## Notes from AAS 234

I attended the 234th meeting of the American Astronomical Society (AAS), held in St. Louis, Missouri, June 9-13, 2019. Here are some highlights from that meeting.

##### Day 1 – Monday, June 10, 2019

Research Notes of the AAS is a non-peer-reviewed, indexed and secure record of works in progress, comments and clarifications, null results, or timely reports of observations in astronomy and astrophysics. RNAAS.

The Bulletin of the American Astronomical Society is the publication for science meeting abstracts, obituaries, commentary articles about the discipline, and white papers of broad interest to our community. BAAS.

We still have many unanswered questions about galaxy formation. The rate of star formation in galaxies and central black hole accretion activity was highest between 10 and 11 billion years ago. This corresponds to redshift z around 2 to 3, referred to as “cosmic high noon”. This is the ideal epoch for us to answer our questions about galaxy formation. Near-infrared spectroscopy is important to the study of galaxies during this epoch, and we are quite limited in what we can do from terrestrial observatories. Space based telescopes are needed, and the James Webb Space Telescope (JWST) will be key.

Galaxies are not closed boxes. We need to understand how inflows and outflows affect their evolution (“galactic metabolism”).

There are five international space treaties, with the Outer Space Treaty of 1967 being the first and most important. The United States has signed four of the five treaties. The Moon Agreement of 1979 which states that no entity can own any part of the Moon does not include the United States as one of the signatories.

U.S. Code 51303, adopted in 2015, identifies asteroid resource and space resource rights, and states that “A United States citizen engaged in commercial recovery of an asteroid resource or a space resource under this chapter shall be entitled to any asteroid resource or space resource obtained, including to possess, own, transport, use, and sell the asteroid resource or space resource obtained in accordance with applicable law, including the international obligations of the United States.”

So, unfortunately, U.S. law does allow a commercial entity to own an asteroid, but you have to get there first before you can claim it. The large metallic asteroid 16 Psyche is highly valuable and will probably be owned by some corporation in the not-too-distant future.

Space law often relies upon maritime law as a model.

Astronomer Vayu Gokhale from Truman State University gave an interesting iPoster Plus presentation on how he and his students are operating three automated and continuous zenithal sky brightness measurement stations using narrow-field Sky Quality Meters (SQMs) from Unihedron. Even measurements when it is cloudy are of value, as clouds reflect light pollution back towards the ground. Adding cloud type and height would allow us to make better use of cloudy-night sky brightness measurements. In a light-polluted area, the darkest place is the zenith, and clouds make the sky brighter. In an un-light-polluted area, the darkest place is the horizon, and clouds make the sky darker.

A number of precision radial velocity instruments for exoplanet discovery and characterization will begin operations soon or are already in operation: NEID, HARPS, ESPRESSO, EXPRES, and iLocater, to name a few.

Dark matter: clumps together under gravity, does not emit, reflect, or absorb electromagnetic radiation, and does not interact with normal matter in any way that causes the normal matter to emit, reflect, or absorb electromagnetic radiation. The ratio between dark matter and normal (baryonic matter) in our universe is 5.36 ± 0.05 (Planck 2018).

What is dark matter? It could be a new particle. If so, can we detect its non-gravitational interactions? It could be macroscopic objects, perhaps primordial black holes. Or, it could be a mixture of both. Another possibility is that a modification to the laws of gravitation will be needed to mimic the effects of dark matter.

How “dark” is dark matter? Does it interact at all (besides gravitationally)? Can dark matter annihilate or decay? Even if dark matter started hot, it cools down rapidly as the universe expands.

Primordial black holes could have masses ranging anywhere between 10-16 and 1010 solar masses. LIGO is possibility sensitive to colliding primordial black holes with masses in the range of a few to a few hundred solar masses. Primordial black holes are a fascinating dark matter candidate, with broad phenomenology.

The Cosmic Microwave Background (CMB) is a nearly perfect blackbody with distortions < 1 part in 10,000. What this tells us is that nothing dramatically heated or cooled photons after 2 months after the Big Bang. Anisotropies are variances in the CMB temperature, and the angular power spectrum is variance of CMB temperature as a function of angular scale. CMB anisotropies are very sensitive to the ionization history of the universe. How the universe recombined plays a key role in CMB anisotropies.

Hydrogen: not such a simple atom.

The CMB is polarized. The polarization is caused by Mie scattering of photons.

At the NASA Town Hall, we learned about current and future missions: TESS, SPHEREx, HabEx, LUVOIR, Lynx, Origins Space Telescope (OST).

The highest image rate of standard CCD and CMOS video cameras for asteroid occultation work is 30 frames (60 fields) per second, providing time resolution of 0.017 seconds per field. Adaptive optics and autoguider imaging devices often have a higher sampling rate, and such a camera could perhaps be easily modified to be used for occultation work. A time-inserter would need to be added to the camera (either on-board or GPS-based), and improvements in quantum efficiency (because of the shorter exposures) would benefit from newer imaging technologies such as a Geiger-mode avalanche photodiode (APD); or the Single-photon avalanche detector (SPAD), which are frequently used in chemistry.

Gregory Simonian, graduate student at Ohio State, presented “Double Trouble: Biases Caused by Binaries in Large Stellar Rotation datasets”. The Kepler data yielded 34,030 rotation periods through starspot variability. However, the rapid rotators are mostly binaries. In the Kepler dataset, many rapid rotators have a spin period of the stars equal to the orbital period of the binary. These eclipsing binaries, also known as photometric binaries because they are detected through changes in brightness during eclipses and transits, need to be treated separately in stellar rotation datasets.

Granulation was discovered by William Herschel in 1801 and are vertical flows in the solar photosphere on the order of 1000 m/s, and 1000 km horizontal scale. Supergranulation (Hart 1954, Leighton et al. 1962) are horizontal motions in the photosphere of 300 to 500 m/s with a horizontal scale on the order of 30,000 km.

The amplitude of oscillations in red giants increase dramatically with age.

We’ve never observed the helium flash event in a red giant star, though models predict that it must occur. It is very brief and would be difficult to detect observationally.

Brad Schaefer, Professor Emeritus at Louisiana State University, gave a talk on “Predictions for Upcoming Recurrent Nova Eruptions”. Typically, recurrent novae have about a 30% variation in eruptive timescales, so predicting the next eruption is not trivial. Due to the solar gap (when the object is too close to the Sun to observe on or near the Earth), we are obviously missing some eruptions. However, orbital period changes (O-C curve) can tell us about an eruption we missed. U Sco and T CrB are well-known examples of recurrent novae. Better monitoring of recurrent novae is needed during the pre-eruption plateau. Monitoring in the blue band is important for prediction.

I had the good fortune to talk with Brad on several occasions during the conference, and found him to be enthusiastic, knowledgeable, and engaging. Perhaps you have seen The Remarkable Science of Ancient Astronomy (The Great Courses), and he is just as articulate and energetic in real life. Among other things, we discussed how the internet is filled with misinformation, and even after an idea has been convincingly debunked, the misinformation continues to survive and multiply in cyberspace. This is a huge problem in the field of archaeoastronomy and, indeed, all fields of study. People tend to believe what they want to believe, never mind the facts.

