Current Events: Astronomers Discover Record-Breaking Star As Small As the Moon but with More Mass Than the Sun

By Stacy Palen

Astronomers set a new record for the densest known white dwarf.

Below are some questions to ask your students about this article.

1). How is a white dwarf typically formed?

Answer: A star less than about 8 times the mass of the Sun loses its outer layers, leaving behind a dense carbon core. (Note: This is NOT in the article, but they should know it from class.)

2). How might a system of two white dwarfs form?

Answer: If a binary star system consists of two stars less than 8 times the mass of the Sun, and both stars evolve, the system will become a binary white dwarf system.

3). The lead author states, “Smaller white dwarfs are more massive.” Is this how normal matter (like cows and people) behaves? What is the name for matter that behaves this way?

Answer: No; degenerate matter.

4). How massive is the combined star?

Answer: 1.35 times the mass of the Sun.

5). Why did this pair of white dwarfs not become a supernova when they merged?

Answer: Because this combined mass is under the Chandrasekhar limit.

6). What comes next for this white dwarf?

Answer: It may become a neutron star, as it is dense enough for charge destruction to take place.

7). How often does a merger like this happen?

Answer: We don’t know, but they are probably common, if there is one nearby.

8). This white dwarf has an extreme magnetic field. How did it develop such a strong magnetic field?

Answer: Nobody knows!


Current Events: "There May Not Be Conflict After All" in Expanding Universe Debate

By Stacy Palen

The two main ways to measure the expansion of the universe have turned in different answers. For the last several years, astronomers have been arguing about whether this disagreement is unimportant (a result of measurement errors) or important (a result of new, unknown physics). This is a terrific example of the process of science. In June 2021, Wendy Freedman published a new review paper arguing that there is not a conflict after all.

Below are some questions to ask your students based on this article.

1). What is the Hubble constant?

Answer: The Hubble constant measures the rate at which the universe is expanding.

2). What are the two ways to measure the Hubble constant?

Answer: The first way is to look at the cosmic microwave background. The second is to measure the velocities and distances of galaxies to make the Hubble law graph.

3). What are the two values of the Hubble constant derived from these two methods?

Answer: 67.4 km/s/Mpc and 72 km/s/Mpc.

4). Historically, the distance to nearby galaxies was determined using Cepheid variables. What is the problem with these measurements?

Answer: They are noisy and more complicated, and the observations may be contaminated.

5). What other objects are now being used to measure the distance to nearby galaxies?

Answer: Freedman is using red giant stars, which always reach the same peak brightness before fading. These observations are less noisy.

6). What does this new method of measuring distances give for the value of the Hubble constant?

Answer: 69.8 km/s/Mpc.

7). Is this new method in better or worse agreement with the method that uses the cosmic microwave background?

Answer: This new method gives a value that is much closer—it cuts the disagreement in half.

8). Review the Scientific Method flowchart in Chapter 1 of the textbook. What part of the flowchart describes the science that’s described in this article?

Answer: The science in the article is on the loop on the left. Previous tests did not agree, so a new hypothesis was suggested (that Cepheid variables are subject to too much noise), and a new experiment (measuring red giants) was devised and performed. Further tests will be in the loop on the right.


Current Events: The Milky Way's Shredded Companion Provides Clues about Dark Matter

By Stacy Palen

The Milky Way is tearing apart the Sagittarius dwarf galaxy. Gaia observations of the resulting tidal streams permit sufficient accuracy to detect the influence of the Large Magellanic Cloud on the interaction. Simulations show that the Milky Way’s dark matter halo is complex.

Below are some questions to ask your students based on this article.

1). Prior to this study, what was known about the shape of the dark matter halo?

Answer: Not much. Different simulations found various shapes, some more symmetric than others.

2). In this study, what is new that allows astronomers to build a more detailed picture?

Answer: The Gaia satellite pinpoints the location of stars with truly unprecedented accuracy.

3). The following questions cannot be answered directly from the article but can be answered with a Google search. The paper discussed in this article was posted to the preprint server arXiv. What is a preprint?

Answer: It is a full draft that has not been peer reviewed.

4). In the process of scientific publication, what comes after the preprint stage?

Answer: The article undergoes formal peer review, and then it might be published.

5). What is the advantage of preprints?

Answer: They give access to the most up-to-date research.

6). Should you treat the results discussed here with more or less skepticism than the results published in a peer-reviewed journal?

