Archive for the ‘astronomy’ Category
what’s that thing called science?
Vera Rubin – check out galaxy rotation rates and dark matter, inter alia
There are people I know who profess no interest in science. Not because they lack intelligence – after all, professing is what professors do, en it? And not because they’re religious – remember Stephen Jay Gould’s silly NOMA (non-overlapping magisteria)? It snared him an audience with the Catholic Papa, and perhaps that’s what it was all about, but the fact remains that The Church is a totalizingly magisterial organisation and always will be.
So, no, it seems to be more about C P Snow’s old notion of ‘the two cultures’. Some quite smart people are happy to be scientifically illiterate and find the whole thing a bore, or worse. A fellow teacher of mine – I used to teach EAP (English for Academic Purposes) to NESB students vying for a place at an English language university – once quoted one of Newton’s laws at me – F=ma perhaps – to prove that it was virtually all kindergarten stuff. I mean, how E=mc² is that? She did know something about Thomas Kuhn’s highly questionable concept of paradigm shifts, which she used as proof that it was all just high-falutin fashion.
So, in defence of science, let me break it down. Science is curiosity. Science is wonder. Like wondering why the sky is blue – when the sun is visible. Why the moon changes shape in the sky. Why some days are hot and some are cold. Why we grow old. Why some people are good to look at, others not so much. Why we get sick. Why we get hungry. Why we get tired. Just about everything is a cause for wonder, and science tries to satisfy that curiosity, though it seems so often to open up more questions. Do all creatures see the same sky that we do? Can all creatures even see? What exactly is seeing? How do eyes work? How long is it possible to live? Why don’t cats and dogs live as long as we do, barring accidents? And so on and so on ad infinitum. Science just never ends. I wonder why?
Here’s my little ‘come to science’ story, probably a just-so story. I was an avid reader from childhood – mostly fiction, but also encyclopaedia entries (mostly history, and biographies from Albert Einstein to Adolf Hitler) and a bit of that soft and very malleable science called psychology, because my mother worked in a mental hospital. And so it went, through to my early to mid twenties when I happened upon a novel by Thomas Mann, called The Magic Mountain. Its central character, Hans Castorp, was about my age, and had been sent to an alpine sanitarium, in an attempt to cure his consumption. He encounters some lively characters, and engages in deep discussion and contemplation, on politics, the nature of time, the origin of life, and even non-life – why is there something rather than nothing? And it was these last, more or less scientific questions that seemed to flick a switch in me. It inspired me to buy my first fully scientific book – The selfish gene, by Richard Dawkins – and a copy of Scientific American every month for the next two or three years, before switching to New Scientist, and then to the various science podcasts and videos available these days.
I may be exaggerating this ‘come to science’ moment – we like to turn our memories into neat narratives – but I do feel that my greater interest in scientific problems and solutions since that time has helped me to ‘rise above myself and grasp the world’, a term attributed to Archimedes which has resonated with me since I first encountered it, not so long ago. And there’s so much about our world that we haven’t yet grasped, and are just discovering. We identified our first exoplanet in the 1990s, and now we know of around 6,000 (in saying ‘we’ I’m reminded again of my teaching colleague, who responded contemptuously to my use of the pronoun – ‘ “We”? – what part did you play in all that?’), and we learned with some certainty in 2010 that Neanderthals and Humans got it on together from time to time. And the Vera Rubin observatory/telescope, which has just come online, will doubtless bring a harvest of new discoveries and conundrums.
And of course the world of science is also a world of scientists, in a multitude of fields, each one more fascinating than the next. These fields are, I must admit, a welcome escape from the horrors, tragedies and stupidities of much that we call global politics, and the workers in those fields seem so much happier in their bubbly spheres of interest. I would have liked to have been one. Better even than being a bonobo, maybe… but I love the way they swing….
References
https://en.wikipedia.org/wiki/Non-overlapping_magisteria
The FIRST images from the RUBIN observatory! (Dr Becky video)
on Lagrange points…
The five Lagrange points in the Earth-Sun system (not to scale obviously). I can only understand L1
So sometimes I just want to understand things – and not just advocate for female domination. For example, what exactly are Lagrange points, why are they important, and who was Lagrange, when he wasn’t Laplace?
First the easy stuff. Joseph-Louis Lagrange (1736-1813) was an Italian-born French naturalist (mathematician/astronomer/physicist). He also has an Italian name, and note that Italy wasn’t a country in his day, and France had quite flexible boundaries. In fact he was born in Turin, which then belonged to the kingdom of Sardinia. Most of his best work was produced in a Prussian city called Berlin. So much for the enduring permanence of nations.
The list of Lagrange’s mathematical contributions is long, and my general mathematical understanding is minuscule, but my fascination with the very sensible notion that there should be a point or region between two massive, gravitationally attracting bodies, such as, say, two planets, where an object would be ‘suspended’ between those two bodies, as their opposite forces (but gravity isn’t a force, they keep telling me), are counter-balanced – that fascination has brought me to attempt to understand, to know more…
So here’s a Wikipedia quote on Lagrange:
He studied the three-body problem for the Earth, Sun and Moon (1764) and the movement of Jupiter’s satellites (1766), and in 1772 found the special-case solutions to this problem that yield what are now known as Lagrangian points.
I’m thinking maybe that my description of a body in a space between two other bodies exerting a more or less equal and opposite gravitational attraction upon it has something to do with this ‘three body problem’ that I’ve heard about only recently. And again, looking at Wikipedia, that magical resource, this seems to be the case:
In celestial mechanics, the Lagrange points… also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the gravitational influence of two massive orbiting bodies. Mathematically, this involves the solution of the restricted three-body problem.
And this is where it gets very complicated, at least for me. The restricted three-body problem seems to be, in essence, a two-body problem, due to the third body’s mass being negligible in the Newtonian scheme of things, such as in the case of a satellite or small ‘planetoid’. In such a situation, at such points, the two large gravitational forces and the centrifugal force are in balance. The centrifugal force is a type of inertial force in Newtonian mechanics. But how can a force be inert? When it’s not a force, obviously. It’s also called a fictitious or pseudo force, but such forces appear to act when viewed in a ‘rotating frame of reference’. And it must be hard to dismiss such rotating frames when we consider that our Earth rotates on its axis, our solar system rotates around its sun and our galaxy rotates around its black hole. And maybe our universe rotates around its centre, if it has one.
But I’m only writing this to avoid the mathematics. Anyway the point about rotating frames of reference is that, if that frame is regular or constant, as is the Earth’s rotation, it will appear to be stationary, and ‘the standard’, which can lead to confusion about other observable bodies, a confusion that lasted for millennia before the likes of Galileo and Newton began to question what had hitherto seemed obvious.
So, Newton’s second law of motion can’t be avoided. I’ll first state it in English words, then… I’m not sure how much further I’ll get:
At any instant of time, the net force on a body is equal to the body’s acceleration multiplied by its mass or, equivalently, the rate at which the body’s momentum is changing with time.
