Astronomy – Cosmic Reflections

Recent arXiv:astro-ph Picks: December 2024

Here are some recent submissions on astro-ph that I found to be especially interesting. Text excerpts below are quoted directly from the articles. My comments are in italics.

Short-Term Evolution and Risks of Debris Cloud Stemming from Collisions in Geostationary Orbit
https://arxiv.org/abs/2412.13586

The geostationary orbit is a popular orbit for communication, meteorological, and navigation satellites due to its apparent motionless. Nearly all geostationary satellites are positioned in a circular orbit with a radius of 42,164 km, making this region particularly vulnerable to space traffic accidents due to the high concentration of objects and the absence of natural debris-clearing mechanisms. The growing population in geostationary region raises concerns about the potential risks posed by fragments stemming from explosions and collisions, particularly following the breakup of Intelsat-33e, which remained operational in geostationary orbit until October 19, 2024.

A breakup event generates a large number of fragments of varying sizes. In the geostationary region, only fragments larger than 1 meter are routinely tracked by the Space Surveillance Network, as the sensitivity of ground-based sensors decreases significantly with distance. However, small, non-trackable fragments can still cause catastrophic damage to spacecraft. The collision velocity of spacecraft in geostationary orbit can reach up to 4 km/s, while micro-meteoroids may hit at speeds of up to 72 km/s.

The impact of a debris cloud is inherently global as it disperses around the entire Earth.

By 2024, over 1,000 objects have been observed near the geostationary orbit (GEO). Nearly all objects exhibit inclinations of less than 15 degrees, with the majority having inclinations of less than 1 degree. Once a fragmentation event occurs, the GEO objects will be exposed to considerable risks, as they are densely clustered along a single ring above the Equator.

More about Intelsat 33e and its breakup:
https://en.wikipedia.org/wiki/Intelsat_33e

Sun-like stars produce superflares roughly once per century
https://arxiv.org/abs/2412.12265

Stellar superflares are energetic outbursts of electromagnetic radiation, similar to solar flares but releasing more energy, up to 10³⁶ erg on main sequence stars. It is unknown whether the Sun can generate superflares, and if so, how often they might occur. We used photometry from the Kepler space observatory to investigate superflares on other stars with Sun-like fundamental parameters. We identified 2889 superflares on 2527 Sun-like stars, out of 56450 observed. This detection rate indicates that superflares with energies >10³⁴ erg occur roughly once per century on stars with Sun-like temperature and variability. The resulting stellar superflare frequency-energy distribution is consistent with an extrapolation of the Sun’s flare distribution to higher energies, so we suggest that both are generated by the same physical mechanism.

Solar flares are sudden local bursts of bright electromagnetic emission from the Sun, which release a large amount of energy within a short interval of time. The increase in short-wavelength solar radiation during flares influences the Earth’s upper atmosphere and ionosphere, sometimes causing radio blackouts and ionosphere density changes. Solar flares are frequently accompanied by the expulsion of large volumes of plasma, known as coronal mass ejections (CMEs), which accelerate charged particles to high energies. When these solar energetic particles (SEPs) reach Earth, they cause radiation hazards to spacecraft, aircraft and humans. Extreme SEP events can produce isotopes, called cosmogenic isotopes, which form when high-energy particles interact with the Earth’s atmosphere. These isotopes are then recorded in natural archives, such as tree rings and ice cores. The total amount of energy released by each flare varies by many orders of magnitude, as determined by a complex interplay between the physical mechanisms of particle acceleration and plasma heating in the Sun’s
atmosphere.

Solar flares have been observed for less than two centuries. Although thousands of them have been detected and measured, only about a dozen are known to have exceeded a bolometric (integrated over all wavelengths) energy of 10³² erg. Among them was the Carrington Event on 1 September 1859, which was accompanied by a CME that had the strongest recorded impact on Earth. Modern estimates of the Carrington Event’s total bolometric energy are 4 × 10³² to 6 × 10³² erg.

It is unknown whether the Sun can unleash flares with even higher energies, often referred to as superflares, and if so, how frequently that could happen. The period of direct solar observations is too short to reach any firm conclusions. There are two indirect methods to investigate the potential for more intense flares on the Sun. One method uses extreme SEP events recorded in cosmogenic isotope data, which have been used to quantify the occurrence rate of strong CMEs reaching Earth over the past few millennia. There are five confirmed (and three candidate) extreme SEP events that are known to have occurred in the last 10,000 yr, implying a mean occurrence rate of ∼ 10⁻³ yr⁻¹. However, the relationship between SEPs and flares is poorly understood, especially for the stronger events.

A second method is to study superflares on stars similar to the Sun. If the properties of the observed stars sufficiently match the Sun, the superflare occurrence rate on those stars can be used to estimate the rate on the Sun.

We found that Sun-like stars produce superflares with bolometric energies > 10³⁴ erg roughly once per century. That is more than an order of magnitude more energetic than any solar flare recorded during the space age, about sixty years. Between 1996 and 2012 twelve solar flares had bolometric energies > 10³² erg, but none were > 10³³ erg. The most powerful solar flare recorded occurred on 28 October 2003, with an estimated bolometric energy of 7 × 10³² erg, which exceeds estimates for the Carrington Event (4 × 10³² to 6 × 10³² erg).

