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The nearest stars: IS Glass, South African Astronomical Observatory.

In this article it is shown how the improvement in the accuracy of star position measurements over the past 400 years led first to the discovery of 'Proper Motions'--the individual movements of the stars--and afterwards to 'Parallaxes' or the measurement of their distances by trigonometry. The pioneering work is described.

At present, the nearest known star is Proxima Cen, discovered by Innes at the Union Observatory in Johannesburg. Though he was right in calling it the nearest star, the accuracy of his data was insufficient to justify his claim. Many further observations were necessary to prove it.

The astrometric satellite Hipparcos has revolutionised this field and there are great expectations for Gaia, scheduled for launch in October 2013.

The progress of position measurements

Fig. 1 (Partly due to E H0g) shows how positional measurements improved over the centuries (by no means all catalogues are represented). In classical times, Hipparchus (ca 150 BC) catalogued a number of stars but no dramatic improvements in accuracy were made until end of the 16th century, when the Danish astronomer Tycho Brahe introduced much improved instruments in his large well-funded observatory located on the island of Hven. The Landgrave of Hesse was a contemporary of Tycho's though his measurements were not published until much later.

The astrometric satellite Hipparcos has revolutionised this field and there are great expectations for Gaia, scheduled for launch in October 2013.

The first southern star catalogue was published by Frederick de Houtman (1571-1627, see Fig. 2) but probably Pieter Dirkszoon Keyser (1540-1596) made most of the observations. The later died during the voyage on which the measurements were made--the 'Eerste schipvaart' (first voyage) that the Dutch made to the East Indies.

Edmund Halley made a small and somewhat improved catalogue of southern stars from the island of St Helena but he had to refer his positions to stars observed by Tycho because of limits imposed by the poor weather he encountered.

In the mid-to-end 17th century, Flamsteed, the first Astronomer Royal, produced a northern hemisphere catalogue accurate to about 10 arcsec.

La Caille and his contemporaries in the mid-eighteenth century could make catalogues to a better standard, achieving errors around 3 arcsec. Though he was interested in searching for proper motions, La Caille did not have the repeated observations needed to find them. It was at this period that positions began to be corrected for the effects of aberration and nutation, just discovered by Bradley. These were significant at the accuracy now being reached.

Giuseppe Piazzi of Palermo used the latest instrument by Ramsden of London around 1800. He was able to make a large catalogue of northern stars (1803) with an accuracy of 1.5 arcsec and, importantly, he repeated it after several years. This was of great importance to later studies, as we will see.

From about 1830 onwards, catalogues with a precision better than one arcsec were the norm as instruments became more and more accurate. Among the most important 19th century surveys were the Bonner Durchmusterung (BD) of Argelander and the Cape Photographic Durchmusterung (CPD) of Gill and Kapteyn. The latter was the first photographic catalogue. At last it was possible to have a permanent record of observations that could easily be verified and did not rely on the eye of an individual.

Discovery of Proper Motion

Halley, who had access to Flamsteed's data, showed in 1717 that three stars, Aldebaran, Sirius and Arcturus, had changed position by 20 to 30 arcmin since ancient times. The effect was so marked that the relatively poor quality of the ancient observations was not important. Their high proper motion tells us they are likely to be nearby. This can be understood simply by looking out of a window: a distant aeroplane may take several minutes to cross the view whereas a nearby bird can fly by quite quickly.

The accuracy of Piazzi's repeated observations, referred to above, was so good that he was able to find a large number of stars with detectable proper motions. One of these, 61 Cygni, moves at over 4 arcsec per year in R.A. and over 3 arcsec per year in Dec and received the sobriquet of 'Piazzi's flying Star',

In the south, permanent observatories began to be established in the 1820s. On St Helena, a young officer of the (British) East India Company, Manuel Johnson, was given the job of erecting a small observatory devoted to positional studies of stars that the Company hoped would be useful to them for navigation. Johnson was in close touch with the Royal Observatory, Cape of Good Hope. By comparing his results to those of La Caille, he found that a Cen has a very large proper motion, around 3.6 arcsec per year. Unfortunately for him, his observatory was shut down soon afterwards. However, before returning to England, he informed His Majesty's Astronomer at the Cape, Thomas Henderson, about it.

