Footnotes

PROLOGUE Standing on the shoulders of giants

1 In fact, it made the job of figuring out that the centre of the Milky Way truly was a black hole a whole lot more difficult. If it had instead been an active black hole – one that is currently growing by ‘eating’ more material – it would have been one of the brightest objects in the Universe. The stars in the southern hemisphere sky would barely be visible for the glare of the Milky Way’s central black hole. I think that’s a world I would quite like to see.

2 Barter Books in Alnwick, Northumberland, UK. I could spend hours in there. Highly recommended.

3 As someone who loves science history, it is both painful and curiously fascinating to watch the rise of ‘flat-Earthers’, who claim that the Earth is flat. They insist that NASA and the US government (with all other space agencies and governments presumably in cahoots) have been perpetuating the lie of a spherical Earth. What’s interesting is that the thoughts and arguments they discuss among themselves are the very same ones that early Greek philosophers had thousands of years ago, but eventually discarded after more experiments and observations. This is the crucial part that ‘flat-Earthers’ struggle with – letting go of an argument they are emotionally attached to when their experiments show them that the Earth is not flat. They somehow cannot end their odyssey of confirmation bias. A society never progresses if it refuses to change its beliefs when presented with overwhelming evidence to the contrary.

1 Why the stars shine

4 Also known as the Big Dipper.

5 So stop blaming your problems on ‘Mercury in retrograde’. Mercury is just happily orbiting the Sun like it has done for the past 4.5 billion years. Earth’s perspective on the position of inanimate rocky objects doesn’t have any influence on your life.

6 Desperate to resolve this problem, scientists of the time even considered the idea that meteors impacting with the Sun could bring extra coal deposits to keep it going for longer.

7 Yes, that is where Sirius Black got his name.

8 This is way off the current estimate of the age of the Earth – around 4.5 billion years. Kelvin didn’t know to take into account the heat given off by radioactive decay in the Earth’s core, as radioactivity hadn’t been discovered yet.

9 14,151,000,000,000,000 / (24,106 × 6,524) = 89,980,422 years. It’s worth noting that this gives the wrong answer because of many incorrect assumptions. For one, the rate at which salt flows through rivers is not constant with time, and for another, because the oceans have long been in a steady state of salinity: rocks on the ocean floor absorb salts as quickly as they are pumped in by rivers.

10 Initially, only Pierre Curie and Henri Becquerel were to be awarded the prize, but a committee member, Swedish mathematician Magnus Gösta Mittag-Leffler, alerted Pierre to the situation, who promptly complained, and Marie Curie’s name was rightly added to the prize. A lesson for us all in how to be an ally.

11 Modern radioactive dating measurements estimate that the Earth is 4.55 billion years old (with an uncertainty of around 0.05 billion years, or 1 per cent).

12 Which also explains why radiation is produced when a heavier unstable element radioactively decays into a lighter stable one.

13 To my fellow Wheel of Time fans; no, I can’t read this paragraph without giggling either. Perrin Aybara: blacksmith, wolfbrother and nuclear physicist.

2 Live fast, die young

14 Although these are also the Greeks who thought that a ‘W’ shape resembled a woman sat on a throne, so named it after Queen Cassiopeia.

15 I can also highly recommend the 2017 film Hidden Figures, which celebrates the contributions of black women computers at NASA during the space race and the Apollo missions, in particular Katherine Johnson, Dorothy Vaughan and Mary Jackson.

16 In 1956, Payne-Gaposchkin also became the first woman to be given the title of professor at Harvard, eventually becoming the Chair of the Department of Astronomy. In doing so, she also became the first woman to head a department at Harvard. She supervised many of her own graduate students in her time, including Frank Drake of Drake-equation fame, which attempts to estimate how many other advanced civilizations there might be in the Milky Way.

17 I feel like this is every physicist’s dream – to come up with a brand new equation and have it named after them. Either that or a very specific graph.

18 5,500°C (or 9,332°F if you must) to use the non-scientific unit of temperature. To convert between kelvin and celsius, just subtract 273.15 from the temperature in kelvin.

