Astronomy 161 - Introduction to Solar System Astronomy

Before we can begin our exploration of astronomy, we need to develop a common language for notating large numbers, and introduce the basic units of length, mass, and time that we will use throughout the quarter. We will first re-introduce the basic metric system, explaining how these units have a physical basis. For measuring lengths in astronomy, we need to introduce two special units: the Astronomical Unit, which is used to discuss interplanetary distances, and the Light Year, used for interstellar distances. We end with a discussion of mass and weight, and the distinction draw in physical measurements that differs (a little) from everyday usage. Recorded 2006 Sep 21 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What are the constellations, and how have they be named and used by many different cultures throughout human history? We will review the most basic feature of the night sky, the 6000 visible stars sprinkled about the sky, and introduce the idea of constellations, reviewing their history and uses. We'll end with a brief discussion of where stars get their names. Recorded 2006 Sep 22 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is the shape and size of the Earth? This lecture traces historical ideas about the shape of the Earth, from ancient flat-Earth models to the compelling demonstrations by Aristotle in the 3rd century BC that the Earth was a sphere. We then discuss ways people measured the size of the Earth, describing the results of Eratosthenes of Cyrene in the 2nd century BC and Claudius Ptolemy in the 2nd century AD, and their impact. Recorded 2006 Sep 25 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Where are we? Where is someplace else? How do I get from here to there? These are questions we need to answer both on the Earth and in the Sky to assign a location to a place or celestial object on the surface of a sphere. We start by introducing angular units, and use them to describe the terrestrial system of latitude and longitude on the spherical Earth. We then define the Celestial Sphere, with its Celestial Equator and Poles, and begin to define an analogous coordinate system on the sky. An important wrinkle is that what part of the sky we see at any given time depends on both where we are on the Earth, and what date/time it is. This gives us the start of the coordinate system we need to begin our exploration of motions in the sky in the next lectures. Recorded 2006 Sep 26 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why do celestial objects appear to rise in the East and set in the West? How does this depend on where you are on the Earth, or the time of year? Today we set the heavens into motion, and look at the two most basic types of celestial motions. Apparent daily motions are a reflection of the daily rotation of the Earth about its axis. The apparent annual motions are a reflection of the Earth's orbit around the Sun. To describe the Sun's apparent annual motion, we introduce the Ecliptic, the Obliquity of the Ecliptic, and four special locations along the Ecliptic: the Solstices and Equinoxes. This will set the stage for much of our discussions in rest of this section. Recorded 2006 Sep 27 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why do we have different seasons? This lecture looks at the consequences of the tilt of the Earth's rotation axis relative to its orbital plane (the Obliquity of the Ecliptic) combined with the apparent annual motions of the Sun around the Ecliptic. The important factor determining whether it is hot or cold at a given location at different times in the year is "insolation": how much sunlight is spread out on the ground. This, combined with the different length of the day when the Sun as at different declinations, determines to total amount of solar heating per day, and drives the general weather. It has nothing, however, to do with how far away we are from the Sun at different times of the year. Finally, the direction of the Earth's rotation axis slowly drifts westward, taking 26,000 years to go around the sky. This "Precession of the Equinoxes" represents a tiny change that is still measureable by pre-telescopic observations, and means that at different epochs in human history there is a different north pole star, or none at all! Recorded 2006 Sep 28 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How does the Moon appear to move through the night sky? This lecture introduces the Moon, and describes the monthly cycle of phases. Topics include synchronous rotation, apogee and perigee, the cycle of phases, and the sidereal and synodic month. Recorded 2006 Sep 29 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Eclipses of the Sun and Moon are among the most glorious spectacles in the sky. This lectures looks at the causes and types of eclipses, and how often they occur. Recorded 2006 Oct 2 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What time is it? This lecture is the first part of a two-part exploration of the astronomical origins of our time-keeping and calendar conventions. Today we will discuss the division of the year into seasons by the motions of the Sun, and the oft-forgotten origins of our holidays in in the solar Quarter and Cross-Quarter days, the division of the year into 12 months based approximately on the cycle of lunar phases, the traditional division of the month into weeks reflecting the seven moving celestial bodies, and the division of the day into hours, minutes, and seconds. We will also discuss the difference between the Solar and Sidereal days, and the introduction of timezones used in modern civil timekeeping. Recorded 2006 Oct 3 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why are there leap years? This lecture explores the astronomical