When Galaxies Collide

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  Last updated October 25, 2018 at 2:37 pm

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From black holes to stars to the fate of our home the Milky Way, a new book from Lisa Harvey-Smith will have you looking at the universe in a whole new way.


An illustration of the predicted merger between the Andromeda and Milky Way galaxies, as seen from Earth. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger


In her brand new book When Galaxies Collide, astronomer Lisa Harvey-Smith explores the history and future of our home galaxy, the Milky Way.


When we look up at the night sky, we see what generations before us wondered at and weaved stories around. It links us with our ancestors.


However, the story will change as the Andromeda Galaxy rushes towards us at 400,000 kilometres an hour.


In When Galaxies Collide, Harvey-Smith peers 5.86 billion years into the future to consider the fate of Earth and its inhabitants. Could we become homeless in space? Could life start afresh on one of the other 100 billion planets out there?


Read an excerpt below, and find an offer for Australia’s Science Channel readers at the bottom of the page.


When Galaxies Collide – Lisa Harvey-Smith


As they gazed into the brilliant night sky, our ancestors had no idea about the riches that lay unseen. As they shared stories around campfires or devised tales of sentient constellations to explain the world, they remained oblivious to the estimated 100 billion galaxies in the universe, which come in a dazzling array of shapes and sizes. Now, in the era of the astronomical telescope, we have photographed literally hundreds of millions of them in vast surveys using Earth-orbiting optical and infra-red telescopes and giant radio telescopes. Like butterfly collectors, we sort and classify them into species and observe their environments to learn more about their lives, their behaviours and their interactions.


We search for rare gems—the platypus and the toucan, the weird and wonderful types. We witness snapshots of galactic collisions between grand spirals, and the creation of brilliant fields of new stars. We see the battered shells of mangled hybrid galaxies and piece together the lives of elliptical galaxies—the bloated elders of the skies. This amazing galaxy menagerie is giving us new insights into the creativity of the cosmos.


Scientific dissection of these samples is beginning to uncover the circuitous path trodden by these giant cradles of stars as they form, interact and transform throughout their billions of years of life.


You and I live in a pretty average example of the most common type of galaxy: the spiral. A single image from the Hubble Space Telescope in any direction reveals hundreds of spiral galaxies, some seen edge-on and appearing thin and serpentine, others proudly displaying their slender arms like an elegant octopus. Spiral galaxies range from around 10,000 to 500,000 light years in size and tumble in all directions in space. Their constituent stars and gas rotate in a near-circular motion that, like pizza dough tossed into the air, causes them to flatten into a thin disc.


Galaxies are classified according to their apparent shape and features. We still use a classification scheme that was devised by Edwin Hubble in the 1930s, albeit with an increasing number of embellishments as new types or subclasses of galaxies are discovered. In this scheme, spiral galaxies are classified as Sa, Sb or Sc, with Sa having the most tightly wound spiral arms and Sc being the most relaxed.


We can tell a lot about a galaxy by looking at the colour of its stars. As the density wave in the spiral arm compresses the gas, gravity is helped along in its quest to clump the gas together in smaller pockets. These clumps of gas become hot and dense and eventually become stars. The passage of a compression wave tends to create a lot of very massive stars that are extremely luminous and dominate the light output of the spiral arms. Hot objects emit more blue light than red (that’s down to Planck’s law of quantum mechanics), so spiral arms with active star formation tend to be blue.


Regions of the galaxy that are devoid of active star formation tend to have fewer massive stars and appear redder. The red colour comes from cooler stars, which emit all colours but are brightest towards the red end of the spectrum. Of course, no stars are really ‘cool’. Red stars have surface temperatures of more than 3000 degrees Celsius, but that is nothing compared with a hot massive blue star, whose surface temperature comes in at a snuggly 30,000 degrees Celsius.


You can easily see the colours of the stars with your unaided eye. Consider the constellation of Orion, for example: it is bright and easily visible from pretty much everywhere on Earth. Take a look next time you get a clear night and you will see that almost every bright star in Orion is blue—that is, except for one star in the corner of Orion’s rectangular body that is bright orange. You’ll find it at the bottom-left corner of Orion if you live in the Southern Hemisphere, or the top-right corner if you’re in the Northern Hemisphere. This is Betelgeuse (often pronounced ‘beetle-juice’), a fantastically bloated red supergiant star nearing the end of its life. Betelgeuse is 1000 times larger than the sun—in fact, if the sun were replaced by Betelgeuse its surface would reach as far as the orbit of Jupiter, and Earth would be totally consumed!


