Image Credit: NASA/Bill Ingalls

By Stacy Palen

Don’t forget to remind your students to watch for the Lyrid Meteor Shower this month. The peak occurs around April 21-22.

This meteor shower comes as Earth passes through the debris left behind by Comet Thatcher. Particles lost from the comet continue to drift in the Solar System, gradually changing their position.

As Earth moves through space, it passes near the trajectory of the comet and runs into collections of these particles. This will happen repeatedly at particular times of the year as Earth returns to the same point in its orbit. The particles burn up, creating meteors as they fall through the atmosphere.

Comet Thatcher has a 415 year orbit, so it is a long-period comet. It will not be back in the inner Solar System until 2276.

To watch a meteor shower, go to a clear dark site where the horizon is not obstructed. Spend about half an hour in the dark, without your cell phone or other bright light in view. This will allow your eyes to adapt to the dark. Then just watch for meteors! They are best seen with the naked eye.

If you are careful and methodical, your observations can contribute to the study of meteors and meteor streams! To learn more, visit the Astronomical League’s Meteor Observing Program website.

By Stacy Palen

LIGO has been busy, and a newly released graphic summarizes many of the exciting discoveries the detector has made in concert with Virgo, its European counterpart.

Summary: Since 2015, the LIGO/Virgo collaboration has detected gravitational waves—ripples in spacetime caused by rapidly accelerating massive objects—from 10 stellar mass binary black hole mergers and one binary neutron star merger. Black holes and neutron stars are both forms of stellar remnants—the final stage of stellar evolution that a star enters when it has burned through its entire fuel supply. This graphic provides a great jumping off point for discussions about masses in the stellar graveyard.

Questions:

1. Consider the final masses of the black hole mergers (larger blue circles). What is the smallest merged mass?

Answer: About 19 solar masses.

2. Consider the masses of black holes that have been detected in X-rays (EM Black Holes, in purple). What is the largest black hole mass that has been detected this way?

Answer: About 23 solar masses.

3. Estimate the average mass of the black holes that have been detected in X-rays.

Answer: About 10 solar masses.

4. Estimate the average mass of the black holes that have been detected in gravitational waves.

Answer: This average looks to be about 25 solar masses.

5. Astronomers make the claim that they are detecting a “new population of black holes” with gravitational waves---—that is, that the type of black holes they are detecting now are different than the ones they were detecting before. Based on your answers to questions one through four, explain why they would say this.

Answer: Even though the two groups of black holes overlap in mass, gravitational waves are detecting more massive black holes, on average, than were detected with X-rays in the past.

6. Compare the number of EM black holes to the number of black holes (before merging) discovered with LIGO/Virgo. How much has LIGO/Virgo contributed to the total sample of known black holes?

Answer: LIGO/Virgo has nearly doubled the number of black holes that have been observed.

7. Is it reasonable, then, to compare the two populations (the pre-merger black holes from the LIGO/Virgo data and the X-ray black holes)?

Answer: Yes, statistically speaking, we know of about the same number of objects in each case.

8. Consider the masses of Neutron stars (yellow). What is the largest neutron star mass that has been detected with light (EM)?

Answer: About 2.1 solar masses.

9. Consider the masses of Neutron stars (yellow). What is the average neutron star mass that has been detected with light (EM)?

Answer: About 1.5 solar masses

10. Theorists predict that we would not expect to observe neutron stars with masses above about 2.14 solar masses. Are these observations consistent with that prediction? What do you think astronomers are wondering about the post-merger object resulting from the merger of two neutron stars?

Answer: The neutron stars observed with light are consistent, but the outcome of the neutron star merger is a little bit too massive. As of this writing, astronomers are still trying to figure out the form of that post-merger object. It could be a black hole, a neutron star collapsing to form a black hole, or a stable neutron star. More data are needed!

By Stacy Palen

Summary: The Kepler mission, after at least one resurrection, has finally come to an end. During its 9.5 year “lifespan,” Kepler discovered more than 2,500 planets around other stars and changed our minds about how common planets actually are.