Apparently the dummy’s version of this is F = ma (force equals mass times acceleration), and the more sciencey versions are:
F = m.dv/dt = ma
F = d/dt.(mv)… where d stands for derivative, v for velocity and t for time.
And there are other versions, I think. It’s this second law that has proved the most controversial and it seems the most fruitful for further research and analysis. But don’t trust me on any of this. What is most interesting is that this classical description of forces has been fruitful enough for later (but not much later!) physicists like Lagrange to work out mathematically certain points in space where satellites and telescopes can hover or circulate well beyond Earth’s atmosphere. We now know of five Lagrange points within the Earth-Sun gravitational system, and another five within the Earth-Moon system. To explain why there are so many would be beyond my current level of competence, but I intend to try an online course in classical mechanics, to get me up to speed, or up to equilibrium.
References
https://en.wikipedia.org/wiki/Lagrange_point
https://en.wikipedia.org/wiki/Joseph-Louis_Lagrange
aspects of climate change – Milankovic cycles
from Wikipedia
I’m currently reading A brief history of the Earth’s climate, by Steven Earle, a Canadian geologist, who provides summaries of the various internal and external forces affecting our planetary atmosphere’s composition and temperature over its history. It’s all very sciencey, which is of course good, but not so good for dumb-funks like me, who have to put it into their own words to get a proper handle on it. So that’s why this piece is on Milankovic cycles, about which I know next to nothing.
In 1941 Milutin Milankovic completed a book entitled Canon of insolation and the ice-age problem, in which, according to Earle,
he argues how the natural variations in the shape of the Earth’s orbit around the sun and in the tilt of the Earth’s rotational axis played a critical role in the timing of glaciations over the past two million years.
A brief history of the Earth’s climate, p63
Insolation is defined as ‘the strength of sunlight shining on the various surfaces of the Earth’, bearing in mind that dark surfaces, such as the oceans and densely vegetated regions, absorb sunlight while ice and snow reflect it. Milankovic’ work went largely unrecognised in his lifetime but he was the first to ‘calculate the effects of insolation and to accurately determine the periods during which those changes would be most likely to contribute to the growth [or shrinkage] of glaciers’.
These periods have everything to do with the Earth’s eccentric (but not too eccentric) orbit, and its wobbling tilt, vis-à-vis its orbital plane. That orbit is elliptical, and the Sun is not at the centre, so the Earth’s distance from the Sun varies seasonally. Most people know this, I hope, but they may not know that the shape of Earth’s orbit varies over time, from slightly elliptical to even more slightly elliptical. But we’re talking about very long periods of time, many thousands of years, between the most and least elliptical orbits. When the orbit is most elliptical, the difference between the Sun at its closest and its farthest from Earth is, of course, greatest. It should be obvious, from what we know of the Sun as essentially our only heat source, that these differences will have a climatic impact.
Now to the wobbling tilt, or the Earth’s obliquity, relative to the plane of its orbit. This tilt is presently 23.5 degrees from vertical, and the degrees vary from 22.1 to 24.5 over a period of about 41,000 years. It basically defines our seasons, as the Northern Hemisphere tilts towards the Sun when the Southern Hemisphere tilts away from it, and vice versa. And the variation in that tilt, the wobble, creates greater or lesser variation between summer and winter seasons.
And now back to Milankovic. He, along with a few colleagues including Alfred Wegener of continental drift fame, made observations about the formation and growth of glaciers:
glaciers grow best at temperate latitudes – in fact at around 65 degrees north or south – and can start growing only on land.
There is in fact relatively little land at 65 degrees in the Southern Hemisphere, but plenty in the north, so that was where Milankovic focussed. He also focussed on the summer insolation, as cooler summers are more a factor in glacier growth than cold winters. As Earle explains, when the summers are cooler, there’s less melting of snow and ice, and when winters are colder, they’re also drier, and less snow falls.
So Milankovic based his cycles on three variables – eccentricity, tilt angle and tilt direction.
Eccentricity, which varies on a 100,000-year cycle, determines the distance between Sun and Earth. A high eccentricity (a greater distance), in conjunction with tilt direction, ‘provides a greater opportunity for the Earth to be pushed from a non-glacial state to a glacial state or vice versa’ (Earle).
Tilt angle, which has a 41,000-year cycle, affects seasonal differences. ‘A lesser tilt angle leads to cooler summers and warmer winters, and that favours the growth of glaciers’.
Tilt direction, which has a 23,000-year cycle, determines which hemisphere, north or south, points to the Sun when the Earth is farthest away from it. ‘Glaciation is favoured when the Earth-Sun distance is greatest during the northern hemisphere summer, leading to cool summers with less melting’.
When Milankovic died in 1958 his insolation theories were far from being accepted by mainstream science. This was largely because, though it was known that glaciers enlarged and reduced over millennia, the timing of these ebbs and flows was much of a mystery. Better measurement techniques were required to verify the Milankovic hypotheses. These came in the sixties and seventies with sea-floor and later ice core samples, as well as measurement of isotopic variations in the history of marine mammals, and their relation to temperature, culminating in a key paper published in 1976, at the end of which the authors wrote:
It is concluded that changes in the Earth’s orbital geometry are the fundamental cause of the succession of Quarternary ice ages.
It’s important to note that these orbital changes were not the cause of the ice ages, it simply explained their timing. The cause was a period of atmospheric cooling over 50 million years until recently, geologically speaking. That atmospheric cooling I’ll (try to!) explain in a follow-up post.
From the 70s onwards, ice core samples from Greenland and Antarctica have been able to be correlated with variations in surface temperatures over 250,000 years, based on measurements of the ratios of hydrogen isotopes in the water molecules from the ice at those sites. To quote Earle:
The correlation between the temperature record and the July insolation levels is reasonably clear. The third-last interglacial, extending from 245,000 to 235,000 years ago, corresponds with a period of high insolation. The following very low insolation initiated the beginning of the second-last glacial period. That was followed by a very high insolation period (at around 220,000 years ago, which led to significant warming but wasn’t enough to break the glacial cycle. Glacial conditions then intensified over the next 90,000 years.
Another period of very high insolation, culminating at around 120,000 years ago, was able to break the cycle, leading to the second interglacial, which lasted from about 127 to 90 thousand years ago. That was followed by a similar cycle of increasingly cold climates and strong glaciation until around 20,000 years ago, when the glacial cycle was again broken by a period of strong insolation.
As Earle further points out, methane levels from the same ice cores are even more closely correlated with the insolation pattern. And there are other positive feedback processes that ‘amplify Milankovic forcing’, as Earle puts it, including carbon dioxide levels and the albedo effect of accumulated ice and snow during cooling periods.
