S3C9: Astronomy

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a) Historical relevance

Astronomy undoubtedly qualifies as the first natural science across the civilizations that have left written records, either by their own markings or through those of other civilizations interacting with them. The reason for this lays not only in the fundamental intellectual attraction it offers for inquisitive minds, but also, more practically, for the distinct advantages it provided to anybody who could map the sky and extract information from the position and perceived movements of the stars.

If we are to distil the various techniques and applications, they all relied on the relationship there exists between three variables: time, position, and motion – and of course being proficient in geometry did help since angles often offered the link permitting to compute the value of the third variable by knowing the first two.

The first use case that comes to mind is probably intra-day timekeeping, knowing what time it is and therefore being able to plan and organize work and social activities. By subtracting T1 from T2, this also gives one the ability to measure the amount of time that has lapsed between said T1 and T2 and thus infer distances provided one keeps the same speed and direction of travel or estimate quantities assuming one knows the rate of flow. So once calibrated, one could use an hourglass to measure time or, even better, a large water clock (also called clepsydra) to both measure and keep the time, an instrument particularly favoured in ancient China. However, the most iconic instrument for marking and reading the hours of the day is the sundial. One only needs to plant a vertical stick called a gnomon on a horizontal plate and, for the plain vanilla case of equatorial sundials where the gnomon is aligned with the Earth’s axis of rotation, each hour corresponds to a 15° arc, being 360° divided by 24 – of course, back then there was no such thing as a standardized hour but the point is that one can tell the time of the day, whatever the local units are being used. Being located on the equatorial line is not a requirement for this, the angle corresponding to the latitude just needs to be replicated off from the horizontal plane. If not, laid horizontally, some geometric formula involving tangent and sine are required whereas for vertical sundials it would be tangent and cosine. I am enclosing the link to the Wikipedia entry for Sundial in the last section of this chapter if you wish to ensure there remains no shadow in your understanding. It should be noted that, compared to the sundial, the water clock had the distinct advantage of also working a night but without additional mechanisms it would start running slow as pressure decreased so that it would need to be constantly replenished, not something everybody could afford.

The second use case, one we may not consider as essential in our manufacturing and service society, is the ability to tell the time of the year. For agricultural societies this was paramount, one had to know when was the right time to plant and harvest as well as when a river like the Nile would flood. Getting it wrong could spell a failed harvest and famine. This is where mapping the sky and using constellations to identify intervals along the elliptical came into play – in case you have not read it yet, we covered the nature of constellations in S3 Section 8.f, the last section of the previous chapter. If the sundial allows an observer to tell the time of the day thanks to the Earth’s rotation upon its axis, then the position of the stars in the sky allows the same observer to deduce the time of the year thanks to the revolution of our planet around the Sun, though at the time it was erroneously thought it was on account of the revolution of the Sun around the Earth.

That is not all however, the position of the stars also provided the means to deduce position, on the surface of our globe, the third main use case of astronomy in ancient times and one which is still of relevance in our technological age. Once more, angles come into play in the most straightforward instances of celestial navigations. Not angles from the centre but above the horizon allowing the observer to compute, using geometric equations, how far one is from the point on the surface of Earth that is located exactly below a reference star – of course this data relating to the reference stars has to be known beforehand or deduced from existing tables. On this basis, it is possible to draw a circle centred on this earthly point with a radius corresponding to the computed distance. Do that again with another reference star and angle combo and you will get a second circle intersecting the initial one in two points. Those two points should be quite far apart so one of them can be ruled out if you already know in which part of the world you are. This is only one example and there are other techniques you may want to read about independently. This ability to put oneself, a caravan or a ship on a map was indispensable when it came to crossing long distances, especially at sea where avoiding reefs was potentially a matter of life and death and when charting new horizons could bring in riches for you and your sovereign.

b) What about astrology?

We talked about constellations, yet isn’t that the domain of astrology? Indeed, it was and still is though not exclusively. Originally astronomy and astrology were one and the same, there was no such thing as sciences and knowing the world was knowing the creation and the realm of the gods. Accordingly, eclipses were taken as omen and specific stars or groups of them, as the product of divinities, were given certain properties.

