From an early age, one of the first things that we all generally taught on astronomy, is that the bright star Alpha Centauri is importantly known as the nearest star to the Sun, lying just 4½ light-years away. This unique and interesting fact has been known for nearly 170 years or so, and since this time no other star has been found closer to us. While such pieces of information are commonplace in learning, we find that very often, little is said of how the original stellar distance was determined. Moreover, even less is said of the very profound effect that the original discovery about the sheer distances of the stars had on all the astronomers of time — directed towards the unbelievable scale of the local region of space, and ultimately the measurable scale of the universe.
Prior to the early 1830s, our direct knowledge of any stellar distances was not known. The only way to achieve this was by measuring the tiny positional shifts against the background stars, using the three hundred million-odd kilometre wide Earth orbit or two astronomical unit as the long base line. Knowing this tiny angular size of this, then simple trigonometry would reveal the elusive stellar distance. As it happened α Centauri was one of the first successful parallax measures to be made. This was achieved by Scottish born Thomas Henderson (1798-1842) of the Royal Observatory at the Cape of Good Hope in South Africa. Born in Dundee, Scotland, Henderson’s early career began as a humble solicitor’s clerk, but his interest and vocation soon changed to astronomy. Around 1830, an astronomical triumvirate of sorts was formed between Thomas Henderson, the German astronomer Friedrich Wilhelm Bessel (1784-1864) of Königsberg Observatory, the Russian double star aficionado, Friedrich Georg Wilhelm Struve (1793-1864) of St. Petersburg’s Dorpat Observatory.
Parallax and Distance
One Astronomical Unit or AU equals a length of 149,597,870.7 kilometres, so the mean baseline of the orbit of the Earth is 2a or 299,195,741.4 kilometres.
For the parallax of one arcsec (3600th of one degree) corresponds to exactly 1.000 parsecs (pc.) (from parallax second) or 3.262 light years (ly.), however, no known star is this close.
d(AU) = 180 × 3600 / π
Each selected one or two particular bright stars to investigate. Logically, the closest stars were likely the brightest ones, but also assuming that these stars had similar luminosities but each lying at different distances. Sirius and Arcturus, and even Aldebaran (then named as Palilicium), were immediately discounted; primarily because of their overwhelming brilliance overshadowing the field stars for their suitable measurements.
The second reason was these star’s known high proper motions, as first determined by Edmond Halley (1656-1742). Halley had compared the positions of stars in Claudius Ptolemy’s book known as the Almagest with the measured astrometric positions found by John Flamsteed. They assumed that high proper motions could add measurement errors to the parallax measures. So on similar lines, Struve chose Wega (being known today as Vega). Henderson decided to pick bright star α Centauri, and seemingly against any logic, Bessel was eventually to selected the much fainter 61 Cygni.
Bessel’s unique pick proved to be a valuable insight. Unlike single stars, 61 Cygni was a known binary star, and this had given Bessel two stars to provide parallax measures. Another important point was that any stars showing very large proper motions would likely be closer to the Solar System than those with small motions. Information on the high proper motion of 61 Cygni was originally discovered by Sicilian astronomer Giuseppe Piazzi (1746-1826) in 1792, who directly christening the star as ‘volatilis asterum’ or The Flying Star.
Henderson’s own southern star selection was based on geography. From 1831, he was royally appointed as the Director of the Royal observatory at the Cape of Good Hope. Making several mural circle observations using (a 17th to 18th century positional telescopic device) between April 1831 and mid-1834 he obtained some useful results but did not immediately act on them. On returning to Scotland, he was promoted to the higher position of Astronomer Royal. After settling into this job, Henderson then began to arduous take his analyse of α Centauri data, and to his surprise, found an initial parallax of 0.76 arcsec — suggesting an astonishingly close distance of 4.2 light-years. Like the Ancients of old, such intimidating distances made some astronomers fear humiliation from their peers and the wider scientific and religious communities. In his hands was the first known stellar distance, but honestly doubting the veracity of the results made him very reluctant to publish. This extraordinarily continued for another four years.
Belatedly in 1835, Struve started measuring his own positions using the high-quality 23cm refractor and his homemade filar micrometer. Struve’s methods proved fairly time intensive, so it was not until mid-1837 that Struve found Wega’s parallax to be 0.125±0.055 arcsec. (26.0±7.9 ly.) This he published in late 1827 “Mensurae Micrometricae”. Unluckily, his chosen star was far more luminous than the Sun, making it notably further away than the precision possible by his observational methods. Struve also doubted his own results, but unlike Henderson, he had legitimately analysed the available data stating:
“We can therefore conclude that the parallax is very small, and it probably lies between 0.07″ and 0.08″. But, indeed, we cannot give it yet absolutely.”
In early 1838, Bessel began his 61 Cygni observations using the 18th Century 11.6cm. equatorial telescopic device known as a heliometer, which has a split objective. By November 1838, he found an accurate parallax of 0.314±0.014 arcsec. and the distance of 10.4±0.95 ly. This fairly precise result was published in December 1838. Having no personal doubt on his methodology, history records Bessel as the discoverer of the first stellar distance. Soon after seeing Bessel’s result, Henderson’s doubt was quashed and he published his results in February 1839. (Here Henderson’ remains a pertinent warning of the “cut-throat” world of both science and professional astronomy “publish or perish.”) However, worrying about “divvying the spoils” in finding these three results cannot reduce the notable importance of these discoveries because 1838 marks the first steps in finding distances beyond the realm of our Solar System.
Comparison with the very recent Hipparcos data reveals that results from all these three observers were very close to current values. These were as follows;
|Star||Parallax (π) in arcsec.||Distance (ly.)||Observer||D (ly.)|
After Sir John Herschel was presented with the Gold Medal of the Royal Astronomical Society in 1841. In his acceptance speech he summarised and prophetically said of these discoveries;
“I congratulate you, and myself, that we have lived to see the great and hitherto impassable barrier to our excursions into the sidereal Universe that barrier against which we chafed so long and so vainly — almost simultaneously overleaped at three different points. It is the greatest and most glorious triumph which practical astronomy has ever witnessed.... Let us rather accept the joyful omens of the time and trust that, as the barrier has begun to yield, it will speedily be prostrated. Such results are among the fairest flowers of human civilization”
In 1848, after the examination of most of the bright stars. α Centauri was deemed the closest. By the early 1850s. new parallax measurements and the ascertaining of the seven main orbital elements changed the original result Henderson of to 0.74 arcsec. giving the often familiar 4.2 or 4.3 light-years. For almost one hundred years, this value remained until more precision was obtained. The literature now gives the distance as 4.396 light years, rounded to 4.4 1y.
Presently the Hipparcos data give the adopted distance as 1.3478±0.002 6pc. or 4.3955±0.0082 ly. from the most accurate parallax known to date of 0.74212±0.00140 arcsec. This distance is now universal adopted, but in reality measurably decreases from year to year for the next few millenia.
Southern Astronomical Delights © (2014)