Article: https://www.sciencenews.org/article/planet-hunting-kepler-space-telescope-dead?fbclid=IwAR0iYMK2_9-tbCgb91JxpFVpLR9MCOgRpC7BxodF69P45Hhtq2_trWv4_4I

Questions for Students:

1. Study the graph of Exoplanet Discoveries. The yellow dots show all the planets discovered by Kepler. Compare the sizes of these planets with those discovered before and after Kepler.

Answer: Kepler discovered smaller planets than those discovered before or after.

2. Study the graph of Exoplanet Discoveries. This graph shows that very few planets have been discovered with orbital periods smaller than one day. Why might this be?

Answer: This is as close as a planet can get, even to a small star, and still be in a stable orbit.

3. Study the graph of Exoplanet Discoveries. This graph shows that few planets have been discovered with orbital periods larger than about 300 days. Why might this be?

Answer: This could be a selection effect. Kepler uses the transit method to detect planets, but planets with large orbits are much less likely to cross in front of the star; our line of sight must lie exactly in the plane of the orbit to see the planet transit. The idea that this is a selection effect is supported by the observation that planets with long periods have been detected by other methods (the blue and gray dots), but not by Kepler.

4. Prior to the Kepler spacecraft, the percentage of stars with planets was unknown. Now that Kepler has completed its mission, do astronomers think this number is large, with many stars having planets or small with few stars having planets?

Answer: This percentage appears to be close to 100%. “…astronomers have used Kepler’s exoplanet haul to predict that every one of the hundreds of billions of stars in the Milky Way should have at least one planet on average."

5. Comment on the impact of the Kepler mission on the Drake Equation.

Answer: The second term in the Drake Equation is the fraction of stars with planets. This term is now quite likely to be nearly one, whereas before the Kepler mission, its value was only speculative.

By Stacy Palen.

Summary: A red dwarf star in the Milky Way barely contains any heavy elements at all. Its age is estimated at 13.5 billion years.

Article: http://www.astronomy.com/news/2018/11/red-dwarf-is-one-of-the-oldest-in-the-universe.

Questions for Students:

1. Why does the lack of heavy elements imply that the star formed very soon after the Big Bang?

Answer: Because since the Big Bang, stars have been making heavy elements and returning them to the interstellar medium. Young stars have more heavy elements than older stars.

2. Why do astronomers think there must have been at least “one ancestor” before this star formed?

Answer: Because it has some heavy elements in it.

3. How is the birth of this small star connected to the first generation of stars, which were probably ALL very massive?

Answer: Supernova explosions from those first stars could trigger the formation of smaller stars.

4. Where would this star lie on an H-R Diagram?

Answer: This star, because it is a very small red dwarf, would lie at the lower right on a H-R diagram.

5. This star is one-seventh (about 0.15 times) the mass of the Sun. Which of the following is a reasonable main sequence lifetime for a star with that mass?
a. 10 million years
b. 100 million years
c. 1 billion years
d. 10 billion years
e. 1 trillion years

Answer: e.

6. Astronomers can confidently state that all stars like this one (with similar mass) are still around, and none have died yet. Why can they state this so confidently?

Answer: Because 1 trillion years is a lot longer than the age of the universe.

by Stacy Palen.

In September of 2018, Jocelyn Bell Burnell won a $3 million prize in recognition of her outstanding discovery of pulsars. This article presents an opportunity to link science and society while recalling and applying information about radio telescopes, the motion of the sky, and pulsars.

Article: https://www.npr.org/2018/09/06/645257118/in-1974-they-gave-the-nobel-to-her-supervisor-now-shes-won-a-3-million-prize

Questions for Students:

1. It may be difficult to visualize the data Bell Burnell was taking from the radio telescope. The chart recorder used to record the data is very similar to a seismometer, a machine that records earthquakes. The radio telescope chart recorder scrolled through 96 feet of paper every day. How much paper did Bell Burnell use for the month of observations between when the blip vanished and when it returned?
   
