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The Star Sweepers

Submitted by Kevin Jardine on 28 January, 2008 - 12:58

While we
Know they are
Enormous suns,
Gold lashing
Seas of heavy
Silver flame,

They look as
Though they could
Be swept
Down, and heaped,
Cold crystal
Sparks, in one
Cupped palm.

Valerie Worth

Ruprecht's Mission

Towards the end of 1963, the Czech astronomer Jaroslav Ruprecht travelled up the slope of Mount Aragats, an extinct volcano in northern Armenia. Ruprecht was carrying out a mission for the International Astronomical Union (IAU) and was on his way to the Byurakan Astrophysical Observatory, the Soviet Union's greatest observatory. There he studied the detailed list of young hot stars and star clusters compiled more than a decade before at the observatory by Beniamin Egishevich Markarian.

Ruprecht's mission was to gather all the available information about OB associations and publish a definitive list. In the days before the Internet and the easy transfer of data, that meant a lot of travel and consultation.

OB associations are enormous agglomerations of young hot stars born together in giant molecular clouds. Their importance in mapping the Milky Way was first pointed out by Victor Amazasp Ambartsumian, who was the founding director of the Byurakan observatory, Markurian's scientific colleague, and, at the time of Ruprecht's mission, the President of the International Astronomical Union.

Ruprecht was the ideal person to gather the information because he was one of the main compilers of the 1958 Catalogue of Star Clusters and Associations and its subsequent supplements and one of the world's leading experts on star clusters.

Ruprecht presented his results during the 12th General Assembly of the International Astronomical Union, in Hamburg, Germany on 26 August, 1964. The President of the IAU Commission on Star Clusters and Associations, and director of the Hamburg Observatory, Hans Haffner, thanked Ruprecht for his "valuable and painstaking work" and the commission voted to publish Ruprecht's list in the IAU transactions. (Sadly, past issues of the IAU transactions are one of the few collections of astronomical documents not available online for public reading. Access is blocked by the German scientific publisher Springer-Verlag. The IAU has recently changed its publisher for new material to Cambridge University Press, which has agreed to make the material public after 18 months. 2018 update: Ruprecht's OB association catalog is finally public and can be seen here:

Ruprecht's work on both star clusters and associations gathered the basic data used by the next generation of astronomers mapping the Milky Way.

Collecting the data

Despite the massive changes introduced by telescopes, satellites and many other tools in the twentieth century, the actual process of astronomy has changed little since the days of Tycho Brahe. Data collection, analysis, and synthesis are all crucial steps in the process. Without the raw data painstakingly collected by Tycho Brahe, Johannes Kepler would never have been able to discover his three laws of planetary motion, and without Kepler's laws, Isaac Newton would never have been able to create the grand synthesis that lasted until the twentieth century.

Mapping the Milky Way is still in its early stages and so the collection of basic data by astronomers like Jaroslav Ruprecht has played a crucial role. Ruprecht's cluster database was carried forward into the computer era by Gosta Lynga and David Leisawitz, and now into the era of the World Wide Web by the Brazilian astronomer Wilton Dias and colleagues.

The Keplerian role of analysing this data has been taken up largely by a remarkable group of female astronomers. Women have long played a crucial role in analysing astronomical data. Annie Jump Cannon, the major creator of the vast HD star catalog, and Cecilia Payne-Gaposchkin, who first established the chemical composition of the stars, are early examples.

Astronomers from the Byurakan observatory in Armenia have played a crucial role in helping us understand the role of luminous stars in determining the structure and development of the Milky Way.

Humphreys and Avedisova

In the 1970s, many astronomers began mapping out the Milky Way using star data. Two women stand out in particular from opposite sides of the planet: Roberta Humphreys from the University of Minnesota and Veta Sergeevna Avedisova from the Institute of Astronomy (INASAN) of the Russian Academy of Sciences. Both women have spent their careers determining the distances and properties of the highly luminous stars, clusters and associations that dominate the Milky Way's spiral arms. Humphrey's classic paper on the hot and luminous stars found in the OB associations defined by Ruprecht appeared in 1978: Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way. Veta Avedisova's important 1984 paper Exciting stars and the distances of the diffuse nebulae does not seem to have been translated into English but as the main table was published in Latin characters, the Russian original is still very useful.

Humphreys continued to develop her database of hot and luminous stars and the most recent version (from 1984), developed with her graduate student Cynthia Blaha, is available here.

Avedisova and her colleagues have created an enormous database of information on over 3200 major star formation regions and over 66 thousand subregions, including many objects visible at only infrared and radio frequencies. The latest version was uploaded to the Vizier astronomical catalog depository in 2002.

Kharchenko and Russeil

Two younger woman astronomers have also made important contributions by further analysing the data for luminous stars.

Nina Kharchenko from the Main Astronomical Observatory of the National Academy of Science of Ukraine has led a team of colleagues to systematically re-analyze the existing star cluster data using the Tycho catalog of bright stars generated by the Hipparcos astrometric satellite from 1989-1993. Kharchenko's work has produced more accurate fundamental parameters, including distance estimates, for 513 open clusters and 7 compact associations, as well as discovered more than 100 new star clusters.

Delphine Russeil from the Laboratoire d'Astrophysique de Marseille has extended the ongoing work by her colleagues Yvon and Yvonne Georgelin to estimate the distances to HII regions. Russeil uses multiple techniques including kinematic analysis that measures the speed of moving gas in the nebulae to photometric analysis that estimates the distances to bright stars. The resulting catalog contains distance estimates to about 500 star formation regions. The latest revisions to her catalog, published in 2007, improve the estimates for 23 star-formation regions in the outer galaxy visible from the northern hemisphere.

Kharchenko and Russeil's work depends on accurate photometric data for bright visible stars. Since much of our view of the Milky Way is blocked by dust clouds which obscure visible light, their work only applies directly to a fraction of the star formation regions in our galaxy. Nevertheless, establishing more accurate properties for these visible regions will help astronomers estimate the distances to other star formation regions visible only at infrared and other non-visible frequencies as well.