We cannot exclude the possibility that there is an inherent difference between flaring and non-flaring stars that was not accounted for by our selection criteria. If so, the flaring stars in the Kepler observations would not be representative of the Sun. Approximately 30% of flaring stars are known to have a binary companion. Flares in those systems might originate on the companion star or be triggered by tidal interactions. If instead our sample of Sun-like stars is representative of the Sun’s future behavior, it is substantially more likely to produce a superflare than was previously thought.

More about the Carrington Event:
https://en.wikipedia.org/wiki/Carrington_Event

ChronoFlow: A Data-Driven Model for Gyrochronology
https://arxiv.org/abs/2412.12244

Gyrochronology is a technique for constraining stellar ages using rotation periods, which change over a star’s main sequence lifetime due to magnetic braking. This technique shows promise for main sequence FGKM stars, where other methods are imprecise. However, models have historically struggled to capture the observed rotational dispersion in stellar populations. To properly understand this complexity, we have assembled the largest standardized data catalog of rotators in open clusters to date, consisting of ~7,400 stars across 30 open clusters/associations spanning ages of 1.5 Myr to 4 Gyr.

Stars in open clusters are all about the same age, so this is highly useful in training models that use stellar rotation periods to determine stellar age.
https://en.wikipedia.org/wiki/Gyrochronology

On The Lunar Origin of Near-Earth Asteroid 2024 PT5
https://arxiv.org/abs/2412.10264

The Near-Earth Asteroid (NEA) 2024 PT5 is on an Earth-like orbit which remained in Earth’s immediate vicinity for several months at the end of 2024. PT5’s orbit is challenging to populate with asteroids originating from the Main Belt and is more commonly associated with rocket bodies mistakenly identified as natural objects or with debris ejected from impacts on the Moon. We obtained visible and near-infrared reflectance spectra of PT5 with the Lowell Discovery Telescope and NASA Infrared Telescope Facility on 2024 August 16. The combined reflectance spectrum matches lunar samples but does not match any known asteroid types—it is pyroxene-rich while asteroids of comparable spectral redness are olivine-rich. Moreover, the amount of solar radiation pressure observed on the PT5 trajectory is orders of magnitude lower than what would be expected for an artificial object. We therefore conclude that 2024 PT5 is ejecta from an impact on the Moon, thus making PT5 the second NEA suggested to be sourced from the surface of the Moon. While one object might be an outlier, two suggest that there is an underlying population to be characterized. Long-term predictions of the position of 2024 PT5 are challenging due to the slow Earth encounters characteristic of objects in these orbits. A population of near-Earth objects which are sourced by the Moon would be important to characterize for understanding how impacts work on our nearest neighbor and for identifying the source regions of asteroids and meteorites from this under-studied population of objects on very Earth-like orbits.

Perhaps the most significant conclusion to finding a second near-Earth object with an apparently Moon-like surface composition is the realization of lunar ejecta as a genuine population of objects. The Quasi-Satellite Kamo‘oalewa has a slightly redder spectrum than 2024 PT5, but the higher quality of our data at longer wavelengths (the Quasi-Satellite was significantly dimmer, so only photometry was obtained beyond ≈ 1.25μm) makes a discussion of how different the two spectra are only qualitative. At the very least, the two lunar NEOs do not look identical. Sharkey et al. (2021) argued that the red spectrum of Kamo‘oalewa was partially due to space weathering – an exposure time of a few million years was likely sufficient to explain its surface properties and was similar to its approximate dynamical lifetime and even the age of the crater that Jiao et al. (2024) suggested it came from, Giordano Bruno. If correct, perhaps 2024 PT5 has a somewhat younger surface than the larger Kamo‘oalewa. In any case, PT5 is smaller than Kamo‘oalewa and thus the craters that are energetic enough to produce an object its size are more common – a more recent ejection age, and thus a ‘younger’ surface might be preferred from that argument as well. (Granted, smaller fragments would be more common than larger ones in cratering events of any size as well.) Further work to study these two objects and to find more lunar-like NEOs will be needed to ascertain the origin of these differences and how they can be related to the circumstances of their creation. At any rate, the smaller size of PT5 means that we are approaching being able to study the impactors and outcomes from the kinds of small impacts seen regularly by the Lunar Reconaissance Orbiter.

For more information about 2024 PT₅ and Kamo‘oalewa:
https://en.wikipedia.org/wiki/2024_PT5
https://en.wikipedia.org/wiki/469219_Kamo%CA%BBoalewa

Call to Protect the Dark and Quiet Sky from Harmful Interference by Satellite Constellations
https://arxiv.org/abs/2412.08244

The growing number of satellite constellations in low Earth orbit (LEO) enhances global communications and Earth observation, and support of space commerce is a high priority of many governments. At the same time, the proliferation of satellites in LEO has negative effects on astronomical observations and research, and the preservation of the dark and quiet sky. These satellite constellations reflect sunlight onto optical telescopes, and their radio emission impacts radio observatories, jeopardising our access to essential scientific discoveries through astronomy. The changing visual appearance of the sky also impacts our cultural heritage and environment. Both ground-based observatories and space-based telescopes in LEO are affected, and there are no places on Earth that can escape the effects of satellite constellations given their global nature. The minimally disturbed dark and radio-quiet sky¹ is crucial for conducting fundamental research in astronomy and important public services such as planetary defence, technology development, and high-precision geolocation.

Some aspects of satellite deployment and operation are regulated by States and intergovernmental organisations. While regulatory agencies in some States have started to require operators to coordinate with their national astronomy agencies over impacts, mitigation of the impact of space objects on astronomical activities is not sufficiently regulated.

