The Visual Milky Way

The Trailing Galaxy (l = 155° to 290°)

Hover cursor over image for object identifiers (these may take several seconds to load). Due to distortions in original image, position of galactic coordinates is approximate. Source: European Space Agency.

List of Labeled Galactic Objects

This last section of the Milky Way panorama provides some of the greatest contrasts of any — from the brilliant Orion star forming region to the far Carina nebula and the dim, distant limits of the Local and Perseus spiral arms as these features extend into the outer limits of the Galaxy disk (diagram, right). From l = 180° we look "upstream" into the Milky Way: all matter we can observe is both outside the Solar circle and behind us in galactic rotation.

As in the other two panorama sections, the Vallée consensus model has provided the toy spiral arms used to calculate the distance of the Carina tangent and the visual extent of the Perseus spiral arm. Following Vázquez & alia (2008), the Local arm has been depicted as a "crossing" structure passing about 630 parsecs ahead of the Sun and extending through the Perseus arm toward a point around l = 230°. Beyond that longitude the nearest spiral structure is over 4600 parsecs away and hidden in a faint scatter of far stars and gas, much of it hidden behind Vela Molecular Ridge.

The galactic band from l = 160° to 230° has the largest number of bright stars of any part of the Milky Way, including the spectacular asterisms of the Hyades, Pleiades, Auriga, Gemini, Orion and Canis Major. In addition, the wide area centered on l = 240° (known as the FitzGerald Window) comprises a dense concentration of naked eye stars and open star clusters that is visually suggestive of the Local arm receding in space. (There are 720 clusters catalogued across this span of the Milky Way.) Because the density of obscuring interstellar medium is extremely low in this direction, we can observe in the visual to great distances — the young (133 million year old) open cluster NGC 2354 is almost 4100 parsecs away. Note that there are very few globular clusters in this direction, as these tend to be concentrated around the galactic nucleus.

The major structural contrasts are evident in the key image (mouseover, above): the obscuring clouds of dust and gas thin considerably between the nearby Taurus and Orion SFRs (from 140 to 400 parsecs distant) and the Vela molecular ridge (700 parsecs distant). As a result there are few large H II regions in proportion to the many galactic star clusters, which are also spread vertically across a larger area of galactic latitude. In fact, the galactic span between l = 215° to 255° contains the most open star clusters of any part of the Milky Way, with particular concentrations around galactic longitudes l = 205° (mostly associated with the Orion star formation complex), l = 240° (our line of sight along the receding Local arm) and 285° (our line of sight into the tangent of the Sagittarius-Carina Arm). With an average age of 705 million years and an average distance of 2550 parsecs, these are among the oldest, faintest and most distant star clusters visible. Due to their age and their orbits in a relatively sparse part of the Galaxy where they will be rarely disrupted by near encounters with giant molecular clouds, they are also among the largest galactic clusters, with an average radius of almost 3.2 parsecs.

This is perhaps the least understood section of the Milky Way. There are substantial infalls of matter from this direction, organized into three spiral like features, one of them clearly the continuation of the Sagittarius-Carina arm, another perhaps a continuation of the Perseus arm, and a third very large feature outside the Perseus extension. The infalling matter from this third feature becomes fragmented into many large clouds which seem to shear across the Perseus arm and continue on a trajectory that is approximately aligned with the Local arm (l = 240° to 260°). But one difficulty in tracing the outward extent of the Local arm is that the galactic content outside the Solar circle rapidly becomes less concentrated and more fragmented with distance. The Local arm thin disk and gas disk profiles continue the "crossing" Local arm model adopted in the previous panorama, but objects that clearly trace a spiral structure into Quadrant IV are few and widely scattered in space. The ongoing Reid & alia survey of distant star forming regions with wide baseline interferometry will survey this area in the future, and may provide clarity on structures that so far have been obscured by ambiguous distance estimates.

Back to the Overview of the Galaxy