Our recent greater understanding of Milankovic cycles allows us to predict their effect on the future climate. We’re entering a period of low ellipticity in the Earth’s orbit, meaning that insolation levels won’t vary much for the next 50,000 years. This means we will have an ‘interglacial’ climate for a long long time to come. So, no cyclical glaciation will arrive any time soon to rescue us from anthropogenic global warming. Add that to the forlorn hopes about other processes touted by climate change skeptics/deniers, such as sunspots and a sudden upsurge of vulcanism….
References
Steven Earle, A brief history of the Earth’s climate: Everyone’s guide to the science of climate change, 2021
cosmology etc, by dummies: we’re going to the moon
screenshot filched from Dr Maggie’s presentation – the orientation of lobate scarps on the moon
Jacinta: So we’re trying to explore more about the mess that is the universe, or more accurately, the mess that’s our understanding of the universe, and, to maintain our bonobo bonafides…
Canto: I like that one.
Jacinta: … we’ll get our info solely from the female experts in the youtubeverse, such as Dr Sabine, Dr Becky and Dr Maggie, and any others we happen to come across.
Canto: So starting with our neighbourhood, I’d heard vaguely about the Artemis missions, something about the moon, but my attention was grabbed, I think by a recent Skeptics Guide podcast, or it might’ve been New Scientist, to the effect that we’re going back to the moon next year!
Jacinta: Yes, I think as part of a series, but next year’s crewed mission will be nowt but a flyby. Anyway, here’s some info from Wikipedia, which is already breaking my promise, unless the article was written by a woman. Anyway the overall programme is called Artemis 2 and this first flyby mission will launch in September next year at the earliest. Four astronauts, one of them a woman, Christina Koch. Slowly getting there. And humans on or near the moon for the first time in 53 years! Or more. Fact is, there have been many delays and changes of plans, so we shouldn’t hold our breath.
Canto: Anyway, let’s focus for now on the moon itself. We know it’s slowly spiralling away from us, and wrote about it years ago. The rate is 3.8 cm a year, a figure we can measure because of devices left on the surface all those years ago by one of the Apollo missions. Don’t know if it’s spiralling away faster – I mean if the distance is increasing – you know, next year it’ll be 3.81 cms further away than this year, and so on…
Jacinta: It makes sense, because as it spirals away, it’s moving that tiny bit from Earth’s centre of gravity – the pull would be ever so slightly less strong… am I right? But then, as that happens, the moon would travel around earth more slowly, just as the outer planets travel more slowly around the sun.
Canto: I think that’s right, but both bodies will probably be destroyed by the sun by the time the moon says goodbye.
Jacinta: Anyway we’ll talk more about Artemis 2 as time goes by, now some things we’ve learned from Dr Maggie Lieu – first that the moon is rusting, which is something of a mystery, since there’s no oxygen there and very little water, the two principal components creating rust – generally iron oxide – on Earth.
Canto: Yes they’ve confirmed the presence of hematite, a type of iron oxide, at the poles in particular, where most of the moon’s water (ice) is. They’ve also found more of it on the side facing Earth, and have proposed that it actually derives from Earth, via our magnetic field:
In 2007, Japan’s Kaguya orbiter discovered that oxygen from Earth’s upper atmosphere can hitch a ride on this trailing magnetotail, as it’s officially known, traveling the 239,000 miles (385,00 kilometers) to the Moon.
Jacinta: Okay enough about rust. So the moon is generally a lot cooler than Earth due its lack of an atmospheric blanket. It’s been cooling ever since it left Earth. Which also means it’s shrinking, as the liquid outer core has solidified, decreasing in volume?
Canto: Yes and that shrinking leads to a wrinkling and breaking up on the moon’s surface, due to thrust faults, with one section of the moon’s crust thrusting over another, something like tectonic plates, but without the tectonic plates. It results in what the cognoscenti call lobate scarps – something to do with the way they look (check them out on Google images, but they’re still not clear to me). Anyway, lots of them in one area tells us that something’s happening beneath the surface. Such stresses can result in moonquakes, measuring up to 5 on the logarithmic Richter scale.
Jacinta: Measuring devices left on the moon during the Apollo missions have connected moonquakes with areas covered in lobate scarps, of which thousands have been discovered. But it’s not really like tectonic plates, with seismic activity occurring at tectonic boundaries – it’s seemingly a much more random sprinkling of these scarps and thrust faults. Remember, huge mountain ranges like the Himalayas and the Andes have been produced by the bashing together of tectonic plates, but there are no mountain ranges as such on the moon.
Canto: The Lunar Reconnaissance Orbiter Camera (LROC) has mapped these lobate scarps and found a striking pattern in their orientations.
Jacinta: Yes, let’s quote Dr Maggie on this:
These [lobate scarps] cover the entire surface of the moon, but when you take a look at their orientations, as indicated on this map [see above] by the black arrows, you’ll notice something interesting. If these faults are caused by the moon shrinking, it should be shrinking the same in all directions, so you would expect these scarps to be in the same direction, but no, at the poles the scarps tend to be oriented perpendicular to the scarps at the equator. It seems that, in addition to the moon’s shrinking, the Earth’s gravitational pull is affecting these scarp lines.
Canto: So enough quoting – how do they come to this conclusion?
Jacinta: Well we know that our tides are affected, or created, by the moon’s gravitational pull, so you’d expect the Earth’s gravitational effects on our moon to be far greater, though of course they can’t be tidal effects, although they’re described as tidal forces, which is a bit confusing. As Dr Maggie reports, our gravity creates moonquakes, of a different type to the shallow moonquakes caused by the moon’s shrinkage. These shallow quakes originate a few kilometres below the surface, and occur at a rate of about 7,000 a year, but tidal forces create deep moonquakes, originating hundreds of kilometres below the surface, at a rate of tens of thousands a year. And they’re not the only moonquakes – meteor impacts can cause them as well as the day-night cycle, which causes freezing and thawing of the surface.
Canto: So yes, the moon is shrinking, very very slowly. Not a big bother, though those moonquakes might be, for the Artemis missions. Presumably they’re not predictable. And I suppose that map they’re making of all those scarpy things might help them to choose a safer landing spot, if they’re going to land women on the moon in the future.
Jacinta: Nice one. Something to explore in future posts – what NASA’s actual plans are for Artemis 2. A human colony or just a bit more surface-scraping…
References
https://en.wikipedia.org/wiki/Artemis_2
Dummies on dark matter 2: there are problems…
from Forbes website, see below
Canto: So there are candidates for dark matter, and there are also those who think that, though there is a serious problem in cosmology, to do with mass and energy, ‘dark matter’ won’t be the fix.
Jacinta: By the way, I was chatting with another dummy on this topic recently, who had the excuse of being much much younger than myself, and she asked if dark matter had anything to do with black holes. I wasn’t able to give a very effective answer, but Sabine Hossenfelder, one of our heroes, says in a video linked below that black holes and brown dwarfs (whatever they are), and other such exotic objects ‘would make too many gravitational lenses, which have not been seen’, to be candidates. Also black holes are so called because they ‘swallow’ light, that’s to say, light-emitting particles. So they really are black, in a sense, whereas ‘dark’ matter is more transparent than anything, according to Hossenfelder.
