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February 2020

Current Events: 7 billion-year-old stardust is the oldest stuff on Earth

By Stacy Palen

I recently stumbled upon this article from The Washington Post about stardust on Earth. Mineral dust in the Murchison meteorite shows traces of neon produced by cosmic rays as the dust traveled through space. The abundance of neon atoms indicates that the dust was formed 7 billion years ago—before the Sun formed.

Here are some questions to ask your students based on the article:

1) What produces neon atoms in grains of interstellar dust?

Answer: Cosmic rays smash into the grain and convert silicon into neon.

 

2) How does the rate of cosmic rays striking the dust change with time?

Answer: It doesn’t. This rate is constant.

 

3) Suppose that one grain of dust has twice as much neon as another grain. What can you conclude about the relative time each grain spent in space?

Answer: The one with twice as much neon was out there twice as long.

 

4) In your own words, describe how astronomers determine the age of a grain of interstellar dust.

Answer: Astronomers count the number of neon atoms and compare that number to the number of neon atoms in a grain of known age. If there are more neon atoms, the dust grain was roaming the galaxy longer.

 

5) How old is the Sun, and how do we know?

Answer: The Sun is about 4.5 billion years old. We know this from measuring isotope abundances in moon rocks.

 

6) Are these dust grains older or younger than the Solar System?

Answer: These dust grains are 2.5 billion years older than the Solar System.

 

7) Is this result consistent with the idea that stars recycle material from the interstellar medium when they form? Explain.

Answer: Yes! Because the Sun and planets formed from material lost from earlier stars (we know this because of the abundance of other materials. Some of that material is still floating in the Solar System, and some of it was lost from stars that died long before the Solar System formed.


Classroom Stories: Another Way to Do the Phases of the Moon

By Stacy Palen

The phases of the Moon are one of those topics that has been extensively studied by the astronomy education research community and is well-known to be more complex than most people think. There’s the change of perspective from Earth-view to space-view. There are multiple motions at once (the rotation of Earth and the Moon, and the revolution of the Moon around Earth). There’s the issue about light rays always traveling in straight lines and not bending. It’s complicated.

Last week, I pulled an old phases-of-the-Moon activity out of the archives, which can be accessed by clicking here, for my students to complete in addition to the activity, “Studying the Phases of the Moon” from the Learning Astronomy by Doing Astronomy workbook. This is not an appropriate activity for Learning Astronomy by Doing Astronomy because it requires students to have Styrofoam balls that have been colored black on one half. (I can’t make the classroom dark, so I can’t use the traditional “balls-on-sticks” approach.) But one thing that I like very much about this activity is that it leads them to figure out how to (approximately) tell time by the Moon, which means that I can ask them a question about it on my zombie-apocalypse midterm—insert evil laugh here!

The activity also asks them to consider the phases of other objects, such as the phases of Earth as seen from the Moon, or the phases of Deimos as seen from Mars or Phobos. Carrying the concept of phases away from Earth seems to help cement the idea that this is a phenomenon that is all about the relative location of the light source and the observer.

I followed this activity the next week with the “Studying the Phases of the Moon” activity from the workbook. I was interested to notice that students finished the activity in record time and were much better prepared for it. The two activities worked well together to really build their picture of how the phases of the Moon actually occur.


Classroom Stories: Energy and Kepler’s Laws: A Surprise for Me about Where the Difficulty Lies

By Stacy Palen

Recently, my students worked on the “Working with Kepler’s Laws” activity from the Learning Astronomy by Doing Astronomy workbook. In this activity, students learn about ellipses, consider the “simple” version of Kepler’s second law (a planet travels faster when nearer to the Sun and slower when farther away), and run some numbers for Kepler’s third law: P2=a3. To my surprise, Kepler 1 and Kepler 3 brought almost no questions from the students (aside from “Am I doing this right?”). It was Kepler’s Second law that brought the most substantive questions.

Over and over they asked “Yes, but WHY does it go faster when it’s closer?”

I used this question as the basis for a whole new activity.