origins of the calendar. We will discuss lunar and solar calendars and their hybrids in history and tradition (for example, the Islamic Lunar Calendar and the Jewish Luni-Solar Calendar), and the Julian and Gregorian Calendar reforms. Recorded 2006 Oct 4 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How do the planets move across the sky? This lecture will review planetary motions, specifically the apparent motions of the five classical planets (Mercury, Venus, Mars, Jupiter, and Saturn) as seen from the Earth. We will discuss the classical division of the naked-eye planets into inferior (Mercury and Venus) and superior (Mars, Jupiter, and Saturn) planets, and describe their main configurations in the sky: conjunction, opposition, maximum elongation, and quadrature. We will then discuss retrograde motion, the apparent westward reversal of motion seen at opposition in the superior planets and inferior conjunction in inferior planets. The quest to describe the very complex motions of the planets marks the birth of science, and will be the central theme of next week's lectures. Recorded 2006 Oct 5 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What are the origins of the Geocentric and Heliocentric models put foward to explain planetary motion? This lecture begins a new unit that will chart the rise of our modern view of the solar system by reviewing the highly influential work by Greek and Roman philosophers who elaborated the first geocentric and heliocentric models of the Solar System. We discuss the various geocentric systems from the simple crystaline spheres of Anaximander, Eudoxus, and Aristotle through the Epicyclic systems of Hipparchus and Ptolemy. We will also briefly discuss what is known of Aristarchus' mostly-lost heliocentric system, which was to so strongly influence the work of Copernicus. The ultimate expression of an epicyclic Geocentric system was that described by Claudius Ptolemy in the middle of the 2nd Century AD, and was to prevail virtually unchallenged for nearly 14 centuries. Recorded 2006 Oct 9 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

In 1543, Nicolaus Copernicus re-introduced the Heliocentric idea of Aristarchus of Samos in an attempt to purge Ptolemy's geocentric system of the un-Aristotelian idea of the Equant. His goal was to derive a model that, in his words, pleased the mind as well as preserved appearances. What he started, without intending, was a profound revolution in thought that was to overturn both Ptolemy and Aristotle within two centuries, and help give birth the the modern world. This lecture looks at the Copernican system, and sets the stage for the scientific revolution of the following generations. Recorded 2006 Oct 10 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

In the generation following Copernicus, the question of planetary motions was picked up by two remarkable astronomers: Tycho Brahe, the brilliant Danish astronomer whose precise measurements of the planets represented the highest expression of pre-telescope astronomy, and Johannes Kepler, the brilliant and tormented German mathematician who used Tycho's data to derive his three laws of planetary motion. These laws were to sweep away the vast complex machinery of epicycles, and provide a geometric description of planetary motions that set the stage for their eventual physical explanation by Isaac Newton a generation later. Recorded 2006 Oct 11 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Tycho did as much as could be done with the naked eye, a new technology was required to extend our vision, the telescope. This lecture introduces Galileo Galilei, the contemporary of Kepler who was in many ways the first modern astronomer, and his discoveries with the telescope. These observations were to electify Europe in the early 17th century, and begin the final intellectual dismantling of the Aristotelian view of the world. Galileo's claims that they constituted proof of the Copernican Heliocentric System, however, were to bring him into conflict with the Roman Catholic Church. Recorded 2006 Oct 12 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

The work of Copernicus, Kepler, and Galileo all contributed to a new way of looking at the motions in the heavens, but did not explain why they move that way. Enter Isaac Newton, who within a few years swept away the last vestiges of the Aristotelian view of the world and replaced with a new, powerfully predictive synthesis, in which all motions, in the heavens and on the Earth, obeyed three simple, mathematical laws of motion. This lecture introduces Newton's Three Laws of Motion and their consequences. We are now ready, next week, to examine the role of Gravity and finally explain the orbits of the planets. Recorded 2006 Oct 13 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is Gravity? This lecture reviews the law of falling bodies first described by Galileo, and then Newton's explanation in terms of his Law of Universal Gravitation. Gravity is a mutually attractive force that acts between any two massive bodies. Its strength is proportional to the product of the two masses, and inversely proportional to the square of the distance between their centers. We then compare the fall of an apple on the Earth to the orbit of the Moon, and show that the Moon is held in its orbit by the same gravity that works on the surface of the Earth. In effect, the Moon is perpetually "falling" around the Earth. Recorded 2006 Oct 16 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why do Kepler's Laws work? This lecture discusses how Newton applied his Three Laws of Motion and the Law of Universal Gravitation to the problem of orbits. Newton generalized Kepler's laws to apply to any two massive bodies orbiting around their common center of mass. We discuss