For example, in Western astrology, the presumption is that the zodiac sign under which one is born influences one’s life whereas in Chinese astrology the five known planets were associated with the five forces or elements (the exact term is “wuxing” and there is no Indo-European perfect translation for it): Mercury goes with Water, Venus with Gold or Metal, Mars with Fire, Jupiter with Wood, and Saturn with Earth or Soil.

Of course, given our current scientific knowledge, astrology need not even be debunked, there is simply no physical basis for it when we know the overwhelming influences of the Earth, Sun and Moon gravity compared to far away stars, the complex workings of our biology including but not limited to the importance of our genetic material, and simply the chaotic nature of the world in which we live, whether it is the weather, our social interactions or the events that befall us, be they lucky or unfortunate.

c) Visible-light astronomy

It seems to us it should have been quite obvious for ancient civilizations who excelled in mathematics, knew certain optical properties including the concept of magnification (which is the enlargement of the apparent size of an object) and were extremely keen on better understanding about the stars and planets, that there was a way to use glass to observe those celestial bodies in much greater details. And yet, the telescope wasn’t invented until the turn of the 17^th century CE.
That’s because we fail to appreciate the incremental changes required not only to theoretical knowledge but also to the hardware and materials involved in developing such a device. For the refractory telescope, it was not sufficient to think about using two lenses, these also had to be made of glass with properties such as transparency, the absence of internal bubbles and an extremely smooth polishing of all four surfaces (two by lens). Hence, it was on the back of the improving quality of glass used for spectacles that the invention of the telescope eventually came about and Galileo was first to record the observations of the namesake moons of Jupiter immediately after building his own version of the device at the end of 1609.

The first aim of the telescope is to gather more light than our human eye naturally can, a refracting telescope does so thanks to a convex objective lens which refracts light and thereby focuses the rays to form an image. The second goal is to magnify this image and “unbend” the divergent rays of light after the focal point so they become parallel again; this is achieved thanks to a second convex lens called eyepiece which is positioned between the focal point and the eye of the observer with the degree of magnification being the ratio of the focal length of the objective over that of the eyepiece. A couple of technical terms you may want to know here: the focal length “f” is the distance between the lens and the focal point, it is positive when the light is being made to converge, and optical power “p” is the reciprocal of the focal length: p=1/f.

However, refracting telescopes have a few flaws, some having to do with the difficulties and cost involved in obtaining perfect glass and others with the nature of light itself, specifically the fact that the refraction index changes with the wavelength of radiations, thus causing chromatic aberrations in the output.

One way to address this is by finding an alternate architecture. The weakness being the large objective lens, we need another way to focus the light. The solution is actually straightforward enough, instead of positioning the light bending device in front of the focal point, one simply can position it behind and reflect the light entering the telescope using a concave surface. So the light enters the device all the way to the bottom where it gets reflected at an angle and focused onto a secondary concave mirror that in turn reflects the light towards the observer – see diagram of a Gregorian telescope in Figure 6A, named after its designer James Gregory in 1663. Alternatively, this second mirror is flat and positioned at an angle so it reflects the focused light towards an observer located on the side of the mirror – see diagram of a Newtonian telescope in Figure 6B, this time named after Isaac Newton who built such a device in 1768. In addition to avoiding the issues of chromatic aberration, this configuration also does away with the need for large transparent and extremely heavy and expensive pieces of glass as well as 4 perfectly polished surfaces. Instead, only one coated reflective surface is required.

Figure 6A: Light path in a Gregorian telescope

Figure 6B: Light path in a Newtonian telescope

Credit for both figures: Krishnavedala (CC BY-SA 4.0)

Nowadays, the majority of telescopes have a reflective architecture and the improvements in angular resolution (the ability to see crisp details of an object from far away) is a function of the aperture (the diameter of the opening) and generally the bigger the better though the diameter required is also a function of the wavelength of light being analysed. Nonetheless, the transparency of the intervening medium between the telescope and the object being resolved also matters. Indeed, the particles in the atmosphere have a distorting effect on electromagnetic radiation, something we have all witnessed in the twinkling of stars. To make matters worse, our urbanizing society is also polluting the night sky with light, which interferes with astronomical observation because the artificial light is reflected within the sky and produces a glow effectively competing with the light from distant planets, stars and galaxies.