Answer: You should treat them with more skepticism.

7). How could you know if the results held up to peer review?

Answer: You could search for the authors’ names or the topic in a database of journal articles.


Classroom Stories: Teaching Interplanetary Distances Using a Human Solar System

By Michael Dunham (State University of New York at Fredonia)

When I first started teaching my Astro 101 course, one of the concepts that I struggled to properly convey to students was the immense change of scale in our Solar System between the spacing of the terrestrial planets and the giant planets. Simply giving the numbers did little to properly convey the spacing to students not used to quantitative thinking. Diagrams also did little to help since, in order to fit a properly scaled Solar System onto it, the terrestrial planets are placed so close together that they are practically indistinguishable. Out of necessity, then, a new idea was born. Here I will describe an activity that I developed to better convey to students the true structure of our Solar System.

When we are just about to start discussing the Solar System, I take the entire class outside for one 50-minute lecture. We first meet in the classroom, where I have an inflatable Solar System consisting of beach ball-sized planets (along with the Sun, Pluto, and the Moon). I ask for 11 volunteers to each take one of the Solar System components, and then we all head outside to an area of the campus that has a straight sidewalk approximately 600-feet long. Before heading out, I do emphasize to students that they should pay careful attention to what they see, as there will be a graded assessment at the end. I have found that it really is necessary to say this, otherwise some students will treat this as a social hour and not pay attention to what they are supposed to be learning.

Once we are outside, I ask the volunteer holding the Sun to stand at the beginning of the sidewalk, and I tell the class that, using a scale where 15 feet is equal to 1 astronomical unit (AU), we are going to place each planet at its proper distance from the Sun. It is worth noting at this point that, while I use an inflatable Solar System that I purchased online, you could just as easily adapt this activity to have students wear planet name tags rather than hold inflatable planets.

Using a 25-foot tape measure that I extend along the ground, I ask the volunteers holding the terrestrial planets to stand at the 6-foot (Mercury), 11-foot (Venus), 15-foot (Earth), and 22.5-foot (Mars) markings on the tape measure. By this point, many students expect that we will place the remaining planets at similar distances and be finished with the activity within the next few minutes. I then announce that, at an average orbital distance of 5.2 AU, Jupiter is located 78 feet from our Sun (or 55.5 feet from Mars). This requires us to move our 25-foot tape measure three times (once to measure the first 25 feet past Mars, once to measure the 50 feet past Mars, and a third time to measure the last 5.5 feet to get to Jupiter).

By the time we place Jupiter, we only have three planets left. However, since Jupiter is at approximately 5 AU from the Sun, whereas Neptune is located at approximately 30 AU from the Sun, we have only traveled one sixth of our total distance. The last three planets end up being placed at 142.5 feet (Saturn), 288 feet (Uranus), and 451.5 feet (Neptune) from the Sun, requiring us to move the tape measure many times. If you decide to place Pluto as well, it ends up being placed at a distance of 592.5 feet from the Sun.

Once we have placed all the planets, I ask the remaining students to spend some time walking back and forth in order to truly take in the differences between the inner and outer planets. The terrestrial planets are so close together that the students can almost reach out and touch each other, whereas Uranus and Neptune are so isolated that the students holding them struggle to hear each other, even when shouting at full volume. I also ask the remaining students to take over for those holding the Solar System objects, especially the Sun and the terrestrial planets, so the volunteers can have a chance to walk down the sidewalk and truly appreciate how far away the giant planets are. Last, I tell all my students to be back in the classroom for the remaining 10 minutes  of class.

Once students have returned to the classroom, I give them the following prompt:

Take out a blank sheet of paper and write your name on it. Then sketch a to-scale diagram of the Solar System, including (at minimum) the Sun and all 8 planets. (You don’t have to label the planets, but you do have to include all 8.) You are not graded on your artistic talent, but you must make a legitimate attempt to show, to the best of your ability, properly scaled distances between the planets. Once you finish, turn in your sketch and you are free to go.

Students absolutely love this activity. It scores very highly in evaluations where I ask students to rank their favorite and least favorite class activities. This activity gets students out of their seats and outside, and it teaches them about the Solar System in a memorable, lecture-free manner. The sketches that students turn in demonstrate that the scale of the Solar System has really sunk in, and high average scores on a nearly identical final exam question 2.5 months later demonstrate that this lesson has not been quickly forgotten. Although class time is very precious and it is always hard to “give up” 50 minutes of lecture, my informal assessments have convinced me that this activity is worth the time it takes.