If you zoom out far enough, spiral galaxies look like (to para- phrase the late English amateur astronomer Patrick Moore) ‘two fried eggs clasped back-to-back’. Their flattened discs are surrounded by a spherical bulge of cooler stars concentrated towards the centre. Galactic bulges come in two flavours. The classical type contains very old stars (typically more than eight to ten billion years old) that orbit the galactic centre in random directions. They contain little or no gas from which to form new stars, and therefore appear quite red. The other type of galactic bulge, discovered more recently using powerful instruments like the Hubble Space Telescope, is more like a mini spiral galaxy within a galaxy. In this type of bulge, the stars generally rotate in the same direction as the galaxy’s disc. These nested bulges contain gas clouds and maintain an active program of star formation, making them appear bluer than their spherical counterparts.


Many spiral galaxies, including the Milky Way, have a straight arm called a ‘bar’ of stars and gas lying across their middles. This always prompts the joke:


Q. Where does an astronaut go to drink?


A. The space bar.


In a barred spiral galaxy, the spiral arms begin their curved paths at each end of the bar. Although the angular shape of a bar might seem out of keeping with the slender curves of a spiral arm, computer simulations of galaxies show that bars will readily form if left to their own devices. They seem to form slowly over time— in a recent study of more than 2000 face-on spiral galaxies, around 70 per cent were estimated to have a bar, but when the universe was half its current age, that figure was closer to 20 per cent. If galaxies are disturbed by interactions with other galaxies, on the other hand, the bars are often disrupted or destroyed.


The presence of a bar seems to affect a galaxy in two ways. First, it can trigger star formation towards the galactic centre. As the galactic bar rotates it acts like a spoon stirring coffee, driving the galaxy’s gas and dust towards its middle and creating stars in its wake. This has the effect of building up the galaxy’s bulge and removing star- forming fuel from the disc, which in turn reddens the spiral arms. Recent research shows that redder galaxies are far more likely to have bars—which corroborates this hypothesis.


It might sound contradictory, but the second effect that this stirring can have is to move some of the gas around in strange orbits. This gas can then pool towards the ends of the galactic bar and the constant stirring motion inhibits star formation in these regions. A study undertaken in 2017 by my PhD student Shaiola Akhter reveals that two large gas clouds near the two ends of the Milky Way’s bar are far more turbulent than similar regions in the inner galaxy. The high random velocities of the gas seem to have inhibited star formation in these regions, possibly by preventing the dense clumps of gas within the clouds from collapsing to form stars. We don’t know if this snapshot view of our inner galaxy is representative of all galactic bars, but one thing is for sure, these galaxy-wide processes are complex. The various effects of the bar on star formation are still under investigation.


At the centre of most galaxies, at the heart of the galactic bulge, lies an invisible object that is around four million times heavier than our sun. It picks up hundreds of errant stars, spins them around like a wizard and wears them as a cloak. If one of the stars dares to get too close, it is ripped apart and devoured in a blaze of fiery sparks. What’s more, this nuclear ninja is incredibly dense—its precise diameter is not known for sure, but at radio frequencies its size has been determined to be less than the distance between Earth and the sun. Furthermore, the object has been monitored for more than a decade, and unlike every other object in the galaxy it has almost zero motion through space. It just sits there. What cosmic creation could behave in such a way?


This is an excerpt from When Galaxies Collide, written by Lisa Harvey-Smith. Published by Melbourne University Press, you can purchase it for RRP $29.99. Enter the promo code ‘ASTRONOMICAL’ to receive a 30% discount off RRP.




About the Author

Ben Lewis
Ben Lewis is Senior Producer at Australia’s Science Channel, and a writer for Cosmos Magazine. He has worked with scientists and science storytellers including Brian Cox, Chris Hadfield, Robert Llewellyn, elite athletes, Antarctic explorers, chefs and comedians. Ben has also been involved in public events around Australia and was co-writer, producer and director of The Science of Doctor Who, which toured nationally in 2014 in association with BBC Worldwide Australia & New Zealand. Want more Ben? You can hear him on ABC radio in Adelaide, regional SA, NSW, and around Queensland, commercial radio in the ACT, and Radio Adelaide. He also speaks at universities around Australia on communicating science to the public. Around the office he makes the worst jokes known to mankind.

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Science and technology is as much a part of our cultural fabric as art, music, theatre and literature. They play a significant role in our daily lives, yet, in a world dependent on science, we often take them for granted. Australia’s Science Channel believes every citizen has a right, and a responsibility, to be informed, and our mission is to create programs to bring that about.


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