In order to circumvent these two issues, major observatories have been set up in locations removed from artificial light sources and high above ground level, where the atmosphere is thin. The first observatory of such kind was completed in 1904 at the Pic du Midi de Bigorre in the French Pyrenees at an altitude of 2,877m and the most famous is perhaps the Mauna Kea Observatory at 4,205m in Hawaii. With dark skies, high altitude and low humidity, the spot is now home to no less than thirteen telescopes – spanning different parts of the electromagnetic spectrum, something we will discuss in the next section.

Of course, pushing the logic to the extreme, the best viewing environment would do completely away with humidity, atmosphere and light pollution. Space beckons. The first space telescope was OAO-2 (Orbiting Astronomical Observatory 2) and the best-known of them is arguably the Hubble Space Telescope, almost a household name thanks to the stupendous quality it delivered not without a serious initial hiccup which required servicing before it could produce correct images. And this is where the drawbacks of space telescopes need to be mentioned: they are expensive to produce, to launch, and to service, with the latter not always being possible.

d) Other techniques and instruments

OAO-2 did not operate in the visible spectrum of light, it focused on ultra-violet, and Hubble main instruments operated in the visible, infrared and UV ranges. In fact, the visible and radio ranges suffer comparatively less filtering and distortion when crossing the atmosphere as compared to other wavelengths so space telescopes tend to be designed to operate in the other parts of the spectrum.

The rationale for operating in different parts of the electromagnetic radiation spectrum is simply that different objects and properties can be detected. Let’s make a quick tour starting from the longest wavelengths (radio) all the way to the highest frequencies (gamma-ray).

As we mentioned in S3 Section 8.d, galaxies with an active nucleus tend to emit a lot of radiation in the radio range and the cosmic microwave background radiations were also detected using radio telescopes. The first key challenge facing such instruments is the reflection caused by the ionosphere (recall that we use it the other way around to reflect radio waves emitted from the surface of the Earth to regions beyond the horizon of emission – this was explained in S3 Section 3.a on the physics of the atmosphere) but unlike optical telescopes they can operate day and night. The second challenge is size: because the wavelength is typically around 3m to 30m, they need to have diameters larger than 100m to provide adequate resolution. Fortunately, the surface need not be continuous and properly spaced wire mesh can do the trick. Necessity being the mother of all inventions, it seemed natural to extrapolate the concept of spaced wire mesh to that of radio interferometry. Essentially, this consists in simulating a large aperture by processing the signals recorded at distant locations with uneven spacing (or baselines) between. This technology replicates the resolution, but not the sensitivity of a telescope of a diameter equivalent to the distance between the furthest receptors. One such example is the famous ALMA, the Atacama Large Millimeter Array located in the arid Atacama Desert of northern Chile at an altitude of 5,059m. It comprises 66 antennae of either 7 or 12 meters in diameter and operates at wavelengths ranging from 0.32mm to 3.6mm, hence the formal name of the interferometer also includes the term submillimetre.

With shorter wavelengths, from 0.3mm down to 0.75 micrometres, we are in the domain of the infrared (IR), between submillimetre waves and visible light. The main reason to use infrared detection is probably because at relatively low temperatures, most of the thermal radiation is emitted within that part of the spectrum. This allows for the discovery of bodies with lower energy levels such as young stars, but it also means these telescopes need to be supercooled to avoid thermal interference from their environment. In addition, infrared light tends to penetrate dust clouds better than light at shorter wavelength so this is the best range for observing formations such as nebulae. And thirdly, the light from those far-away redshifted galaxies moving away from us at cosmic speed will tend to have stretched waves (the reason for the redshift) so infrared instruments come in handy for their observation.

