A Magical Mystery Tour of the Universe
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A MAGICAL MYSTERY TOUR OF THE UNIVERSE By Mike McPhee [Text of an Address to the Sydney Unitarian Church on 13 May 2018.] It must be at least two years since I last addressed this congregation on a scientific topic, so I’ll try to make up for that today. There are many strange and beautiful things in this Universe of ours, for which reason I have always wondered why some people feel the need for immaterial things to inspire their lives. However, 13.7 billion light-years is a long way to go, so we should get started straight away. On our way out of the Solar System, I’ll just show you a few things that you probably haven’t seen before. I’m hoping most of these pictures are real – even if some are computer-enhanced – as that hasn’t always been clear from the sources I found. Mercury is an unusual planet in many respects, not least because its core makes up 55% of its volume. Long thought to be in locked tidal rotation, like our Moon, it is actually in a resonant state wherein it rotates three times in two revolutions. This picture shows the topography of Mercury’s northern hemisphere, with a colour scheme from the deepest regions (violet) to the highest points (red). While they are not visible at his magnification, the surface has many parallel ridges formed as the core and mantle cooled and shrank, causing the crust to crumple. There is also a rift known as the Great Valley, which is 100 km long, 400 km wide and 3.2 km deep – larger than any such feature on Earth. And here is what Venus would look like if we could see through its hopelessly murky atmosphere. It has two major terrains (highlands), with mountains as high as 11 km above the average surface elevation, and 167 volcanos over 100 km across. The pristine condition of its many impact craters indicates that its entire surface was renewed between 300 and 600 million years ago. Venus also has the distinction of rotating once every 243 days – longer than its 225-day year – and that slow rotation may explain its weak magnetic field. However, no mountain or volcano on Venus or Earth can compare to Olympus Mons on Mars. While it has the spread-out shape of a shield volcano, it rises 22 km above its surroundings and its diameter is 600 km. It is thought to have formed at least 2 billion years ago, making it the youngest of Mars’ many volcanos. Similarly. Mercury’s Great Valley has nothing on Valles Marineris, which runs 4000 km along Mars’ equator – about one-quarter of the planet’s circumference. It is up to 200 km wide and 7 km deep in places, probably widened by erosion after being formed by seismic activity. And now we come to the Asteroid Belt which, for all its extent, has only 4% of the mass of our Moon. Further, the four largest asteroids account for 50% of its total mass. The largest asteroid, Ceres, was recently reclassified as a dwarf planet, meaning that it has sufficient mass to attain a spherical shape. With a diameter of 945 km, it has a rocky core and an icy mantle, under which may lie an ocean of liquid water. Ceres also has a cryovolcano called Ahuna Mons and several bright spots, whose high reflectance is attributed to crystalline minerals in the ice. The second largest asteroid is Vesta, which is visible non-spherical and has a mean diameter of 525 km. Despite its much smaller mass, it has a differentiated interior characteristic of protoplanets. The main reason for Pluto’s demotion to dwarf planet status is that it is smaller than Ceres; our own Moon, four of Jupiter’s and one of Saturn’s are also larger, as is at least one body in the Kuiper Belt. Pluto’s highly inclined orbit, which brings it closer to the Sun than Neptune at times, and its 2-to-3 orbital resonance with that planet strongly indicate that it is not a ‘charter member’ of the planetary system. The Kuiper Belt consists mainly of balls of frozen water, ammonia and methane, though the largest have small rocky cores. This system extends from Neptune’s orbit to almost twice that distance from the Sun – twenty times the width of the Asteroid Belt and 20–200 times as massive. The Kuiper Belt is also quite thick, spanning 100 transversely above and below the plane of the Solar System. Outside of that is the hypothesised Oort Cloud, a spherical body of icy planetesimals that may extend to two light-years from the Sun – effectively, to the limit of the Sun’s gravitational range. This region has never been observed directly, but it is thought to go back to the formation of Solar System’s protoplanetary disc. The Oort Cloud is also seen as the source of most long-period comets, whereas the short-period comets (with orbits under 200 years) are from the Kuiper Belt. So, now we come to our nearest stellar neighbour, Alpha Centauri, all of 4.37 light-years away. It’s actually a triple-star system with two stars of roughly solar mass convolving with a period of 80 years at a mean separa- tion equivalent to the distance between the Sun and Uranus. Much further out is a red dwarf companion, Alpha Centauri C, often called Proxima Centauri because it is closest to our Sun – that won’t always be the case but, with an orbital period of 550,000 years, there won’t be any noticeable change in our lifetimes. (Whoever produced this diagram must have thought the major stars might have some planets – in fact, only Proxima has a confirmed planet about as massive as our Earth and in its habitable zone.) Further out, we find a number of red dwarf stars, some of which have been discovered fairly recently. We also find a few stars that are visible to the naked eye, such as the white giants, Sirius and Procyon (both of which have white dwarf companions), the Sun-like Tau Ceti and two smaller orange stars, Epsilon Eridani and Epsilon Indi. There are also a number of brown dwarfs, designated by ‘L’. It should be added that Tau Ceti has as many as five planets, two in the habitable zone; also, Epsilon Indi has a gas giant planet 2.7 times more massive than Jupiter and two brown dwarfs in remote orbits. The Sun is in the Orion Arm, a relatively dense stellar region 10,000 light-years long and 3500 light-years across. Despite the name, this is considered a ‘branch’ rather than a proper spiral arm like those on either side. (It is quite difficult to map our galaxy from the inside.) These arms may not be permanent structures, as they may just stand out due to an abundance of bright young stars. While the Perseus and Sagittarius Arms are named for their directions, the Orion Arm actually contains a number of that constellations brightest stars Blue-hot young stars are indeed plentiful in the Orion Arm, mostly found in open clusters of between 100 and a few thousand stars. The best-known such cluster is the Pleiades in the constellation Taurus which, at 440 light-years, is so close that its nine brightest stars are visible to the naked eye. The cluster is perhaps 100 million years old and has at least 1000 stars in an approximate sphere of radius 43 light-years. Further out at 577 light-years is the Beehive Cluster in the constellation Cancer. Of similar size to the Pleiades and visible to the naked eye, this cluster is much older at 600 million years old. At that age, the more massive stars congregate toward the centre, while those on the periphery have begun to drift away. Moreover, this is enough time for the largest stars to complete their life-cycles, becoming red supergiants that explode to form white dwarfs. However, these new star clusters form in nebulas, literally ‘clouds’ of gas and dust that condense over time to form protostars. There are basically two varieties – bright and dark– but the only real difference between them is that the former are illuminated by existing stars within them. Our region has some famous examples of each kind, starting with the Orion Nebula, which is seen by the unaided eye as the middle star in his sword. At 1340 light-years, it is the closest nebula to our Sun; further, it is 24 light-years across and contains 2000 solar masses of material. Also, in Orion’s Belt we find the dark Horsehead Nebula, 1500 light-years away. The dust here is extremely thick, blocking the light from the stars behind it, and there are indications of various organic molecules. As with the Horsehead, some nebulas have both bright and dark components. The North America Nebula in the constellation Cygnus (the Swan) is another, where the ‘Gulf of Mexico’ is unilluminated. It is 1600 light-years away and covers an area of space four times larger than a Full Moon, though it is too faint for us to see more than a foggy patch with the aid of binoculars. And then, there is the Coalsack Nebula in the Southern Cross, which covers such a huge area in the sky that it can be easily seen by the unaided eye. Indeed, some Aboriginal peoples identified it as the head of ‘the Emu in the Sky’, using other dark nebulas to make up its body.