Prior to the early 1830’s, direct knowledge of stellar distances was not known. The only way to achieve this was by measuring tiny shifts in position against the background stars, using Earth”s three hundred million kilometre orbit as a base line. If the size of this tiny angle is known, simple trigonometry would reveal the elusive stellar distance. As it happened, one of the first successful parallax measures was of α Centauri by Thomas Henderson (1798-1842). 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 the German astronomer Friedrich Wilhelm Bessel (1784-1864) of Königsberg Observatory, Thomas Henderson at the Cape of Good Hope, and the Russian double star aficionado - Friedrich Georg Wilhelm Struve (1793-1864) of St. Petersburg”s Dorpat Observatory.
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 Palilicium), were immediately discounted - primarily because of their brilliance overshadowing the field stars for their suitable measurements. The second reason was their high proper motions - as first determined by Edmond Halley. when he compared Claudius Ptolemy’s Almagest data against John Flamsteed”s measured astrometric positions. On similar lines, Struve chose Wega (today”s Vega), Henderson chose α Centauri, and seemingly against logic, Bessel eventually selected 61 Cygni. Bessel’s pick proved to be a valuable insight, because unlike the single stars, 61 Cygni was a known binary star - and this gave Bessel two stars to provide parallax measures. Another importan issue was that stars having very large proper motions would likely be close to the Solar System. Information on the high proper motion of 61 Cygni was discovered by Sicilian astronomer Giuseppe Piazzi (1746-1826) in 1792. who christening it “volatilis asterum” - “The Flying Star”.
Henderson”s southern star selection was based on geography, as in 1831, he was royally appointed as the Director of the observatory at the Cape of Good Hope. Making several observations using a mural circle (a 17th-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 of analyse his α Centauri data, and to his surprise, found an initial parallax of 0.76″arcsec. - suggesting an astonishingly 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 doubting the veracity of the results made him very reluctant to publish. This 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 π = 0.125±0.055″arcsec. (26.0±7.9 ly.) This he published in “Mensurae Micrometricae” in late 1837. Unluckily, his chosen star was far more luminous than our 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 a 11.6cm. heliometer (An 18th Century equatorial telescopic device which has a split objective). By November of the same year, he found an accurate parallax of π = 0.314±0.014 arcsec. and the distance of 10.4±0.95 ly. This quite 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. (Henderson”s remains a warning in 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 importance of these discoveries - as 1838 marks the first steps in finding distances beyond the realm of the Solar System.
Comparison with the very recent Hipparcos data reveals that results from all three observers were very close to current values. These were as follows;
|Star||Parallax (π) ″arc sec.)||Distance (ly.)||Observer||D (ly.)|
|Alpha Centauri||0.128 93±0.000 55||25.30 1±0.054 0||Struve||25.0|
|Wega (Vega)||0.742 12±0.001 40||4.395 5±0.008 3||Henderson||4.2|
|61 Cygni||0.286 61±0.001 51||11.395±0.073 0||Bessel||10.4|
After Sir John Herschel was presented with the Gold Medal of the Royal Astronomical Society in 1841. his speech 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 1850”s. new parallax measurements and the ascertaining of the seven orbital elements changed Henderson”s original result to 0.74″arcsec. giving the often familiar 4.2 and 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 1.347 8±0.002 6pc. or 4.395 5±0.008 2 ly. from the most accurate parallax known to date of 0.742 12±0.00140″arcsec. This distance is universal adopted, but in reality decreases measurably from year to year.
Each star seen by the naked eye is moving at tremendous velocity through space. Yet the vast distance lying between the stars is so large, that over an average human’s life-span. all stars remain essentially fixed in the sky. It is only after many centuries or millennia have elapsed, that the outlines of the constellations begin to change. Knowledge of this was first found by Halley in 1718, after noticing that from Ptolemy’s given position of the first magnitude star Arcturus it had moved more than 1o in the sky. Subsequently he found that Sirius had also moved about the diameter of the Moon (about ¼o) after 1 400 years or so.
Evidence of such significant stellar motions in the southern hemisphere would be α and β Centauri, which are now separated by 4.4o in the sky. Throughout the centuries of recent history their separation has been visually diminishing. In the centuries that are to follow, an observer would notice an obvious difference in positions around the year 2150 AD and by 4000 AD, the stars at 2.3o separation will point to the exact centre of the Southern Cross. Closest approach of these two bright stars will occur in the year 6200 AD, when a Centauri will pass within 23′ arcmin of β Centauri - making a brilliant spectacle in the night sky.
Compared with our Sun’s own motion, the distribution of stellar velocities can vary significantly. Most stars are either receding or approaching us at tens of kilometres per second. A few travel at nearly 250 kms-1, and are often referred to as high velocity stars or runaway stars. and are believed to be ejections from past violent cataclysmic events within the Milky Way. Three examples include, Mu Columbae, AE Aurigae and 53 Arietis. each having velocities exceeding 200 kms-1 and whose point of origin seems to be the Great Nebula in Orion. The highest velocities are sometimes caused by stars having galactic rotations opposing the Galaxy’s clockwise motion.
Astronomers have measured such motions using transit telescopes, by measuring stellar motion across the line of sight - the common proper motion (c.p.m.) or simply proper motion. For most stars, proper motion amounts to small movements that seldom exceed 0.1’ arcsec. per year. Proper motion is also dependent on stellar distance. For example. the angle made by two stars that have the same velocity through space. but one being twice as far as the other, will show haft the proper motion. Another measured motion is the stellar velocity relative to the Sun. obtained using a spectroscope or spectrograph. Such instruments observe certain spectral lines produced by stellar atmospheres. whose relative motion is detected by the so-called Doppler shift - similar in principle to the radar used by the police in catching speeding motorists. Called the radial velocity. it is measured from the positional differences between spectral lines observed on Earth and from the star. A negative value indicates movement towards us, positive is movement away. Using positional data from the last two centuries with the radial velocity, the true direction of motion relative to the Sun can be determined. This true speed of motion is called the transverse velocity. Applying this to the nearby stars. it is possible to use the information to calculate the true space motion relative to the Sun and r esultant changes in distance and brightness.
Average values for α Centauri AB measure -22.7 kms-1 for radial velocity and about 9’arcsec. per year for proper motion. This information combined with the Hipparcos satellite's parallax finds the closest approach occurring in the year 31,240±1320 AD. at 2.970’0.012 ly. from the Sun. Then it will be -1.28 magnitude. slightly fainter than Sirius. and will appear in the constellation of Hydra.
(NOTE: A summary of these changes appears in “Immediate History of Alpha Centauri” in Part 7 of this text.)
Another example of significant stellar motion is Barnard’s Star (BD +4o 3561) (17581+0434) which has the largest proper motion of 10.31’arcsec. each year. Although 5.986 light years from the Sun, this 9.5 magnitude double star visually changes its position over a human’s life-span roughly the apparent diameter of the Moon once every 183 years! Like most of the runaway stars, Barnard’s star is travelling at 111 kms-1 towards us. By the year 48,000 AD, this star makes its closest approach of a mere 3,761 light years, moving with respect to the Sun at some 143 kms.-1. As the magnitude peaks at 8.5, the star will remain invisible to the unaided eye.
Determination of the motion of the Sun relative to the Milky Way has also been made by analysing the true motions of hundreds of stars. The “local” velocity is about 20 kms.-1 while the galactic velocity is 230 kms.-1.