Introduction to the Milky Way Explorer

This IRAS infrared image from the Milky Way Explorer shows the expanding ring of clouds surrounding Lambda Orionis. Betelgeuse is visible as the bright white star just to the upper left of the ring.

You can find the Milky Way Explorer here, but you may wish to read this introduction before using it.

This chapter describes the Milky Way Explorer, a set of multiwavelength maps of our galaxy as seen from the Earth. If you would like maps of the Milky Way as seen from a star ship hovering far above the galactic nucleus, visit the Face-on map overview.

Seeing through the dust

Perhaps the most difficult problem in mapping the Milky Way galaxy is that our view of much of it is blocked by large clouds of dust. As a result, the standard lists of visible nebulae and intensely luminous stars are sadly incomplete.

The Milky Way Explorer is intended to help solve this problem by combining images and data from outside the visual spectrum to produce a new view of our home galaxy. These non-visual frequencies are transmitted through the Milky Way's dust clouds and enable us to see what lies beyond them.

Major data sets

The Milky Way Explorer combines three major infrared surveys: IRAS, MSX and Spitzer, along with lower resolution images at radio, microwave and hydrogen-alpha frequencies. Overlaid on this data are markers showing the locations of the star formation regions listed in the Catalog of Star-Forming Regions in the Galaxy, produced by Veta Avedisova and her colleagues at the Centre for Astronomical Data in Moscow, as well as the supernova remnants from Dave Green's Catalogue of Galactic Supernova Remnants. One of the hydrogen-alpha images in addition contains detailed information from several data sources on ionising stars. The full sources of the data used to create the Explorer are described in the Sources section at the bottom of this page.

The MSX data are currently the most detailed complete images of the galactic plane available outside the visual spectrum. The Spitzer data are still more detailed but cover only the inner galactic plane. The lower resolution IRAS data cover almost the entire sky. The Avedisova catalog is the most comprehensive list of star forming regions (including regions not detectable in visual light) with 3229 major regions, and a total of 66885 observations referenced.

The data sets work well together, with the Avedisova catalog forming a guide to many of the major objects visible at infrared and other frequencies.

Explorer development

In June 2007, I (Kevin Jardine, the creator of this site), downloaded the complete A-band (8.28 μm) MSX dataset and used it to produce a single large colour map of the galactic plane. Like many astronomical images, the MSX A-band measures the intensity of a single frequency of radiation. Compressing this data into 256 shades of gray would lose most of the detail in the data, so I created a 24-bit false colour scheme designed to show off as much of the detail in the data as possible.

The complete MSX dataset forms a 216000 x 5998 pixel image. As this image is obviously too large to view at once, I originally created a Flash application to view the data at two levels of resolution - the full resolution and one reduced to a tenth of the size in height and width (that is, a 21600x600 pixel image).

In June 2008, NASA released the stunning Spitzer mosaic of the inner galactic plane. This was available in several formats but the one that caught my attention was a Google Maps interface. This offered a full screen interface that loaded much more quickly than my original Flash application.

Between September and November 2008, I converted my application to the Google Maps interface and added IRAS and radio data and well as more comprehensive versions of the hydrogen-alpha and microwave data included in my original application. I also took advantage of the Google Maps interface to make accessing the star formation region and supernova remnant information much easier.

All of the map tiles (including the ones NASA created for the Spitzer map as well as my own) were created using the very useful Google Map Image Cutter.

Using the Milky Way Explorer

Unlike the face-on map of the Milky Way also available from this site, the Milky Way Explorer shows the sky as we would see it from Earth, albeit with eyes that could detect infrared, radio and microwave frequencies. By default, the Explorer shows an infrared IRAS image centred on the Milky Way's nucleus. You can then pan and zoom from this using the familiar Google Maps interface.

Two sets of coordinates identify your location. The first row shows the current location of your mouse pointer in galactic coordinates. The second row shows the location of the centre of the map. You can move the centre of the map to another location by clicking "Edit". You can then enter new galactic coordinates in the second row and click on the "Go" button to recentre the map at the new location.

The SIMBAD link opens a separate window that shows all the SIMBAD objects within 20 arcseconds.

The Prawn Nebula (RCW 116, Gum 56), a part of the Sco OB1 association, in infrared at full MSX resolution.

The link labelled "LINK" gives you a direct link to the Explorer with your current map settings. You can use this to bookmark a particular location or to embed a link to it in a document or website.

Markers showing star-formation regions (blue) and supernova remnants (red) appear at the higher zoom levels. Hovering over the blue markers gives a list of significant objects found within the star formation region followed by the number (in square brackets) of SIMBAD references to the most referenced object. This will give you some idea how well-studied the region has been. Clicking on any marker will bring up an information box which will allow you to find more information about the region or supernova remnant.

There are three IRAS views: a darker view that brings out details in the inner galactic plane, a medium view that best shows objects in the outer galaxy and the local Gould belt, and a brighter view that shows off faint details in the infrared cirrus clouds. The inner galaxy and a few other bright spots appear as a negative image in the brighter view. This is a little strange but actually more useful than simply showing a burnt out white area, so I left it in.

There are several other map images available for comparison with the infrared views. The data sources are described in more detail in the "Sources" section below. The Effelsberg/Parkes radio view reveals supernova remnants and other radio-bright objects in the galactic latitude range -5° < b < +5°. The LAB HI view gives a low resolution but all-sky map of atomic hydrogen (HI) gas.

It is especially interesting to switch back and forth between the brighter IRAS and LAB HI views. This reveals that many of the infrared cirrus clouds are the warmer parts of larger HI clouds.