¹We refer to the radio-quiet sky as simply the ‘quiet sky’

To address this issue, the CPS [International Astronomical Union (IAU) Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS)] urges States and the international community to:

1) Safeguard access to the dark and quiet sky and prevent catastrophic
loss of high quality observations.

2) Increase financial support for astronomy to offset and compensate the impacts on observatory operations and implement mitigation measures at observatories and in software.

3) Encourage and support satellite operators and industry to collaborate with the astronomy community to develop, share and adopt best practices in interference mitigation, leading to widely adopted standards and guidelines.

4) Provide incentive measures for the space industry to develop the required technology to minimise negative impacts. Support the establishment of test labs for brightness and basic research into alternate less reflective materials
and reduction of unwanted radiation in the radio regime for spacecraft manufacturing.

5) In the longer term, establish regulations and conditions of authorization and supervision based on practical experience as well as the general provisions of international law and main principles of environmental law to codify industry best practices that mitigate the negative impacts on astronomical observations. Satellites in LEO should be designed and operated in ways that minimise adverse effects on astronomy and the dark and quiet sky.

6) Continue to support finding solutions to space sustainability issues, including the problem of increasing space debris leading to a brighter sky. Minimising the production of space debris will also benefit the field of astronomy and all sky observers worldwide.

The elephant in the room—not specifically mentioned in this report—is that countries and companies should be sharing satellite constellations as much as possible to minimize the number of satellite constellations in orbit. This is analogous to the co-location often required for terrestrial communication towers. Our current satellite constellation predicament illustrates yet another reason why we need a binding set of international laws that apply to all nations and are enforced by a global authority. The sooner we have this the better, as our cultural survival—if not our physical survival—may depend upon it.

A New Method to Derive an Empirical Lower Limit on the Mass Density of a UFO
https://arxiv.org/abs/2412.12142

I derive a lower limit on the mass of an Unidentified Flying Object (UFO) based on measurements of its speed and acceleration, as well as the infrared luminosity of the airglow around it. If the object’s radial velocity can be neglected, the mass limit is independent of distance. Measuring the distance and angular size of the object allows to infer its minimum mass density. The Galileo Project will be collecting the necessary data on millions of objects in the sky over the coming year.

Any object moving through air radiates excess heat in the form of infrared airglow luminosity, L. The airglow luminosity is a fraction of the total power dissipated by the object’s speed, v, times the frictional force of air acting on the object. The radiative efficiency depends on the specific shape of the object and the turbulence and thermodynamic conditions in the atmosphere around it. If the object accelerates, then this friction force must be smaller than the force provided by the engine which propels the object. The net force equals the object’s mass, M, times its acceleration, a.

In conclusion, one gets an unavoidable lower limit on the mass of an accelerating object. The object’s mass must be larger than the infrared luminosity from heated air around it, divided by the product of the object’s acceleration and speed.

This limit provides an elegant way to constrain the minimum mass of Unidentified Flying Objects (UFOs), also labeled as Unidentified Anomalous Phenomena (UAPs). To turn the inequality into an equality, one needs to know the detailed object shape and atmospheric conditions around the object.

The first Galileo Project Observatory at Harvard University collects data on ∼ 10⁵ objects in the sky every month. A comprehensive description of its commissioning data on ∼ 5 × 10⁵ objects was provided in a recent paper (Dominé et al. 2024). The data includes infrared images captured by an all-sky Dalek array of eight uncooled infrared cameras placed on half a sphere.

Within the coming month, the Galileo Project’s research team plans to employ multiple Daleks separated by a few miles, in order to measure distances to objects through the method of triangulation.

If the measured velocity and acceleration of a technological object are outside the flight characteristics and performance envelopes of drones or airplanes, then the object would be classified by the Galileo Project’s research team as an outlier. In such a case, it would be interesting to calculate the minimum mass density of the object. If the result exceeds normal solid densities, then the object would qualify as anomalous, a UAP. Infrared emission by the object would be a source of confusion, unless the object is resolved and the emission from it can be separated from the heated air around it.

All flying objects made by humans have a volume-averaged mass density ⟨ρ⟩ which is orders of magnitude below 22.6 g cm⁻³, the density of Osmium – which is the densest metal known on Earth. A UFO with a higher mass density than Osmium would have to carry exotic material, not found on Earth.

By summer 2025, there will be three Galileo Project observatories operating in three different states within the U.S. and collecting data on a few million objects per year. With new quantitative data on infrared luminosities, velocities and accelerations of technological objects, it would be possible to check whether there are any UFOs denser than Osmium.

I admire the author, Avi Loeb, Harvard astrophysics professor, for his creative approaches to interesting problems outside the mainstream that many of his colleagues tend to avoid. Lately, he’s been focusing a lot on technosignatures, and I imagine he has a keen interest in the recent spate of unexplained nighttime drone sightings in New Jersey and elsewhere. For more about Loeb and the Galileo Project:
https://en.wikipedia.org/wiki/Avi_Loeb
https://en.wikipedia.org/wiki/The_Galileo_Project

Beyond CCDs: Characterization of sCMOS detectors for optical astronomy
https://arxiv.org/abs/2409.16449

