Yearbook of Astronomy 2021

Brian Jones

• Publisher: White Owl
• Year: 2020
• ISBN:
  9781526771889
• Topic: Ciencias físicas
• Subtopic: Astronomía y astrofísica

The annual treasury for sky-watchers and stargazers, including references and a variety of fascinating articles. The Yearbook of Astronomy series is renowned for its comprehensive jargon-free monthly sky notes and authoritative sky charts that enable backyard astronomers and sky-gazers everywhere to plan their viewing of the year's eclipses, comets, meteor showers, and minor planets, as well as detailing the phases of the moon and visibility and locations of the planets throughout the year. Every annual edition also includes a variety of entertaining and informative articles. Among the wide-ranging articles in this 2021 edition are:

• Male Family Mentors for Women in Astronomy
• Henrietta Swan Leavitt and Her Work
• Solar Observing
• Obsolete Constellations
• Lunar Volcanism
• Pages From the Past: Collecting Vintage Astronomy Books
• Maori Astronomy in Aotearoa-New Zealand, and more

In addition you'll find the first in a series entitled Mission to Mars: Countdown to Building a Brave New World, scheduled to appear in the Yearbook throughout the 2020s to keep you fully up to date with the ongoing investigations, research, and preparations that are already underway, as well as those in the planning phase, geared towards sending a manned mission to Mars around the end of the decade. We are at the start of what promises to be an exciting journey—and the Yearbook of Astronomy continues to be an essential companion.

Article Section

Astronomy in 2020

Rod Hine

Planck Legacy Data – a Twist in the Tail?

The Planck Observatory, which operated from 2009 to 2013, produced the definitive data on the Cosmic Microwave Background radiation (CMB) which is one of the fundamental pillars of the “Standard Model of Cosmology”. Widely accepted, the model explains how the universe was created from pure energy some 13.8 billion years ago and is now formed from a mixture of dark energy, cold dark matter and ordinary matter in very specific proportions. To explain the detailed path from the instant of creation to the currently observed distribution of stars and galaxies there are many assumptions and theories to make the model work satisfactorily, few more important than the “period of inflation”.

It was American theoretical physicist and cosmologist Alan Harvey Guth, then working at Cornell University, USA, who first shared his ideas at a seminar in 1980 to show how a very short but intense episode of exponential expansion of the universe could perhaps explain some puzzling features of the CMB. Guth and others continued to develop the theory further and were able to show that it accounted for the isotropic nature of the universe; it showed why the CMB was distributed so evenly across the sky and why the universe appears to be flat. In this context, flatness means that there is no curvature and presumably the universe is infinite in extent. For a prosaic demonstration that we live in a flat part of the universe we can merely observe that the interior angles of a triangle add up to 180º – as simple as that.

Given that a flat universe is so much a part of the Standard Model, some recent work by Alessandro Melchiorri at the Sapienza University of Rome has caused some consternation. Melchiorri and his colleagues studied the recently released Planck Legacy 2018 final data set for signs of gravitational lensing. This phenomenon arises where light rays are bent and focused by passing close to massive objects located between the source and the observer, and in the CMB data gives a measure of the amount of dark matter. Their paper, published in Nature Astronomy on 4 November 2019, shows that there could be a lot more dark matter than hitherto assumed, and that this extra dark matter could pull the universe into a finite sphere. They assign a confidence level of more than 99% that the results favour a closed universe with positive curvature.

The local geometry of the universe is determined by whether the density parameter Ω is greater than, less than, or equal to 1. From top to bottom: a spherical universe with Ω > 1; a hyperbolic universe with Ω < 1; and a flat universe with Ω = 1. These depictions of two-dimensional surfaces are merely easily-visualizable analogs to the three-dimensional structure of (local) space. (NASA Official: Gary Hinshaw)

If this is correct, and the universe is not flat after all, then numerous other problems arise for cosmologists. It has been difficult enough in a flat universe to account for the increasing rate of expansion, attributable to dark energy, and will be even tougher for a closed spherical universe. For many cosmologists, the hope is that this observation is some kind of statistical fluke and can be rebutted.

There are no other observations to back it up so far, but it may not be the end of the story when the Simons Observatory in Chile begins operations in the early 2020s. Funded by a generous grant of $40 million from the Simons Foundation of New York, USA, together with contributions from many participating universities, the site in Cerro Toco, Atacama Desert, Chile, will host a set of radio telescopes to measure the CMB with utmost precision at frequencies from 27 GHz to 280 GHz. The CMB peaks at 160 GHz so the new telescopes will be able to cover the most interesting parts of the spectrum but with much finer resolution than the Planck satellite. In addition to the main Large Aperture Telescope (LAT) with a 6 metre mirror there will be three Small Aperture Telescopes (SAT) of 0.5 metre aperture to form an array. Overall the resolution will be at least an order of magnitude better than Planck with its under 2 metre primary mirror. The instruments in the LAT will be housed in a huge cryostat to reduce noise and increase sensitivity. The unusual design, the so-called “crossed Dragone” configuration, allows a compact fully-steerable mirror to deliver the signals to the horizontally mounted cryostat fixed on the structure and therefore more accessible and protected than conventional mounting at the prime focus of a paraboloid.

