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Submitted by Kevin Jardine on 12 March, 2005 - 23:08

This chapter describes the sources for the face-on maps of the Milky Way available on this site. For the sources for the Milky Way Explorer (the Earth-centred maps), see the Milky Way Explorer introduction.

Every one of the 8978 objects currently on the face-on Milky Way maps has a position, distance, and either size or luminosity determined from the scientific literature. At the fourth zoom level, clicking on an object shows the distance in parsecs, the position in galactic longitude and latitude and the source of the data (and may show more depending on the object).

You can download the basic map data (including distances, positions and sources) as an Excel spreadsheet. You can also download more detailed data on the stars here. (Most of the time the star class data is split into separate spectrum and luminosity columns, but not always.)


The major sources of the star data are the "Catalog of Luminous Stars in Associations" and the "Clusters Catalog of Luminous and Early-Type Stars in the Field". These catalogs were released by Roberta Humphreys, Cynthia Anne Blaha and Douglas McElroy in 1984 and are based in part on Roberta Humphreys' original luminous star catalog, published in "Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way" in 1978. Oddly, none of these catalogs are available from the VizieR depository of astronomical catalogs, but the first two can be downloaded from NASA.

The "Catalog of Luminous Stars in Associations" does not provide explicit distances for each star, but instead lists the OB association the star is part of. An additional table provides size and distance estimates for these associations.

Simply mapping all the stars that belong to an association at the estimated distance of the association would force them all unrealistically into a narrow arc. To avoid this, I have randomly perturbed the distances of these association stars, however keeping them within the estimated borders of their association (assuming that the stars for each association fall within the boundary of a circle in the galactic plane). In some cases, however, a cluster of these stars are indeed concentrated in the association and these can be seen on the map as a line of stars with identical longitude and latitude coordinates.

This information has been supplemented by van der Hucht's "The Seventh Catalogue of Galactic Wolf-Rayet stars", published in 2001, and "The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way", published by Kaltcheva and Hilditch in 2000.

The colour of each star as displayed on the galaxy map depends on its spectral type (essentially its temperature). The O-stars are purple, B-stars are blue, A are white, F are yellow-white, G are yellow, K are orange and M are red. Wolf-Rayet stars are pink.

The size of the star is determined by its absolute bolometric magnitude. In most cases this was supplied in the catalog. For Wolf-Rayet stars I have applied a bolometric correction of 4.5 as suggested by Smith and Maeder in "The bolometric corrections and the M(dot):L relation for Wolf-Rayet stars" in 1985. The dimmest star on the map has a bolometric magnitude of -2.93, which is about 1200 times brighter than our Sun. The brightest stars on the map are about one million times brighter than our Sun.

Molecular Clouds

The ordinary matter in the Milky Way galaxy is dominated by large clouds of hydrogen gas. One form of this gas, called atomic hydrogen, consists of a proton orbited by at most one electron. Large clouds of atomic hydrogen gas are called HI regions and can be detected in the radio frequencies around 1420 MHz. The first major exploration of the entire Milky Way galaxy accessible from the northern hemisphere was carried out by Gart Westerhout using the Dwingeloo radio telescope (set up near a Dutch national park) in the 1950s. Westerhout published his famous paper "A survey of the continuous radiation from the Galactic System at a frequency of 1390 Mc/s" in 1958. Westerhout identified 81 major sources of galactic radiation at this frequency. Many of these are now known to be major areas of star formation and are still referenced by their Westerhout numbers (eg. W51).

Hydrogen gas also comes in the form of two hydrogen atoms bound together to form a molecule. Molecular hydrogen is extremely difficult to detect directly. Fortunately, molecular hydrogen clouds often contain molecules of many other substances. In particular, carbon monoxide radiates at several frequencies, including 115 and 230 GHz. Many studies have been done using the detection of carbon monoxide to plot the location of these clouds in various regions of the galaxy. The first comprehensive survey of carbon monoxide clouds was carried out by Columbia University astronomers Pat Thaddeus and Tom Dame using a small radio telescope mounted on a roof in Manhattan (later moved to Cambridge, Massachusetts). To complete the study, they mounted a similar telescope at the Inter-American Observatory in Cerro Tololo, Chile. The galaxy map includes clouds from two of their published papers "A wide-latitude CO survey of molecular clouds in the northern Milky Way" from 1985 and "A composite CO survey of the entire Milky Way" from 1987.