Modern scientific complementary metal-oxide semiconductor (sCMOS) detectors provide a highly competitive alternative to charge-coupled devices (CCDs), the latter of which have historically been dominant in optical imaging. sCMOS boast comparable performances to CCDs with faster frame rates, lower read noise, and a higher dynamic range. Furthermore, their lower production costs are shifting the industry to abandon CCD support and production in favour of CMOS, making their characterization urgent. In this work, we characterized a variety of high-end commercially available sCMOS detectors to gauge the state of this technology in the context of applications in optical astronomy. We evaluated a range of sCMOS detectors, including larger pixel models such as the Teledyne Prime 95B and the Andor Sona-11, which are similar to CCDs in pixel size and suitable for wide-field astronomy. Additionally, we assessed smaller pixel detectors like the Ximea xiJ and Andor Sona-6, which are better suited for deep-sky imaging. Furthermore, high-sensitivity quantitative sCMOS detectors such as the Hamamatsu Orca-Quest C15550-20UP, capable of resolving individual photoelectrons, were also tested. In-lab testing showed low levels of dark current, read noise, faulty pixels, and fixed pattern noise, as well as linearity levels above 98% across all detectors. The Orca-Quest had particularly low noise levels with a dark current of 0.0067±0.0003 e⁻/s (at −20^∘C with air cooling) and a read noise of 0.37±0.09 e⁻ using its standard readout mode. Our tests revealed that the latest generation of sCMOS detectors excels in optical imaging performance, offering a more accessible alternative to CCDs for future optical astronomy instruments.

The Hamamatsu Orca-Quest CP15550-20UP, simply called Orca-Quest, is advertised as being a quantitative CMOS detector with extremely low noise levels and photoelectron counting capabilities. It features a custom 9.4-megapixel sensor with 4.6 × 4.6 μm pixels. The Orca-Quest has two scan modes that were characterized: standard and ultra-quiet. The ultra-quiet mode has a much lower frame rate at 5 frames per second (fps) compared to the standard mode’s 120 fps, which allows for much lower read noise. Also characterized was the ‘photon number resolving’ readout mode which claims to report the integer number of incident photoelectrons based on a proprietary calibrated algorithm using the ultra-quiet scan. The Orca-Quest has a detector-imposed temperature lock at −20^◦C when air-cooled. The standard and ultra-quiet modes are 16-bit, with a saturation limit of 65536 ADU while the photon number resolving mode has a saturation limit of only 200 ADU. The Orca-Quest boasts a peak quantum efficiency of 85%.

Unlike CCDs, which use a single global amplifier with a shift register, sCMOS pixels have individual readout electronics, requiring each pixel to be tested as an independent detector. Historically, this led to high fixed pattern noise in CMOS detectors, but we found negligible fixed pattern noise in almost all the detectors we analyzed pixel-wise.

4534 Rimskij-Korsakov

On Sunday afternoon, October 13, 2024, I attended a wonderful concert by the Southern Arizona Symphony Orchestra (SASO) that included a rousing performance of Scheherazade by the Russian composer Nikolai Rimsky-Korsakov.

Early that evening, I was the first person in the world to observe the composer’s namesake asteroid 4534 Rimskij-Korsakov passing in front of a distant star and, briefly, blocking its light. As a classical music lover, that made me very happy.

The 0.5-second occultation of the 13.6-magnitude star UCAC4 558-003434 by the asteroid
4534 Rimskij-Korsakov on 14 Oct 2024 2:23:46 UT as seen from Tucson, Arizona
using an 8-inch telescope

4534 Rimskij-Korsakov was discovered on 6 Aug 1986 by the Russian astronomer Nikolai Chernykh (1931-2004) at the Crimean Astrophysical Observatory near the small settlement of Nauchnyi on the Crimean peninsula, part of Ukraine but illegally occupied by Putin’s Russian forces since 2014.

At the time of its discovery, this asteroid received its preliminary designation 1986 PV₄. As is the custom, the discoverer gets to choose a name for the asteroid if they so desire, and Nikolai Chernykh decided to name his discovery after Nikolai Rimsky-Korsakov (1844-1908). This name was approved by the IAU and published in Minor Planet Circular 23352 on 25 Apr 1994.

4534 Rimskij-Korsakov is not a large asteroid. Its average diameter is estimated to be just 9.9 miles. Had I been right on the centerline of the asteroid’s shadow, I should have seen the star disappear for about 1.2 seconds. Given that I had to use an integration time of 0.27^s due the faintness of the occulted star, the 0.5-second event I recorded had only two data points in the “dip” where the 13.6 magnitude star disappeared leaving only the sky background since the asteroid’s estimated magnitude was just 17.5^m. Normally, one likes to have at least three data points in the dip, but two is better than one and the event happened at exactly the predicted time.

Nikolai Rimsky-Korsakov wrote a lot of great music, and he was a master of orchestration and orchestral “colors”. Here are my favorite works. If you don’t already know them, give them a listen!

Capriccio espagnol
Le Coq d’Or, Suite [arranged by Alexander Glazunov (1865-1936) & Maximilian Steinberg (1883-1946)]
Russian Easter Festival Overture
Scheherazade
Suite from The Snow Maiden
Symphony No. 2, “Antar”
The Tale of Tsar Saltan, Suite

Comet Tsuchinshan-ATLAS (C/2023 A3)

A bright comet with a long tail is just now emerging into our evening sky after passing perihelion, and today around 9:08 a.m. MST it passed closest to the Earth at a distance of 0.47 AU.

Even though there is currently moonlight interference and the comet’s head is on the WSW horizon at the end of astronomical twilight (here in Tucson), the tail may be visible even as early as tonight, and each evening going forward Comet Tsuchinshan-ATLAS will be rising higher in the WSW sky.