these new, generalized laws of orbital motion, introducing the families of open and closed orbits, circular and escape velocity, center-of-mass, conservation of angular momentum, and how orbital mechanics is used to measure the masses of astronomical objects. Recorded 2006 Oct 17 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why are there two high tides a day? This lecture examines another of the consequences of gravity, the twice-daily tides raised on the Earth by the Moon. Tides are a consequence of differences in the gravity force of the Moon from one side to the other of the Earth (stronger on the side nearest the Moon, weaker on the side farthest from the Moon). The Sun raises tides on the Earth as well, about half as strong as Moon tides, giving rise to the effect of Spring and Neap tides that strongly correlate with Lunar Phase. We also look at body tides raised on the Moon by the Earth, and how that has led to Tidal Locking of the Moon's rotation, which is why the Moon always keeps the same face towards the Earth. We then explore the combined effects of tidal braking of the Earth, which slows the Earth's rotation and increases the length of the day by about 23 milliseconds per century, and causes the steady Recession of the Moon, which moves 3.8cm away from Earth every year. Tidal effects are extremely important to understanding the Dynamical Evolution of many bodies in the Solar System, as we'll see time and again in the second half of the class. Recorded 2006 Oct 18 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How do we prove physically that the Earth rotates on its axis and revolves around the Sun? Newtonian physics was so compelling that it was mostly accepted before there were ironclad physical demonstrations of the Earth's daily rotation about its axis and annual revolution (orbit) around the Sun. This lecture reviews three of these demonstrations: the Coriolis Effect, the Foucault Pendulum, and Stellar Parallaxes. This ties up the last loose-end of the Copernican Revolution. Recorded 2006 Oct 19 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is Light? This lecture reviews the basic properties of light, introducing the inverse square law of brightness and the Doppler Effect. Recorded 2006 Oct 23 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is Matter? This lecture reviews the nature of matter from subatomic to atomic scales, and introduces the ideas of atomic structure, atomic number (number of protons), the elements, isotopes, radioactivity, and half-life. We conclude with a brief overview of the four fundamental forces of nature: gravitation, electromatgnetic, and the strong and weak nuclear forces. Recorded 2006 Oct 24 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How do matter and light interact? This lecture is the first of a two-part lecture on the physical basis of spectroscopy. Today we will discuss the Kelvin Absolute Temperature scale, which provides a measure of the internal energy content of matter, and Kirchoff's empirical Laws of Spectroscopy, along with the Stefan-Boltzmann Law and the Wein Law to describe the continuous emission from a blackbody. We will end by briefly describing the suggestive properties of emission- and absorption-line spectra, whose explanation in the details of atomic structure will be the topic of the next lecture. Recorded 2006 Oct 25 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why does each chemical element have its own unique spectral-line signature? How do emission- and absorption-line spectra work? This lecture is the second part of a two-part exploration of the interaction between matter and light, today discussing how the unique spectral-line signatures of atoms are a reflection of their internal electron energy-level structure. We will discuss energy level diagrams for atoms, excitation, de-excitation, and ionization, and do a short demonstration with gas-discharge tubes and slide-mounted diffraction gratings. For podcast listeners, the last portion of the class is the demo, which we do not, unfortunately, have the resources to videotape. Recorded 2006 Oct 26 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Telescopes, equipped with advanced electronic cameras and spectrographs, are the primary tools of the astronomer. This lecture reviews the types of telescopes and observatory sites, and discusses radio and space telescopes, and reviews briefy the observing facilities at Ohio State. Recorded 2006 Oct 27 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How old is the Earth? This lecture reviews the idea of cyclic and linear time, since how you view time determines whether the question of the age of the Earth is even meaningful. We then review various ways people have estimated the age of the Earth, starting with historical ages that equate human history with the history of the Earth proper, and then see how various physical estimates, which do not make an appeal to human history, were made. This brings us to the technique of radioactive age dating of the oldest rocks, leading to our current best estimate of 4.5+/-0.1 Billion years for the age of our planet. Recorded 2006 Oct 30 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is the structure of the Earth? What better place to begin our exploration of the Solar System then with the best-studied planet, the Earth. This lecture discusses the interior structure of the Earth, introducing the idea of differentiation, how geologists map the interior of the Earth using seismic waves, and the origin of the Earth's magnetic field. We then discuss the crust of the Earth, which is divided into 16 tectonic plates, and explore how plate motions driven by convection in the upper mantle have shaped the visible surface of our planet over its dynamic history. Recorded 2006 Oct 31 