A Meditation on Science: Bed-time Story Astronomy, part II
Bed-time Story Astronomy, part II
[Image: NASA’s “Astronomical Photograph of the Day” for 1996-01-24. A part of the first Deep Field image.]
Update 2022-11-29: Interview with Geoff Marcy added to Additional Resources.
Part I of this meditation concludes with Galileo Galilei's publication of Sidereus Nuncius which serves as a hallmark for the arrival of the scientific method and one of the final nails in the coffin for the geocentric model of astronomy.
It had taken around 2 000 years for the first western proponent of this idea, Aristarchus of Samos to be proven correct. Part II surveys the next four hundred years, meeting significant contributors to Astronomy and the arrival of the next major revolutions of our understanding of our place in the "heavens" or cosmos.
In Part I we saw the Greek, Danish, Italian and German/Austria contributions to Astronomy. As the "scientific revolution" kicks off in Europe we will see the English, Scottish, French and finally the USA join the picture as science and astronomy in particular becomes a global endeavour.
I wish to re-state that this "history" is dominated by "the west". We know that the ancient Chinese and Egyptian civilization made accurate astronomical observations. Indeed, thinkers from the ancient Greek city states learned much by visiting Egypt. Similarly, excellent records and advances were made during the periods of the Islamic caliphates from 900 CE onwards. I have already mentioned the amazingly accurate calendars of central and southern american civilizations.
A Meditation: Part II
Three invaluable gifts were to be offered to Astronomers by a single man, Sir Isaac Newton. Everybody who has taken any physics class at high school will have heard of Newton and his laws of motion. They are fundamental and remain valid to this day (except for massive bodies moving close to the speed of light). In the field of entertainment one knows that one has "made it" when one has a single name. Newton's most famous work is known in modern science simply as "Principia".
From this work and its laws of gravitation one can better calculate the orbits of the planets. Combined with the understanding of the heliocentric model more reliable observations could be made.
Another work by Newton was Optics. The publication of this was delayed until the death of Robert Hooke who took a great dislike to some of its ideas. The year following Hooke's death in 1703 Opticks was published in English with a Latin translation issued two years later. The major advantage to astronomers here would not really show up for a few centuries. It would require quantum mechanics and the emergence as chromophotography. In the meantime, the increased study in light would enable lens makers to improve their work.
The third gift from this man who "stood on the shoulders of giants" was an improved design for a telescope. He was working off ideas proposed by others but which had never been realised. Newton did actually build a telescope of his design. The new design replaced the two lenses of the then current type of telescope, the refracting telescope, with two mirrors, a reflecting telescope. Improvements on this design, but retaining the core idea, remain the best design for both optical and radar telescopes to the present.
Newton's contribution to astronomy was nothing short of stellar.
While improvements in telescope design and manufacture occurred, particularly in the methodology to produce parabolic mirrors, the next major advance was some century and a half away. It comes from one of the most amazing scientists whose work is poorly known outside of the communities of physicists, astonomers and electrical engineers. Indeed, the implications of his major work would lead Einstein to develop his Theory of Special Relativity. The Scotsman's name was James Clerk Maxwell.
[Image: James Clerk Maxwell, from Wikipedia.]
The background to Maxwell's work were the efforts of Michael Faraday, Volta and others in the fields of electromagnetism and electrochemistry. These essentially lead to things like batteries, electric engines and alternators. The field was a "hot topic" in the early to mid nineteenth century. Maxwell's magnum opus was "A Dynamical Theory of the Electromagnetic Field" and took what was known, that electricity and magnetism were related and described their interrelationship mathematically. He also described light as an electromagnetic wave and predicted the existence of radio waves. 80 years later Radio Astronomy would extend astronomy into entirely new methods of observation.
Maxwell's work also extended that of Louis-Jacques-Mandé Daguerre who developed the first commercially viable stable method of taking and recording a detailed, photographic image. Daguerre's process introduced in 1839 was improved in utility in 1860. Maxwell's description of light as an electromagnetic phenomena and his equations for electromagnetism were published in 1865, just before Faraday's death a couple of years later.
It was to be the improvement in telescope design and mirror fabrication combined with improved photography which would lead to the next revolutions in Astronomy.
A Standard Candle
One of the fundamental questions of astronomy is how far away a source of light is. There is an easy way to calculate this if one knows the luminosity of the star. The relevant law is, like so many others, based on an inverse square law, which is to say the the brightness dims with the square of the distance from the source. So, if one knows the "natural" brightness, one can calculate the distance. Two other methods exist for determining astronomical distances, parallax and triangulation. However, their utility is limited to short distances of a few hundred light years. Our galaxy is more than 100 000 light-years in diameter.
Using optical telescopes astronomers had identified objects which were blurry rather than being a single point of light. They were assigned the name nebulae, the Latin for clouds. By this stage astronomers had identified that the visible stars all seemed to lie within a range of distances and were confined to the Milky Way galaxy. This in itself is a significant moment in astronomy. We had moved from the earth is at the center of the heavens to the sun is at the centre of the heavens to the sun is a part of a galaxy and the center of that would seem the center of the cosmos.
There was, however, a nagging question. Were there any other galaxies or was the Milky Way the only one?