Our first chance this month to see Comet Tsuchinshan-ATLAS at least 10° above the horizon in a sky free of twilight and moonlight will come next Saturday evening, but you should definitely make an effort to get out of the city to a dark rural location free of light pollution to get the best view.

Here’s a dark-sky ephemeris for Comet Tsuchinshan-ATLAS for Tucson, Arizona for October and November. Since the comet is moving away from both the Sun and the Earth, the sooner you make an effort to see this spectacular comet, the better!

Comet Tsuchinshan-ATLAS (2023 A3) 202410 Download

How Far the Sun

How do we know our Sun is 93 million miles (150 million km) away¹?

The ancient Greek astronomer and mathematician Aristarchus of Samos, who lived around 2,300 years ago, was probably the first person who made a reasonable attempt to determine the distance to the Sun.

Using a method of geometric analysis developed by Euclid (trigonometry had not yet been invented), Aristarchus measured the angle between the half-lit Moon and the Sun and determined that the Sun is 18 to 20 times farther away than the Moon. Though he fell far short of the actual value of 389 due to the extreme difficulty of making accurate measurements using the instruments and methods available to him, Aristarchus showed the way for future generations of astronomers to determine the true distance to the Sun.

Determining the actual distance (and not the relative distance) to the Sun had to wait for Kepler’s Third Law of planetary motion that relates a planet’s orbital period to its distance from the Sun, the invention of the telescope, and Isaac Newton’s laws of motion and gravitation.

$P^{2}\propto a^{3}$

Distances within the solar system can be determined using trigonometry and parallax, which is the apparent shift of an object against the distant background stars as seen from different locations.

Hold your thumb at arm’s length and alternate between right and left eye open to see the parallactic shift.
Bring your thumb closer, and the shift is greater.

Measuring the parallax to a Sun-orbiting object (such as Mars) from two different locations on the Earth’s surface allows us to measure its distance and, thanks to Kepler and Newton, sets the scale for the entire solar system. The true distance of each planet from the Sun can then be mathematically determined. This was first accomplished in 1672, and has been done many times since, with ever-improving accuracy.

Observations of the position of Mars by Giovanni Cassini at Paris and Jean Richer at Cayenne
allowed the first determination of the distance to Mars using trigonometric parallax in 1672.

Today, we have even better methods to determine the scale of the solar system: timing radar reflections off of solar system objects, and measuring travel time for radio communications between Earth and spacecraft. Both radar and radio signals travel at the speed of light, which is very well determined.

¹Approximate average distance

Hale, Hooker, Hubble, Humason

Clockwise from upper left: George Ellery Hale, Edwin Powell Hubble, Milton La Salle Humason, John Daggett Hooker, Hooker 100-inch telescope at Mt. Wilson Observatory

Edwin Powell Hubble (1889-1953) was born in Marshfield, Missouri, nine years after a devastating F4 tornado destroyed most of the town, killing 99 people and injuring 100. The Hubble family moved to Wheaton, Illinois (near Chicago) the year Edwin was born.

After receiving a B.S. degree from the University of Chicago in 1910, Hubble spent three years at Oxford University as a Rhodes Scholar. The experience must have made quite an impression on young Hubble, as he returned to the U.S. with an affected British accent and other mannerisms (such as smoking a pipe) that stayed with him (and sometimes irritated others) for the rest of his life.

George Ellery Hale (1868-1938) offered Hubble a job at the Mount Wilson Observatory in 1919, and that same year also hired a talented man who would soon become Hubble’s assistant, Milton Humason (1891-1972), just as Mt. Wilson’s 100-inch Hooker telescope (the largest in the world at that time) started to see regular use.

Hubble identified Cepheid variables in M31, the Andromeda Nebula (and some other spiral nebulae), using the 100-inch in 1922-1923. From those observations, Hubble determined without a shadow of doubt that the Andromeda Nebula is in fact another galaxy of stars lying far beyond our own Milky Way galaxy. Up until this time, there was great debate about whether “spiral nebulae” like M31 were within our own galaxy or beyond it. Many thought that our galaxy was the entire universe. Thanks to Edwin Hubble and those who followed him, we now know that our galaxy is but one of many billions in this unimaginably vast universe we are lucky enough to explore.

How did Hubble use the faint Cepheid variables to determine the distance to M31? Cepheid variables are very luminous yellow giant and supergiant stars whose luminosity is directly related to the period of time it takes for the star to vary in brightness from brightest to dimmest to brightest again. The longer the period, the brighter the star really is. Knowing the apparent brightness of a star (dependent on distance), and knowing its true brightness (not dependent on distance), we can easily calculate the distance to the star. In the case of M31, the Andromeda Galaxy, we now know its distance to be 2.48 ± 0.04 million light years. M31 and the Milky Way are comparable in size and mass, and are by far the two largest galaxies of the Local Group, which contains at least 80 members. M31 and our Milky Way are moving towards each other due to gravitational attraction, and they will “collide” in about 4 to 5 billion years, probably leading to the formation of a giant elliptical or lenticular galaxy. But no one on Earth will witness this event. Due to the warming Sun, the surface of the Earth will become lifeless in a billion years or so.

