The Visual Milky Way

The Local Arm (l = 45° to 180°)

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

Although visually much darker than the central Galaxy, this section of the Milky Way is perhaps the most dramatic. Here we look toward the large scale structures nearest to the Sun: the star clusters and dark clouds associated with the Local arm (sometimes called the Orion arm) sweeping ahead of us in galactic rotation.

The most remarkable fact about the Local Arm is that after a century of astronomical study its limits, structure and content are still unclear. Originally believed to be a minor spur extending from the Sagittarius-Carina arm at a pitch angle of perhaps 25°, it has recently been described as a massive "crossing" structure that pierces the Perseus arm to connect with infalling matter from Quadrant III, as a massive "interarm" structure concentric with the Perseus arm at a pitch angle of around 14° — or as an illusion produced by windows through obscuring clouds along lines of sight toward l =60° and 240°.

For purposes of illustration, the angular profile of the Local arm and its thinner gas disk have been projected in the key diagram (mouseover, above) according to the Vázquez & alia "crossing" or Orion arm conception of the structure (yellow contours in the diagram, above right), assuming a thin disk height of 350 parsecs and a width of about 800 parsecs, and a gas disk height of 50 parsecs. The center of this hypothetical arm passes by the Sun at a near distance of about 600 parsecs (toward l = 150°), and continues into the Perseus arm at a pitch angle of about 30°. This places the Orion SFR at the trailing edge of the structure and the Sun in a void situated behind it. (The alternative Reid & alia "interarm" model is outlined in blue in the diagram above.) In either model, the long wall of obscuring giant molecular clouds appears to correspond to the concentrated dark lanes that are often visible in other galaxies along the "upstream" side of a spiral arm inside the spiral pattern corotation radius or in the "valley" between widely separated spiral arms.

Depending on the model you prefer, the Sun appears to be located at or just inside the trailing boundary of this Local arm (diagram, above right), so it is remarkable that the arm appears generally faint and broken to the naked eye. This is partly because we observe it through an enormous and turbulent screen of obscuring gas and dust, stretching in a continuous but tattered wall from Aquila through Taurus (mouseover image, above). However, it is also because the Local arm is not a concentration of Solar type stars, like the Scutum-Crux arm: it is instead a loose aggregation of dark clouds, star forming regions and the young stars and star clusters that indicate recent star forming activity.

The thin disk projection of this arm suggests that the visual Milky Way should enlarge and brighten considerably up to a maximum around l = 150°, but the extent of Milky Way stellar brightness does not follow this contour (even along the lower edge where obscuration is minimal). This indicates the lack of naked eye evidence for a large scale and nearby structure. Instead, there is obviously an increase in stellar density in the direction of Cepheus and into Cygnus, including the Cygnus star cloud. Most models of the Local arm place its origin or vanishing point within the Summer Triangle. The downward curve of the distant Cygnus clouds allows us an unimpeded and nearly lengthwise view through the Local arm contents at distances of over 2000 parsecs — more than twice the distance of the near point of the Sagittarius-Carina arm. This foreshortened view of the Local arm appears to the naked eye as the Cygnus star cloud. This feature, stretching from β to γ Cygni (Albireo to Sadr), is among the most beautiful telescopic star fields in the Galaxy, spangled with bright, closely spaced stars, many double stars, and several extended H II regions including NGC 7000, the "North America" nebula.

The turbulence wihin the curtain of dark clouds is apparent in its undulating profile, which originates below the galactic equator in Cygnus, then sweeps upward in tendrils that reach above the galactic equator in Cepheus and Cassiopeia (and extend from Cepheus as far as Polaris), and finally plunges below the equator again in Taurus. As we look into higher galactic longitudes we observe clouds that are closer to the Sun, from the Cygnus rift at 2500 parsecs to the nearby Taurus-Auriga SFR only 140 parsecs away. The large Northern Rift between galactic longitudes l = 140° to 160°, like the Aquila Rift, is dense enough and close enough (~250 pc) to produce a drop in the number of visible open star clusters (about 750 are catalogued within this third of the Milky Way panorama). Beyond these clouds lie the star forming regions of the Local arm and, because spiral arms consist almost entirely of empty space, we can see through the Local arm to the farther Perseus arm. Indeed, some of the best known star clusters in this direction — the Double Cluster NGC 884/869 (h & χ Persei), NGC 2099 (M 37) and NGC 1912 (M 38) — are located at the distance of the Perseus arm, well beyond any boundaries that have been proposed to define the Local arm.

The Local arm experienced a burst of star forming activity about 3 billion years ago, and it has remained one of the most active regions of the Galaxy visible to us, with recent or ongoing star formation in Cygnus, Cepheus, Cassiopeia and Taurus, and many galactic bubbles, radio sources, planetary nebulae and expanding gas fragments that indicate several recent nova and supernova explosions (including those observed in 1054, 1572 and 1604).

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