by Stacy Palen.
Here is a nice little article from NRAO that corresponds to material in Chapter 13 of Understanding Our Universe and Chapter 18 in 21st Century Astronomy: https://public.nrao.edu/news/neutron-stars-fall.
Questions for Students:
- Make a sketch of this triple-star system to show how the three objects move in their orbits as time passes.
Answer: A sketch with a pair of stars in a small orbit around each other and the combined system making a much larger orbit around a third body.
Anne Archibald says that they can “account for” every pulse since they began their observations. What does she mean by that: does she mean they observed every pulse or they can calculate the time of every pulse?
Answer: The astronomers can calculate the time of every pulse.
Think back to Ole Roemer’s observations of the speed of light. Roemer observed that the moons of Jupiter passed behind the planet sooner than expected when Jupiter was closer to Earth in its orbit because light did not have as far to travel from Jupiter to Earth. In addition, he observed that the moons passed later than expected when Jupiter was farther from Earth in its orbit. That’s because light had a greater distance to cover. From this, he was able to measure the speed of light to fair accuracy. The experiment conducted in the article used a different type of “clock”, created not by orbiting moons, but by a rotating neutron star. Explain how the experiment described in the article is related to Roemer’s experiment. Remember, we now know the speed of light quite precisely.
Answer: This experiment solves the problem “backwards”. It used the known speed of light with the early arrival of a pulse to determine that the pulsar is closer. A late arrival means the pulsar is farther away.
“Gravitational binding energy” can be thought of as analogous to “nuclear binding energy”. Where in this course have you seen “nuclear binding energy”?
Answer: Nuclear binding energy appears in discussions of nucleosynthesis, the proton-proton chain, the CNO cycle and the enrichment of the galaxy in heavier elements.
Why is it important to test a scientific idea over and over again?
Answer: It’s important to repeatedly test a scientific idea because there may be limits in which the idea fails. These limits become more accessible over time as technology improves.
Suppose that the result had been different. Imagine if the neutron star fell differently than the inner white dwarf. What would astronomers conclude about Einstein’s Equivalence Principle?
Answer: Astronomers would conclude that the Equivalence Principle might be wrong for very dense objects. They would test this again in another system, if possible, as well as further test some of the alternative ideas mentioned in the article.
What other questions would you ask your students, based on this article? Feel free to leave suggestions in the comments!