Maria Mitchell: America’s First Female Astronomy Professor

Maria (pronounced Ma-RYE-ah) Mitchell (1818-1889), America’s first female professor of astronomy, was born August 1, 1818 on Nantucket Island (Massachusetts). Her interest in astronomy was encouraged by her father, and she assisted him with his research at a time when few women were allowed an opportunity to do scientific research. She discovered a comet in 1847 at the age of 29, and this brought her fame as one of America’s few women scientists. She was employed for many years as a computer (a person who performs lengthy mathematical calculations), and then taught astronomy at Vassar College for many years (1865-1888), a women’s college in Poughkeepsie, New York. At Vassar, she was also the director of the Vassar College Observatory. A devoted teacher, she believed that students learn best by doing real research projects. In 1869, she traveled to Burlington, Iowa with six of her students to observe a total solar eclipse.

Seven years after the death of Maria Mitchell, her sister, Phebe Mitchell Kendall, (1828-1907) compiled a book, Maria Mitchell: Life, Letters, and Journals (1896).

The Maria Mitchell Observatory was established on Nantucket Island in 1908, and today continues its long legacy of public outreach and undergraduate research.

“When we are chafed and fretted by small cares, a look at the stars will show us the littleness of our own interests.”

“We travel to learn; and I have never been in any country where they did not do something better than we do it, think some thoughts better than we think, catch some inspiration from heights above our own.”

“Question everything.”

“The best that can be said of my life so far is that it has been industrious, and the best that can be said of me is that I have not pretended to what I was not.”

Quotes by Maria Mitchell

Star-Shy Asteroids

Thanks to Gaia, many star positions (and proper motions) and minor planet positions (orbits) have improved so much that those of us who try to observe stellar occultations by minor planets have recently seen a vast improvement in our likelihood of success. These occultation events are an excellent way to discover minor planet satellites as well as double stars. At the very least, they provide highly accurate minor planet astrometric positions that lead to more accurate orbits, and if several observers record an event, the size and shape of the minor planet can be more accurately determined.

Perhaps surprisingly, a number of low-numbered (and thus generally larger) minor planets have never been observed to occult a star. Here are the ten lowest-numbered minor planets still awaiting their first-observed stellar occultation event.

To predict future stellar occultation events for any given minor planet (and so much more!), use the latest version of Occult – Occultation Prediction Software by David Herald.

Last Updated: December 16, 2024

157 Dejanira
Main-belt Asteroid. Diameter 19.959 ± 2.476 km.
Discovered 1875 Dec 1 by A. Borrelly at Marseilles.
Named for the second wife Dejanira (Greek: Deianeira) of Heracles; Megara was the first. She unwittingly killed Heracles by sending him a garment steeped in the poisoned blood of the centaur Nessus. This garment, Nessus has said, had power to reclaim a husband from unlawful loves.
https://en.wikipedia.org/wiki/157_Dejanira

180 Garumna
Main-belt Asteroid. Diameter 23.440 ± 0.414 km.
Discovered 1878 Jan 29 by J. Perrotin at Toulouse.
Named for the Garonne river on which the city of discovery is situated. Garumna is the ancient name.
https://en.wikipedia.org/wiki/180_Garumna

183 Istria
Main-belt Asteroid. Diameter 32.927 ± 0.168 km.
Discovered 1878 Feb 8 by J. Palisa at Pola.
Named for the {now Croatian} peninsula at the northern end of the Adriatic sea, containing Trieste and the city of discovery. Named by Vice-Admiral B. Freiherr von Wüllerstorf who was the commander of the first Austrian circumnavigatory adventure with the frigate Novara.
https://en.wikipedia.org/wiki/183_Istria

228 Agathe
Main-belt Asteroid. Diameter 9.30 ± 0.8 km.
Discovered 1882 Aug 19 by J. Palisa at Vienna.
Named in honor of the youngest daughter of Theodor von Oppolzer (1841-1886), professor of astronomy in Vienna.
https://en.wikipedia.org/wiki/228_Agathe

244 Sita
Main-belt Asteroid. Diameter 11.077 ± 0.022 km.
Discovered 1884 Oct 14 by J. Palisa at Vienna.
Named possibly for the wife of Rama in the Sanskrit epic The Ramayana. It is a symbol of the ideal spouse and of everlasting faith.
https://en.wikipedia.org/wiki/244_Sita

262 Valda
Main-belt Asteroid. Diameter 14.645 ± 0.141 km.
Discovered 1886 Nov 3 by J. Palisa at Vienna.
Any reference of this name to a person or occurrence is unknown. Name proposed by the Baroness Bettina von Rothschild.
https://en.wikipedia.org/wiki/262_Valda

263 Dresda
Main-belt Asteroid. Diameter 23.952 ± 0.213 km.
Discovered 1886 Nov 3 by J. Palisa at Vienna.
The planet is named to honor the German city of Dresden.
https://en.wikipedia.org/wiki/263_Dresda

281 Lucretia
Main-belt Asteroid. Diameter 11.036 ± 0.145 km.
Discovered 1888 Oct 31 by J. Palisa at Vienna.
Named in honor of Lucretia Caroline Herschel (1750-1848), sister of the discoverer (1781) of Uranus, Sir William Herschel (1738-1822), whom she assisted, beginning in 1772. She independently discovered seven or eight comets. After her brother’s death, she returned from England to Hannover, Germany and constructed a catalogue of the nebulae and clusters discovered by him. She received the Gold Medal of the Royal Astronomical Society in 1828.
https://en.wikipedia.org/wiki/281_Lucretia

291 Alice
Main-belt Asteroid. Diameter 10.456 ± 0.419 km.
Discovered 1890 Apr 25 by J. Palisa at Vienna.
Name of unknown origin. Named by the Société Astronomique de France at the invitation of the discoverer. Independently discovered by A. Charlois at Nice one night later.
https://en.wikipedia.org/wiki/291_Alice

296 Phaëtusa
Main-belt Asteroid. Diameter 8.196 ± 0.100 km.
Discovered 1890 Aug 19 by A. Charlois at Nice.
Names for one of the daughters of Apollo and Klymene, changed by Zeus into poplars after the death of their brother Phaethon.
https://en.wikipedia.org/wiki/296_Pha%C3%ABtusa

References
Schmadel, Lutz D. 2012. Dictionary of Minor Planet Names. 6th ed. Berlin, Germany: Springer. https://doi.org/10.1007/978-3-642-29718-2.