Canto: Well, getting back to Peter Fisher’s Royal Institution talk, he talks about the 1980s as a time of confusion and excitement in theoretical physics and cosmology when so many things weren’t adding up. At the same time a concept called super-symmetry was being mooted. It ‘predicted all kinds of heavier particles that we wouldn’t have observed in accelerators because they weren’t powerful enough’, according to Fisher. He also presented, what I’ve heard before, a conjecture that there must be this ginormous halo of dark matter surrounding galaxies to make up the missing mass and to account for the behaviour of visible matter at the edge of galaxies. In other words, this dark matter must have a gravitational effect on the outer arms of these galaxies.
Jacinta: I know that Hossenfelder is no great fan of bigger and more expensive accelerator-colliders in the hope of discovering more teensy-tiny but ultra-ultra numerous particles to fit the dark matter bill, but Fisher also goes on to talk about the Standard Model and how effective it has been, without dark matter screwing it up…
Canto: Yes it’s been very effective for accounting for some 4% of the mass-energy of the universe. Anyway Fisher helped to debunk a theory regarding ‘heavy neutrinos’ as a candidate for dark matter in the late eighties, which seems to this dilettante like an absurdity – neutrinos being near-massless, which presumably helps them to pass through planets as if they’re not there.
Jacinta: I think this heavy neutrino thing might’ve morphed, in theory, into the idea of weakly interacting massive particles, or WIMPs, and they’re still looking for em, for example on the International Space Station. They’re also looking for theoretical particles called axions, using special detectors. No luck so far for WIMPs or axions.
Canto: Fisher describes another source, black holes, via work done by Stephen Hawking, but I found it difficult to follow, so I’ll try roughly quoting:
In the beginning of the universe there was all this mass around, it’s very dense, and in a regime where quantum mechanics is very important, so the density is… fluctuating and changing, and [Hawking] thought, could it be that the density would be high enough to form a black hole? He did some rough calculations and found that, yes they could collapse into a black hole, and there’d be a lot of black holes, but there must be a way of getting rid of them, because we don’t see them. Over time, he invented a mechanism by which black holes radiate light at a very low level, a concept now called Hawking radiation, a remarkable notion, as it suggests the only connection we currently have between gravity and quantum mechanics.
The connection with dark matter is that there still may be ‘primordial’ black holes with a lot of mass but tiny in size. No luck in finding them either, needless to say.
Jacinta: So now let’s focus on Sabine Hossenfelder’s RI talk. It seems to me she goes into a lot more detail about the anomalies in what we observe, ascribed to the missing matter. For example, structure formation in the universe – which I remember being fascinated by when Carl Sagan presented it image-wise in his Cosmos series. Here are two points she makes on structure formation:
- Dark matter cannot build up radiation pressure and therefore starts forming structures sooner than normal matter
- normal matter on its own does not produce sufficient structures on short scales to be compatible with observation
And I have no idea what they mean. She mentions things that we can see, such as ‘galactic filaments, and so on and forth’. So, thinks me, wtf are galactic filaments? Well, Wikipedia calls them galaxy filaments, and they’re the largest known structures in the universe…
Canto: This is actually exciting – how could I have lived so long without knowing about these things?
Jacinta: Haha well Wikipedia is pretty good on this stuff:
In cosmology, galaxy filaments are the largest known structures in the universe, consisting of walls of galactic superclusters. These massive, thread-like formations can commonly reach 50/h to 80/h Megaparsecs (160 to 260 megalight-years) — with the largest found to date being the Hercules-Corona Borealis Great Wall at around 3 gigaparsecs (9.8 Gly) in length — and form the boundaries between voids. Due to the accelerating expansion of the universe, the individual clusters of gravitationally bound galaxies that make up galaxy filaments are moving away from each other at an accelerated rate; in the far future they will dissolve.
Galaxy filaments form the cosmic web and define the overall structure of the observable universe.
Canto: Great. What’s a void?
Jacinta: Cosmic voids, doncha know, are those vast spaces between filaments that contain few or no galaxies. But to return to Hossenfelder, who’s a theoretical physicist, and a lot more proficient in maths than we are, as we’ll see. She’s also something of a dark matter skeptic, it seems. She highlights four problems that dark matter doesn’t solve, and we should try to understand them:
- the brightness of galaxies is strongly correlated with the (asymptotic) rotational velocity (‘Tully-Fisher Law’). Dark matter doesn’t explain this
- dark matter leads to density peaks in galactic centres which badly fits with observations (‘galaxy cusps’)
- dark matter predicts too many dwarf galaxies
Canto: Okay let’s start with asymptotic rotational velocity. Asymptotic analysis, in mathematics, is about describing behaviour of functions as they approach a limit, such as infinity. So galactic velocity presumably has some sort of limit, which can be calculated mathematically. The Tully-Fisher Law, or Relation, from Wikipedia:
is a widely verified empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its asymptotic rotation velocity or emission line width. Since luminosity is distance-dependent, the relationship can be used to estimate distances to galaxies from measurements of their rotational velocity.
So the point Hossenfelder makes here, I think, is that rotational velocity correlates well with brightness, which correlates with distance, as measured from Earth. Dark matter appears to be irrelevant to these calculations. Of course I may be getting this all wrong.
Jacinta: I wouldn’t know. But it seems that dark matter and its supposed halo should be interfering with orbital velocities and so interfering with calculations, but it isn’t?
Canto: Hmmm.. Anyway, second point. Galaxy cusps takes me to the ‘cuspy halo problem’, which I’ll try to explain in my own words. It’s also called the core-cusp problem, which, broadly speaking, is a discrepancy found in small, low-mass galaxies, according to different measurement systems or predictions. ‘Cuspy’ represents an energy-mass distribution which is denser at small radii, whereas most dwarf galaxies have a more flat ‘core’ distribution.
Jacinta: These are all just the beginnings of our explorations of this topic. In a hundred years or so we’ll be fully conversant with the issues. As to dark matter predicting too many dwarf galaxies, aka the dwarf galaxy problem, apparently we’ve observed and identified some 38 of these dwarf galaxies in our Local Group (a dumbbell-shaped group of galaxies with the Milky Way and Andromeda forming the two lobes), instead of the 500 or so predicted by dark matter simulations, and that’s just around the Milky Way.
Canto: Okay, we’re doing well, sort of. Final problem – the alignment of satellite galaxies. Essentially, they form a disk rather than a halo. Perhaps surprisingly, the Forbes website has what seems to me an excellent article on the subject. Dark matter simulations produce halos merging together in spiral formations surrounded by sub-halos in a variety of orientations. But that’s not what we see – we see satellite galaxies in the same orientation as their ‘hosts’, and co-rotating with them. This has been observed for the Milky Way, Andromeda and most satellite galaxies observed, such as Centaurus A. So what accounts for these discrepancies?
