon?

 

 

 

14. Compare and contrast chemical reactions from nuclear reactions, giving an example of each. What are the similarities? What are the main differences?

 

 

 

15. Carbon, nitrogen, and oxygen are three of the elements most important to life as we know it. By what astronomical processes were they formed?

 

 

 

28. Compare and contrast the s-process with the r-process. Which elements are they responsible for creating? What are the major differences between the two processes? What observational evidence do we have for the events?

 


 

 

True or False:

1. True or False: A star's central temperature is usually cooler than the surface temperature. _____ Explain your answer.

 

 

2. True or False: Generally speaking, the Sun is more luminous than a typical protostar. _____ Explain your answer.

 

 

3. True or False: Stars with much more than the mass of the Sun take longer to become main-sequence stars. _____ Explain your answer.

 

 

4. True or False: In order to be able to start the contraction process leading to star birth, an interstellar cloud must be very hot. _____ Explain your answer.

 

 

5. True or False: A high-mass star spends most of its existence in the throes of its violent death. _____ Explain your answer.

 

 

6. True or False: The "helium flash" lasts up to 10 million years. _____ Explain your answer.

 

 


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