Current Events: First Results from Mars InSight

By Stacy Palen

The Mars InSight lander is using marsquakes to probe the interior of Mars. In July 2021, the first clutch of papers on the results were published.

Below are some questions to ask your students based on this article.

1). What is a marsquake?

Answer: It’s like an earthquake, but on Mars. While on Earth, quakes are caused by the movement of tectonic plates, on Mars, quakes are caused by stresses as the planet cools.

2). How many marsquakes had InSight observed as of the date of this article?

Answer: 733, but only 35 of them provided data for the papers discussed here.

3). How many interior layers was Mars predicted to have?

Answer: Mars was predicted to have three layers: a crust, a mantle, and a core.

4). How many layers were found by Mars InSight? Were they as predicted?

Answer: Three layers were observed. The core was the size that was predicted, but the crust was thinner than expected. Logically, we conclude that the mantle would be thicker than expected.

5). Were there any other surprises in the observations?

Answer: Yes. The biggest quakes come from one area: Cerberus Fossae, which has “recently” been volcanically active. But no quakes have been observed from the giant Tharsis region, which might be a result of Mars InSight’s location in the “shadow” of the core.

6). Is the mission still ongoing, or has Mars InSight finished its work?

Answer: The mission is still ongoing.


Current Events: Venus Lacks Plate Tectonics, But It Has Something Much More Quirky

By Stacy Palen

A reanalysis of Magellan images has led to the hypothesis that Venus has “campi,” or blocks of rock that float on the mantle, shimmying and bumping into each other like packs of ice.

Below are some questions to ask your students based on this article.

1). Describe plate tectonics on Earth.

Answer: On Earth, a small number of very large plates float on top of the mantle, bumping into each other, sliding under or along each other’s boundaries, and creating geological features.

2). Why is liquid water required for plate tectonics?

Answer: Water lubricates the plates, permitting them to break bend and flow.

3). What happened to Venus’ liquid water?

Answer: It was lost during some kind of apocalyptic event that heated Venus to temperatures too high for liquid water to persist. This event happened about a billion years ago.

4). How is the process with campi, described in the article, different from that of plate tectonics?

Answer: Campi don’t flow past, rise over, or slide under each other. The campi are much smaller than the tectonic plates on Earth.

5). What is the evidence that this process might still be ongoing?

Answer: The observed campi are in the lava-covered lowlands, which are geologically young.

6). How will scientists explore whether this process is still actually occurring?

Answer: Several spacecraft are heading to Venus over the next few years. These spacecraft have higher-resolution radars than Magellan's and will compare the current positions of the campi with the positions observed by Magellan.


Classroom Stories: Teaching Climate in Astronomy Class

By Stacy Palen

This year, in particular, feels like a year in which we might be able to move the needle a little bit on the public understanding of climate change. The effects are starting to capture the attention of ordinary citizens who are infinitely distracted by…everything. Between the fires in the West, the extreme heat, and Hurricane Ida, ordinary citizens are starting to wake up to the fact that climate change matters to them.

Climate change is a thread that runs through my astronomy class, with a day devoted to it during my discussion of planetary atmospheres, and a lengthy revisit to it in our astrobiology discussion. But I also mention it when we talk about telescopes and atmospheric opacity (if the IR light can’t get down to the ground, it can’t get out to space, either, which is interesting because it has consequences that we talk about later in class). And I mention it when I talk about molecular bonds. And I also talk about it when we talk about “going to Mars” and whether there are fossil fuels there. In fact, I mention it matter-of-factly every time I see a connection that even remotely makes sense.

I also happen to teach a more advanced course in which we discuss climate and energy issues in gory physical detail, which means that I’m always looking for simulations and interactive sites that I can build activities around for students to use as they develop an intuition for the scope and complexity of the problem. (These activities often don’t make it into my Norton textbooks because they use resources we don’t control, so I can’t rely on them to be available, or to work the same way, for more than a semester at a time.) 

Earlier this year, the Climate Reality Project pulled together six of these interactive tools, with explanations about each. I found the list useful, and it might be useful to you, too!

Additionally, I have used the En-ROADS climate simulator for years, and it keeps getting better and more powerful (although that also means more complicated). I have an activity where students work in groups to negotiate how to adjust the world economy to try to control climate change. Hilarity, and sometimes intense arguments, ensue. Sometimes, they mention that this is one of the most meaningful activities of the whole semester because it reveals how complex these issues are.