Jacinta: You don’t know? Well, we’ll have to look at that next time, or not. I suspect there might end up being hundreds of these dark matter posts. We might even have to learn some maths….
References
Is Dark Matter Real? – with Sabine Hossenfelder (Royal Institution video)
What is dark matter? – with Peter Fisher (Royal Institution video)
https://en.wikipedia.org/wiki/Galaxy_filament
https://en.wikipedia.org/wiki/Void_(astronomy)
https://en.wikipedia.org/wiki/Tully–Fisher_relation
https://en.wikipedia.org/wiki/Cuspy_halo_problem
https://en.wikipedia.org/wiki/Local_Group
Journey into dark matter , iTelescope Webinars, with Dr Maggie Lieu
Dummies on dark matter 1 – the missing galactic masses
Jacinta: So here’s where we try to educate ourselves on dark matter, just for the fun of it.
Canto: But we’re serious. Our blog’s motto is ‘Rise above yourself and grasp the world’, supposedly from the wisdom of Archimedes, and he meant the universe, or would’ve.
Jacinta: Well, smarter guys than us are trying to grasp dark matter, with limited success it seems.
Canto: Yeah but they’ve been smart enough to recognise that there’s this missing matter, when they look at galaxies and find they’re spinner faster than their observed mass suggests they should, because physical laws apparently tell us that the greater the mass, the greater the gravitational effect, which would cause a greater spin towards the black hole sun, I think. But when astrophysicists measure a galaxy’s mass – which must surely be a tricky process, but I think it’s about measuring light spectra – different molecules give off different electromagnetic waves, though how they manage to measure all that is beyond me – anyway when they measure the galaxy’s mass and its spin, the numbers are off by orders of magnitude. I think.
Jacinta: Galaxies contain hundreds of billions of stars, I hear, so they must’ve built some impressive measuring technology. Okay, first research – from Physics LibreTexts:
The mass of [our] Galaxy can be determined by measuring the orbital velocities of stars and interstellar matter.
The article I’m quoting from focuses on our galaxy as a more or less typical example. Our sun is orbiting the galaxy’s centre (a black hole presumably?) at 200 kilometres per second, and it’s calculated that it takes about 225 million years for a full orbit.
Canto: Orbital velocity sounds so much more legitimate than spin, methinks.
Jacinta: So I want to put this explanation of the Milky Way’s proposed mass in my own terms to try to understand it better. Imagine the Sun’s roughly circular orbit around the galaxy, which is shaped somewhere between a sphere and a more or less two-dimensional disc. As Newton worked out, in the case of a sphere like Earth, gravity acts to pull everything towards the centre. Our Sun, which lies in an outer arm of the galaxy, gets whipped around at great speed due to the large mass between it and the galaxy’s centre (some 26,000 light years away). Kepler’s third law comes into play here…
Canto: His third planetary law (from the NASA Science website):
The squares of the orbital periods of the planets are directly proportional to the cubes of the semi-major axes of their orbits. Kepler’s Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit.
This was presumably simply an observational measurement from Kepler, but Newton and others found that, the further out from the Sun, the faster the planet moved, due to the greater accumulated mass in between. I think.
Jacinta: Uhh no. The period increases rapidly, not the velocity. The word ‘rapidly’ has led you astray. Naughty NASA Science website, it should have said that the period becomes longer, and not just because there’s a greater distance to travel but because the planet itself is revolving more slowly. Don’t forget that’s there’s relatively little, in fact very very very little mass, relatively speaking, added between Jupiter and the Sun than there is between Earth and the Sun. And here’s some figures: Earth is revolving round the Sun at about 107,000 km/h, Jupiter at around 47,051 km/h and Mercury at 170,500 km/h.
Canto: Okay, so galaxies are quite different from solar systems. So in the centuries after Kepler and Newton, astrophysicists used their theories, along with further developments, to measure the Milky Way’s mass, generally maintaining the theory that the vast majority of that mass lay between the Sun and the centre. This was assumed to be reasonable because:
The number of bright stars and the amount of luminous matter (meaning any material from which we can detect electromagnetic radiation) both drop off dramatically at distances of more than about 30,000 light-years from the galactic center.
But this assumption has turned out to be wrong, because we now know that there’s lots more matter well out from the centre, but it happens to be invisible – to us at least, and maybe to our instruments, but then if it’s undetectable how do we know it exists?
Jacinta: Well, Physics LibreTexts gives a good explanation:
We can understand how astronomers detected this invisible matter by remembering that according to Kepler’s third law, objects orbiting at large distances from a massive object will move more slowly than objects that are closer to that central mass. In the case of the solar system, for example, the outer planets move more slowly in their orbits than the planets close to the Sun. There are a few objects, including globular clusters and some nearby small satellite galaxies, that lie well outside the luminous boundary of the Milky Way. If most of the mass of our Galaxy were concentrated within the luminous region, then these very distant objects should travel around their galactic orbits at lower speeds than, for example, the Sun does. It turns out, however, that the few objects seen at large distances from the luminous boundary of the Milky Way Galaxy are not moving more slowly than the Sun.
So I added some italics to help us. If all the mass, or a vast majority, was located at the galaxy’s centre, as our sun with its massive mass is located at the centre of our local system, then this handful of luminous objects that are clearly part of our galaxy but further, sometimes much further, from the centre, should be travelling more slowly than the Sun, but instead they’re travelling faster.
Canto: Which presumably means unaccounted for mass, something to do with e = mc², more mass provides more energy, giving more velocity…
Jacinta: Yeah, something like. But light is a kind of energy, like electromagnetic energy, which presumably dark matter doesn’t have, but it must have energy if it has mass. Or must it?
Canto: Andrew Pontzen, in a Royal Institution lecture, tells us that, according to current calculations there should be five times more dark matter than visible matter, and it’s streaming through the planet, and our bodies, as we write…
Jacinta: Like neutrinos? Are neutrinos a kind of dark matter?
Canto: Ha, well that’s a good thought, maybe. Here’s a quote from an article in Nature India in early 2022:
Physicists have developed a mathematical model that may shed light on the identity of dark matter, the mysterious substance that far outweighs visible matter in the universe. Dark matter is not made of atoms or other known fundamental particles and doesn’t interact with any form of light or electromagnetic radiation, making it difficult to detect. The new model showed that a non-interacting or sterile neutrino is probably a dark matter particle and contributes to the mass of dark matter.
Jacinta: A sterile neutrino?
Canto: Yes, aka an inert neutrino – currently hypothetical, so outside of the Standard Model. The Wikipedia article on this is pretty comprehensive and complex… I’ll quote from the very beginning of the article, and then we can discuss it, hahaha:
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term sterile neutrino is used to distinguish them from the known, ordinary active neutrinos in the Standard Model, which carry an isospin charge of ±+1/ 2 and engage in the weak interaction. The term typically refers to neutrinos with right-handed chirality (see right-handed neutrino), which may be inserted into the Standard Model.