Astrobites is a daily astrophysical-literature blog written by graduate students in astronomy around the world. The goal of Astrobites is to present one interesting paper from astro-ph per day in a brief format accessible to its target audience: undergraduate students in the physical sciences who are interested in active research.

Helioseismology can be done both from space (all) and the ground (some). Active regions on the far side of the Sun can be detected with helioseismology.

All HMI (Helioseismic and Magnetic Imager) data from the Solar Dynamics Observatory is available online.

A good approach to studying solar data is to subtract the average differential rotation at each point/region on the Sun and look at the residuals.

The Wilcox Solar Observatory has been making sun-as-a-star mean magnetic field measurements since 1975.

It is possible to infer electric currents on the Sun, but this is much more difficult than measuring magnetic fields.

Future directions in solar studies: moving from zonal averages to localized regions in our modeling, and the ability through future space missions to continuously monitor the entire surface of the Sun at every moment.

Systematic errors are nearly always larger than statistical uncertainty.

##### Day 2 – Tuesday, June 11, 2019

It is probably not hyperbole to state that every star in our galaxy has planets. About 1/5 of G-type stars have terrestrial planets within the habitable zone. Life is widespread throughout the universe.

Gas-grain interaction is at the core of interstellar chemistry. Interstellar ices, charged ices, surface chemistry – there is more time for interactions to occur on a dust grain than in a gas. Grain collisions are important, too.

Hot cores are transient regions surrounding massive protostars very early in their evolution. Similar regions are identified around low-mass protostars and are called corinos.

Methanol (CH3OH) is key to making simple organic molecules (SOM). Evaporating ice molecules drive rich chemistry. Dust plays a key role in the chemistry and in transporting material from the interstellar medium (ISM) to planetary systems.

The Rosetta mission detected amino acids on comet 67P/Churyumov–Gerasimenko.

JUICE (JUpiter ICy moons Explorer) is an ESA mission scheduled to launch in 2022, will enter orbit around Jupiter in October 2029 and Ganymede in 2032. It will study Europa, Ganymede, and Callisto in great detail.

The gravitational wave event GW170817 (two infalling and colliding neutron stars) was also detected as a gamma-ray burst (GRB) by the Fermi gamma-ray space telescope, which has a gamma-ray burst detector that at all times monitors the 60% of the sky that is not blocked by the Earth.

The time interval between the GW and GRB can range between tens of milliseconds up to 10 seconds.

The Milky Way galaxy circumnuclear disk is best seen at infrared wavelengths around 50 microns. Linear polarization tells us the direction of rotation. The star cluster near the MW center energizes and illuminates gas structures. Gravity dominates in this region. The role of magnetic fields in this region has been a mystery.

Pitch angle – how tightly wound the spiral arms are in a spiral galaxy.

Are spiral arms transient or long lived? They are probably long lived. There may be different mechanisms of spiral arm formation in grand design spirals compared with other types of spiral galaxies.

In studying spiral galaxies, we often deproject to face-on orientation.

The co-rotation radius is the distance from the center of a spiral galaxy beyond which the stars orbit slower than the spiral arms. Inside this radius, the stars move faster than the spiral arms.

The Sun is located near the corotation circle of the Milky Way.

The origins of supermassive black holes (SMBH) at the centers of galaxies are unclear. Were they seeded from large gas clouds, or were they built up from smaller black holes?

The black holes at the centers of spiral galaxies tend to be more massive when the spiral arm winding is tight, and less massive when the spiral arm winding is loose.

Spiral Graph is in review as a Zooniverse project and has not yet launched. Citizen scientists will trace the spiral arms of 6,000 deprojected spiral galaxies, and 15 traces will be needed for each galaxy. Spiral arm tracings will provide astronomers with intermediate mass black hole candidate galaxies.

Barred spiral galaxies are very common. 66% to 75% of spiral galaxies show evidence of a bar at near-infrared wavelengths.

Magnetic fields in the inner regions of spiral galaxies are scrambling radio emissions to some extent, but radio astronomers have ways to deal with this.

For me, the plenary lecture given by Suvrath Mahadevan, Pennsylvania State University, was the first truly outstanding presentation. His topic was “The Tools of Precision Measurement in Exoplanet Discovery: Peeking Under the Hood of the Instruments”. His discussion of the advance in radial velocity instrumentation was revelatory to me, as his starting point was Roger F. Griffin’s radial velocity spectrometer we used at Iowa State University in the 1970s and 1980s, giving us a precision of about 1 km/s. My, we have come a long way since then!

To discover our Earth from another star system in the ecliptic plane would require detecting an 8.9 cm/s velocity shift in the Sun’s motion over the course of a year.

Precision radial velocity measurement requires we look at the displacement of thousands of spectral lines using high resolution spectroscopy.

The two main techniques are 1) Simultaneous reference and 2) Self reference (iodine cell). Also, externally dispersed interferometry and heterodyne spectroscopy can be used.

Griffin 1967 ~ km/s → CORAVEL 1979 ~300 m/s → CORALIE/ELODIE 1990 ~ 5-10 m/s → HARPS 2000 ~ 1 m/s → ESPRESSO/VLT, EXPRES/DCT, NEID/KPNO, HPF/HET.

We cannot build instruments that are stable over time at 10 cm/s resolution or less.

You can track the relative change in velocity much better than absolute velocity because of the “noise” generated by stellar internal motions.

Measuring the radial velocity at red or infrared wavelengths is best for M dwarfs, and cooler stars.

High radial velocity precision will require long-term observations, and a better understanding of and mitigation for stellar activity. Many things need to be considered: telescope, atmosphere, barycentric correction (chromatic effects can lead to 1/2 m/s error), fibers, modal noise, instrument decoupled from the telescope, calibrators, optics, stability, pipeline, etc. Interdisciplinary expertise is required.

NEID will measure wavelengths of 380 – 930 nm, and have a spectral resolution of R ~ 90,000.

Pushing towards 10 cm/s requires sub-milli-Kelvin instrument stability high-quality vacuum chambers, octagonal fibers, scrambling, and excellent guiding of the stellar image on the fiber to better than 0.05 arcseconds.

Precision radial velocity instruments such as NEID and HPF weigh two tons, so at present they can only be used with ground-based telescopes.

Charge Transfer Efficiency (CTE): need CCDs with CTE > 0.999999. Other CCD issues that don’t flat field out accurately: CCD stitch boundaries, cross hatching in NIR detectors, crystalline defects, sub-pixel quantum efficiency differences. Even the act of reading out the detector introduces a noise source.

10 cm/s is within reach from a purely instrumental perspective, but almost everything has to be just right. But we need to understand stellar activity better: granulation, supergranulation, flares, oscillations, etc. We may not be able to isolate these components of stellar activity, but we will certainly learn a lot in the process.