19 Bethe’s mother was Jewish, and in 1933 Bethe found himself dismissed from his research post at the University of Tübingen, due to the newly elected Nazi party’s anti-Semitic and racist Law for the Restoration of the Professional Civil Service. After a short stint at the University of Manchester in the UK, in 1935 he moved to the US permanently to a professorship at Cornell University. During the Second World War he then found his nuclear physics knowledge had earned him a position as head of the theoretical division at Los Alamos laboratory, developing the first atomic bombs, such as the one dropped on Nagasaki in 1945. In later life he campaigned alongside Albert Einstein against nuclear testing and the nuclear arms race.

20 If you can’t remember or don’t know what a proton, a neutron or an electron is, don’t worry, we’ll get to that in the next chapter.

21 Also excepting ever heavier elements, that become unstable without a whole load of extra neutrons to hold them together against radioactively decaying into lighter elements.

22 We now know that stars with around the Sun’s mass or less are actually dominated by the proton-proton chain reaction, and only those stars heavier than the Sun power themselves using the CNO-cycle.

23 He also vehemently argued against the Big Bang theory for the origin of the Universe. He even coined the term ‘Big Bang’ on a BBC radio programme as a visual description of the theory for the listening British public. Instead, Hoyle insisted that the Universe had always existed and would continue to exist in a steady, unchanging state. He was eventually proven wrong and the Big Bang theory, which he named, came out on top.

24 Note that the Sun won’t ever do this, as it’s not massive enough. In 5 billion years or so it will swell into a red giant, swallowing up the Earth and perhaps even Mars, but won’t reach quite the same level of onion-like status as more massive stars. It’s not massive enough for gravity to apply enough force to the core to trigger fusion of carbon and oxygen into heavier elements. At that point, the core is so hot it will push back the outer layers of the Sun’s red giant atmosphere in more of a fizzle than a spectacular supernova.

3 Mountains high enough to keep me from getting to you

25 ‘Thanks to Hamilton, our cabinet’s fractured into factions’ – that one’s for my fellow Hamilton fans.

26 A circle is just a very special case of an ellipse where the furthest and closest positions are equal.

27 Spirograph was one of my favourite games as kid. I obsessively produced every variety of Spirograph with every different-smelling and -coloured gel pen I could get my hands on.

28 The organisation of this expedition also allowed Eddington to avoid conscription into the British army during the First World War at the age of thirty-four. He claimed to be a conscientious objector due to his Quaker beliefs.

29 I particularly enjoy the fact that part of the headline reassured people that there was nothing to worry about.

30 Similar to how Hoyle’s ‘Big Bang’ analogy eventually made it into the scientific lexicon.

31 But here’s a secret for you to keep until Chapter 7: they’re not technically dark.

4 Why black holes are ‘black’

32 I love that Galileo is widely known by only his first name (a mononymous person). It puts him in such interesting company, with the likes of Hercules, Boudicca, Michelangelo, Madonna and Beyoncé. Now there’s a dinner party I’d like to be at.

33 You can test your reaction times with various different websites online. I just tested mine (I procrastinate a lot when I write) and it averaged out over five tries to about 0.263 seconds.

34 Fun fact, Rømer also invented what we’d recognise as the modern thermometer, showing the temperature between the freezing and boiling point of water.

35 This is now a definition for the speed of light – we no longer measure it. The speed of light is a universal constant, but the metre is a human construct whose length is completely arbitrary. So we no longer measure the speed of light, but have defined it as 299,792,458 m/s, and instead measure the length of a metre to extreme precision.

36 Instead of p=mv, there’s once again another term: . So for everyday speeds, ends up as a really small number, so the bottom of that fraction just ends up being 1 and you get back the normal p = mv. But for speeds close to the speed of light you end up having to divide by a small number, increasing your momentum. When v = c then you end up dividing by zero to get infinite momentum.

5 A teaspoon of neutrons helps the star collapse down!

37 I’m sure many of you will argue with my use of the word lucky in this context.

38 There’s a plaque commemorating Thomson’s discovery outside the old Cavendish Laboratory building in Cambridge where the discovery was made. It’s on Free School Lane, right in the very centre of town, an unassuming yet quintessential university town side street and well worth a visit.

39 Fun fact: Rutherford’s daughter Eileen Mary Rutherford married the physicist Ralph Fowler, who realised the implications of the ionisation of gases being linked to absorption in stars. We’ll meet him again later in this chapter.

40 Marsden was born in Britain but lived the majority of his life in New Zealand. Rutherford did the opposite; having been born in New Zealand he lived the majority of his life in the UK.