A cross section through the Simons Observatory Large Aperture Telescope showing the mirrors housed in the elevation structure. The white cylinder on the right is the 2.4 meter diameter cryostat. (Wikimedia Commons/Sdicker – Own Work)

Gravitational lensing of the CMB is high on the observing priority for the Simons Observatory and it will be fascinating to see whether Melchiorri’s results hold good or can be dismissed. There are several other anomalies in the Standard Model, for example the unexplained variation in the Hubble Constant when calculated from different metrics. Another major spanner in the works could undermine the Standard Model and cause a genuine cosmological crisis.

The Mystery of Variable Quasars

Since the early 1960s the Santa Catalina Mountains close to Tucson, Arizona, USA, have been home to an observatory run by the University of Arizona. Largely inspired by the famous Gerard Kuiper of that same University, the observatory was moved from its original site on Mount Bigelow to Mount Lemmon in 1971. At that time there were two telescopes on Mount Lemmon, one of 1.5 metre aperture and another of 1 metre. In 1998 a team came together for a new project called the Catalina Sky Survey (CSS). Led by staff scientist Steve Larson and two undergraduates, Tim Spahr and Carl Hergenrother, they managed also to secure access to the unused 0.7 metre Schmidt Telescope that remained on Mount Bigelow and began an extensive photographic survey of objects at higher ecliptic latitude. With help from NASA they began to search for NEOs (Near Earth Objects) and eventually established the Siding Spring Survey in New South Wales, Australia, also dedicated to NEO studies.

In addition to discovering over three hundred comets and numerous asteroids, the same telescopes provide data for the Catalina Real-time Transient Survey (CRTS), founded in 2007 and operated from California Institute of Technology (Caltech), which notifies observers immediately of any optical transient events detected in the images.

This artist’s impression of one of the most distant, oldest, brightest quasars ever seen is hidden behind dust. The quasar dates back to less than one billion years after the big bang. The dust is also hiding the view of the underlying galaxy of stars that the quasar is presumably embedded in. (NASA/ESA/G.Bacon, STScI)

A recent publication by a group from Caltech led by Matthew Graham has used the CSS and CRTS data to try to resolve one of the strangest features of quasars. Conventional wisdom describes quasars as supermassive black holes at the core of incredibly distant galaxies, devouring gas and dust from its surroundings and shining as a beacon across billions of light-years. Such a scenario should take tens of thousands of years to deplete the local supply of material yet some quasars have been observed to fade out in timescales of as little as a year, leaving an unexceptional ordinary-looking galaxy in its place. Some other quasars have suddenly flared into existence from otherwise normal galaxies, again with suspiciously short timescales.

A group at JPL, Pasadena, led by Chelsea MacLeod of Harvard Smithsonian Centre for Astrophysics, Cambridge, Massachusetts, had already compiled a list of more than 200 highly variable quasars and active galactic nucleus (AGN) objects that could be described as “changing-look quasars” (thus adding CLQ to the list of astronomical acronyms). Published in early 2019 in the Astrophysical Journal, they speculated on the many possible explanations and specifically on whether the variations were intrinsic to the quasar or extrinsic, i.e. arising from the quasar’s surroundings.

Graham and his colleagues systematically searched the CSS and CRTS data for quasars which exhibited different variability behaviours such as periodic variability, major flaring episodes and extreme variations in spectral line properties. By studying such objects in all possible wavelengths from infrared to visible, they have observed that when the visible light from a quasar changes the same change is echoed afterwards in the infrared. Since the visible light mostly emanates from the accretion disk and the infrared comes from the more distant surrounding clouds of cooler dust this strongly suggests that the visible light warms the dust which then re-radiates the energy in the infrared, and so the rapid changes are due to changes in the accretion disk itself, as unlikely as that seems. Previous explanations had suggested that quasars might have been eclipsed by dust or magnified by chance gravitational lensing events but, for these quasars at least, that now seems unlikely. The extreme variations are indeed intrinsic to the quasar.

Graham has gone so far as to classify at least 73 quasars as “changing-state quasars” (CSQ) from hundreds of candidate CLQs and has shown that this model can account for the rapidly-varying brightness by heating and cooling wave-fronts propagating through the accretion disk in the observed timescales. It still isn’t clear exactly what causes the rapid variations in the first place, and there may be several different causes according to John Ruan of McGill University, Montreal, Canada. Based on the wide diversity of the examples he says, “I would not be surprised at all if changing-look quasars are due to a variety of things.” Ruan was not involved in the study and is a post-doctoral Fellow at McGill specialising in AGN variability and accretion disk transitions.

First Results from NASA’s Parker Solar Probe

Launched in August 2018, the Parker Solar Probe, named for Emeritus Professor Eugene Parker of the University of Chicago, has sent back results of its first hazardous flyby of the Sun. In 24 successive approaches over the next seven years or so, the spacecraft will fly ever closer, using Venus’s gravity to dive through the Sun’s intense heat and radiation to within 3.8 million miles (about 6 million kilometres) of the photosphere. At its closest approach it will be travelling at 430,000 mph (700,000 km/h) and the heat shield will reach 1,377ºC. Key to the spacecraft’s survival is the 12 centimetre thick heat shield which will always be facing the Sun and will keep the rest of the craft at sensible room temperature.

This almost suicidal mission will make measurements of magnetic fields, radiation, the corona and the solar wind to try to discover exactly how the outer atm...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Editor’s Foreword
• Preface
• About Time
• Using the Yearbook of Astronomy as an Observing Guide
• The Monthly Star Charts
• Monthly Sky Notes and Articles 2021
• Article Section
• Miscellaneous
• Astronomical Organizations
• Our Contributors
• Glossary Brian Jones and David Harper