1s time resolution is required to properly apply barycentric corrections.

NASA’s Universe of Learning : Connecting Learners to the Subject-Matter Experts of NASA Astrophysics: https://www.universe-of-learning.org/

The OpenStax Astronomy Text: https://openstax.org/details/astronomy

Andrew Fraknoi gave an update on the OpenStax Astronomy text.

• about 70 people have been involved in its development and vetting
• each chapter includes collaborative group activities
• math examples are in separate boxes
• it is estimated that 500+ institutions have adopted this online and free introductory astronomy textbook, and ~200,000 students have used it, including ~30,000 amateur astronomers
• multiple choice question bank for registered instructors
• short videos with each chapter
• available to everyone

Open Educational Resources (OER): https://www.oercommons.org/

International Lunar Observatory Association (ILOA); http://www.spaceagepub.com/ ; https://galaxyforum.org/

The surface of the Moon has a thinner atmosphere than low-Earth orbit.

Kenneth Gayley, University of Iowa, gave an interesting short talk, “The Real Explanation for Type Ia Supernovae and the Helium Flash”. Here’s the abstract: https://ui.adsabs.harvard.edu/abs/2019AAS…23422404G/abstract . I’m looking forward to reading the entire paper.

Gene Byrd, University of Alabama, gave an interesting short presentation, “Two Astronomy Demos”. The first was “Stars Like Grains of Sugar”, reminiscent of Archimedes’ The Sand Reckoner. And “Phases with the Sun, Moon, and Ball”. He uses a push pin in a golf ball (the golf ball even has craters!). Morning works best for this activity. The Sun lights the golf ball and the Moon and they have the same phase—nice! Touching as well as seeing the golf ball helps students understand the phases of the Moon. Here’s a link to his paper on these two activities.

Daniel Kennefick, University of Arkansas, gave a short presentation on the 1919 eclipse expedition that provided experimental evidence (besides the correct magnitude of the perihelion precession of Mercury) that validated Einstein’s General Relativity. Stephen Hawking in his famous book A Brief History of Time mis-remembered that the 1979 re-analysis of the Eddington’s 1919 eclipse data showed that he may “fudged” the results to prove General Relativity to be correct. He did not! See Daniel Kennefick’s new book on the subject, No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein’s Theory of Relativity.

Brad Schaefer, Louisiana State University, gave another engaging talk, presenting evidence that the Australian aborigines may have discovered the variability of the star Betelgeuse, much earlier than the oft-stated discovery by John Herschel in 1836. Betelgeuse varies in brightness between magnitude 0.0 and +1.3 quasi-periodically over a period of about 423 days. It has been shown that laypeople can detect differences in brightness as small as 0.3 magnitude with the unaided eye, and with good comparison stars (like Capella, Rigel, Procyon, Pollux, Adhara, and Bellatrix—not all of which are visible from Australia—for Betelgeuse). It is plausible that the variability of Betelgeuse may have been discovered by many peoples at many different times. The Australian aborigines passed an oral tradition through many generations that described the variability of Betelgeuse. https://ui.adsabs.harvard.edu/abs/2019AAS…23422407S/abstract.

As a longtime astronomical observer myself, I have actually never noticed the variability of Betelgeuse, but Brad has. After his presentation, I mentioned to Brad that it would be interesting to speculate what would lead early peoples to look for variability in stars in the first place, which seems to me to be a prerequisite for anyone discovering the variability of Betelgeuse. His response pointed out that all it would take is one observant individual in any society who would notice/record the variability and then point it out to others.

During the last plenary session of the day, it was announced that the Large Synoptic Survey Telescope (LSST), which is expected to see first light in 2020, is expected to be renamed the Vera Rubin Survey Telescope. Tremendous applause followed! https://aas.org/posts/news/2019/06/lsst-may-be-renamed-vera-rubin-survey-telescope .

If you haven’t looked at the NASA/IPAC Extragalactic Database (NED) lately, you will find new content and functionality. It has been expanded a great deal, and now includes many stellar objects, because we don’t always know what is really a star and what is not. There is now a single input field where you can enter names, coordinates with search radius, etc. NED is “Google for Galaxies”.

I noticed during the 10-minute iPoster Plus sessions that there is a countdown timer displayed unobtrusively in the upper right hand corner that helps the presenter know how much time they have remaining. I think this would be a great device for anyone giving a short presentation in any venue.

Galactic archaeology is the study of the oldest stars and other structures in our galaxy to better understand how our galaxy evolved.

Day 2 ended with an evening presentation of “Cielo”, a documentary film by Alison McAlpine. Highly recommended!

I noted that “Cielo” was presented on the Documentary Channel in Canada. Too bad we do not have a channel like that here in the U.S.!

##### Day 3 – Wednesday, June 12, 2019

Day 3 began with what for me was the finest presentation of the entire conference: Joshua Winn, Princeton University, speaking on “Transiting Exoplanets: Past, Present, and Future”. I first became familiar with Josh Winn through watching his outstanding video course, The Search for Exoplanets: What Astronomers Know, from The Great Courses. I am currently watching his second course, Introduction to Astrophysics, also from The Great Courses. Josh is an excellent teacher, public speaker, and presenter, and it was a great pleasure to meet him at this conference.

Transits provide the richest source of information we have about exoplanets. For example, we can measure the obliquity of the star’s equator relative to the planet’s orbital plane by measuring the apparent Doppler shift of the star’s light throughout transit.

Who was the first to observe a planetary transit? Pierre Gassendi (1592-1655) was the first to observe a transit of Mercury across the Sun in November 1631. Jeremiah Horrocks (1618-1641) was the first to observe a transit of Venus across the Sun in November 1639. Christoph Scheiner (1573-1650) claimed in January 1612 that spots seen moving across the Sun were planets inside Mercury’s orbit transiting the Sun, but we know know of course that sunspots are magnetically cooled regions in the Sun’s photosphere and not orbiting objects at all. Though Scheiner was wrong about the nature of sunspots, his careful observations of them led him to become the first to measure the Sun’s equatorial rotation rate, the first to notice that the Sun rotated more slowly at higher latitudes, and the first to notice that the Sun’s equator is tilted with respect to the ecliptic, and to measure its inclination.

An exoplanet can be seen to transit its host star if the exoplanet’s orbit lies within the transit cone, an angle of 2R*/a centered on our line of sight to the star. R* is the star’s radius, and a is the semi-major axis of the planet’s orbit around the star.

Because of the geometry, we are only able to see transits of 1 out of every 215 Earth-Sun analogs.

Space is by far the best place to study transiting exoplanets.

If an exoplanet crosses a starspot, or a bright spot, on the star, you will see a “blip” in the transit light curve that looks like this:

Are solar systems like our own rare? Not at all! There are powerful selection effects at work in exoplanet transit statistics. We have discovered a lot of “hot Jupiters” because large, close-in planets are much easier to detect with their short orbital periods and larger transit cones. In actuality, only 1 out of every 200 sun-like stars have hot Jupiters.