The first successful observations of parallax

Henderson occupied the post of His Majesty's Astronomer at the Cape for a very short time only, namely March 1832 to May 1833. On hearing from Johnson, he set about observing a Alpha Cen more intensively, using a transit telescope to measure Right Ascension and a mural circle to measure declination. These were state-of-the-art instruments of the time.

To detect a parallax, which simply means the angle subtended by the radius of the Earth's orbit as viewed from a star, one observes the position of a star relative to another close by in direction but actually much more distant. This is repeated six months later, when the Earth is on the opposite side of its orbit. The angle between the star of interest and the distant one will have changed and trigonometry can be used to find out how far away it is, based on the known Earth-Sun distance. Measurements of this kind had been impossible before the 1830s because the effect was too tiny to be measured with the equipment then available.

Henderson's observations were successful. It is not clear if he realized this straight away or if he put his notebooks aside until after he heard of Bessel's success but fact remains, that he did not publish them immediately. His reluctance to publish may have been due to a fear of being proved wrong--several other astronomers had in recent years 'found' parallaxes that had been shown to be non-existent--and he would not have wished to look foolish.

Meanwhile, Friedrich Wilhelm Bessel (1784-1846) started work on parallax in Konigsberg, East Prussia. He had been informed of the measurement by Piazzi of the very high proper motion of 61 Cyg.

Bessel used a very fine split-objective telescope called a 'Heliometer' that had been made by Fraunhofer (Fig. 3). Though again, the technique was to measure the position of 61 Cyg versus a distant star in a nearby direction, the method was quite different from Henderson's. Henderson had to measure each star separately in declination and then in Right Ascension, using two separate instruments. With a heliometer, the two halves of the objective lens were rotated and moved sideways until the two images overlapped. The separation was much more accurately determined because it resulted from a single observation.

Bessel started his observations of 61 Cyg only in September 1834 (after Henderson!). On 23 October 1838 he announced a parallax of 0.31 arcsec, through the Royal Astronomical Society. His results astounded the whole astronomical world. Here at last was a proof that Copernicus was right--the Earth really does go around the Sun! According to John Herschel, "Such results are among the fairest flowers of civilization".

Henderson made his own announcement on 11 January 1839, but he had lost priority to Bessel. However, they remained good friends! Henderson obtained for Alpha Cen the value 1.16 +/- 0.11 arcsec. This value is now known to be too high and has been reduced to 0.742 arcsec, but Alpha Cen is sill the nearest star other than its possible companion Proxima Cen. Based on this figure, its distance is 4.396 light-years.

The discovery of Proxima Cen

RTA Innes, a young astronomer at the Royal Observatory, Cape of Good Hope, had been given the job of analysing data from the Cape Photographic Durchmusterung. He found a star that Kapteyn had thought was missing--it turned out to have a high proper motion and had moved a significant amount since its position had been measured some time before in Cordoba, Argenina. It is now called Kapteyn's star.

A few years later, after he had become director of the Union Observatory, Innes had the idea of looking for high proper motion stars near ones that were already known. While comparing two photographs of the region of sky near a Cen, he found a faint star that had a high proper motion similar to that of a. This is the star he afterwards called 'Proxima'.

Innes had no suitable telescope for doing parallax work, but he nevertheless tried, using a 9-inch refractor with a micrometer eyepiece. Though his results were not sufficiently accurate to be certain, he nevertheless declared that 'Proxima' is the nearest star.

Quite a few other observers over the following decades attempted to get parallaxes of Proxima but the results concerning its true proximity often seemed marginal or contradictory. [The history can be read in Glass (2008)].