Below the visible spectrum in terms of wavelength (or above in terms of frequency) we enter the domain of ultraviolet astronomy, from 320 down to 10 nanometres. Our atmosphere shields us from those high energy photons, as it does with x-rays and cosmic rays, so UV telescopes operate in space and were the first instruments onboard OAO-2 as mentioned at the outset of this section. Contrary to IR, UV telescopes will thus favour the detection and observation of bodies emitting high energy particles such as igniting and dying stars, phenomenon such as auroras, and generally the behaviour of plasma and the composition of the interstellar medium.

Moving down the list is X-ray astronomy, it is used for wavelengths from 10nm down to 10 picometers (0.01nm) although the energy units of about 100 keV down to 100eV are often used instead since we are dealing with high-energy particles emitted by astronomical objects recording temperatures in excess of one million kelvins. If you do not know or recall about the electronvolt, you may want to take a look at S1 Section 6.e. Thus, the focus would be regions such as the Sun’s corona and celestial bodies like neutron stars, the accretion disks surrounding black holes, and supernovae.

Finally, photons with wavelength under 10 picometers (a picometer is 1×10^-12m) and energy levels above 100kEV fall under the purview of gamma-ray astronomy. These gamma rays are created by energy-intensive bodies and celestial events such as the collision of neutron stars, solar flares, pulsars and active galaxy nuclei. This type of observation is limited by the relative scarcity of these high-energy photons compared to other types of radiations and resolution is problematic because of the difficulty experienced in focusing this type of rays.

If you wish to know, the now famous James Webb Space Telescope which entered service in July 2022 orbits the L2 Lagrange point of the Sun-Earth system, operates towards the orange to mid-infrared range of the electromagnetic radiation spectrum, and its 6.5m in diameter main mirror is actually built of 18 adjacent hexagonally-shaped gold-plated beryllium.

e) The future of astronomy

The James Webb Space Telescope did deliver stupendous images, however the total cost for the project came up to US$10bn, quite a hefty bill. There lies one of the main issues for the future of astronomy: cost. Big science is expensive with funding often tied not simply to academic interests but also to national considerations. Granted, the cost of launching payloads into space is decreasing, except this is only a fraction of the overall development and manufacturing cost. Exacting precision and calibration over large pieces of hardware calls for a lot of time and expensive technologies and materials.

Perhaps the saving grace lies with the developments happening in the realm of space exploration, the topic of Chapter 10. Having forward bases or even full-blown colonies in space would allow for the manufacturing, deployment and even servicing of more powerful telescopes.

In any cases, progress in astronomy will always go hand in hand with advances in theoretical research. New observations can lead to new hypotheses and conversely, the unresolved questions theorists are grappling with may one day be addressed by new observational instruments up in space. Unfortunately this symbiosis suffers from the lag involved in developing anything big-science: it takes time to get the funding, plan, design, manufacture and launch. This means today’s theoretical questions will sometimes have to wait a couple of decades before they can be tested with new technologies.

If I were to venture an opinion, I would say the main thrust of astronomy in the decades to come will be oriented towards the search for habitable planets and extra-terrestrial life with astrobiology the companion science that will experience a huge leap in popularity and funding.

f) Searching for life outside Earth

The Encyclopedia Britannica defines astrobiology as “a multidisciplinary field dealing with the nature, existence, and search for extra-terrestrial life.”.] This is a very broad field indeed so here I will only briefly discuss the search aspect as opposed to the conditions for life to emerge elsewhere in our Solar System or on the billions of exoplanets (a term referring to any planet outside of the Solar System) that are very likely to be out there in the Milky Way and other galaxies, if we extrapolate from the observations made so far. Nor will I explore the processes under which life emerge; if you are keen to know more about this you may want to read about abiogenesis, I have included the link to the relevant Wikipedia article in the next section.

And so, the search for extra-terrestrial life is on and in particular the search for intelligent life. There is of course an acronym for it: SETI. The primary means of detection of any life form is the spectroscopy of the atmosphere of a planet, looking for biosignatures such as the presence of certain chemical compounds indicating some metabolic process is happening on the planet. Considering Earth, it would be the presence of a large ratio of oxygen in our atmosphere for example but also other features such as the presence of cities and other obvious signs of technological developments pointing to the existence of intelligent life on our planet. This type of clue is called techno-signature and these would include radio wave emissions or the sending of probes, whether they contain golden records for alien consumption or not.