Jacinta: Well, just to complicate this apparently dire situation of masses of matter or stuff we know nothing about, there’s also dark energy, which according to another Royal Institution presentation, this time by Peter Fisher in 2022, makes up even more of the unknown stuff in the universe than dark matter, leaving us in the dark about 96% of universal stuff. How can we be so incompetent?
Canto: Well, it’s an increasingly informed incompetence I’m sure. We’ve learned, as you say, that all the particulate matter-energy that we know about makes up only 4% of all there is, so now we (I mean you and I) need to learn how we (the astrophysics community) learned this.
Jacinta: About 73% of the unknown stuff is dark energy, 23% is dark matter. So let’s try to regurgitate Fisher’s talk in our own words, so that we can sound more informed.
Canto: Right so we start with the ‘big bang’ 13.7 billion years ago, followed by ‘inflation’, a very rapid expansion over a relatively short period, resulting in the cosmic background afterglow, after which the universe slows down markedly in its expansion thenceforward, with first stars and then galaxies developing. But then, after about half the life of the universe, it starts to expand slightly more rapidly, apparently due to dark energy.
Jacinta: Yes, according to an illustration I’ve seen before, and which we’ll post here, dark matter dominates in the first half of universe’s history, and dark energy dominates the second half.
Canto: And why might that be? That is presumably an unsolved mystery. Which we will continue to explore…
References
https://science.nasa.gov/resource/orbits-and-keplers-laws/
https://en.wikipedia.org/wiki/Sterile_neutrino
What is dark matter? With Peter Fisher (Royal Institution lecture, 2022)
physics by a dummy – what’s this thing about dark matter?
So why is there a dark matter problem, or is there? This putative stuff apparently doesn’t interact with light. My uneducated guess is that we’ve tried to measure the mass of the universe (if we ever have) through measuring light given off by stars/galaxies and found not enough to correlate with some sophisticated mathematical theories of universal mass/energy we’ve constructed. According to the PBS Space Time guy, who always sounds super smart about this stuff, recent findings by the Just Wonderful Space Telescope (JWST) of super-massive galaxies over 30 billion light years away (don’t ask) have raised speculation about ‘dark stars’ powered by dark matter – even though these galaxies are really shiny bright. So, shiny bright dark matter – what gives?
Well needless to say, all this raises oodles of problems. First, to make a dark star you likely need a new type of particle, one that can’t ‘interact with itself’ [particle masturbation?], which apparently is a rule for dark matter. And the PBS guy goes on:
That means one dark matter particle can’t bounce off another without getting super close. That enables dark matter to avoid collapsing easily under its own gravity, which is needed to explain how it remains as a giant puffy cloud surrounding nearly all galaxies.
Slam on the brakes, I think I’ve learned something? Dark matter forms a giant cloudy stuff around all galaxies! Or remains there, a sort of remnant? I cling to words, as I don’t know anything else. For example, I don’t really understand matter collapsing under its own gravity…
So this is how stars form… Gravitational contraction and collapse is fundamental to ‘structure formation in the universe’. First we get accretion, where gaseous matter, presumably of a simple sort (hydrogen? or proto-hydrogen?) is pulled into an accretion disc, which after reaching some sort of gravitational tipping point collapses in on itself to create pockets of density like black holes and stars. But what is this gravitational tipping point? I know I’m moving away from dark matter here. Anyway, this collapse, contraction or compression raises the temperature to the point where thermonuclear fusion occurs. But somehow dark matter avoids all that.
Anyway getting back to JWST, it has been given a number of missions or tasks, and the relevant one here is JADES (the JWST Advanced Deep Extragalactic Survey), which is an attempt to gain as much info as possible on the first galaxies or whatever to form in the universe. JWST apparently works – by design – particularly well in the infrared section of the electromagnetic spectrum:
It can see stars whose energetic ultraviolet and visible light has been stretched far into infrared wavelengths as it travelled to us through an expanding universe.
So I gather from that sentence that infrared is longer wavelength light, and that the expansion of the universe actually stretches the wavelength of initial bursts of radiation over space-time…
It’s estimated that these ‘dark stars’ or the radiation from them, date to a period some 400 millions years after the birth of the universe. And their brightness suggests ‘super-galaxies’, common enough in the universe, but not from way back then, because there doesn’t seem to have been enough time for them to form. So these discoveries have sent cosmologists into a spin. Here’s another interesting quote from our very interesting PBS Space Time Guy:
After all, these are the cosmic dark ages we’re peering into, a time when the ocean of pristine hydrogen forged in the Big Bang shrouded our vision across much of the electromagnetic spectrum. It’s a time when that same pristine hydrogen was able to form stars many thousands of times more massive than today.
So there seems a slight contradiction – not enough time for super-galaxies (or super-anything?) to form, yet ‘pristine hydrogen’ could form super-massive stars. Mais, continuons. The visual with this depicts Aldebaran (I have to notice everything), a star in the Taurus constellation and one of the biggest stars visible to we near-blind humans. It’s 44 times the diameter of our sun.
So these JWST discoveries have spawned scientific papers, of course, with some suggestion that they’re ‘super-bright dark stars’ (it’s theoretical cosmology, get over it). The theory, I think, is that under certain circumstances dark stars may form via dark matter particle annihilation. The particles are annihilated by their anti-particles – except that it’s more weird than that, as it’s theorised, in a recently published paper, that the particles are their own anti-particles, causing a process of self-annihilation.
Clearly we don’t know what dark matter actually is – one proposed candidate is a WIMP, a weakly interacting massive particle – but if we assume, with the PBS Space Time guy, that there’s lots of dark matter in the early universe, seasoned with a fair measure of hydrogen and helium, with uneven densities, accreting and pulling stuff in as mentioned before, creating structure, possibly at gigantic scales…. Well, here’s where I’ll quote the Space Time guy again, coz I don’t really get it:
The seeds of the first giant stars would have been so-called mini-halos with masses of millions to hundreds of millions of times the Sun’s mass. The dark matter part would have a hard time collapsing due to being weakly interacting. However the gas in that halo would fall towards the centre, perhaps en route to building a star, depending on how large this halo was.
So, I’m not quite sure where the halo idea came from, but mea culpa. Here’s some useful info from Phys.org:
The largest gravitationally bound objects in the universe are galaxy clusters that form at the intersection of cosmic web filaments. These entities are shaped and grow through massive collisions as material streams into their gravitational pull. Within the heart of some galaxy clusters are mysterious and little known radio mini-halos. These rare, dispersed, and steep-spectrum (brighter at low frequencies) radio sources surround a bright central radio galaxy and are highly luminous at radio wavelengths.
This, so far, isn’t taking me anywhere clear, but I’ll continue on in later posts, using Canto and Jacinta as my guide… But the next post will likely be on determinism (in human affairs).