Planet statistical properties was the main goal of the Kepler mission. Here are some noteworthy discoveries:

Kepler 89 – two planets transiting at the same time (only known example)

Kepler 36 – chaotic three-body system

Kepler 16 – first known transiting exoplanet in a circumbinary orbit

Transiting Exoplanet Survey Satellite (TESS) – Unlike Kepler, which is in an Earth-trailing heliocentric orbit, TESS is in a highly-elliptical orbit around the Earth with an apogee approximately at the distance of the Moon and a perigee of 108,000 km. TESS orbits the Earth twice during the time the Moon orbits once, a 2:1 orbital resonance with the Moon.

TESS has four 10.5 cm (4-inch) telescopes, each with a 24˚ field of view. Each TESS telescope is constantly monitoring 2300 square degrees of sky.

TESS is fundamentally about short period planets. Data is posted publicly as soon as it is calibrated. TESS has already discovered 700 planet candidates. About 1/2 to 2/3 will be true exoplanets. On average, TESS is observing stars that are about 4 magnitudes brighter than stars observed by Kepler.

The TESS Follow-Up Observing Program (TFOP) is a large working group of astronomical observers brought together to provide follow-up observations to support the TESS Mission’s primary goal of measuring the masses for 50 planets smaller than 4 Earth radii, in addition to organizing and carrying out follow-up of TESS Objects of Interest (TOIs).

HD 21749 – we already had radial velocity data going back several years for this star that hosts an exoplanet that TESS discovered

Gliese 357 – the second closest transiting exoplanet around an M dwarf, after HD 219134

TESS will tell us more about planetary systems around early-type stars.

TESS will discover other transient events, such as supernovae, novae, variable stars, etc. TESS will also make asteroseismology measurements and make photometric measurements of asteroids.

The James Webb Space Telescope (JWST) will be able to do follow-up spectroscopy of planetary atmospheres.

Upcoming exoplanet space missions: CHEOPS, PLATO, and WFIRST.

Hot Jupiter orbits should often be decaying, so this is an important area of study.

Sonification is the process of turning data into sound. For example, you could “listen” to a light curve (with harmonics, e.g. helioseismology and asteroseismology) of a year’s worth a data in just a minute or so.

Solar cycles have different lengths (11-ish years…).

Some predictions: 2019 will be the warmest year on record, 2020 will be less hot. Solar cycle 24 terminate in April 2020. Solar cycle 25 will be weaker than cycle 24. Cycle 25 will start in 2020 and will be the weakest in 300 years, the maximum (such as it is) occurring in 2025. Another informed opinion was that Cycle 25 will be comparable to Cycle 24.

Maunder minimum: 1645 – 1715

Dalton minimum: 1790 – 1820

We are currently in the midst of a modern Gleissberg minimum. It remains to be seen if it will be like the Dalton minimum or a longer “grand minimum” like the Maunder minimum.

Citizen scientists scanning Spitzer Space Telescope images in the Zooniverse Milky Way Project, have discovered over 6,000 “yellow balls”. The round features are not actually yellow, they just appear that way in the infrared Spitzer image color mapping.

Yellow balls (YBs) are sites of 8 solar mass or more star formation, surrounded by ionized hydrogen (H II) gas. YBs thus reveal massive young stars and their birth clouds.

Antlia 2 is a low-surface-brightness (“dark”) dwarf galaxy that crashed into our Milky Way galaxy. Evidence for this collision comes from “galactoseismology” which is the study of ripples in the Milky Way’s disk.

The Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and the Sagittarius Dwarf Galaxy have all affected our Milky Way Galaxy, but galactoseismology has shown that there must be another perturber that has affected the Milky Way. Antlia 2, discovered in November 2018 from data collected by the Gaia spacecraft, appears to be that perturber.

Gaia Data Release 2 (DR2) indicates that the Antlia 2 dwarf galaxy is about 420,000 ly distant, and it is similar in extent to the LMC. It is an ultra-diffuse “giant” dwarf galaxy whose stars average two magnitudes fainter than the LMC. Antlia 2 is located 11˚ from the galactic plane and has a mass around 1010 solar masses.

A question that is outstanding is what is the density of dark matter in Antlia 2? In the future, Antlia 2 may well be an excellent place to probe the nature of dark matter.

Gravity drives the formation of cosmic structure, dark energy slows it down.

Stars are “noise” for observational cosmologists.

“Precision” cosmology needs accuracy also.

The Vera Rubin telescope (Large Synoptic Survey Telescope) in Chile will begin full operations in 2022, collecting 20 TB of data each night!

We have a “galaxy bias” – we need to learn much more about the relation between galaxy populations and matter distribution.

Might there be an irregular asymmetric cycle underlying the regular 22-year sunspot cycle? The dominant period associated with this asymmetry is around 35 to 50 years.

The relationship between differential rotation and constant effective temperature of the Sun: the Sun has strong differential rotation along radial lines, and there is little variation of solar intensity with latitude.

Solar filaments (solar prominences) lie between positive and negative magnetic polarity regions.

Alfvén’s theorem: in a fluid with infinite electric conductivity, the magnetic field is frozen into the fluid and has to move along with it.

Some additional solar terms and concepts to look up and study: field line helicity, filament channels, kinetic energy equation, Lorentz force, magnetic energy equation, magnetic flux, magnetic helicity, magnetohydrodynamics (MHD), meridional flow, polarity inversion lines, relative helicity, sheared arcade, solar dynamo.

Filamentary structures: barbs, Hα, dextral, sinistral.

We would like to be able to predict solar eruptions before they happen.

1. Magnetic helicity is injected by surface motions.
2. It accumulates at polarity inversion lines.
3. It is removed by coronal mass ejections.
##### Day 4 – Thursday, June 13, 2019

Cahokia (our name for it today) was the largest city north of Mexico 1,000 years ago. It was located at the confluence of the Mississippi, Missouri, and Illinois Rivers. At its height from 1050 – 1200 A.D., Cahokia city covered 6 square miles and had 10,000 to 20,000 people. Cahokia was a walled city. Some lived inside the walls, and others lived outside the walls.

Around 120 mounds were built at greater Cahokia; 70 are currently protected. Platform mounds had buildings on top, and some mounds were used for burial and other uses.

Monks mound is the largest prehistoric earthwork in the Americas. Mound 72 has an appalling history.

Woodhenge – controversial claim that it had an astronomical purpose. Look up Brad Schaefer’s discussion, “Case studies of three of the most famous claimed archaeoastronomical alignments in North America”.

Cahokia’s demise was probably caused by many factors, including depletion of resources and prolonged drought. We do not know who the descendents of the Cahokia people are. It is possible that they died out completely.

The Greeks borrowed many constellations from the Babylonians.

One Sky, Many Astronomies

The neutron skin of a lead nucleus (208Pb) is a useful miniature analog for a neutron star.

