Quick viewing(Text Mode)

A Magical Mystery Tour of the Universe

A Magical Mystery Tour of the Universe

A MAGICAL MYSTERY TOUR OF THE

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 , 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 in many respects, not least because its core makes up 55% of its volume. Long thought to be in locked tidal rotation, like our , it is actually in a resonant state wherein it rotates three times in two revolutions. This picture shows the topography of ’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 .

And here is what 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 . 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 Belt which, for all its extent, has only 4% of the mass of our Moon. Further, the four largest account for 50% of its total mass.

The largest asteroid, , was recently reclassified as a , 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 ’s demotion to dwarf planet status is that it is smaller than Ceres; our own Moon, four of ’s and one of ’s are also larger, as is at least one body in the . Pluto’s highly inclined orbit, which brings it closer to the than 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 , 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 , 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 . 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 – 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 planet 2.7 times more massive than Jupiter and two brown dwarfs in remote orbits.

The Sun is in the 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 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 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 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 , 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.

We could look at nebulas all day but, for now, we’ll stop with the Eagle Nebula in the constellation Serpens (the Serpent). It is 7000 light-years away and surrounds a large open cluster of about 8100 stars, even as more protostars are forming as we watch. The cluster is estimated to be 1–2 million years old and you can see the ‘eagle’ in the centre of the picture

Just below the centre is an amazing feature known as the ‘Pillars of Creation’, which was only discovered in 1995 from pictures taken by the Hubble Space Telescope. This is a large region of star formation in which dark globules of protostars can be seen but, unfortunately, later evidence indicates that a supernova may have destroyed the pillars. Light from that explosion would have reached Earth at least 1000 years ago but it will be another millennium before we see what the shock wave has done to the pillars.

There are also what are called ‘planetary nebulas’, which are really the remains of supernovas. Among the best-known is the in the constellation Lyra (the Lyre), whose star exploded at least 1600 years ago leaving a white dwarf in the centre. This nebula is 2300 light-years away and 2.6 light-years across, still expanding at about 25 km/s and radiating at 200 times the luminosity of our Sun.

Also in the is the in the constellation Vulpecula (the Fox), 1360 light-years away and 1.44 light-years across. Its progenitor star exploded at least 10,000 years ago and, again, there is a white dwarf in the centre.

A supernova that was actually observed by Chinese astronomers in 1054 produced the Crab Nebula in Taurus. It is about 6500 light-years away and 13 light-years across, expanding at 1500 km/s. This must have been a truly massive explosion, as the supernova could be seen in broad daylight and the progenitor star became a pulsar that was only discovered in 1968. The gas in the nebula contained elements as heavy as iron – further evidence that the star was a red supergiant – and its temperature ranges from 11,000 to 18,000 degrees.

Abell 78 in Cygnus is truly remarkable, as its parent star has exploded twice. The second explosion was more violent, sending matter outward fast enough to collide with the gas from the first explosion and heating it to over a million degrees – hot enough to emit X-rays.

Moving further out, the band of light known as the ‘’ was observed in ancient times. It took Galileo to determine that it was really a mass of faint – and therefore very distant – stars. As long ago as 1785, the British astronomer, William Herschel, undertook a detailed count of the stars in various directions and concluded (wrongly) that the Sun was in the approximate centre of a lenticular body of stars.

That turned out to be only one portion of our galaxy, as Herschel couldn’t see through the Sagittarius Arm that obscures the rest of it. In a sense, our galaxy was only discovered after other spiral galaxies had been observed by more advanced telescopes. While there is still not a full consensus, it is generally thought that the Milky Way has two main spiral arms, meaning that the Sagittarius and Perseus Arms are connected.

Similarly, our galaxy is 100,000 to 180,000 light-years across and contains between 100 and 400 billion stars. Our Sun is about 28,000 light-years from its centre and revolves around it over 240 million years, which is probably the rotational period of the whole system. The central nucleus (‘bulge’) is about 10,000 light-years across and presumably contains the oldest stars – more importantly, it is centred on a of 4.1–4.5 million solar masses.

Rather like our Solar System, the galaxy must have condensed from a roughly spherical cloud of material that began to rotate, even as much of its matter gravitated toward the centre. This rotation would have produced the relatively flat , even as it left a spherical ‘halo’ of isolated stars whose radius is at least 100,000 light-years. There is also a gaseous halo that extends much further out.

However, the halo also contains globular clusters containing hundreds of thousands of very old stars. Like the galactic core, these are very dense collections of stars with typical radii of 100 light-years. What became the Milky Way after subsequent mergers with other galaxies is thought to be almost as old as the Universe and these clusters probably go back a similar length of time.

Most of our galaxy’s 150 globular clusters are within 100,000 light-years of the centre, though a few are almost twice as far away. They orbit the galaxy, so the closest ones must pass through the galactic disc at times.

The Milky Way is part of a small galactic cluster known as the , most of whose 54 members are dwarf galaxies. Many of those are satellites of our galaxy or of the other two major members. The entire system is 10 million light-years across and all but five of its members are dwarf galaxies.