References
https://en.wikipedia.org/wiki/Gravitational_collapse
https://phys.org/news/2017-08-brighten-perspective-mysterious-mini-halos.html#
reading matters 13: the glass universe
Canto: So The glass universe, published in 2016, has a cute title, referring as it does to the ‘glass ceiling’, another clever term for that invisible barrier up there that appears to prevent women from rising in politics, business and science, but also to the glass photographic plates upon which were recorded the spectrographic signatures of a vast arrays of stars, clusters and the like, in the decades of the late nineteenth century and early twentieth century, by a somewhat less vast array of human computers – the name given to the largely underpaid female stargazers and recorders of Harvard College observatory and elsewhere.
Jacinta: Yes, Dava Sobel, author of the fascinating little book Longitude, as well as Galileo’s Daughter, which is in a stack of books here waiting to be read, has brought to life a group of dedicated women and their male supporters over a period when the higher education of women was just starting to be addressed.
Canto: Yes, it all started with the Drapers, a wealthy and well-connected couple in the 1870s. Henry was a leading astronomer of the day, and ‘Mrs Henry’, aka Anna, a socialite and heiress. Their social evenings were mostly science-focused, with guests including the inventor Thomas Edison, the zoologist Alexander Agassiz, and Prof. Edward Pickering, of Harvard. Henry Draper was working on the chemical make-up of stars, using ‘a prism that split starlight into its spectrum of component colours’, for which he’d won great acclaim, when he died suddenly of a flu-like illness in his mid-forties. His devoted and rich widow, keen to continue his legacy, helped finance, along with Pickering, a continuation of his ground-breaking research.
Jacinta: And so the computers of Harvard College Observatory were born. We need to explain – or try to – the science of spectrographic analysis, but I’d like to first briefly describe some of the women who did this work. They include Williamina (Mina) Fleming, a canny Scotswoman frae Dundee (our birthplace), whose first American job was as the Pickering’s maid but who soon proved her worth as a star spotter and tracker, classifier, and organiser, leading the team of computers in the early decades. In 1899 she was given the title of ‘curator of astronomical photographs’, becoming the first titled female in the university’s history. As such she presided over 12 women ‘engaged in the care of the photographs; identification, examination and measurement of them; reduction of these measurements, and preparation of results for the printer’.
Canto: Far from just bureaucratic work – this would’ve involved a lot of learning and conjecture, noting patterns and anomalies and trying to account for them.
Jacinta: Absolutely. Antonia Maury, Annie Jump Cannon, Cecilia Payne-Gaposchkin, Henrietta Leavitt and the tragically short-lived Adelaide Ames were among the most noteworthy of these computers, and I should stop using the term, because they weren’t machines and they all made lasting contributions to the field…
Canto: And they all have their own Wikipedia pages. What more evidence do we need?
Jacinta: They contributed to academic papers, often without attribution, especially in the early years, and had their findings read out in academic institutions to which they were barred. Over time they became established teachers and lecturers, in the women’s colleges which started to become a thing in the twenties. But let’s get onto the daunting stuff of science. How were these glass plates created and what did they reveal?
Canto: So spectroscopy became a thing in the 1860s. Spectroscopes were attached to telescopes, and they separated starlight into ‘a pale strip of coloured light ranging from reddish at one end through orange, yellow, green, and blue to violet at the other’. I quote from the book. But what these changing colours meant exactly, as well as the ‘many black vertical lines interspersed at intervals along the coloured strip’, this was all something of a mystery, a code that needed to be cracked. Henry Draper had captured these spectral lines and intervals on photographic plates, which were bequeathed to Harvard by his widow. They formed the beginning of the collection.
Jacinta: The term spectrum was first used by Isaac Newton two centuries earlier, and he correctly claimed that this coloration wasn’t due to flaws in glass and crystals but was a property of light itself. The dark lines within the stellar spectra on Draper’s plates are called Fraunhofer lines, after a Bavarian lens-maker, Joseph von Fraunhofer, who built the first spectroscope. He at first thought the dark lines between the rainbow of colours his instrument produced were somehow artificial, but continued work convinced him that they were a natural effect. He gave them alphabetical labels according to their thickness, including the letter D for a double line in the pale orange region. He mapped hundreds of them, though today we’ve detected many thousands of them in sunlight. He didn’t understand what they were, though he realised they were something significant. Later in the 19th century Robert Bunsen and Gustav Kirchov conducted experiments with various chemical elements and found that they burned in colours around those black lines, which we now know as absorption lines.
Canto: Yes, it was Kirchov who connected the colours created by burning elements to the spectral lines that the sun’s light could be separated into, concluding that this great fireball of gases producing white light in the sky was actually a mixture of burning elements, or elements being transformed into other elements. As to the absorption lines, Sobel puts it this way:
As light radiated through the sun’s outer layers, the bright emission lines from the solar conflagration were absorbed in the cooler surrounding atmosphere, leaving dark telltale gaps in the solar spectrum.
These absorption lines, which together with emission lines, are spectral lines in the visible spectrum which ‘can be used to identify the atoms, elements or molecules present in a star, galaxy or cloud of interstellar gas’, to quote from this Swinburne University site.
Jacinta: So we’ll try to keep within the confines of the book, and the scientific developments of the period which these women, in particular, contributed to. So, rather, surprisingly to us modern wiseacres, these revelations about the sun as a super-hot fireball and a producer of elements was a bit hard for 19th century folk to take in, but scientists were excited. Henry Draper described spectral analysis as having ‘made the chemist’s arms millions of miles long’, and in 1872 he began photographing the spectra of other stars. It was long known that they had different colours and brightnesses – called ‘apparent luminosities’ – but spectral analysis provided more detailed data for categorisation, and sets of photographs revealed changes in luminosity and colour over time. Williamina Fleming, Harvard’s principal computer, took charge of Draper’s thousands of plates, which provided the most detailed spectral data of stars up to that time, and was able to analyse them into classes, via their absorption lines, in new and complex ways. It was cutting edge science.
Canto: There was also an interest in throwing more light, so to speak, on variable stars. They were so numerous and complex in their variability that Pickering needed more computers to track them. Lacking funds, he advertised for volunteers, emphasising the role of women in particular, whose effectiveness he’d seen plenty of evidence for.
Jacinta: Not to mention their willingness to work for less, or effectively nothing. These were often siblings or partners of astronomers or other scientists, with unfulfilled scientific ambitions. Later, though, came from the newly created ‘Ladies’ Colleges, such as Radcliffe and Wellesley.