I have also used the Climate Time Machine, which makes it easy to run as a demonstration during lecture. You slide the slider to see, for example, the impact of sea-level rise on various geographic areas.

There’s also a very nice Footprint Calculator on the list. There are lots of these around, but this particular one runs by sliders and dials, which makes it simple to use in a classroom situation where you don’t want students to get stuck on the details of one particular issue. The calculator ends by answering two questions: a). “On what day of the year have you used up your share of resources?” and b). “How many Earths would we need if everyone lived like you?” This offers a really great framing of the subject that students intuitively understand. If the answer to a). is before December 31, and the answer to b). is more than one Earth, then we’re in big trouble.

Feel free to check out these interactives and let me know if you end up using any of them in the classroom! I’m always looking for new ideas to help make this issue more concrete for students, and I always hate to leave them feeling helpless and wondering, “Yes, but what can I do?” These interactives help them find a meaningful way forward.


Classroom Resources: Two Active-Learning Explorations for Introductory Astronomy

By Tabitha Buehler (University of Utah)

I consider my introductory astronomy class (The Universe) at the University of Utah to be an active-learning class. To me, this means that my students don’t sit and passively listen to a lecture for the entire class period—I sprinkle in activities that engage their senses besides hearing among short bits of lecture. Alongside others, these activities include two-minute writing reflections, think-pair-share clicker questions, group worksheet activities, and get-up-out-of-your-chair-and-do-something activities. My students and I particularly enjoy the latter type, and I try to incorporate them when I feel that we have the time. Two of these activities are what I would call explorations—one of which examines the Earth-Moon distance, and the other the H-R Diagram.

Earth-Moon Distance Exploration

I like to both begin and end the semester of my class with the theme, so eloquently stated by Douglas Adams, that “space is big.” In class, we discuss unfathomably large (and, sometimes, small) sizes and distances that it really gets near impossible to have a feel for the true scale of things. This first-day-of-class activity is an attempt to begin to impress upon students the scale of astronomical sizes and distances.

Supplies needed:

Intended Learning Outcome:

  • Relate the size of the Earth, the size of the Moon, and the Earth-Moon distance

I divide my class into groups of four and give each group a cutout of the Earth and a cutout of the Moon. I ask them to do this activity without looking anything up online. I tell my students that the sizes of the Earth and Moon are at the correct scale with respect to each other. I give the groups about three minutes to guesstimate how far apart they think the pair of worlds is with respect to their sizes and to place them at appropriate distances apart somewhere in the classroom. I get a wide range of distance estimates, and, after the students look around the room at the guesses of all of the groups, they are curious to know the answer! I then reveal that the Moon is about 30 Earth-diameters away from the Earth and ask for a volunteer group to set their Earth and Moon at this distance for everyone to see. Most students are surprised at how far apart the Earth and the Moon are.

For remote instruction during the COVID-19 pandemic, I adapted this activity in such a way that it would also be useful to an instructor who wanted to devote less time to it or did not have a large enough space for it. For the adaptation, I created a multiple-choice question that could be used as a think-pair-share activity or could still be discussed and answered by a four-person group. The question asks, “Which of the following images (Figure 1, Figure 2, Figure 3, and Figure 4) best represents the Earth-Moon distance?”

H-R Diagram Exploration

My motivation behind creating this activity was to allow students to discover the elegance of the H-R diagram. This was partly because I wasn’t detecting the same level of excitement from my students that I feel (and try to share) when we would discuss the H-R diagram.

Supplies needed:

  • Whiteboard space for each group
  • At least one whiteboard marker for each group
  • A table that lists stars, sizes (small, medium, large), solar luminosities, and surface temperatures
  • Cutouts of circles of different sizes and colors to match the stars in the table; 1 set for each group
  • Scotch tape for each group

Intended Learning Outcomes:

  • Find the position of a star on the H-R diagram based on its luminosity and surface temperature
  • Identify trends in stellar characteristics among a group of stars plotted on an H-R diagram

I have done this activity twice so far, and both times I did it before we had an in-class discussion of the H-R diagram. My students were supposed to have read about the H-R diagram before coming to class that day, but it might be interesting to do this activity before they even do the reading. Either way, it sounded like it was still new enough to my students that they still experienced a nice level of exploration.