This is a transparent reference to the active SETI efforts that saw us embark samples of Earthly cultures and knowledge on Voyager 1 and 2. Nevertheless, not everybody agrees this is a clever move, and I am firmly in the camp of those who think it was a dangerous, irresponsible act though given the unrelenting volume of radiations our modern technologies emit we should be pretty easy to spot if other alien civilizations have their own SETI program. Why is this dangerous? Well, simply because we have no idea about the intentions of such civilizations and if they can come to us, this means they are much more advanced than we are technologically and we would be no match if they do not come in peace. Furthermore, there is a game-theory element to this: they would not know about our behaviour or intentions either and would thus run a risk by letting us develop along a technological continuum. Following this logic, one could argue it is better to be safe than sorry and that pre-emptive strikes are the best remedy. Even assuming a more benevolent stance, which is contrary to the evolutionary principles of survival first and the necessity of finding food sources, the appropriate strategy is to literally remain under the radar and not give any external sign of intelligent life so as to remain undetected.

This “dark forest” hypothesis, named after Liu Cixin’s namesake novel The Dark Forest, is only one of many possible explanations to the Fermi paradox, or rather it is one of the many arguments that show the Fermi paradox isn’t one at all, just an interesting question based on debatable predicates. In short, Fermi asked “where is everybody?” if there are this many planets harbouring intelligent life out there. Undoubtedly, he was right in that, considering the billions of galaxies and the billions of planets in each of those galaxies, even filtering down the number of habitable exoplanets so they meet the goldilocks criteria such as temperature, presence of liquids and an interface with rocks, a magnetic field, and geological activity providing a heating source, we still end up with tens of millions if not billions of probable candidates. At some stage, some of them will experience the development of life and a fraction of these will progress to the stage of complex life and ultimately intelligence accompanied by technological development. The last stage happened on Earth in a blink of an eye, by cosmic time standards.

The first main flaw in the chain of reasoning is to assume that surely interstellar travel is possible. Possible by probes certainly, possible for living organisms, this is not a given. The second flaw is to imply that if it is possible then it must have been done. Here, let’s not forget we are dealing with supremely intelligent creatures, probably much wiser than we are, and as I will discuss in the next chapter (S3 Section 10.e) it is not obvious this would be the case. Thirdly, and this is the dark forest point, he posited that such alien civilizations would actively be looking for other forms of intelligent life, whether by visiting worlds with techno-signatures themselves or via unmanned probes, thereby revealing their existence. The fourth and final major flaw is declaring this had not happened. Really? I don’t believe there is any evidence this is the case but maybe it did happen a few thousand or million years ago and these aliens did not find anything they thought was genuinely interesting, or they do know about us and are quietly keeping an eye on the situation. How would we know?

For the record, I think the first three challenges are strong enough to make the fourth one unnecessary.

g) Trivia: the Observable Universe

The concept of observable universe is a direct consequence of the assumption that nothing can travel faster than light. Using this as a starting point and combining it with the estimated age of the universe of 13.79 billion (Earth) years, we are not able to see further than a horizon from where light would have been emitted exactly more than 13.79 billion years ago. We are not able, and we will never be able to. As explained in S3 Section 7.c, since there is no centre and the universe looks to us to be the same and unbounded in every direction, then this horizon forms a sphere with Earth at the centre. This is our observable universe and it would be a different one if we were located in a different stellar system or galaxy.

Now, you may remember that in the same Chapter 7 we also mentioned the universe is expanding, possibly due to the properties of dark energy. As a result, any point in space that was 13.79 billion light years away from us has in the meantime moved further away… this is called co-moving and this dynamic works out to a co-moving distance not of 13.79 billion light-years but of 46.6 billion light-years. Hence, the observable universe is a sphere surrounding us with a diameter of just over 93 billion light-years.

h) Further reading and listening (S3C9)

Suggested podcast:

Season 3 of the Scientific Odyssey: https://thescientificodyssey.libsyn.com/