Infalling binary neutron stars, such as GW 170817, undergo tidal deformation.

SmallSats

• Minisatellite: 100-180 kg
• Microsatellite: 10-100 kg
• Nanosatellite: 1-10 kg
• Picosatellite: 0.01-1 kg
• Femtosatellite: 0.001-0.01 kg

CubeSats are a class of nanosatellites that use a standard size and form factor. The standard CubeSat size uses a “one unit” or “1U” measuring 10 × 10 × 10 cm and is extendable to larger sizes, e.g. 1.5, 2, 3, 6, and even 12U.

The final plenary lecture and the final session of the conference was a truly outstanding presentation by James W. Head III, Brown University, “The Apollo Lunar Exploration Program: Scientific Impact and the Road Ahead”. Head is a geologist who trained the Apollo astronauts for their Moon missions between 1969 and 1972.

During the early years of the space program, the United States was behind the Soviet Union in space technology and accomplishments. The N1 rocket was even going to deliver one or two Soviet cosmonauts to lunar orbit so they could land on the Moon.

Early in his presidency, John F. Kennedy attempted to engage the Soviet Union in space cooperation.

Chris Kraft’s book, Flight: My Life in Mission Control is recommended.

The Apollo astronauts (test pilots) were highly motivated students.

The United States flew 21 robotic precursor missions to the Moon in the eight years before Apollo 11. Rangers 1-9 were the first attempts, but 1 through 6 were failures and we couldn’t even hit the Moon.

Head recommends the recent documentary, Apollo 11, but called First Man Hollywood fiction, saying, “That is not the Neil Armstrong I knew.”

The Apollo 11 lunar samples showed us that the lunar maria (Mare Tranquillitatis) has an age of 3.7 Gyr and has a high titanium abundance.

The Apollo 12 lunar excursion module (LEM) landed about 600 ft. from the Surveyor 3 probe in Oceanus Procellarum, and samples from that mission were used to determine the age of that lunar maria as 3.2 Gyr.

Scientists worked shoulder to shoulder with the engineers during the Apollo program, contributing greatly to its success.

Apollo 11 landed at lunar latitude 0.6˚N, Apollo 12 at 3.0˚S, Apollo 14 at 3.6˚S, and Apollo 15 at 26.1˚N. Higher latitude landings required a plane change and a more complex operation to return the LEM to the Command Module.

The lunar rover was first used on Apollo 15, and allowed the astronauts to travel up to 7 km from the LEM. Head said that Dave Scott did remarkable geological investigations on this mission. He discovered and returned green glass samples, and in 2011 it was determined that there is water inside those beads. Scott also told a little fib to Mission Control to buy him enough time to pick up a rock that turned out to be very important, the “seat belt basalt”.

In speaking about Apollo 16, Head called John Young “one of the smartest astronauts in the Apollo program”.

Harrison Schmitt, Apollo 17, was the only professional geologist to go to the Moon, and he discovered the famous “orange soil”. This is the mission where the astronauts repaired a damaged fender on the lunar rover using duct tape and geological maps to keep them from getting covered in dust while traveling in the rover.

When asked about the newly discovered large mass under the lunar surface, Head replied that it is probably uplifted mantle material rather than an impactor mass underneath the surface.

Radiometric dating of the Apollo lunar samples have errors of about ± 5%.

One of the reasons the Moon’s albedo is low is that space weather has darkened the surface.

The South Pole-Aitken basin is a key landing site for future exploration. In general, both polar regions are of great interest.

Smaller objects like the Moon and Mars cooled efficiently after their formation because of their high surface area to volume ratio.

We do not yet know if early Mars was warm and wet, or cold and icy with warming episodes. The latter is more likely if our solar system had a faint young sun.

Venus has been resurfaced in the past 0.5 Gyr, and there is no evidence of plate tectonics. The first ~80% of the history of Venus is unknown. Venus probably had an ocean and tectonic activity early on, perhaps even plate tectonics. Venus may have undergone a density inversion which exchanged massive amounts of material between the crust and mantle. 80% of the surface of Venus today is covered by lava flows.

A mention was made that a new journal of Planetary Science (in addition to Icarus, presumably) will be coming from the AAS soon.

I attend a lot of meetings and lectures (both for astronomy and SAS), and I find that I am one of the few people in attendance who write down any notes. Granted, a few are typing at their devices, but one never knows if they are multitasking instead. For those that don’t take any notes, I wonder, how do they really remember much of what they heard days or weeks later without having written down a few keywords and phrases and then reviewing them soon after? I did see a writer from Astronomy Magazine at one of the press conferences writing notes in a notebook as I do. I believe it was Jake Parks.

Anyone who knows me very well knows that I love traveling by train. To attend the AAS meeting, I took a Van Galder bus from Madison to Chicago, and then Amtrak from Chicago to St. Louis. Pretty convenient that the AAS meeting was held at the Union Station Hotel, just a few blocks from Amtrak’s Gateway Station. It is a fine hotel with a lot of history, and has an excellent on-site restaurant. I highly recommend this hotel as a place to stay and as a conference venue.

The bus and train ride to and fro afforded me a great opportunity to catch up on some reading. Here are a few things worth sharing.

astrometry.net – you can upload your astronomical image and get back an image with all the objects in the image astrometrically annotated. Wow!

16 Psyche, the most massive metal-rich asteroid, is the destination for a NASA orbiter mission that is scheduled to launch in 2022 and arrive at Psyche in 2026. See my note about 16 Psyche in the AAS notes above.

The lowest hourly meteor rate for the northern hemisphere occurs at the end of March right after the vernal equinox.

A tremendous, dynamic web-based lunar map is the Lunar Reconnaissance Orbiter Camera (LROC) Quickmap, quickmap.lroc.asu.edu.

I read with great interest Dr. Ken Wishaw’s article on pp. 34-38 in the July 2019 issue of Sky & Telescope, “Red Light Field Test”. Orange or amber light is probably better that red light for seeing well in the dark while preserving your night vision. You can read his full report here. Also, see my article “Yellow LED Astronomy Flashlights” here.

Jupiter and Saturn will have a spectacular conjunction next year. As evening twilight fades on Monday, December 21, 2020, the two planets will be just 1/10th of a degree apart, low in the southwestern sky.

An oblate spheroid with axes a = b > c is called a Maclaurin spheroid. If all three axes have different lengths a > b > c, then you have a Jacobi ellipsoid.

The light curve of a stellar occultation by a minor planet (asteroid or TNO) resembles a square well if the object has no atmosphere (or one so thin that it cannot be detected, given the sampling rate and S/N), and the effects of Fresnel diffraction and the star’s angular diameter are negligible.

Astronomer Margaret Burbidge, who turns 100 on August 12, 2019, refused the AAS Annie Jump Cannon Award in 1972, stating in her rejection letter that “it is high time that discrimination in favor of, as well as against, women in professional life be removed, and a prize restricted to women is in this category.” In 1976, Margaret Burbidge became the first woman president of the AAS, and in 1978 she announced that the AAS would no longer hold meetings in the states that had not ratified the Equal Rights Amendment (ERA).