I divide my class into groups of four and give them the supplies they will need. I show them this blank plot of an H-R diagram and ask them to recreate it on their whiteboards. I give them about 15 minutes to do this, and, after, I prompt them to use the scotch tape to plot the circles they are given on the H-R diagram based on the stars in the table. When they are done plotting, I ask them to discuss with their groups to see if they can spot any trends in their diagrams. I ask for volunteers to share the trends that they find with the class.

In the two times that I’ve done this activity, I actually observed some enthusiasm regarding these trends, as my students discovered them for themselves! We then commenced a discussion of luminosity, surface temperature, size, main-sequence mass, and (briefly) evolutionary trends in the diagram. I do not discuss cluster ages until a little later in the semester, after we have gone through stellar evolution in more detail. For my next use of this activity, I intend to add a follow-up assessment that would include multiple-choice, think-pair-share questions regarding stellar characteristics on the H-R diagram.

I also adapted this activity during the COVID-19 pandemic, in which I put each student group into a Zoom breakout room and had them share a Zoom whiteboard. They worked together to recreate the blank H-R diagram plot and drew their own colored circles on the plot to represent the stars in the table.

My students have responded positively to both of these activities, and I am eager to more thoroughly assess the intended learning outcomes of the second activity, in particular, for future semesters. I hope you and your students enjoy these active-learning explorations as well!


Classroom Stories: Helping Students Interpret Magnetic-Field Images

By Stacy Palen

Over the last year or so, there have been a number of extraordinary images of astronomical objects with an overlay of magnetic fields in the news. One of these, from the Event Horizon Telescope, has caught extra attention, but Sofia’s HAWC+ imager has also been capturing polarization in the far-infrared. And, of course, there are existing famous images, such as the one taken by Planck that maps the magnetic field of the Milky Way.

When I showed these images to my students, I found that I needed to spend some time explaining how to interpret them.

Here are the points that I needed to make explicit to them:

  • Places without magnetic fields shown may just be places with no data. The magnetic field is not necessarily zero in those regions.
  • The “streamlines” are along the direction of the magnetic field; they are not, for example, contour lines connecting places where the magnetic field strength is constant. These streamlines neglect any component of the magnetic field that is towards or away from the observer. This component cannot be measured using polarization studies of this kind. There are several metaphors that you could use here to help students distinguish these directions. For example, you might reference proper motion versus radial motion; or, you might reference radial velocities from the discussion of exoplanets.
  • Places where the streamlines are close together indicate a stronger magnetic field than places where streamlines are farther apart.
  • The colors of streamlines are often meaningless. They are chosen to provide contrast with the background image, and also to look pretty.
  • Magnetic field lines often parallel flows of material, but not always. For example, in a galaxy, the magnetic field tends to be parallel to the bipolar outflows, but, in a star-forming region, they may be perpendicular to the direction that the infalling material is moving. In brief, this is because, sometimes, the magnetic field is directing the material, and, sometimes, the material is dragging the magnetic field. Untangling the interactions between magnetic fields and the movement of material is the main reason that these kinds of images are interesting to astronomers.

Even physicists have difficulty imagining what magnetic fields look like and how they are distributed, so it is helpful to have these extraordinary images with the magnetic fields in overlay. If we remember to slow down and explain how to interpret these invaluable images in class, we can help our students understand what they are seeing so much better.


Current Events: The Tides of the Moon and the Suez Canal

By Stacy Palen

How delightful! The phases of the Moon were in the news in late March, giving all of us an opportunity to teach students about the practical applications of astronomy in the modern world. You will likely recall the giant container ship that was stuck in the Suez Canal for almost a week, disrupting supply chains around the world. The arrival of spring tide helped float the ship off the bank where it was wedged, setting it free.

CNN reported about this news, although there are other outlets as well. 

I have included some snippets about this news with some questions on my final exam, asking students to sketch the relative positions of the Earth, Moon, and Sun during spring tide, and to make a sketch that demonstrates why so-called “supermoons” result in extra-high spring tides. Sketches like these are very quick to grade, so I like to use them during any times of the semester when I have a large grading load.

I can also code these questions as addressing the “Science and Society” general-education learning outcome, so that the crowd that does our general-education assessment will be able to check the box on their report.

I have in mind that I could build an entire assignment around this event, for future semesters, but I haven’t done it yet. Like you, I am just trying to put one foot in front of the other to get to the end of this semester!