41 He also has the ‘Pauli effect’ named after him, whereby technical equipment seems to break around certain people. There were many anecdotes of fellow physicists complaining that their demonstrations would always fail when Pauli was around. The German-American physicist Otto Stern reportedly went as far as banning his friend Pauli from his lab. Perhaps I should’ve cited the Pauli effect when explaining to my chemistry teacher at Bolton School Girls’ Division why boiling tubes and beakers always ended up smashed after I used them in a lesson.

42 Remember, the one who married Eileen Rutherford? Rather than William Fowler of B²FH paper fame (see here). Ralph Fowler was one of many twentieth-century physicists who were caught up in the First World War; he served in the Royal Marine Artillery of the British Army. His shoulder was wounded during the Gallipoli campaign, after which his physics talents were put to good use studying the aerodynamics of spinning anti-aircraft bombs.

43 In his first paper on the limit in 1931, Chandrasekhar incorrectly concluded the limit was 0.910 times the mass of the Sun. A nice reminder of ‘if at first you don’t succeed, try, try again.’

44 Eddington went about this in an academically brutal way; the minutes of this particular meeting of the Royal Astronomical Society read like a soap opera. Many have questioned whether Eddington’s behaviour was motivated by race, but there are similar stories of scientific clashes with other junior researchers like Edward Arthur Milne (who studied how temperature changes in the atmospheres of stars) and James Jeans (who was one of the founders of modern cosmology).

45 Seriously, what did they not do there?!

46 Of Manhattan Project infamy in the Second World War; Oppenheimer was one of the few who observed the Trinity Test in 1945 when the first atomic bomb was detonated. Again, knowledge of nuclear physics and neutrons has many applications.

47 In 2018, Dame Jocelyn Bell Burnell was awarded the Special Breakthrough Prize in Fundamental Physics, worth $3 million. She donated all the money to a grant ‘to fund women, under-represented ethnic minority and refugee students to become physics researchers’, which I think best summarises the wonderful person Jocelyn is. When I arrived in Oxford on my first day as a PhD student I was told that if I had any worries or concerns that I couldn’t talk to my supervisor or college about, Jocelyn was the astrophysics department’s ‘ombudsman’, and her door was always open for a friendly chat. You can just tell that she genuinely cares.

48 Martin Hewish went on to win the Nobel Prize in 1974 for his role in this discovery, along with Martin Ryle for their work pioneering radio astronomy. There is a rather large controversy around the fact that Bell Burnell was not included in the prize, especially given the fact the prize can be shared between a maximum of three people, but was only split between Hewish and Ryle. However, Bell Burnell herself said in 1977: ‘I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases, and I do not believe this is one of them.’ I disagree with Jocelyn here; hindsight and science history have shown us that her discovery truly was one of those exceptional cases.

49 According to Bell Burnell, ‘pulsars’ was an invention of The Daily Telegraph’s science reporter Anthony Michaelis. He suggested during an interview that since they’d been trying to study quasars (shortened from quasi-stellar objects) at the time, why not shorten ‘pulsating radio objects’ to ‘pulsars’? And the name stuck.

50 Henry Russell (of the Hertzsprung–Russell diagram) wrote his obituary for the Astrophysical Journal.

6 Funny, it’s spelled just like ‘escape’

51 That’s a really big deal.

52 To the dismay of physics students around the world, there is no equivalent to the ‘quadratic formula’ – for Einstein’s field equations. If only.

53 Another one for all my Hamilton fans out there.

54 ‘Why do you write like you’re running out of time?’ I can’t stop. Love you Lin-Manuel.

7 Why black holes are not ‘black’

55 The other two stars in Orion’s Belt are 1,260 and 2,000 light years away. A reminder that although the stars in constellations appear close together when projected onto the two-dimensional night sky (appearing as if they are points on the inside of a sphere), in reality, in three dimensions, they are literally light years apart.

56 Even up until the late 1940s, shoe shops would offer free X-rays so customers could see the bones in their feet.

57 Today, the Fulbright Program is the largest international cultural exchange in the USA, operating in over 155 countries worldwide with over 294,000 scholarships awarded in the past sixty years to students wishing to study or teach abroad in a huge range of subjects. It has an enormous legacy, with eighty-eight alumni having received a Pulitzer Prize for journalism, sixty alumni winning Nobel Prizes (in physics, chemistry, medicine, literature and/or peace), thirty-eight having served as a head of state and one as Secretary General of the United Nations.