During the days following the conference when I was writing this report, I received the happy news from both the AAS and Sky & Telescope that AAS was the winning bidder of S&T during a bankruptcy auction of its parent company, F+W Media. I believe that this partnership between the AAS and Sky & Telescope will benefit both AAS members and S&T readers immensely. Peter Tyson, Editor in Chief of Sky & Telescope, stated in the mutual press release, “It feels like S&T is finally landing where it belongs.” I couldn’t agree more!

## Direct Imaging of Exoplanets Through Occultations

Planetary orbits are randomly oriented throughout our galaxy. The probability that an exoplanet’s orbit will be fortuitously aligned to allow that exoplanet to transit across the face of its parent star depends upon the radius of the star, the radius of the planet, and the distance of the planet from the star. In general, planets orbiting close-in are more likely to be seen transiting their star then planets orbiting further out.

The equation for the probability of observing a exoplanet transit event is

$p_{tra} = \left (\frac{R_{\bigstar}+R_{p}}{a} \right )\left (\frac{1}{1-e^{2}} \right )$

where ptra is the transit probability, R* is the radius of the star, Rp is the radius of the planet, a is the semi-major axis of the planetary orbit, and e is the eccentricity of the planetary orbit

Utilizing the data in the NASA Exoplanet Archive for the 1,463 confirmed exoplanets where the above data is available (and assuming e = 0 when eccentricity is unavailable), we find that the median exoplanet transit probability is 0.0542. This means that, on average, 1 out of every 18 planetary systems will be favorably aligned to allow us to observe transits. However, keep in mind that our present sample of exoplanets is heavily biased towards large exoplanets orbiting close to their parent star. Considering a hypothetical sample of Earth-sized planets orbiting 1 AU from a Sun-sized star, the transit probability drops to 0.00469, which means that we would be able to detect only about 1 out of every 213 Earth-Sun analogs using the transit method.

How might we detect some of the other 99.5%? My admired colleague in England, Abdul Ahad, has written a paper about his intriguing idea: “Detecting Habitable Exoplanets During Asteroidal Occultations”. Abdul’s idea in a nutshell is to image the immediate environment around nearby stars while they are being occulted by asteroids or trans-Neptunian objects (TNOs) in order to detect planets orbiting around them. While there are many challenges (infrequency of observable events, narrow shadow path on the Earth’s surface, necessarily short exposure times, and extremely faint planetary magnitudes), I believe that his idea has merit and will one day soon be used to discover and characterize exoplanets orbiting nearby stars.

Ahad notes that the apparent visual magnitude of any given exoplanet will be directly proportional to the apparent visual magnitude of its parent star, since exoplanets shine by reflected light. Not only that, Earth-sized and Earth-like planets orbiting in the habitable zone of any star would shine by reflected light of the same intrinsic brightness, regardless of the brightness of the parent star. He also notes that the nearer the star is to us, the greater will be a given exoplanet’s angular distance from the occulted star. Thus, given both of these considerations (bright parent star + nearby parent star = increased likelihood of detection), nearby bright stars such as Alpha Centauri A & B, Sirius A, Procyon A, Altair, Vega, and Fomalhaut offer the best chance of exoplanet detection using this technique.

Since an exoplanet will be easiest to detect when it is at its greatest angular distance from its parent star, we will be seeing only about 50% of its total reflected light. An Earth analog orbiting Alpha Centauri A would thus shine at visual magnitude +23.7 at 0.94″ angular distance, and for Alpha Centauri B the values would be +24.9 and 0.55″.

Other considerations include the advantage of an extremely faint occulting solar system object (making it easier to detect faint exoplanets during the occultation event), and the signal boost offered by observing in the infrared, since exoplanets will be brightest at these wavelengths.

A distant (and therefore slow-moving) TNO would be ideal, but the angular size of the TNO needs to be larger than the angular size of the occulted star. However, slow-moving objects mean that occultation events will be rare.

The best chance of making this a usable technique for exoplanet discovery would be a space-based observatory that could be positioned at the center of the predicted shadow and would be able to move along with the shadow to increase exposure times (Ahad, personal communication). It would be an interesting challenge in orbital mechanics to design the optimal base orbit for such a spacecraft. The spacecraft orbit would be adjusted to match the position and velocity of the occultation shadow for each event using an ion drive or some other electric propulsion system.

One final thought on the imaging necessary to detect exoplanets using this technique. With a traditional CCD you would need to begin and end the exoplanet imaging exposure(s) only while the parent star is being occulted. This would not be easy to do, and would require two telescopes – one for the occultation event detection and one for the exoplanet imaging. A better approach would be to use a Geiger-mode avalanche photodiode (APD). Here’s a description of the device captured in 2016 on the MIT Lincoln Labs Advanced Imager Technology website:

A Geiger-mode avalanche photodiode (APD), on the other hand, can be used to build an all-digital pixel in which the arrival of each photon triggers a discrete electrical pulse. The photons are counted digitally within the pixel circuit, and the readout process is therefore noise-free. At low light levels, there is still noise in the image because photons arrive at random times so that the number of photon detection events during an exposure time has statistical variation. This noise is known as shot noise. One advantage of a pixel that can digitally count photons is that if shot noise is the only noise source, the image quality will be the best allowed by the laws of physics. Another advantage of an array of photon counting pixels is that, because of its noiseless readout, there is no penalty associated with reading the imager out frequently. If one reads out a thousand 1-ms exposures of a static scene and digitally adds them, one gets the same image quality as a single 1-s exposure. This would not be the case with a conventional imager that adds noise each time it is read out.

References
Ahad, A., “Detecting Habitable Exoplanets During Asteroidal Occultations”, International Journal of Scientific and Innovative Mathematical Research, Vol. 6(9), 25-30 (2018).
MIT Lincoln Labs, Advanced Imager Technology, https://www.ll.mit.edu/mission/electronics/ait/single-photon-sensitive-imagers/passive-photon-counting.html. Retrieved March 17, 2016.
NASA Exoplanet Archive https://exoplanetarchive.ipac.caltech.edu.
Winn, J.N., “Exoplanet Transits and Occultations,” in Exoplanets, ed. Seager, S., University of Arizona Press, Tucson (2011).

## Exoplanets with Deep Transits

The list above shows the 35 stars presently known to dip in brightness by 0.02 magnitudes or more due to a transiting exoplanet.

The change in the star’s magnitude during transit is given by

$\Delta m = 2.5\log_{10}\left ( 1+\delta \right )$

where Δm is the drop in magnitude, and δ is the transit depth

The time between transits for these exoplanets ranges between 0.79 and 5.72 days, with a median period of 2.24 days.  You can generate your own ephemeris for any of these transiting exoplanets at:

https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TransitView/nph-visibletbls?dataset=transits

The transit duration for these exoplanets ranges between 1.08 and 3.11 hours, with a median duration of 2.11 hours.