Solar System Dynamics. (Downloaded 16 Dec 2024). (Small-Body Database Lookup). https://ssd.jpl.nasa.gov

Solar Siblings

When our Sun formed 4.6 billion years ago, it almost certainly was a member of an open star cluster. Over several hundred million years, most of the stars in this cluster would have dissipated. Is there any hope, then, of finding some of our solar siblings?

I ran a query against the Gaia DR3 database to find stars with radial velocities and proper motions that are zero, within the measurement uncertainties. In other words, their space motions appear to be similar to that of the Sun. Could some of these stars be our long lost solar siblings?

First, some caveats.

4.6 billion years is a lot of time, and dynamical evolution may lead to solar siblings no longer having comparable space motions to the Sun.
Error bars for the radial velocities, proper motions, and distances of many of these stars are large enough that subsequent more precise measurements may show that they are not co-moving with the Sun.
Though radial velocities are not affected by increasing star distance, proper motions are; therefore, proper motion in right ascension and declination will approach zero with increasing stellar distance
Some co-moving stars will be coincidental, especially if they are at large distances

I found 230 candidate stars in Gaia DR3 that appear to be co-moving with the Sun. They are listed in the table below.

Gaia DR3 Zero Space Motion

wdt_ID	wdt_created_by	wdt_created_at	wdt_last_edited_by	wdt_last_edited_at	Gaia DR3 SOURCE_ID	Other Catalog	RA (2016)	Dec (2016)	G Mag	Distance (ly)
1	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	5534600793005666944	TYC 7663-2637-1	08 05 30	- 40 05 11	10.63	2,100
2	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	4044381556633823232	HD 321719	18 25 18	- 34 39 16	10.91	3,776
3	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	5933186123279263872	TYC 8323-81-1	16 15 34	- 52 29 35	11.30	2,930
4	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	4069457877771166464		18 00 01	- 22 47 10	11.33	5,673
5	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	5926323972473953792	TYC 8349-1491-1	17 19 13	- 50 14 57	11.85	999,999
6	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	1816548038377615872	TYC 1639-1018-1	20 22 27	+ 20 06 07	11.86	1,208
7	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	3403073120299336960	UCAC4 557-018920	05 44 22	+ 21 14 45	12.00	1,720
8	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	5316984970605614208		08 46 50	- 54 57 33	12.08	1,192
9	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	2224937958644193920	V898 Cep	22 38 02	+ 67 27 58	12.19	1,998
10	do18559252	30/04/2024 09:51 AM	do18559252	30/04/2024 09:51 AM	4103489613769523712		18 42 09	- 14 55 00	12.21	2,988
					Gaia DR3 SOURCE_ID	Other Catalog	RA (2016)	Dec (2016)	G Mag	Distance (ly)

Please note that a distance of 999,999 ly (light years) indicates a Gaia parallax that is negative, meaning that the star is so far away that a reliable parallax cannot be measured. In other words, it is zero. Also, the farther away the star is, the more uncertainty there is in the distance.

19 of these 230 stars are bright enough, important enough, or lucky enough to have entries in the SIMBAD database. The nearest of these is TYC 8312-3134-1 which is 518 ly away in the constellation Norma.

We can do a simple BOTEC to determine how fast TYC 8312-3134-1 would have to be moving relative to the Sun to travel 518 ly in 4.6 Gyr. The answer is just 0.03 km/s = 30 meters/second. This is much less than the typical space motion of stars in the solar neighborhood relative to the Sun, which is on the order of many kilometers per second. It is therefore completely plausible that solar siblings could now be at a distance of at least 500 ly and even many times further than that.

Reference

SELECT TOP 2000   gaia_source.source_id,gaia_source.ra,gaia_source.dec,gaia_source.parallax,gaia_source.pmra,gaia_source.pmdec,gaia_source.ruwe,gaia_source.phot_g_mean_mag,gaia_source.bp_rp,gaia_source.radial_velocity,gaia_source.radial_velocity_error,gaia_source.phot_variable_flag,gaia_source.non_single_star,gaia_source.has_xp_continuous,gaia_source.has_xp_sampled,gaia_source.has_rvs,gaia_source.has_epoch_photometry,gaia_source.has_epoch_rv,gaia_source.has_mcmc_gspphot,gaia_source.has_mcmc_msc,gaia_source.teff_gspphot,gaia_source.logg_gspphot,gaia_source.mh_gspphot,gaia_source.distance_gspphot,gaia_source.azero_gspphot,gaia_source.ag_gspphot,gaia_source.ebpminrp_gspphot
FROM gaiadr3.gaia_source 
WHERE (gaiadr3.gaia_source.radial_velocity-gaiadr3.gaia_source.radial_velocity_error <= 0)
  and (gaiadr3.gaia_source.radial_velocity+gaiadr3.gaia_source.radial_velocity_error >= 0)
  and (gaiadr3.gaia_source.pmra-gaiadr3.gaia_source.pmra_error <= 0)
  and (gaiadr3.gaia_source.pmra+gaiadr3.gaia_source.pmra_error >= 0)
  and (gaiadr3.gaia_source.pmdec-gaiadr3.gaia_source.pmdec_error <= 0)
  and (gaiadr3.gaia_source.pmdec+gaiadr3.gaia_source.pmdec_error >= 0);

