Abstract
It was a clear and windless winter’s evening in early July when this author first saw α Centauri. Brought into sharp focus by a telescope at the Stardome Observatory Planetarium in Auckland, New Zealand, its light was of a cold-silver. The image was crisp and clear, a hard diamond against the coal-black sky. The view was both thrilling and surprising. To the eye α Centauri appears as a single star – the brightest of ‘the pointers.’ Indeed, to the eye it is the third brightest ‘star’ in the entire sky, being outshone only by Sirius and Canopus (see Appendix 1).
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Notes
- 1.
Ptolemy refers to the right foot, since his imagined view is that of a god-like observer looking down on the sphere of the heavens. For us mortals, on Earth, α Centauri appears as the left front foot.
- 2.
SIMBAD = Set of Identifications, Measurements, and Bibliography for Astronomical Data.
- 3.
A waka is a long, narrow-beamed canoe.
- 4.
In terms of apparent magnitude ranking (see Appendix 1 in this book), α Cen A is the third brightest star in the sky, with m = –0.27, α Cen B is the 21st brightest star with m = +1.33, while β Centauri (Agena) is the 10th brightest star, with m = +0.60.
- 5.
At first glance it might appear that α Centauri is featured on the Australian flag, but the large seven-pointed star under the defiled union jack symbolizes the federation of the seven Australian states.
- 6.
Somewhat confusingly, the location for the origin point for right ascension is no longer in the constellation of Aries; rather it is now located in the constellation of Pisces.
- 7.
A bark is a three-masted, square-rigged, ship.
- 8.
The distant stars are not actually fixed in space, of course. Rather, their vast distance away from us means that the time required to accumulate any measurable shift in the sky is determined on a timescale of many millennia.
- 9.
Barnard’s Star has the largest known proper motion, moving across the sky at a rate of 2.78 times faster than α Centauri. Of the stars within 5 pc of the Sun, Kaptenyn’s Star has the highest space velocity of 293 km/s.
- 10.
In the modern era the characteristics of a telescope are typically defined according to the diameter of its object lens or mirror. This makes sense since it is the size of the objective that determines the light-gathering power and resolution. Early refracting telescopes were usually defined in terms of the focal length, however, and in many cases these were many tens of feet in length. The primary reason for such long focal lengths was to reduce the image-degrading effects of chromatic aberration, and because it is more straightforward (although still a definite skill) to grind and polish long focal length lenses.
- 11.
William Doberck; the quotation is taken from the paper: “On the orbit of η Coronae Borealis,” Astronomishe Nachricten, 141, 153 (1896).
- 12.
John Mitchell (1724–1793) is perhaps better known in modern times for his suggestion that some stars might be so massive that their associated escape speed would be greater than the speed of light – making them what we would now call black holes. It was also Mitchell who developed the experiment, eventually carried out by Henry Cavendish, to determine the value of the universal gravitational constant (G ≈ 6.67 × 10−11 Nm2/kg2).
- 13.
Captain William Stephen Jacobs first traveled to India in 1831 and was soon subscripted into the great India survey, then under the directorship of George Everest. Retiring from the survey due to ill health, however, he turned to astronomy, eventually taking over directorship of the Madras Observatory operated by the East India Company. Jacob specialized in observations of double stars and in the computation of binary star orbits. In 1855 he suggested that the motion of the binary star 70 Ophiuchi indicated it might have a planetary companion. It was in the following year, in a letter written to the editor of the Monthly Notices of the Royal Astronomical Society in London, that he made the same claim for α Centauri. In each case, however, it turned out that there were no specific planets – at least as envisioned by Jacob.
- 14.