58 It’s not very well understood why the atmosphere of the Sun is so much hotter than its surface. Hypotheses have ranged from the magnetic field of the Sun being to blame, to escaping radiation from tiny sunspots on the surface. I think it’s a nice reminder that although we now know so many things about our wider Universe, there’s still so much we don’t know about even our own Sun.

59 Remember, stars in constellations are actually light years apart, like in Orion’s Belt. Something being in a constellation doesn’t mean it’s nearby to the other stars in the constellation, just that it’s in the same direction in that part of the sky from Earth’s perspective. Constellations are just used by astronomers as handy marker points for navigating the sky, to give general directions of objects.

60 Both Shklovsky and Sagan shared Ukrainian-Jewish heritage.

61 Uhuru is the Swahili word for freedom. The satellite was named to honour Kenya, after being launched from near Mombasa. The closer you are to the equator for space launches the better; the equator is spinning faster than the Earth’s poles, so you get an extra boost of energy. Anywhere with an Eastern coast is also preferable, due to the direction of the Earth’s rotation; if anything goes wrong, your rocket crashes into the sea rather than onto land.

8 When 2 become 1

62 Unfortunately, they’re just a little bit too faint to see with the naked eye, but with binoculars and a map of the constellations of Scorpius and Cassiopeia, you should be able to find both in clear, dark skies.

63 Before the realisation that red dwarf stars are far more common than first thought, astronomers were skewed by the fact that the more obvious, brighter, massive stars had a companion more often than not and believed that the majority of stars in the Milky Way were in multi-star systems. Instead, since red dwarfs make up the majority of stars, only a third of stars in the Milky Way are in multi-star systems.

64 The Arecibo telescope sustained a lot of damage during Hurricane Maria in 2017. Two subsequent cable failures in August and November 2020 led to the telescope being safely decommissioned. Before work could start, though, the telescope collapsed and was damaged beyond repair.

65 Hulse and Taylor didn’t realise the other star was another neutron star at the time, even though there was no visible companion. The nature of the other star was eventually confirmed by other groups of researchers studying the system.

66 Nobel Prizes can be shared by a maximum of three people. Lee Fowler unfortunately passed away in 1983, at the age of thirty-two, in a rock-climbing accident. I don’t know why McCulloch wasn’t also awarded the prize. Perhaps because the interpretation that the energy was being lost as gravitational waves wasn’t quite agreed upon at the time the prize was awarded.

67 This is also how we precisely measure the distance from the Earth to the Moon. There are five ‘retroreflectors’ that have been left on the surface of the Moon, which are mirrors that make sure the light is bounced back in the same direction it came from (like a ‘cat’s eye’ in the centre of a road at night). Three were left by NASA’s Apollo missions and two by the Soviet Union’s un-crewed Luna missions. With these retroreflectors and a very powerful laser, astrophysicists have been able to work out that the Moon is moving away from the Earth at around 4 centimetres per year.

68 Gravitational wave detectors built in this way can therefore only detect a certain frequency of gravitational wave; it depends on the wavelength of laser you use, and the distance between the laser and mirror. It’s nothing to do with the amplitude of the gravitational wave.

69 A dream that might come true in the twenty-first century thanks to NASA’s plan for the Laser Interferometer Space Antenna (LISA), to be launched in 2037 (at the earliest).

70 He is notable in the twenty-first century for scientifically advising on Christopher Nolan’s 2014 sci-fi epic Interstellar.

71 A lot of my research field uses data from the VLT in Chile, so I’m rather grateful that this was also funded!

72 Shout out to my fellow postdocs. We’ve got this.

9 Your friendly neighbourhood black hole

73 All of which are places us astronomers are more than happy to travel to, especially if we can tack on a few days holiday at the end of an observing trip.

74 In 2000, Greg Buchwald, Michael Dimario and Walter Wild (three amateur astronomers) reported another ‘pre-discovery’ of Pluto in photographic plates taken in August 1901 at the Yerkes Observatory in Williams Bay, Wisconsin. This is the earliest known ‘pre-discovery’, along with fourteen other observations of Pluto from observatories around the world. These extra observations are incredibly important to our understanding of Pluto’s orbit. Pluto takes almost 248 Earth years to complete one orbit around the Sun, so has only moved along about 37 per cent of its orbit since its discovery in 1930. Extra observations back to 1901 take that to almost half of its orbit, allowing us to understand Pluto’s orbit with greater precision.