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is the composition and structure of the Earth's atmosphere? Why is it as warm as it is, and how did it form? These are the questions for today's lecture. The Earth's atmosphere is a complex, dynamic, and evolving system. We will discuss the composition and structure of the atomsphere, the nature of the different thermal layers, the Greenhouse Effect, and the Primordial Atmosphere and atmospheric evolution. This will give us a basis for comparison when we begin to examine other planetary atmospheres in future lectures. Recorded 2006 Nov 1 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is the structure of the Moon, and what physical processes have shaped its surface? In this lecture we turn to our nearest celestial neighbor, the Moon, to see a world quite different than the dynamic Earth. We will discuss the surface features of the Moon (the Maria and cratered highlands), see how crater density tells us the relative ages of terrain, and look at the composition of Moon rocks returned by astronauts and robotic probes. We also discuss the interior of the Moon, and review what we know about lunar history and formation. Recorded 2006 Nov 2 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

We start our exploration of the Solar System with a quick overview of its constituent parts. I will take as my starting point that Pluto, Eris, and Ceres are Dwarf Planets according to the 2006 IAU decision. This decision, which is not without controversy, will be one of the questions we will revisit during these lectures. Recorded 2006 Nov 6 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How did the Solar System form? This lecture examines the clues in the present-day dynamics (orbital and rotation motions) of the planets and planetary composition to the formation of the solar system. We will then describe the accretion model, where grains condense out of the primordial solar nebula, grains aggregate by collisions into planetesimals, then gravity begins to work and planetesimals grow into protoplanets. What kind of planet grows depends on where the protoplanets are in the primordial solar nebula: close to the Sun only rocky planets form, beyond the Frost Line ices and volatiles can condense out, allowing the growth of the gas giants. The whole process took about 100 million years, and we as we explore the solar system we will look for traces of this process on the various worlds we visit. Recorded 2006 Nov 7 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Mercury is the innermost of the planets, a hot, dead world that has been heavily battered by impacts. This lecture reviews the basic properties of Mercury, particularly its surface and interior. Recorded 2006 Nov 8 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Venus, the second planet from the Sun, is perpetually veiled behind opaque clouds of sulfuric acid droplets atop a hot, heavy, mostly carbon dioxide atmosphere. In size and apparent composition, however, it is a near twin-sister of the Earth. Why is it do different? This lecture reviews the basic properties of Venus, and examines the similarties and differences with the Earth. Recorded 2006 Nov 9 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Mars, fourth planet from the Sun, is a cold desert planet with a thin, dry carbon-dioxide atmosphere. The geology of Mars, however, shows signs of an active past, with hot-spot volcanism, and tantalizing signs of ancient water flows. While a cold, dead desert planet today, Mars' past may have been warmer and wetter, with liquid water during the first third of its history. This lecture will review the properties of Mars, and discuss the evidences of its active past. Recorded 2006 Nov 13 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Having completed our tour of the terrestrial planets, we want to step back and compare their properties. In particular, we want to look at the processes that drive the evolution of their surfaces, their interiors, and their atmospheres. Recorded 2006 Nov 14 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Jupiter and Saturn are the largest planets in the Solar System, and the prototype of the Jovian Gas Giant planets. This lecture focusses on the planets themselves, looking at their composition, atmospheres, and internal structures. We will leave discussion of their fascinating systems of rings and moons for next week. Recorded 2006 Nov 15 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Uranus and Neptune are the smallest and outermost of the 4 Jovian planets. While superficially similar to Jupiter and Saturn, there are substantial differences. Uranus and Neptune have smaller rocky cores surrounded by deep, slushy ice mantles and relatively thinner hydrogen atmospheres, quite different from the massive cores and deep metallic hydrogen mantles of Jupiter and Saturn. We will also ask why they appear blue, look at their internal energy and weather, and then review the properties of the Jovian planets as a group. Recorded 2006 Nov 16 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Jupiter is surrounded by a solar system in miniature of 63 known moons. Most (59) are tiny, irregular bodies that are a combination of captured asteroids and comets. The 4 largest are the giant Galilean Moons: Io, Europa, Ganymede, and Callisto. Each is a fascinating world of its own, with a unique history and properties: volcanically active Io, icy Europa which may hide an ocean of liquid water beneath the surface, the grooved terrain of Ganymede, and frozen dirty Callisto with the most ancient surface of the four. Recorded 2006 Nov 20 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Saturn is attended by a system of 56 known moons and bright, beautiful rings. The Moon system is the focus of our attention