Galaxy Map

A detail from the face-on map of the Milky Way. The full map poster is is available on the web here.

Sometimes dreams do come true. Today I can announce a detailed map of the Milky Way out to 3000 parsecs or about 10 thousand light years from the Sun.

I developed this map with help from scientists with the European Space Agency's Gaia mission and researchers at Leiden and Heidelberg universities. It includes star density isosurfaces mapping the major concentrations of the hotter O, B and A class stars in the Gaia DR2 release, about 5000 extremely hot ionizing stars, dust clouds and HII regions. Even better, it is available both in a face-on form viewed from above the Milky Way and in a true 3D version in the latest version of Gaia Sky.

A more detailed description of the map is up at the ESA Gaia mission website here.

A printable pdf is available here. (Warning, even at A0 size, the smallest labels are tiny. Needs sharp eyes, a magnifying glass or printing at a custom size with a width greater than one metre.)

A zoomable pannable web version is available here.

There is a gray background in the face-on versions of the map that shows the approximate locations of the inner and outer galaxy.

You can download the latest version of Gaia Sky with 3D mesh support here.

A Youtube trailer showing a flight through the map, animated in Gaia Sky, is available here:

Many people have helped make this map possible. I'd like to mention a few now.

Anthony Brown, a professor at Leiden University and the chair of Gaia's Data Processing and Analysis Consortium (DPAC), has been an enthusiastic supporter of this project and was always available with suggestions and comments.

Toni Sagristà Sellés, the principal developer of Gaia Sky, added numerous features to Gaia Sky to support visualizing and animating the 3D meshes.

Stefan Jordan, a professor at Heidelberg University, the sponsor of Gaia Sky, and Manager for Outreach and Education for DPAC, has also provided support and encouragement.

And at the European Space Agency itself, Jos de Bruijne and Tineke Roegiers, two Gaia Mission scientists, provided advice and support at key points in this project.

Finally, I'd like to thank Bob Benjamin, professor at the University of Wisconsin - Whitewater, who has long encouraged my interest in galactic cartography and made useful suggestions for positioning the HII regions on the current map.

The map depends upon data from many researchers. Here are some key credits:

Gaia Data Release 2 data as available from the Gaia Archive.

The isosurface technique was first pioneered for Hipparcos by H. Bouy and J. Alves, as explained in "Cosmography of OB stars in the Solar neighbourhood".

The map also includes dust density isosurfaces computed by dust extinction values provided in a preprint article "3D maps of interstellar dust in the Local Arm: using Gaia, 2MASS and APOGEE-DR14" by Lallement et al.

Positions for OB associations are based on the computed median distances to their members in the data set as described by R.M. Humphreys and D.B. McElroy in "The initial mass function for massive stars in the Galaxy and the Magellanic Clouds".

Positions for HII regions are computed using the median distances for known ionizing stars as described here.

The map also includes an overlay of about 5000 known ionizing stars. Distances for these stars are derived from a catalogue associated with a preprint article by C.A.L. Bailer-Jones et al in "Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2".

Of course all errors are mine. As with any map in the early days of exploration, it is bound to have missing sections and regions with incorrect data. But I think that it is a good start. I'm excited about the current map and even more looking forward to the many new features and improvements that we'll be adding in the future.

I've been running this site for almost 14 years now but today feels like a new beginning.

A face-on map of the Milky Way within 3kpc (about 10 thousand light years). The full revised map is is available as a PDF here.

I have revised the face-on map I published in my last blog post. As before, this is based on the Humphreys/Blaha data set of ionizing and highly luminous stars and a (now expanded) list of ionizing stars for HII regions. For this revision, I used the distance estimates for Gaia DR2 stars provided by Bailer-Jones and his colleagues at Germany's Max Planck Institute for Astronomy. These distances provide more accurate distance estimates for many stars than a simple reciprocal calculation, and allow distances to be estimated for a much larger set of stars.

In order to provide an additional check on the HII region distances, I have compared the Bailer-Jones distances for the ionizing stars with existing photometric and kinematic distance estimates for the associated HII regions. If a photometric or kinematic distance estimate provided an error range, I used that, or otherwise used a range of +/- 25% of the published value.

If an ionizing star had a Bailer-Jones distance outside the error range for any published distance estimate for the associated HII region, I considered it an outlier and filtered it out. I then computed a filtered distance for the HII region as the median distance for the filtered ionizing stars. If 50% or more of the ionizing stars for an HII region made it through the filtering process, I used the filtered distance value for the revised map. If less than 50% of the ionizing stars for an HII region made it through the filtering process, I excluded the HII region from the map.

For a small number of HII regions with very bright ionizing stars I used Hipparcos stars with err/plx < 0.2 when Gaia DR data was not available. I used the reciprocal method to compute the distance and then applied the same filtering process described above.