The Milky Way’s most prominent satellites are the Large and Small , which are visible to the naked eye. Both are small irregular galaxies, though it appears that they once had spiral structures which may have been disrupted by gravitational distortion from our galaxy. The Magellanic Clouds are respectively 160,000 and 200,000 light-years away and 17,000 light-years apart. The smaller galaxy orbits the larger one and both are thought to be on a collision course with the Milky Way

The largest member of the cluster is the Andromeda Galaxy, 2.5 million light-years away and 220,000 light- years across. With as many as a trillion stars, it is at least twice as massive as our galaxy. Like our galaxy, Andromeda has a supermassive black hole in its nucleus, spiral arms and about 460 globular clusters around it. It also has 14 known satellites and a huge spherical cluster of a million stars. Too big to be a globular cluster, this is thought to be the remains of a dwarf that passed through the disc of the main galaxy.

Lastly, we have the Triangulum Galaxy, three million light-years away and possibly a satellite of Andromeda. This is a small , only 60,000 light-years across and containing 40 billion stars, at the most. Triangulum is predicted to collide with Andromeda in 2.5 billion years, as will our Milky Way 1.5 billion years later – the resulting galaxy will be truly huge.

And our Local Group is only a small part of the , a system 110 million light-years across that contains at least 100 clusters and groups. Its gravitational centre appears to be the , which is 54 million light-years away and contains as many as 200 galaxies in a 5 million light-year radius. The next largest members are the Fornax and Eridanus Clusters, roughly 70 million light-years away.

Clusters like Virgo aren’t much to look at but some of their members are absolutely amazing. We only have time for a small selection of these but you’ll quickly see what I mean.

As we know, galaxies come in all shapes and sizes. In addition to normal spiral galaxies, there is a class of barred spirals that have elongated nuclei. A classic example is the Great Barred Spiral Galaxy in the Fornax Cluster, 56 million light-years away. Properly speaking, even our Milky Way and Andromeda – long thought to be an elliptical galaxy – are not proper spirals, though their bars are very short. These bars are thought to be early stellar breeding grounds that become shorter as the galaxy ages.

But even the one-third of spiral galaxies that don’t have bars can look very different when seen edge-on. Con- sider the Sombrero Galaxy in the Virgo Cluster, 31 million light-years away – while it is only 30% of the size of our galaxy, the density of the galactic disc is clearly evident, as is the halo.

Very strange things happen in space, though, as a result of galactic collisions and mergers. The Black Eye Galaxy in Coma Berenices, 17 million light-years away, has its nucleus obscured by a dark nebula. It consists of an inner ring of stars rotating in one direction and an outer belt of stars and dust rotating in the opposite direction. The most likely explanation for this is that a satellite galaxy was shredded to form the outer belt.

However, the true extent of intergalactic violence is more apparent when we look at the elliptical galaxy in Pisces, just known as NGC474. (The initials just refer to the New General Catalogue, successor to the early Messier list of clusters, galaxies and nebulas.) Elliptical galaxies are typically large and old, lacking spiral arms and open clusters because star formation has effectively ceased. However, this one is being torn apart by the gravitation of a spiral galaxy behind it.

If you think that’s bad, try the Porpoise Galaxy in the constellation Hydra. Here we have a spiral galaxy so de- formed by a larger neighbour that the ‘eye’ of the porpoise is actually its nucleus, while its ‘nose’ is a region where new stars are being formed. Miraculously enough, it is anticipated that the two galaxies will merge in a billion years or so to re-emerge as a normal galaxy.

Lastly, we have the Cartwheel Galaxy, 500 million light-years away in the Sculptor Supercluster and 150,000 light-years in diameter. It was once a lenticular galaxy until a smaller satellite (one of the two seen on the left) passed straight through it. That caused a massive shock wave that blew its gas and dust outward, triggering new star formation that has lit up the ‘rim’ of the ‘wheel’ while the nucleus remained intact.

Well, of course, there are millions of other galaxies in countless superclusters further away from us. The largest-scale picture I can give you is this, compiled from a great many infrared photographs. The blue arrow at the bottom right points to the ‘Great Attractor’, an unidentified gravitational anomaly thousands of times more massive than the Milky Way that is dragging the whole Virgo Supercluster toward it.

Needless to say, the further outwards in space – and, therefore, back in time – we look, the poorer is the resolution we get even with the best of telescopes. What we do know is that stars and even galaxies were forming in the first billion years after the ‘Big Bang’. Only in recent weeks was it revealed that a collection of 14 ancient galaxies had been observed through a freak of gravitational lensing – not only were they amazingly well formed for that early time (not that this is a real picture) but they were in the process of a mega-merger!

So, this is as far as we can go and I just hope you didn’t get lost on the way. What I hope you’ll remember after this is the old adage: “Not only is the Universe stranger than we think, it is stranger than we can think.”

Top View