Canto: The Orion Nebula was a particularly rich source of these variable stars, and Pickering found an ideal computer, Henrietta Leavitt, a Radcliffe graduate, to explore them. Within six months, she’d confirmed previous identifications of variables in the nebula and added more than 50 others, afterwards confirmed by Fleming. Then, using a combination of negative and positive glass plates, she found hundreds more, in the Orion Nebula and the Small Magellanic Cloud. As Pickering pointed out, due to the lack of resolution in the plates, this number was likely the tip of the iceberg. In writing up a report of her findings, Leavitt described a pattern she’d found: ‘It is worthy of notice… that the brighter variables [aka cepheid variables] have the longer periods’. This brightness (or luminosity) and its relationship to periodicity (the time taken to go through a full cycle of change) is now known as the Leavitt Law, though of course it took decades for Henrietta Leavitt to receive full recognition for discovering it.
Jacinta: Yes, it’s worth noting that these women worked painstakingly on data analysis, developing new and more rigorous classification systems, studying and theorising about anomalies, and communicating their findings to leading astronomers and researchers around the world. And it’s also worth noting that they were supported and highly appreciated at Harvard by Edward Pickering and his successor as Director of the Harvard College Observatory, Harlow Shapley – though of course there were plenty of naysayers.
Canto: Okay so we’ve spoken of two or three of the computer stars’, and there were many more, but let’s finish with the work of Antonia Maury.
Jacinta: Well we must also mention Annie Jump Cannon (great name), star classifier and photographer extraordinaire, suffragist and generally formidable persona, in spite of being almost completely deaf. She classified around 350,000 stars and contributed greatly to the Harvard Classification Scheme, the first international star classification system. Antonia Coetana de Paiva Pereira Maury (I’m not kidding), a graduate of Vassar College, was a niece of Henry Draper.
Canto: Not what you know but who you know?
Jacinta: It is partly that – and that cliché is worth a whole book to itself – but Maury was no slouch, she was a keen and observant star observer and systemiser. One important discovery she shared with Pickering was one of the first known binary star systems, in the handle of the Big Dipper. This required months of careful observation from 1887 through 1889, as they noted one spectral line separating into two then the lines merging again, then separating, with one line shifting slightly to the red end of the spectrum and the other to the blue. Once they recognised that they were dealing with binary star systems, others were soon found. And once these systems were confirmed, Maury carefully calculated their orbital periods and speeds.
Canto: There were many other important breakthroughs. Spectral colours, as we’ve pointed out, were connected to particular chemical elements, and Cecilia Payne, whose major focus was the measurement of stellar temperatures, found a superabundance in the elements hydrogen and helium, which confounded other experts and soon made her doubt her own calculations. Payne wrote up her findings in the Proceedings of the National Academy of Sciences in 1925, ‘admitting’ that the percentages of hydrogen and helium were ‘improbably high’ and ‘almost certainly not real’.
Jacinta: Yes, it’s well worth noting that the knowledge we have of stars today, which seems almost eternal to us, is in fact very recent. The book also covers the dispute between Harlow Shapley and Edwin Hubble – with many on either side of course – as to whether other galaxies existed. That dispute was only resolved in the thirties, and now we count other galaxies in the trillions. So the period covered in Sobel’s book was a truly transformative period in our understanding of the universe, as well as transformative in terms of women’s education and women’s participation in the most heavenly of all the sciences.
Canto: Whateva.
References
The glass universe, by Dava Sobel, 2016
https://science.nasa.gov/astrophysics/focus-areas/what-are-galaxies
https://en.wikipedia.org/wiki/Annie_Jump_Cannon
https://en.wikipedia.org/wiki/Antonia_Maury
https://en.wikipedia.org/wiki/Williamina_Fleming
reading matters 1
The universe within by Neil Turok (theoretical physicist extraordinaire)
Content hints
– Massey Lectures, magic that works, the ancient Greeks, David Hume and the Scottish Enlightenment, James Clerk Maxwell, quantum mechanics, entanglement, expanding and contracting universes, the square root of minus one, mathematical science in Africa, Paul Dirac, beauty and knowledge, the vitality of uncertainty, Mary Shelley, quantum computing, digital and analogue, Richard Feynman, science and humanity, humility, education, love, collaboration, creativity and thrill-seeking.
water on Earth – no problemo
So, as described in my last post, H2O in its various forms is plentiful in our solar system as well as beyond it. But, being more or less scientifically illiterate – despite decades of reading stuff on science – I can’t quite work out how liquid water is so abundant on the Earth’s surface. The story has long been told of water-iced asteroids in the time of the heavy bombardment being responsible, with the major proof being that these carbonaceous chondrite asteroids have, or had, the same signature of heavy (deuterium-rich) water as the water we find on Earth. While this seems a strong argument to me, how did the Earth manage to hold on to that water during those super-heated days?
I’ve looked at this in a previous post, sort of, but I’m still not clear on the atmospheric conditions that brought about our soggy planet (much more soggy during the Mesozoic though). In any case, I’ve recently read that bonafide researchers on this topic have also been mystified about the sheer volume of water on Earth.
Enter a new (to me) hypothesis, published in the Journal of Geophysical Research: Planets a little over a year ago. It argues – and other astrophysicists appear to be impressed by the reasoning and the detailed analysis in the paper – that the water came not only from asteroids but also from the solar nebula.
Solar nebula? Never heard of it, but apparently the concept has a long history. The so-called nebular hypothesis for the formation of our solar system was first proposed by Emanuel Swedenborg in the 1730s, and further elaborated by such luminaries as Immanuel Kant and Pierre-Simon Laplace later in the 18th century. Surprisingly for such an early contention, it has stood the test of time and survives today, though the details are still argued, and there are a few competing hypotheses. In any case, without going into too much detail, a nebula of dust and gas began to form around 4.6 billion years ago, and collapsed in on itself due to gravitational forces, spinning around a newly-formed sun. Out of this material, protoplanets gradually formed.
Water in the Earth’s oceans has approximately the same D/H (deuterium to hydrogen) ratio as that of the above-mentioned asteroidal carbonaceous chondrites, so it has always seemed a safe bet that most if not all water came from those asteroids. Yet the sheer volume of water was still a problem. Jun Wu, the lead author of the recent paper, had this to say about the theoretical situation:
The solar nebula has been given the least attention among existing theories, although it was the predominant reservoir of hydrogen in our early solar system.
What has apparently added credence to the new hypothesis is that samples of hydrogen near the core of the Earth have significantly less deuterium and may fit better with the ratio of hydrogen in the solar nebula. Also the isotopic signatures of the noble gases helium and neon found in the Earth’s mantle fit the signatures of these gases from the time of the solar nebula. The explanation of how the lighter hydrogen found itself drawn to the Earth’s centre, in a process called isotropic fractionation, is provided in the paper, apparently. It’s a very interesting story, if true, and it may have implications for liquid water on habitable-zone exoplanets. That’s to say, there’s no reason for it not to be quite common. Here, to finish, are a couple of thought-provoking comments from members of the research team.
… there’s another way to think about sources of water in the solar system’s formative days. Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have served as the origin of Earth’s water.
Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems.
References
How did Earth get its water?
https://www.britannica.com/science/solar-nebula
a little about the chemistry of water and its presence on Earth