The galaxy map also includes clouds from more detailed catalogs, including

In some cases these catalogs show the concentrations of the radioactive variant of carbon monoxide, containing the carbon-13 isotope. This tends to show the locations of the dense cores of molecular clouds.

The distance estimates in these molecular cloud catalogs are only rough estimates and often many clouds are assigned the same distance. To better display these clouds, I have randomly perturbed these distances within the range of +/- 25 parsecs.

I have included all the clouds with defined distances in the catalogs except in the case of the huge catalog of more than 10 thousand clouds associated with "The Equilibrium State of Molecular Regions in the Outer Galaxy". In this case, the authors of the paper warn that clouds with velocity >= - 20 km/s have highly inaccurate distances because they are close and may be affected by localised gas streaming. I only included clouds with velocity 270 K km pc2/s. According to the paper, these clouds contain 90 percent of the total luminosity. These two restrictions reduce the size of the catalog to about 625 clouds and I have placed these on the map.

Molecular clouds are shown as translucent green spheres.

HII regions

Extremely hot stars (O-stars, Wolf-Rayet stars or hot B-stars) ionize the hydrogen gas in any clouds surrounding them, stripping electrons away from the proton nucleus and causing the gas to glow a dull red. Even more importantly, these HII regions also emit characteristic radio waves, making them detectable even on the opposite side of the galaxy.

Stewart Sharpless published "A Catalogue of H II Regions" in 1959. This included 313 regions (called Sh 2 regions) visible from the northern hemisphere. Sharpless did not provide distance estimates, but in 1982, Blitz, Fich, and Stark analyzed carbon monoxide clouds that appeared to be associated with the Sharpless HII regions and added 62 new BFS regions of their own in "Catalog of CO radial velocities toward galactic H II regions". They were able to provide distance estimates for many of these regions and I have included these in the map.

Sharpless's catalog was soon matched by a southern hemisphere version listing 182 HII regions based on data from Australia's Mount Stromlo observatory. Rodgers, Campbell, and Whiteoak published "A catalogue of H-alpha-emission regions in the southern Milky Way" in 1960. These are called RCW regions. Numerous studies have been done since that estimated the distances of one or sometimes a few RCW regions, but there has been no systematic attempt to determine distances to all HII regions visible from the southern hemisphere until recently.

More information on the creation of the Sharpless, RCW and related catalogs can be found here.

Over the last few years, studies appeared that attempted to derive distances for a large number of Milky Way objects. In 2003, Delphine Russeil published "Star-forming complexes and the spiral structure of our Galaxy and Roberta Paladini along with many colleagues published "A radio catalog of Galactic HII regions for applications from decimeter to millimeter wavelengths" of 1442 HII regions derived from 24 previous catalogs.

Although the Paladini et. al. "master catalog" of HII regions did not itself include distance estimates, in 2004, Paladini, Davies, and DeZotti published a paper, "Spatial distribution of Galactic HII regions" describing the techniques they used to derive distance estimates for more than 500 HII regions. In 2005, Roberta Paladini was kind enough to send me a personal communication with these estimates and I have placed these objects on the galaxy map.

HII regions are shown as red spheres.

Star clusters

Unlike the other object types, data on optically visible star clusters has long been centrally catalogued. Gosta Lynga began a catalog of all optically visible star clusters in 1977 ("Plans for a Computer-Readable Cluster Catalogue") and it has since been continued by others, most recently in the "New catalogue of optically visible open clusters and candidates" by Dias, Alessi, Moitinho, and L├ępine. This catalog continues to be updated and the latest version is available here.

The galaxy map includes all the star clusters with defined distances from version 2.7 of the Diaz et. al. cluster database (27 October 2006).

In addition to optically visible star clusters, there are now several hundred optically invisible star clusters known from infrared observations. (For example, see "New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS".) Infrared star clusters will be added to a future version of the galaxy map where distance estimates are available.

Optically visible star clusters are shown as gold circles.