Fortunately, almost all the major HII regions within 3kpc made it through the filtering process. The ones that did not were mostly small regions with few known ionizing stars. I have added a table below listing the HII regions that had at least one ionizing star with a Bailer-Jones distance compatible with a previously published HII region distance. I provided links to the relevant distance papers in the last column of the table.

The full revised map is is available as a PDF here. You will need to zoom into about 200% in the PDF to read the labels for each HII region.

You can also see the list of ionizing stars I used for the HII regions.

A detail from a face-on map of the Milky Way within 3kpc (about 10 thousand light years). The full map is is available available as a PDF here.

Now that Gaia DR2 is available, it is possible to do an enormous amount of mapping and stellar analysis, much of which is announced on Twitter via the #GaiaDR2 hash tag.

I have used the two data sets I mentioned in my last two blog posts (The Humphreys/Blaha catalog of luminous stars and the expanded Avedisova catalog of ionizing stars for HII regions) to create a simple map of the Milky Way out to 3kpc (about ten thousand light years). It is available as a PDF here.

There is a lot of detail on the map, so you will need to zoom into 100% to see it. In general I used the Sharpless, Gum, Georgelin and RCW names for the HII regions, following Avedisova, which are not necessarily the most commonly used names today. At some point I will do a version with expanded labels.

Blue dots are OB and Wolf Rayet stars. Magenta dots are cooler stars. The yellow dot at the centre is the Sun. Red circles are HII regions.

I filtered the GaiaDR2 cross matches to only select stars with err/parallax < 0.2. This found roughly two-thirds of the stars in the data sets. If I spent more time on the cross matches I could probably find more.

This is very much a draft map. There are many possibilities for errors with this kind of map: errors in the ionizing stars for specific HII regions, bad cross matches, etc.

Also, a better map would be based on Gaia DR2 star density and not just a specific list of stars. And would be 3D. All of that is under development by myself and others. Expect to see some amazing Milky Way maps over the next few weeks and months!

A detail from the catalog of ionizing stars for HII regions. A full all sky image in galactic coordinates is available here.

In 1984, Avedisova and Kondratenko published an important paper Exciting stars and the distances of the diffuse nebulae [ADS link: 1984NInfo..56...59A] in which they listed several hundred ionizing stars for HII regions.

Although the paper is written in Russian, the tables are provided in Roman characters.

Over the last few years I have expanded the Avedisova ionizing star catalog to include data from about 90 other scientific papers as part of a project to determine the ionizing stars for the major visible HII regions. I am making this expanded catalog available today in preparation for the Gaia DR2 release (expected in just a few hours now!). It contains more than 500 stars.

You can download the ionizing star catalog as a zip here. The catalog includes a reference to the scientific paper for each star, a SIMBAD identifier, the HII region it ionizes and, crucially, a cross match identifier for 2MASS or Tycho-2 so that parallaxes can be retrieved via the Gaia DR2 cross match tables.

In addition, the zip includes a table giving basic parameters for each HII region, including the galactic longitude and latitude, and the radius of the region in the sky in arcminutes.

Interestingly, the Avedisova catalog of ionizing stars was finished in the same year (1984) as the Humphreys/Blaha catalog of luminous stars I mentioned in my last blog post. This may not be entirely a coincidence. In some ways these catalogs were a culmination of a period from about 1970 to 1985 when astronomers were intensely interested in mapping optically visible objects in the Milky Way. After 1985, interest moved to non-visual frequencies and more astrophysical topics like dark matter or the age of the universe.

It seems clear that the release of Gaia DR2 today is likely to spark a new period of interest in mapping the Milky Way, making the Avedisova and Humphreys/Blaha catalogs very relevant again.

If you are interested in learning more about luminous stars or HII regions, please read The Cloud Hunters and The Star Sweepers for more information.

A detail from the Humphreys catalog of luminous stars. A full all sky image in galactic coordinates is available here.

Roberta Humphreys first published her catalog of luminous stars in her highly cited 1978 paper, Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way [ADS link: 1978ApJS...38..309H].

The catalog was further refined by Cindy Blaha and colleagues and divided into two sections on luminous stars in associations and field stars. There are about 5000 stars in total.

The catalog was completed in 1984 and eventually made available for download from several locations, including NASA and ADS servers. However, over time the catalog disappeared from these servers and for some reason does not appear to have been uploaded to Vizier.

You can click here to download a zip of the catalog files here, along with a SIMBAD cross match I did recently. The cross match includes ids for existing star catalogs like 2MASS, Tycho and UCAC4. The Gaia DR2 dataset will include best neighbour cross matches for these catalogs when it is released, so the cross match file can be used to look up parallaxes for these stars.

I was able to find cross matches for almost all the stars in the Humphreys catalog and will put up a simple face on map of the data set after Gaia DR2 is released. For now you can click here to download an image of the data in the sky with galactic coordinates. Blue dots are O, B and Wolf-Rayet stars. Red dots are cooler stars.