The Coldest Place on Earth

The coldest temperature ever reliably recorded on the surface of the Earth occurred on July 21, 1983, when a temperature of -128.6° F was recorded at Vostok Station, Antarctica (φ = 78° 27′ 52″ S, λ = 106° 50′ 14″ E, elevation 11,444 ft.). Located at the center of the East Antarctic ice sheet, Vostok Station is prone to extremely cold temperatures given its high elevation and location far inland (~868 miles) from the moderating influence of the ocean. Other contributing factors to the low temperature are the extremely low humidity (water vapor retains heat near the surface) and the high albedo of the snow and ice which reflects much solar radiation back out into space.

Vostok Station is the most isolated of all the established research stations on the Antarctic continent. Only about 30 scientists and engineers reside at Vostok Station during the summer months, but during winter that number dwindles to about 15.

The monthly average temperature at Vostok is as follows: April -84.6°F, May -86.4°F, June -85.5°F, July -88.1°F, August -90.2°F, September -86.8°F, October -70.8°F, November -44.7°F, December -25.2°F, January -25.6°F, February -47.7°F, March -72.2°F. The warmest temperature ever recorded at Vostok was +6.8° F on January 5, 1974.

For the current temperature at several locations within Antarctica, including Vostok, see https://www.timeanddate.com/weather/antarctica [apologies for the display ads, but out of my control]. For a Vostok weather forecast, visit https://www.yr.no/en/forecast/daily-table/2-6620791/Antarctica/Vostok%20Station. Planning a trip soon?

Vostok is a desert, averaging just 0.9 inches of snowfall each year. Does any non-dormant indigenous life exist at Vostok Station? No. Except for the human presence there, on the surface it is lifeless. But, fortuitously, Vostok Station sits above a giant freshwater lake called, appropriately, Lake Vostok, 13,100 feet under the ice. Scientists believe that life exists there, but they want to be very, very careful not to biocontaminate the lake as they begin exploring it in earnest.

It is interesting for us to ponder the possibility that sub-surface life exists on Mars and some of the satellites in the outer solar system. Though far more difficult than Lake Vostok to explore, someday we will.

Incidentally, at higher elevations along the Eastern Antarctica Plateau (specifically, along the ridge between Dome Argus and Dome Fuji), satellite measurements between 2010 and 2013 indicate that even colder surface temperatures than at Vostok Station have been reached, perhaps even as low as -144°F. However, since these are not surface temperature measurements, the current Vostok Station record of -128.6° F still holds as the coldest temperature ever recorded on Earth. And with anthropogenic global warming, that record is not likely to be broken anytime soon.

Factino: Did you know that it takes a lot more energy to cool down humid air than to cool down dry air? Air conditioners cool much more efficiently in Arizona and New Mexico than they do in Florida and Louisiana. Moreover, evaporative coolers in desert areas can reduce energy use by 80% or more over refrigerative air conditioning, but they only work well in dry climates.

Land of the Long Twilights

The first (and only!) sunset¹ this year at Amundsen–Scott South Pole Station in Antarctica occurs on March 22 at 0615 UTC (using “astronomer’s time” as time zone has no meaning so close to the South Pole).

The year’s first and only end of civil twilight (when the geometric center of the Sun lies 6° below the horizon) occurs on April 4 at 1153 UTC. That’s 13^d05^h38^m after sunset.

The year’s first and only end of nautical twilight (when the geometric center of the Sun lies 12° below the horizon) occurs on April 21 at 0409 UTC. That’s 16^d16^h16^m after the end of civil twilight.

The year’s first and only end of astronomical twilight (when the geometric center of the Sun lies 18° below the horizon) occurs on May 11 at 0521 UTC. That’s 20^d01^h12^m after the end of nautical twilight, and 49^d23^h06^m after sunset. That’s one heck of a long twilight!

Night lasts from May 11 at 0521 UTC until astronomical twilight begins on July 31 at 1916 UTC. A duration of 81^d13^h55^m.

Nautical twilight begins on August 21 at 0024 UTC. That’s 20^d05^h08^m after the beginning of astronomical twilight.

Civil twilight begins on September 6 at 2121 UTC. That’s 16^d20^h57^m after the beginning of nautical twilight.

The first and only sunrise of the year occurs on September 20 at 0945 UTC. “Morning” twilight lasts a total of 50^d14^h29^m.

The Sun remains above the horizon continuously until sunset on March 22, 2025 at 1100 UTC. Daylight “hours” last 183^d01^h15^m.

Strange place!

¹Sunrise and sunset. For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of the center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun’s center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes).
[Reference: https://aa.usno.navy.mil/faq/RST_defs, but see here:
https://digitalcommons.mtu.edu/etdr/697/]

Note: SkySafari 6 Pro, Version 6.8.2 (6820) for MacOS was used to determine these dates and times. The location coordinates used for Amundsen–Scott South Pole Station were 89° 58′ 59.9″ S, 139° 16′ 01.2″ E, 2835 m.

Fun Fact: Did you know that there is a seismic station near the south pole, and that it has been operating since 1957?