Approaching this question as an energy problem, I had the students throw a ball straight up in the air and make pie charts representing how much kinetic energy, gravitational potential energy, or thermal energy the ball had at various points in its trajectory. Then they threw the ball to a friend and made similar pie charts (in this case the velocity is never zero, so the kinetic energy is also never zero). Then I had them consider a planet in orbit around the Sun and make a third set of pie charts.

Wow! This was so much harder for them than I expected!

First, it turned out that pie charts are a concept that most (but not all) of my students have in common. Who knew?

Second, we ran into the issue about where to put the “zero” of gravitational potential energy. This information was in the Background section, so it was invisible.

Third, we faced our biggest issue: Convincing students that when they threw the ball straight up into the air, the ball had zero speed at the apex of the trajectory. That alone was a 20-minute conversation!

Finally, even though I told them to describe what happens to the ball between the moment after it left their hand to the moment before they caught it, many students turned all the energy into thermal energy. I’ve edited the activity to try to correct these problems and will use it again in the fall in search of perfection.

Despite these problems, I was very happy about the conversations that I overheard as I moved around the room. Some students were completely unfamiliar with the conservation of energy. They made progress simply by learning how the energy transformations occur for a ball thrown in the air!

Other students rocked that part but were stuck when the questions about orbits showed up; this was often because they drew the Sun at the center of the orbit instead of at a focus. What a great opportunity to correct this problem!

Finally, some students spent a very long time arguing about whether they needed to account for energy lost to thermal energy in our current Solar System.

Overall, I was pleased by what I learned about how they think about energy as well as how well they grappled with this material. And I’ve now set them up to have a spark of recognition when they learn about planet migration later in the semester. This activity is a work in progress, but I will definitely try it again!

You can access the activity by clicking here!


Classroom Stories: Classroom Calculators

By Stacy Palen

Here’s the thing: all students have a calculator in their phone. And for a long time, I've thought, “They should use the calculator in their phone so they know how to use the calculator in their phone!” But here’s the other thing: a lot of those calculators are terrible. They don’t all do the order of operations the same way. They don’t all have the same “buttons” on them. They don’t all use the same notation. Therefore, any time I have students do any math at all in the classroom, I spend most of the time running around and helping them figure out how to put the “times ten to the” into their calculator. iPhone calculators are pretty good, but Samsung calculators don’t have the same functionality. Students must painstakingly type “(3 X 10 ^ 8)” rather than “3EE8.” That may not seem so bad, but if they forget the parentheses, the calculator doesn’t see their input as one number. So, if the problem includes division, the student is stymied. In addition, students' having their phone in their hand is distracting to the point of incompetence.

This semester, I had the idea to invest in a classroom set of calculators. I found a fairly simple solar-powered calculator that I could buy for less than $7, and I begged the chair of my department to use some of our lab fees to buy 60 of them.

When we have an activity involving a math problem, I invite students to borrow one, and I use the document camera to show them exactly how to punch things into the calculator. For Kepler’s Third Law, I show them how to square a number and how to take a cube root. For multiplying powers of 10, I show them how to put in “3 X 108” so the calculator interprets it as a single number (3EE8 or 3EXP8).

So far, this has been revolutionary. I spend far less time helping students with their calculators and far more time helping them think about their answers. Students can now help each other with the calculators, too, and they don’t need to wait for me to come around to them. Generally, this seems to be helping them be more patient.

Even for the many students who have their own favorite calculator, they sometimes don’t know how to use it for our specific purpose—it’s set up for stats, for example. While they have the option to borrow a calculator or use their own, I still spend some time helping these students find the “EE” or “EXP” key for scientific notation, but if they otherwise know how to use the calculator, they seem to remember this new function more easily. Looking back, I estimate that I could have bought only half as many calculators for the 70 students in the room, and even fewer if they work in pairs.

As I go along, I’m compiling a list of calculator instructions that I can print and tape to the cover. I may make a large poster of this information, instead, which might work better once we are back in our usual teaching space.

I have been pleasantly surprised by how much easier it is to manage the classroom when all my students have the same tool. In retrospect, it seems obvious that this method would be easier, but it took me nearly 20 years to think of it…