There are two combined infrared/radio views that respectively overlay the "darker" and "medium" IRAS images on top of a heightmap determined by the Effelsberg/Parkes radio data, producing "mountains in the sky" terrain images. These are especially useful for distinguishing between warm (thermal) radio sources and cold (non-thermal) radio sources. The non-thermal radio sources are often supernova remnants.

The microwave/CO view reveals molecular clouds in the galactic latitude range -35° < b < +35°.

There are several hydrogen-alpha views based on Douglas Finkbeiner's wonderful all-sky hydrogen-alpha map, added as a visual frequency comparison. One of these has an overlay of detailed information on ionising stars. At the highest zoom level, hovering your mouse over each star will give you the name, spectral type and luminosity of the star, and clicking on the star will bring up an information window that allows you to search for more information on that star. Other views overlay information on the location of nebulae in the Gum, Sharpless, RCW and BFS catalogs.

I recommend that you put your browser in full screen mode to view as much of the Explorer interface as possible.

Other data sets

In July 2009 I added several other tile sets for parts of the galactic plane to the Milky Way Explorer, including SuperCOSMOS hydrogen-alpha, several more detailed radio maps and some views of the galactic centre region.

You need to be located in the appropriate place in the galactic plane to see this data.

You can find more information for each data set by clicking the Info button.

Downloading complete map images

In addition to viewing them in the Explorer, you can download any of the 21600x600 galactic plane images below for off-line viewing. On Windows, you would do this by right-clicking on the image and selecting "Save link as ...". You can also view the images directly in your browser by clicking on the image (although viewing images this large may crash some browsers.)

MSX 8μm infrared

Radio

Microwave

Hydrogen-alpha

Composite of radio, infrared and hydrogen-alpha

Sources

Images

The IRAS colour composite images were created by combining all four IRAS frequencies (12μm - violet, 25μm - cyan, 60μm - green, 100μm - red). The data is taken from the IRIS - Improved Reprocessing of the IRAS Survey site and reprojected to the galactic cartesian projection used for all the Milky Way Explorer maps. As the IRIS site mentions, there are a few artifacts in the data, including some holes, inconsistent data from overlapping images taken at different times, and zodiacal dust visible from our local solar system.

The MSX infrared data is described here and the complete set of FITS data can be downloaded here.

The Spitzer data is a composite of several infrared bands from two Spitzer Space Telescope surveys, MIPSGAL and GLIMPSE, of the inner galactic plane, as explained in more detail here.

The Effelsberg/Parkes radio data is from two separate sources. I started with the Effelsberg 11 cm (2.7 GHz) data from here and supplemented it with the lower resolution Parkes 2.4 GHz data from here. Because the Parkes data is lower resolution and has a somewhat different frequency, there are obvious transition points in the radio map. There does not appear to be a complete galactic plane radio map at the Effelsberg resolution or better, so this somewhat awkward compromise will have to do for now.

The LAB HI data is derived from the Leiden/Argentine/Bonn galactic HI survey and was created from a velocity integrated (-250 km/s < v < 250 km/s) FITS file kindly supplied to me by Tom Dame from the Harvard-Smithsonian Center for Astrophysics.

The microwave data is from the FITS file available from the Milky Way in Molecular Clouds page here.

The Cassiopeia A supernova remnant and nearby nebulae in the direction of the Cas OB2 association, at radio frequency.

The hydrogen-alpha data is from the highest resolution data available from Douglas Finkbeiner's all-sky hydrogen-alpha page here.

The radio, microwave, hydrogen-alpha and MSX images were constructed from the single frequency data using 24-bit colour schemes of my own devising in which the hue shifts from a dark blue to a light red with the emission intensity. An exception is the LAB HI image which uses a more hue-restricted temperature colour scheme that seemed to best bring out the details of many of the huge HI clouds.

The SuperCOSMOS data used to create the SuperCOSMOS Explorer is taken from the blocked-down SuperCOSMOS data (1/16 the width and height of the full AAO/UKST H-alpha survey), then reprojected to galactic coordinates and mosaicked together using Skyview-in-a-jar. I've rendered this as black and white as this seems to best reveal the subtle fine details in the original images.

The SuperCOSMOS data is not calibrated. I've used the constant median offset de-edger from Skyview-in-a-jar to reduce some of the abrupt plate transitions, but they are still visible. I've tried other mosaicking packages like Swarp but they have not improved the mosaic significantly. Some of the difficulties involved in calibrating this data are described on the SuperCOSMOS site.

Overlays

Veta Avedisova's Catalog of Star-Forming Regions in the Galaxy can be downloaded from Vizier here.

The supernova remnants were taken from the catalog on Dave Green's website.

The ionising star data were taken from Cameron Reed's Catalog of Galactic OB Stars (the version dated 24 July 2007). This is much more current than the 1984 Blaha and Humphreys data used to construct the face-on map of the galaxy elsewhere on this website. The Reed data lacks distance estimates but these were not necessary for the Milky Way Explorer. The Reed data is quite hard to find and can only be downloaded one file at a time. The five files are obcatalog.dat, obubvbeta.dat, ob4color.dat, obmk.dat and obnotes.doc.

The ionising star cluster data were taken from Nina Kharchenko's star cluster catalog and the related new cluster catalog. I selected the 143 clusters which had at least one ionising star (Wolf-Rayet, O-star or B star hotter than B3) as a probable member.

The OB association boundaries are taken from the Blaha-Humphreys luminous star catalog.

Thank you!

Thanks for reading this introduction. You can find the actual Milky Way Explorer here. If you enjoy using it or have questions or suggestions for improvements, you can email me at kevinjardine (at) yahoo.com.