75 Slipher was the first person to observe and record the redshifted light of galaxies in 1912, the first experimental evidence for the expansion of the Universe. Edwin Hubble is often wrongly credited for these observations; Hubble combined his own measurements of distances to galaxies with Slipher’s observations of redshift to show there was a correlation between the two in 1929. It was George Lemaître who had predicted this correlation two years earlier (using Einstein’s general relativity equations) and asserted that if this was the case then the Universe must be expanding. According to Allan Sandage (who used the correlation found by Hubble to derive the first accurate estimate for the age of the Universe in 1958), Hubble himself was always doubtful of the expansion-of-the-Universe interpretation of his results.

76 Most languages also use the name Pluto, with some using the literal translation for the ‘God of the Underworld’ in their own languages. For example, in Hindi, Pluto is known as Yama, after Yamarāja, the Hindu, Sikh and Buddhist deity of death and the underworld. Similarly, in Māori, Pluto is known as Whiro after Whiro-te-tipua, the embodiment of all evil who inhabits the underworld in Māori mythology.

77 Die-hard Pluto fans often complain that this definition should surely rule out the likes of Jupiter as well, since Jupiter has a collection of asteroids that have clumped together in front of and behind it in its orbit (known as the Trojan asteroids). But the mass difference between the goliath of Jupiter and a bunch of tiny asteroids is vast. Whereas the mass of the detritus of objects in the Kuiper Belt compared to Pluto is very similar. There’s no comparison.

78 There are many research projects that need help classifying huge amounts of data at https://www.zooniverse.org/, which has over 2.3 million volunteers worldwide. The Zooniverse started with the Galaxy Zoo project which was set up by British astrophysicist Chris Lintott at the University of Oxford to originally classify the 1 million galaxy images from the Sloan Digital Sky Survey. Chris also happened to be my PhD supervisor, and during that time I used data from Galaxy Zoo project to do ‘big picture’ galaxy evolution studies. My PhD was made possible by the efforts of 300,000 volunteers around the world classifying the shapes of galaxies, and I will be for ever grateful for their efforts. If you were one of those 300,000 – thank you.

79 A title which is practically the scientific equivalent of clickbait: I don’t think I’ve ever clicked on a newly published paper so fast.

10 Supermassive-size Me

80 ‘So casually cruel in the name of being honest.’ My fellow Swifties know.

81 As Gimli so eloquently puts it about dwarves in Tolkien’s The Lord of the Rings.

82 I’ve been lucky enough to observe at the same observatory. I was very excited about the trip as observatory locations are obviously chosen for their very clear dark skies. I had plans to lay myself down on a blanket outside and gaze at the stars in the warm Californian night air while the telescope was taking the thirty-minute exposure images of the galaxies I was studying. I arrived at the observatory to find signs everywhere telling me to beware of the mountain lions. The observatory staff told me not to worry, they were rare and only ever seen when their prey were around: deer. On the first night I decided to be brave and head outside to see the stars, but after five minutes of incessant nervous glancing at the dark trees around me I heard a rustle and saw three deer bound out of the tree line in the starlight. It was a sight that should have taken my breath away. Indeed, it did leave me gasping for breath, as I turned tail and sprinted back up the steps of the telescope building to get away from the mountain lion I was convinced was about to follow them. I spent the remainder of the trip cloistered indoors until the observatory staff told me about the balcony running around the edge of the telescope dome. After convincing myself that a mountain lion couldn’t possibly jump that high, I finally found the perfect spot to sit down, kick my feet over the edge and lean back to gaze at the stars.

83 This is exactly how I measured the masses of supermassive black holes in the centre of some galaxies during my PhD, after observing them with a telescope in the Canary Islands on the island of La Palma. It blows my mind not only that was I able to do that as part of my job, but also that we as humans are capable of it. The fact that collectively we have been able to piece together all the scraps of knowledge from chemistry, quantum physics and astrophysics to be able to measure the masses of supermassive black holes billions of light years away is something I will never get over, no matter how many times in my career I might do it.