   Answer: 30 days * 96 feet per day = 2,880 ft
   
   
2. Why did Hewish think the signal must be man-made?
   
   Answer: He thought it must be a man-made radio interference because the signal disappeared and then reappeared.
   
   
3. Bell Burnell figured out the signals were coming from space. What observation about the pulses led her to that conclusion?
   
   Answer: Bell Burnell observed that the source moved at the same speed as the stars.
   
   
4. Prior to Bell Burnell’s discovery, astronomers thought that neutron stars might not be observable. Why might neutron stars be difficult to observe?
   
   Answer: Neutron stars might be difficult to observe because they are incredibly small. Even if they are very hot, they will not be very bright.
   
   
5. What is it about Bell Burnell’s discovery that earned her the Breakthrough Prize?
   
   Answer: No one had ever dreamed that an object could act in this way.
   
   
6.  Some people in the scientific community see this award as righting a long-standing wrong. Does Bell Burnell see it that way?
   
   Answer: No, actually. She seems to be perfectly fine with it. But then, she’s giving all the prize money to promote diversity and fight unconscious bias. So maybe she’s just being graceful.
   
   
7. According to Bell Burnell, why did she not receive the Nobel Prize in 1974?
   
   Answer: Bell Burnell says that at that time, the committee was not awarding early career scientists.
   
   
8. Do you think that was a fair decision of the Nobel committee?
   
   Answers will vary.
   
   

Share your own questions in the comments!

by Stacy Palen.

Until the result discussed in the article linked below, only two distinct neutrino sources were known: the Sun and Supernova 1987a. Now there is a third: a distant blazar.

This article complements material about active galactic nuclei, neutrinos, scientific instrumentation, and the process of science. Following are some questions that I thought of as I read the article. Share your own (with answers!) in the comments.

Article: https://www.eso.org/public/blog/pinpointing-the-source/

1. What is a blazar?
   
   Answer: A blazar is a particular kind of active galaxy in which the jet points at Earth.
   
   
2. A blazar is a little bit similar to a pulsar, but not exactly the same. Compare and contrast the two objects.
   
   Answer: A blazar is detected by the emission coming from its jet. In this way, it is something like a pulsar, which is observed when its jets point toward Earth. The pulsar, however, is much smaller and spins rapidly so that the jet points toward Earth only some of the time.
   
   
3. You have learned that there are many, many neutrinos passing through a human body in one second: 100 trillion, just from the Sun. How many neutrinos were detected from this blazar?
   
   Answer: Only one neutrino was detected! It is somewhat surprising that one neutrino out of so many can be important.
   
   
4. How many different regions of the electromagnetic spectrum were observed in this project? What are they?
   
   Answer: Five regions of the electromagnetic spectrum were observed: gamma ray, radio, infrared, optical, and X-ray.
   
   
5. This discovery is an example of what astronomers sometimes call "multi-messenger astronomy." What do they mean by that? If the neutrino had not been detected, would the discovery still be "multi-messenger?"
   
   Answer: Multi-messenger astronomy means that astronomers are getting information about an object from light (electromagnetic radiation) AND another source, like neutrinos or gravitational waves. If the neutrino had not been detected, this would not have been multi-messenger because all the other detections were made from observations of light.
   
   
6. Why has a blazar like this never been discovered before? Do you expect to see more discoveries in the future? Why or why not?
   
   Answer: Astronomers did not have equipment capable of discovering these neutrinos until IceCube became operational just a few years ago.
   
   
7. This discovery took many people working together, at many different facilities. The end of the article focuses on some of the difficulties and advantages of this approach. Describe one difficulty and one advantage of involving many scientists, particularly different kinds of scientists, in a scientific project.
   
   Answers will vary.

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 21^st Century Astronomy: https://public.nrao.edu/news/neutron-stars-fall.

Questions for Students:

1. 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.

2. 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.

3. 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.

4. “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.

5. 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.

6. 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!