Edwin Hubble worked at the Mount Wilson Observatory in California, USA. Around 1919 the Hooker Telescope designed to use photographic plates to record its imagery and having a 100 inch primary, parabolic mirror was completed. This telescope was the largest in the world at this time. Hubble was attempting to answer this gnawing question of whether the Milky Way was alone in the universe.
A decade earlier, another unsung hero, or heroine in this case, was Henrietta Leavitt.
[Image: Henrietta Leavit. The source again is the relevant Wikipedia page.]
I would love to write an entire article on this remarkable woman, but a short summary will have to suffice. She worked as a "human calculator", meaning she was astonishingly good at mathematical calculations, at the Harvard College Observatory. In the course of her duties she extended the two distance methods which were limited with a calculation which was limited only by the ability of a telescope to see the key object. She had found a "standard candle".
There is a type of star called a Cepheid variable. It issues an extra strong light in a beam from its center moving to its equator which causes the star's brightness to grow and diminish on a regular periodic basis as the star rotates. Her discovery by reviewing the photographic plates at the college observatory was the relationship between the period of the rotation and the luminosity of the star. Thus, if one measures the period, one can determine the brightness and thus derive the distance. This can only be described as a magnificent contribution to astronomy.
Hubble was studying (photographing) the Andromeda Nebula. On one of the most famous photographic plates in the history of astronomy he wrote "VAR!".
[Image: on the left is an image of Hubble’s original photographic plate. On the right are images of the same star showing its variable brightness (luminosity). Source NASA.]
He had identified a Cepheid variable star in the nebula and thus its distance could be calculated. He came up with about 900 000 light-years distant. He was out by a little over a factor of two, but this mattered little. This was far beyond what was understood to be the limits of our galaxy at the time. It still is. The Milky Way has a width of around 100 000 light-years and Andromeda is about 2 500 000 light years away, or 25 times the width of our galaxy distant.
2 Trillion Galaxies
A century later, the USA launched the world's first space telescope, the Hubble Space Telescope (HST), into orbit. This would remove the frustration of weather and atmospheric variations which annoy optical astronomers to no end. Thus, everyone would have wished to use it, and therefore a committee needed be establish to assess the merits of applications for its use.
Some bright spark proposed to this committee that the space telescope be pointed at a tiny very dark corner of the sky where there was "literally nothing to see". Whoever proposed this crazy idea must have had quite some backing as the proposal was accepted. The HST over the period of 10 days took 342 exposures to capture the equivalent of a long time exposure.
The result is the Hubble Deep Field in which three of the four quadrants of the combined image are not fouled by a single star. We see thousands of coloured lights every one of which is a Galaxy. Hubble would have been most proud. It is thoroughly fitting that his name was on the telescope which took this signature image. I’d like the image to be known as the Hubble-Heavitt Deep Field.
[Image: a part of the eXtreme Deep Field. Published by the European Southern Obervatory.]
So informative, shocking even, was the original Hubble Deep field that two later versions have been performed with more exposure and more details, the Ultra and eXtreme Deep Fields. It is these and other astronomical investigations which provide the basis for the current estimate of the number of galaxies in the visible universe.
Current astronomical estimates are that there are 2 trillion galaxies. This is for the "observable universe". Does the universe extend beyond this? Nobody knows, but it seems likely.
Our galaxy has between 200 and 400 billion stars in it. Another question yearning to be answered was how common are planetary systems around stars? The very aptly named Kepler Space Telescope was assigned the task of beginning to answer this question. The hunt for Exoplanets began. It has been followed my numerous other missions, many space based, to provide further data to help answer this question. The basic answer currently is that many if not most stars in our galaxy seem to have planets orbiting them.
Thus, one can make a back-of-the-napkin calculation for the number of planets in the universe. Let's assume 4 (why not) planets per star and that 50% of stars have planets and that the Milky Way has 200 billion stars. This would seem a conservative estimate. We then assume that these numbers extend to all observable galaxies:
1000000000000000000000000 planets, roughly, give or take quite a lot.
That’s 25 zeros, or ten trillion trillion planets.
[If you are interested in how astronomers managed to discover planets, see the interview with Geoff Marcy below.]
The primary lesson from this story is a restatement of Newton's "standing on the shoulders of giants". Science is a team endeavour, and one's team includes luminaries who have been deceased for centuries or millennia.
Richard Feynman described science as a "satisfactory philosophy of ignorance". To Feynman, doubt is fundamental to science. One of his famous quotes summarizes this. In the style of Oscar Wilde or Mark Twain he declared:
I would rather have questions that can't be answered than answers that can't be questioned.
The body of theoretical science could be described as the collection of theories which make useful predictions and which have yet to be disproven by experiment. Theories are not correct.
They are employed if they are useful. They are waiting for experiment to disprove them and improvements to be conceived.
The Hubble Deep Field: Looking Back In Time, NASA Goddard, 2021-08-02
Professor Brian Cox Our Place in the Universe, Science Den, 2017-07-02 (the event) uploaded 2021-08-29
Skip to 00:05:11 to jump the pleasantries. The event is a celebration of Stephen Hawking’s contribution to Astrophysics.
The Search for Exoplanets and Life Elsewhere in the Universe | Geoff Marcy, Lawrence Krauss interviews Geoff Marcy, The Origins Podcast, 2022-07-29
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