84 You may think of Manchester as a terrible place to put a telescope, considering it’s one of the rainiest cities in England (damn that relief rainfall over the Pennines – the rainclouds from the Atlantic hit the barrier of the Pennine Hills running down the middle of England and abruptly stop, resulting in them dumping out all the water they picked up from the Atlantic over the North West – a phenomenon I am all too familiar with after growing up in Chorley, Lancashire). But that’s the beauty of radio astronomy: you don’t need clear skies to do it. Radio waves easily go through clouds, otherwise we’d never get any signal on our favourite radio stations on overcast or rainy days. Hell, you can even observe during the day with a radio telescope if you’re clever about it – although it’s still a good rule of thumb not to point a radio telescope at the Sun, since they’re designed to focus tiny scraps of light, not the telescope-melting amount of light from the Sun.

85 Figuring out the shape of the Milky Way wasn’t an easy task for astronomers either, because we’re stuck inside of it. Imagine trying to make a map of your city without being able to leave your house!

86 Lynden-Bell is another BNIP (Big Name in Physics), who served as President of the Royal Astronomical Society and as the first director of the Institute of Astronomy at Cambridge University, when it formed from the merger of Hoyle’s Institute for Theoretical Astronomy and the Cambridge Observatories in 1972.

87 Mauna Kea is another place that I have been fortunate enough to visit in my time as an astronomer. I spent six days observing with the Caltech Sub-millimetre Telescope (affectionately referred to as the golf ball) and then two days snorkelling at sea level (if I hadn’t become an astrophysicist I’d be a marine biologist). Mauna Kea is 4,207 metres tall (13,800 feet) and so altitude sickness really starts to kick in. Falling asleep at night (or during the day since you observe during the night and sleep all day on an observing trip) is nigh on impossible because your body constantly thinks you’re not getting enough oxygen due to the thin air. You know that feeling when you’re going to sleep and you jerk awake because you thought you were falling? Turns out your body does that when it’s short on oxygen too (it’s known as a myoclonic jerk). When I made it back down to sea level I slept for fifteen hours straight. The lack of oxygen at that altitude also affects your eyes, so when you step outside of the telescope building to look at the stars, you find you can’t see as many as you thought because your brain has redirected the precious oxygen it can get to your internal organs. Breathing in from an oxygen canister causes a practical explosion of lights in front of your eyes as thousands of fainter stars come into view. It’s magical. But probably not recommended by health and safety.

88 This is the size of the region inside the orbit of the closest star to the centre. The actual event horizon of the supermassive black hole is just seventeen times bigger than the Sun’s diameter.

89 However, there was still a lot of debate between astronomers – as there had been since the early 1990s – about whether it was one single black hole or a swarm of black holes. It is in fact one supermassive black hole, because a swarm would be completely unstable with black holes flying off in all sorts of directions. But if I’m being honest, I’m sort of disappointed a swarm of black holes doesn’t exist!

11 Black holes don’t suck

90 King is a disputed title in this house. Saturn is my personal favourite.

91 From The Hitchhiker’s Guide to the Galaxy.

92 Thanks to The Lion King, an elephant graveyard is always the scariest thing I can think of.

93 JAXA reported that the force felt by IKAROS’s solar sail was 1.12 millinewtons – equivalent to the same force a pinch of salt feels from the Earth’s gravity. This constant force from radiation pressure means the craft is constantly accelerating and accumulating speed. After six months with its solar sail deployed, IKAROS had increased its speed by 100 metres per second (about 360 kilometres per hour) to a top speed of 1,440 kilometres per hour by the time it arrived at Venus. For comparison, the rocket-fuelled Parker Solar Probe arrived at Venus in less than two months after launch and reached it with a speed of approximately 60,000 kilometres per hour.

94 This is why the X-rays from accretion disks around supermassive black holes were spotted way before any visible light from the billions of stars in the galaxies around them. Trillions >>> billions.

12 The old galaxy can’t come to the phone right now. Why? Because she’s dead

95 Although there is a very long-running joke in the astronomy community that three data points make a line. It stems from the scarcity of observations available to people historically.

96 Every astrophysics researcher who studies galaxies will have a copy of James Binney and Scott Tremaine’s Galactic Dynamics. It’s a bible of sorts for us. Arguments are settled with a quick: ‘What do Binney and Tremaine say?’

97 I’m beginning to realise just how odd it is to write a book about your own colleagues.

98 Although it’s interesting to note that even as recently as 1998, Magorrian referred to these as Massive Dark Objects: MDOs. It’s a sobering reminder that my field of astrophysics is still in its infancy.

99 The probability of any two stars physically colliding in a galaxy merger is vanishingly small, because once again, space is just very, very big.