The exoplanets with the deepest transits, HATS-6 b at 0.035 magnitudes and Kepler-45 b at 0.034 magnitudes, cross stars that are 15.2 and 16.9 magnitude, respectively, so these events might be out of reach for most amateur photometrists.  The only other star hosting a transiting exoplanet with a Δm ≥ 0.03m is Tycho 5165-481-1 in Aquila (WASP-80 b) which at visual magnitude 11.9 is a better candidate for smaller instruments.  The brightest star on our list (by far) is HD 189733 in Vulpecula, magnitude 7.7, with a drop in brightness that is almost as good at 0.026 magnitudes.

References
Fakhouri, O. (2018). Exoplanet Orbit Database | Exoplanet Data Explorer. [online] Exoplanets.org. Available at: http://exoplanets.org/ [Accessed 11 Dec. 2018].

## Einstein, Brahms, and Exoplanets

What do Albert Einstein, Johannes Brahms, and exoplanets have in common?  They are all great courses provided by The Great Courses.

Call me old fashioned, but I love a great lecture presented by an expert in the field.  What a wonderful way to get introduced to a new subject, or refamiliarize yourself with an old subject, or deepen your knowledge about a subject with which you are already familiar.

I recently finished watching the magnificent course “Albert Einstein: Physicist, Philosopher, Humanitarian” by Don Howard, Professor of Philosophy at the University of Notre Dame, former Director of Notre Dame’s Graduate Program in History and Philosophy of Science, and a Fellow of the University of Notre Dame’s Reilly Center for Science, Technology, and Values.

I have taken an interest in Einstein since I first encountered relativity in my early teens, and of course being a physics major in college I became much more familiar with Einstein’s remarkable scientific contributions.  But this course surprised and delighted me with many details about Einstein himself.  Howard obviously has a much deeper understanding of Einstein the person than most physicists do, and his enthusiasm for his subject comes through in every lecture.  I doubt you will find a more thorough treatment of Einstein anywhere short of reading a biography.  Recommended!

As luck would have it, while I was nearing the end of this course, Time came out with an updated reissue of its special edition, “Albert Einstein: The Enduring Legacy of a Modern Genius”.  Great photographs, great text.  Well worth every penny!

Robert Greenberg is music historian-in-residence with San Francisco Performances and has produced a lot of high-quality music courses for The Great Courses.  I am in the process of watching all of them (yes, really, they’re that good!).  Recently, I finished his course on Johannes Brahms, who is probably my all-time favorite composer.

The music of Brahms is well known by many, but how much do you know about Johannes Brahms the person, and the events of his life?  This course is the perfect introduction to those subjects, as well as his extraordinary compositions.

It is amazing to me that no one has yet made a feature-length film about the life of Johannes Brahms (1833-1897).  A historically accurate dramatic portrayal could easily become one of the most significant musical film biographies ever made.  Brahms was one of the greatest composers who ever lived, and he had an interesting life—there is much material to draw upon for the making of this movie.  Greenberg’s course is a great place to begin, and I would also recommend the definitive biography, “Brahms: His Life and Work” by Karl Geiringer.

You’ve just got to love The Great Courses.  This is what television could have been.  PBS is the only thing that even comes close.  I recently completed “The Search for Exoplanets: What Astronomers Know” presented by Joshua Winn, now Professor of Astrophysical Sciences at Princeton University.  Not since Carl Sagan or Neil deGrasse Tyson have I been this excited about an astronomy presenter.  Josh Winn presents his exoplanets course with enthusiasm, precision, and a delivery that really draws you in to the subject.  I hope we see much more of him in the future.

## Separating Observer from Observed

One of the most difficult things to do in observational science is to separate the observer from the observed.  For example, in CCD astronomy, we apply bias, dark, and flat-field corrections as well as utilize median combines of shifted images to yield an image that is, ideally, free of any CCD chip defects including differences in pixel sensitivity and zero-point.

We as observers are constrained by other limitations.  For example, when we look at a particular galaxy, we observe it from a single vantage point in space and time, a vantage point we cannot change due to our great distance from the object and our existence within an exceedingly short interval of time.

Yet another limitation is a phenomenon that astronomers often call “observational selection”.  Put simply, we are most likely to see what is easiest to see.  For example, many of the exoplanets we have discovered thus far are “hot Jupiters”.  Is this because massive planets that orbit very close to a star are common?  Not necessarily.  The radial velocity technique we use to detect many exoplanets is biased towards finding massive planets with short-period orbits because such planets cause the biggest radial velocity fluctuations in their parent star over the shortest period of time.  Planets like the Earth with its relatively small mass and long orbital period (1 year) are much more difficult to detect using the radial velocity technique.  The same holds true for the transit method.  Planets orbiting close to a star will transit more often—and are more likely to transit—than comparable planets further out.  Larger planets will exhibit a larger Δm than smaller planets, regardless of their location.  It may be that Earthlike planets are much more prevalent than hot Jupiters, but we can’t really conclude that looking at the data collected so far (though Kepler has helped recently to make a stronger case for abundant terrestrial planets).

Here’s another important observational selection effect to consider in astronomy: the farther away a celestial object is the brighter that object must be for us to even see it.  In other words, many far-away objects cannot be observed because they are too dim.  This means that when we look at a given volume of space, intrinsically bright objects are over-represented.  The average luminosity of objects seems to increase with increasing distance.  This is called the Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982).

## Stars Like Our Sun

The spectral type of our Sun is G2V, that is to say, a G2 main-sequence star.

Sun
Zodiacal Constellations
mv = -26.75, mb = -26.10, B-V = 0.65
Ecliptic
0.0000158 ly
Single star

Here are the brightest stars visible in the nighttime sky that have the same spectral type and therefore are, arguably, most like our Sun.  All have an apparent visual magnitude brighter than +6.00.

Rigil Kentaurus A, Alpha Centauri A (α Cen A)
Centaurus
mv = 0.01, mb = 0.72, B-V = 0.71
α2000 = 14h 39m 36s, δ2000 = -60° 50′ 02″
4.30 – 4.34 ly
Triple star system

Alula Australis B, Xi Ursae Majoris B (ξ UMa B)
Ursa Major
mv = 4.73, mb = 5.38, B-V = 0.65
α2000 = 11h 18m 11s, δ2000 = +31° 31′ 46″
28 – 30 ly
Quintuple star system

HR 4523 A
Centaurus
mv = 4.88, mb = 5.55, B-V = 0.67
α2000 = 11h 46m 31s, δ2000 = -40° 30′ 01″
30.0 – 30.1 ly
Binary star system; exoplanet

Eta Coronae Borealis A & B (η CrB A & B)
Corona Borealis
A: mv = 5.577, mb = 6.123, B-V = 0.546
B: mv = 5.95, mb = 6.48, B-V = 0.53
α2000 = 15h 23m 12s, δ2000 = +30° 17′ 18″
57 – 59 ly
Triple star system