today. Saturn has one giant moon, Titan, which is the 2nd largest moon in the Solar System, and the only one with a heavy atmosphere. On Titan, the atmosphere is mostly nitrogen and methane, but the temperature and pressure are such that methane plays the same role that water plays on the Earth: it can be either a solid, gas, or liquid. I will review tantalizing evidence from the Cassini and Huygens probes that there is, in fact, liquid methane and maybe even liquid methane lakes on Titan. Most of the other moons are ancient, icy, and heavily cratered - geologically dead worlds - but one, Enceladus, is a big surprise. The shiniest object in the Solar System, Enceladus has spectacular fountains - cryovolcanos - that spew water vapor from reservoirs created in its tidally-heated interior. This ice repaves much of the surface of Enceladus, giving it a young, shiny surface, and builds the E ring of Saturn. Recorded 2006 Nov 21 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

All Jovian planets have rings. We are most familiar with the bright, spectacular rings of Saturn, but the other Jovian planets have rings systems around them. This lecture describes the different ring systems and their properties, and discusses their origin, formation, and the physics - resonances and shepherd moons - that govern their evolution. Recorded 2006 Nov 22 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Asteroids are the leftover rocky materials from the formation of the Solar System that reside mainly in a broad belt between the orbits of Mars and Jupiter. Meteoroids are fragments of asteroids or bits of debris from passing comets that occasionally pass through our atmosphere as meteors, and even more rarely survive the fiery passage to reach the ground as a meteorite. This lecture reviews the physical and dynamical (orbital) properties of Asteroids and Meteoroids, and discusses the role of Jupiter and orbital resonances in dynamically sculpting the Main Belt. Recorded 2006 Nov 27 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Beyond the orbit of Neptune is the realm of the icy worlds, ranging in size from Triton, the giant moon of Neptune, and the dwarf planets Pluto and Eris, all the way down to the nuclei of comets. This lecture discussed the icy bodies of the Trans-Neptunian regions of the Solar System, discussing the basic properties of Triton (the best studied such object), Pluto, Eris, and the Kuiper Belt, introducing the dynamical families of Trans-Neptunian Objects that record in their orbits the slow migration of Neptune outwards during the early history of the Solar System. The Kuiper Belt is the icy analog of the main Asteroid Belt of the inner Solar System: both are shaped by their gravitational interaction with giant gas planets (Jupiter for the asteroids, Neptune for the KBOs), and are composed of leftover raw materials from the formation of their respective regions of the Solar System. Recorded 2006 Nov 28 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Comets are occasional visitors from the icy reaches of the outer Solar System. This lecture discusses the orbits, structure, and properties of comets, and introduces the "dirty snowball" model of a comet nucleus. The end of class was a demo where I created a model of a comet nucleus from common household and office materials. Imagine a twisted combination of Alton Brown and Emeril Lagasse with a PhD in Astrophysics and you get the idea. We were not able to arrange for a videographer to come, but we did get some stills before the batteries died on the digital camera. The pictures are on the lecture webpage. The lecture is slightly abbreviated because we did the student evaluation of instruction surveys before class started. Recorded 2006 Nov 29 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is a planet? Is Pluto a planet? This lecture traces the debate on the nature of what it means to be a planet by taking an historical approach, looking at how the question has arisen with the discovery of the asteroids and later Pluto and the Kuiper Belt. Many of the issued raised at the 2006 IAU General Assembly meeting were raised two centuries before after the discovery of Ceres and Pallas. We will end with the new definition of a planet, and why Pluto is better understood as a Dwarf Planet, among the two largest objects of the class of small icy bodies of the outer solar system, than as the smallest of the planets. Recorded 2006 Nov 30 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Are there planets around other stars? Are there Earth-like planets around other stars? Do any of those harbor life? Intelligent life? We'd like to know the answers to all of these questions, and in recent years we've made great progress towards at least answering the first. To date, more than 200 planets have been found around other stars, most in the interstellar neighborhood of the Sun, but a few at great distance. This lecture reviews the search for ExoPlanets, discussing the successful Doppler Wobble, Transit, and Microlensing techniques. What we have found so far are very suprising systems, especially Jupiter-size or bigger planets orbiting very close (few hundredths of an AU) from their parent stars. The existance of a significant population of so-called "Hot Jupiters" may be telling us that planetary migration can be much more extreme that we saw in our own Solar System, or that these planetary system formed in a very different way than ours. It seems appropriate to end this class with more questions than answers, but that's where the science becomes most exciting. Recorded 2006 Dec 1 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

A new podcast, Astronomy 141, Life in the Universe, is available for those interested in continuing an exploration of topics in modern astronomy.