With reference to Fig. 1.11 we have already written down that the distance to a star with parallax P is tan P = 1 au/D. Simple algebra now gives D = 1 au/tan P, and according to the parsec definition we find D = 1 pc ≡ 1 au/tan 1”.0. In the modern computer-dependent world this is where the calculation would stop; the distance would be determined by simply inputting the number for a measured parallax into a calculator. Going back just 50-years from the present, however, electronic calculators were rare things; indeed, there were only a handful of computers in the entire world. Historically, say, going back 100 years, the evaluation of a tangent and then its inverse required the use of mathematical tables, a pen, some paper and a lot of hard graft. The calculation can certainly be made, but it is tedious and time consuming. Astronomers, like most people, being conscious of both time and effort, wondered, therefore, if there might be some shortcuts to the mathematics. Luckily there is a shortcut, but before taking this shortcut, we must first consider some new mathematics, since the useful dodge that can be employed relates to angles measured in radians. The radian measure is simply another way of measuring angles, and by definition there are 2π radians in a circle. The connection with circles is revealed, of course, by the number π appearing in the definition, and the 2π term comes about since it is the circumference of a circle of radius r = 1. The important and useful point about radians is that when the angle φ, measured in radians, is very small, then the tangent operation simplifies to tan φ ≈ φ (radians), with the approximation becoming better and better as φ gets smaller and smaller. The point of all this is that mathematically speaking, for small angles we can now write D = 1 au/P (radians), and this saves us from having to calculate the tangent of an angle. (Remember this was useful when there was no such thing as an electronic calculator.) The practical problem for astronomers, however, is that it is not possible to construct a measuring scale in radians. This, however, is not an insurmountable problem in that the angle of parallax can be measured in arc seconds and then mathematically converted to radians. In this manner, given that there are 2π radians in 360° and 3,600 s of arc in 1°, so 1 s of arc = 2 π/(360 × 3,600) ≈ 1/(206, 265) radians. We now recover the result D = (206,265) × 1 au/P (arc sec) ≡ 1 pc/P (arc sec). From the latter relationship it can be seen that when the angle of parallax is 1 s of arc, then the distance is, by definition, 1 parsec (1 pc), and that 1 pc is equivalent to a distance of 206,265 AU.
- 15.
Sir James South (1785–1867) was one of the founding members of the Astronomical Society of London – later to become the Royal Astronomical Society. South was involved in a veritable soap opera-like lawsuit with famed instrument maker Edward Troughton concerning the construction of a new observatory.
- 16.
See also, Kervella, P., and Thévenin, F. “Deep imaging survey of the environment of a Centauri. II. CCD imaging with the NTT-SUS12 camera” (Astronomy and Astrophysics, 464, 373, 2007).
- 17.
The various age estimates for α Centauri will be discussed in greater detail later.
- 18.
Joan George Eradus Gijsbertus Voûte is a rather obscure figure. It is known that he was an assistant at the Cape Town Observatory, and that he later took charge of the time-keeping section of the Meteorological Office in Jakarta, Indonesia. A brief obituary is provided in the Quarterly Journal of the Royal Astronomical Society (5, 296–297, 1964).
- 19.
Such planets are thought to have grown within the gas/dust disks associated with newly forming proto-stars. The (rogue) planets are then later ejected from the host system as a result of gravitational interactions with other, larger planets.
- 20.
These results are based on a straightforward simulation of the possible VW Ophiuchi encounter conditions. The simulation considered the closest encounter distance and time of encounter for 100,000 VW Ophiuchi clones, with proper motion and initial parallax values chosen at random from within the presently allowed range of observational uncertainties.
- 21.
This calculation assumes that the galactic disk has a ring-like structure with an inner radius of 2 kpc and an outer radius of 16 kpc, with a uniform thickness of 1 kpc. This gives a ‘disk’ volume of just under 800 million cubic parsecs. The solar neighborhood volume exhibited in the Fig. 1.18 amounts to that of a sphere of radius of 3.8 pc – giving a volume of just under 230 cubic parsecs.
- 22.