100 In the Horizon-AGN simulation team, including Garreth Martin, Sugata Kaviraj, Julien Devriendt, Marta Volonteri, Yohan Dubois, Christophe Pichon and Ricarda Beckmann. Ricarda and I also did our PhDs together in Oxford; we were roommates for two years and now collaborate together on our research, as well as remaining good friends.

101 Remember, science needs time. And funding; if anyone out there at a university wants to offer me a fellowship or permanent professorship to figure this out? I know, I know, I’m a shameless postdoc.

102 I say beck and call, but putting together a proposal to use a professional telescope is a very lengthy process and there’s no guarantee of time when telescopes are horrendously over-subscribed. The VLT in Chile, for example, is over-subscribed by a factor of eight on average in each round of proposals.

103 Named after the Alfred P. Sloan Foundation, which was set up in 1934 by Alfred P. Sloan Jr., who was then the president and chief executive officer of General Motors. The foundation awards grants to projects across science, technology and engineering disciplines.

104 The technical term is ‘elliptical’, but I prefer blob. Especially because I hear it in Rowan Atkinson’s voice in my head.

105 Jerry Ostriker is the husband of celebrated American poet Alicia Ostriker, known for her Jewish feminist poems.

106 Frenk, Lacey, Baugh and Cole all taught me an area of physics while I was an undergraduate at Durham University. That’s one of the wonderful things about being a student – getting taught by experts who are at the cutting edge of research. Not that you’re fully aware of it at the time.

107 For most of the twentieth century, all red galaxies were thought to be blob shaped. It was the work of British astrophysicist Karen Masters and the Galaxy Zoo team using images from the Sloan Digital Sky Survey that showed that around 30 per cent of red galaxies are actually spiral shaped; you don’t need a merger to shut off your star formation.

108 The brainchild of American astrophysicist Kevin Bundy, now an Assistant Professor at UC Santa Cruz, who is a brilliant champion of all us working in the MaNGA collaboration. I first met Kevin at a conference on the Mexican island of Cozumel that was being held at an all-inclusive resort. There was a pool with a swim-up bar at this hotel, and us PhD students (as I was at the time), desperate to make a good impression with the more senior academics, diligently ignored the bar and attended all the research sessions instead. However, we quickly realised that the best way to network at this conference was not to attend the sessions, but to show up at the bar, because that was where all the senior academics had obviously drifted off to. I remember grabbing a Piña Colada, swimming up to a group of people chatting and introducing myself to the nearest person: ‘Hi, I’m Becky!’ – ‘Hello, I’m Kevin Bundy’ – I nearly choked on my drink.

13 You can’t stop tomorrow coming

109 Space is hard, words are harder.

110 Patent pending.

14 Well, Judy, you did it. She’s finally full

111 Any excuse to drop it into conversation.

112 It almost sounds like a beatbox percussion. Lin-Manuel (I’ve mentioned him so many times in these footnotes we’re on first name terms now), I am patiently waiting for a black hole hip-hop musical that can make possible a beatbox number all about the ISCO.

113 Rossa Matilda Richter, also known by her stage name Zazel, was the very first person to be shot out of a cannon at the age of seventeen in 1877 at the Royal Aquarium, London. She toured Europe and America with Barnum & Bailey’s travelling circus, aka ‘The Greatest Show on Earth’. Fans of the recent Hugh Jackman film The Greatest Showman should be familiar.

15 Everything that dies, someday comes back

114 Bekenstein also developed the ‘no-hair theorem’ of black holes; that no matter what the black hole is made of on the inside (i.e. what it has accreted over the years), it can be described by three things: its mass, its electric charge and how fast it is spinning. No other information is needed (‘hair’ being a metaphor for this extra information) to completely characterise the black hole: ‘the black hole has no hair’. I guess another way of looking at it would be that black holes don’t rely on any hairography to wow us. They are bald.

115 I’m not talking about string theory here, just using strings on a violin as an analogy.

EPILOGUE Here at the end of all things

116 WMAP stands for Wilkinson Microwave Anisotropy Probe. It’s named in honour of American astrophysicist David Wilkinson, who pioneered the study of the cosmic microwave background through the 1970s. He was a member of the science team for the WMAP project, and managed to see the satellite launch in 2001, but not the new science results it revealed after he unfortunately passed away in 2002 after a seventeen-year battle with cancer.