It was really the Hipparcos satellite that settled the question. Fig. 4 shows its proper motion and parallax. As seen from the Earth, Proxima moves around a small parallax ellipse once per year. But, because of proper motion, the ellipse itself is moving at about 3.85 seconds of arc per year in R.A. and 0.8 seconds per year in Declination. The amplitude of the parallax at 0.772 arcsec is still less than one second of arc, or less than 1 part in 1 296 000 of a complete circle. Proxima's distance is thus 4.225 light-years.

The types of the nearest stars Once we know its distance and apparent brightness we can figure out the luminosity (wattage) of a star. In the case of a binary, we can study its orbit and figure out the masses of its two components using Kepler's and Newton's laws. This is the only way to find out the exact masses of stars.

Alpha Cen is in fact a binary [when we speak of its proper motion and parallax we are actually referring to the centre of gravity of the system] and each of its components, a G2 and a K1 dwarf, somewhat resembles our Sun, a G2 dwarf. Proxima, which may or may not be gravitationally bound to a, is a much smaller and cooler M5.5 dwarf. The other nearest stars fall into several categories: (1) very cool M dwarfs, (2) Brown dwarfs (which have the size of Jupiter but have masses from 13 to 75 or 80 times Jupiter's mass. They are not hot enough to burn hydrogen), (3) cooler L-type brown dwarfs (which show metal hydride bands in infrared) and (4) T-type brown dwarfs (coolest of all, showing methane, and water in their infrared spectra). Some of the nearby stars are so cool and thus faint in visible light that they were in fact discovered during infrared surveys.

However, Proxima is sill the nearest star known!

Hipparcos satellite

Hipparcos stands for HIgh Precision PAR-allax COllecting Satellite. This was a very fruitful project of the European Space Agency that ran for 3% years (Fig. 5). Hipparcos always looked at two fields simultaneously as it spun around its axis. Sorting and cross-correlating the resulting images was an enormous computational task. The final catalogue contained 118 000 stars at 0.001 arcsec accuracy. Each one had been measured many times so that both proper motions and parallaxes could be determined.

Of course, with this degree of precision there are few stars that do not show some degree of proper motion, so it had to be assumed that each one of them might have a measurable proper motion and parallax that had to be solved for. No star in the Milky Way can be regarded as fixed. Only objects beyond the Milky Way, such as quasars, can be taken as positional standards. In future the motions of even these may be a problem.

One of the results of Hipparcos was the verification that Proxima really is more distant than [alpha] Cen.

The Gaia satellite

The newest astrometric satellite, Gaia, will be launched in October 2013 for a 5-year mission. It is expected that it will find the positions, motions and temperatures of 100 billion stars. It will be many times more accurate than Hipparcos, i.e. about 5 microarc-sec, and will collect 10 000 times more data than it. Its precision will be such that it will be able to measure the distances of stars at the Galactic Centre with an accuracy of 20%.

Gaia (Fig. 6) will be located at the "L2" point. This is a position on the opposite side of the Earth from the Sun at a distance of 1.5 million km. It will orbit the Sun with the same one-year period as the Earth. Its sunscreen, incorporating its solar cells, and having a diameter of 10m, will always face sunwards. Gaia will spin on its axis of symmetry in order to scan the sky in great circles in a similar manner to Hipparcos.

Gaia will be able to reach to much greater distances than Hipparcos, enabling us to learn more about the shape of our own galaxy, the Milky Way. It will become possible to study the motions of stars around the centre of the Galaxy and classify them according to their orbits. Their motions are related to age and should reveal information on the formation history of the Milky Way.

References

De Houtman, F., 1603. The Earliest Star Catalogue for the Southern Hemisphere, Reprinted 1927, University Observatory, Oxford.

Glass, I.S., 2008. Proxima, the Nearest Star (Other than the Sun), Cape Town, Mons Mensa.
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Title Annotation:presidential address 2013
Publication:Monthly Notes of the Astronomical Society of Southern Africa
Geographic Code:6SOUT
Date:Oct 1, 2013
Words:2333
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