The data relating to observed supernova rates indicate that something like 19 supernovae (of all types, but mostly Type II) will occur within our Milky Way Galaxy every 1,000 years. These supernovae will occur at random locations within the disc of the galaxy, and if we imagine each supernovae having a circular region of devastation with a radius of 10 pc, then something like 2.5 million supernovae would need to occur before one is likely to be placed within 10 pc of the Sun. (As with Note 21, the disk is taken to have an inner radius of 2 kpc and an outer radius of 16 kpc.) To achieve this disk coverage, and Solar System-threatening location, would require about 132 million years of supernovae explosions. Accordingly, given the Solar System is 4.5 billion years old, so of order 34 supernovae must have occurred within 10 pc of the Sun. Likewise, over the next two billion years some 15 additional supernovae will likely occur within 10 pc of the Sun. A review of observed supernova characteristics and the possible consequences of a close supernova explosion to the Solar System is given by the author in the article, “The past, present and future supernova threat to Earth’s biosphere” (Astrophysics and Space Science, 336, 287, 2011).
- 23.
It has been argued, for example, that during the times of spiral arm crossing not only is the supernova threat enhanced, as a result of there being more massive stars within spiral arm regions, but so, too, is the likelihood that some form of Oort Cloud disruption enhanced. This latter effect comes about due to the enhanced density of giant molecular clouds in the spiral arm. Additionally, the enhanced number of gravitational perturbations, it has been argued, should result in an enhanced cometary influx to the inner Solar System and accordingly to more impacts on Earth. Furthermore, the cosmic ray flux will be higher at times of spiral arm crossing, and this will potentially result in enhanced atmospheric ozone depletion. Another effect, first discussed by William McCrea (University of Sussex) in 1975, is that accretion of material by the Sun, if it chances to pass through a particularly dense region of the interstellar medium, could result in its luminosity increasing and precipitating thereby a dramatic warming of Earth. Although it is acknowledged that all of these various phenomena and effects could happen, there is no clear and unambiguous evidence to indicate that they actually have happened or will. Certainly the Solar System is not disconnected from events and phenomenon occurring in the rest of the galaxy, but it is far from clear what effects, if indeed any, the connections might have had upon Earth’s historical past (and indeed, will have upon its future).
- 24.
The precise details concerning this remarkable star have only recently been deduced, and are based upon parallax observations made with the Hubble Space Telescope. The details of this study are given by Howard Bond (Space Telescope Science Institute) and co-workers at http://hubblesite.org/pubinfi/pdf/2013/08/pdf.pdf.
- 25.
This catalog is historically more infamous for the fact that in 1795 the observers working on its content twice recorded the position of Neptune, but failed to notice that it had moved and, indeed, that it was not a star but a planet. This missed discovery is a little surprising given that William Herschel had serendipitously discovered the planet Uranus just 14 years earlier. The ‘official’ discovery of Neptune did not take place until it was swept up by Johann Galle at the Berlin Observatory on September 23, 1846.
- 26.
Sirius B was first observed by American astronomer Alvan Clark in 1862 while testing the newly constructed 18.5-in. diameter refractor at Dearborn Observatory – then the largest telescope in the world.
- 27.
It is perhaps a little ironic that it was George Gatewood who showed that Peter van der Kamp’s analysis of the proper motion shown by Barnard’s star didn’t support the presence of any planets. More recent studies of both Barnard’s star and Lalande 21185, using the Doppler monitoring technique, have failed to find any evidence for the existence of associated planets.
- 28.
The present group of spacecraft heading into deep space are the Voyager 1 and 2 probes, Pioneer 10 (launched in 1972) and Pioneer 11 (launched in 1973). The probe that has traveled furthest from the Sun is Voyager 1. This select group of objects will eventually be joined, in the next several decades, by the New Horizons spacecraft (launched in 2006) currently on its way towards the dwarf planet Pluto and the Kuiper Belt region beyond.
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Beech, M. (2015). Discovery, Dynamics, Distance and Place. In: Alpha Centauri. Astronomers' Universe. Springer, Cham. https://doi.org/10.1007/978-3-319-09372-7_1
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