HR 8323
Grus
mv = 5.58, mb = 6.18, B-V = 0.60
α2000 = 21h 48m 16s, δ2000 = -47° 18′ 13″
51.9 – 52.5 ly
Single star

Mu Velorum B (μ Vel B)
Vela
mv = 5.59, mb = 6.10, B-V = 0.51
α2000 = 10h 46m 46s, δ2000 = -49° 25′ 12″
116 – 119 ly
Binary star system

HR 7845 A
Capricornus
mv = 5.65, mb = 6.34, B-V = 0.69
α2000 = 20h 32m 24s, δ2000 = -09° 51′ 12″
79 – 80 ly
Binary star system

HR 3578
Hydra
mv = 5.86, mb = 6.39, B-V = 0.53
α2000 = 8h 58m 44s, δ2000 = -16° 07′ 58″
68 – 69 ly
Single star

HR 2007
Orion
mv = 5.97, mb = 6.61, B-V = 0.64
α2000 = 5h 48m 35s, δ2000 = -4° 05′ 41″
49.2 – 49.8 ly
Single star with exoplanet

The Eta Coronae Borealis system is noteworthy in that its two primary components are both G2V stars orbiting each other every 41.6 years.  The third component of this system is a distant infrared dwarf, spectral type L8V.

Two of these G2V stars host at least one exoplanet: HR 4523A in Centaurus and HR 2007 in Orion.

HR 4523A has a planet midway in mass between Uranus and Neptune orbiting every 122 days between 0.30 and 0.62 AU from the star (similar to orbital distance of the planet Mercury in our own solar system).  The other stellar component of this system. HR 4523B, is a distant M4V star currently orbiting about 211 AU from HR 4523A.

HR 2007, a single star like the Sun, has a planet about 78% more massive than Neptune, orbiting every 407 days, more or less.  If this planet were in our own solar system, it would range between the orbits of Venus and Mars, roughly.

## Metallicity

No, it’s not the name of a rock band. Astronomers (unlike everybody else) consider all elements besides hydrogen and helium to be metals. For example, our Sun has a metallicity of at least 2% by mass (Vagnozzi 2016). That means as much as 98% of the mass of the Sun is hydrogen (~73%) and helium (~25%), with 2% being everything else.

Traditionally, elemental abundances in the Sun have been measured using spectroscopy of the Sun’s photosphere.  In principle, stronger spectral lines (usually absorption) of an element indicate a greater abundance of that element, but deriving the correct proportions from the cacophony of spectral lines is challenging.

A more direct approach to measuring the Sun’s elemental abundances is analyzing the composition of the solar wind, though the material blown away from the surface of the Sun that we measure near Earth’s orbit may be somewhat different from the actual photospheric composition.  The solar wind appears to best reflect the composition of the Sun’s photosphere in the solar polar regions near solar minimum.  The Ulysses spacecraft made solar wind measurements above both the Sun’s north and south polar regions during the 1994-1995 solar minimum.  Analysis of these Ulysses data indicate the most abundant elements are (after hydrogen and helium, in order of abundance): oxygen, carbon, nitrogen, magnesium, silicon, neon, iron, and sulfur—though one analysis of the data shows that neon is the third most abundant element (after carbon).

The elephant in the room is, of course, are the photospheric abundances we measure using spectroscopy or the collection of solar wind particles indicative of the Sun’s composition as a whole?  As it turns out, we do have ways to probe the interior of the Sun.  Both helioseismology and the flux of neutrinos emanating from the Sun are sensitive to metal abundances within the Sun.  Helioseismology is the study of the propagation of acoustic pressure waves (p-waves) within the Sun.  Neutrino flux is devilishly hard to measure since neutrinos so seldom interact with the matter in our instruments.  Our studies of the interior of the Sun (except for sophisticated computer models) are still in their infancy.

You might imagine that if measuring the metallicity of the Sun in our own front yard is this difficult, then measuring it for other stars presents an even more formidable challenge.

In practice, metallicity is usually expressed as the abundance of iron relative to hydrogen.  Even though iron is only the seventh most abundant metal (in the Sun, at least), it has 26 electrons, leading to the formation of many spectral lines corresponding to the various ionization states within a wide range of temperature and pressure regimes.  Of the metals having a higher abundance than iron, silicon has the largest number of electrons, only 14, and it does not form nearly as many spectral lines in the visible part of the spectrum as does iron.  Thus defined, the metallicity of the Sun [Fe/H] = 0.00 by definition.  It is a logarithmic scale: [Fe/H] = -1.0 indicates an abundance of iron relative to hydrogen just 1/10 that of the Sun.  [Fe/H] = +1.0 indicates an abundance of iron relative to hydrogen 10 times that of the Sun.

The relationship between stellar metallicity and the existence and nature of exoplanets is an active topic of research.  It is complicated by the fact that we can never say for certain that a star does not have planets, since our observational techniques are strongly biased towards detecting planets with an orbital plane near our line of sight to the star.

References
Vagnozzi, S. 2016, 51st Recontres de Moriond, Cosmology, At La Thuile

Many times each week, all manner of asteroids and trans-Neptunian objects pass in front of stars, casting shadows a few miles wide all over the Earth.  There are several potential events each week at any particular location.  I use the word “potential” because there is still significant uncertainty in the paths for many of these events.  The orbits of most small solar system objects are not yet precisely known, and, to a lesser extent, there is some uncertainty in the position of the occulted (obscured) star.

On Sunday evening, November 20, I got lucky.  Not only did I record a 1.02 second occultation event, but I was lucky to see it at all as I was significantly south of the predicted path.

The star affected was Tycho 5182-758-1 (also known as BD -3° 5037) in Aquarius and the object that moved in front of it was the asteroid 430 Hybris, a space rock about 20 miles across that orbits once around the Sun every 4.8 years.  Many asteroids have interesting names, and Hybris is no exception.  In Greek mythology, Hybris is a spirit of insolence, violence, and outrageous behavior.  It is also an alternative form of the word hubris.  All quite appropriate given the outcome of the U.S. presidential election less than two weeks earlier.

Here is the video I recorded of the event:

Occultation of the star Tycho 5182-758-1 in Aquarius by the asteroid 430 Hybris

And here is the light curve I derived from the video which clearly shows the event:

Steve Messner (near Northfield, Minnesota) and I were the only ones to observe this event.  It was a miss for Steve, and he was much closer to the predicted path!

Why do we do it? Even a single positive observation can greatly improve our knowledge of the orbit of the asteroid or trans-Neptunian object.  More than one positive observation gives us valuable information about its size and shape.  We can discover asteroid/TNO satellites and even rings!  But that’s not all.  These occultation events can also give us valuable information about the star.  Its size, position, and the separation and position angle of new or known companion stars.  Someday, we may even be able to use these events to discover exoplanets!

If you love observational astronomy and would like to contribute scientifically valuable observations by observing occultation events, contact me and I will help you get started.  The more observers we have, the more valuable our scientific contribution will be.