For the most part, the stars that lie along this section of the Milky Way, as well as the dark clouds that decorate it, lie within the Sagittarius-Carina Arm
of our galaxy.
When astronomers realized that the Sagittarius arm (found in the northern sky) and the Carina arm (in the south) were part of a single, larger structure, they became known as the Sagittarius-Carina arm
. Similarly, since Dame and Thaddeus believe the new arm is an extension of Scutum-Centaurus, "we suggested 'Outer Scutum-Centaurus' as a more logical name," Dame says.
Beyond their eastern end we look across an interarm gap, poor in dust and open clusters, at the next spiral feature in from ours: the Sagittarius-Carina Arm, which extends from the Scutum Star Cloud on the northeast to the Eta Carinae region on the southwest.
Several windows through the dust clouds of the Sagittarius-Carina Arm give us glimpses of the next spiral feature toward the galactic bulge: the Norma Spiral Arm (sometimes called the Scutum-Centaurus Arm).
One plausible explanation is that toward Vela we look along a bridge that joins our Orion-Cygnus Arm to the outcurving arc of the Sagittarius-Carina Arm. Or perhaps the entire Orion-Cygnus Arm is bending inward here.
Here we would expect to see a dust-and cluster-poor interarm gap between the outcurving arcs of the Orion-Cygnus and Sagittarius-Carina arms. Instead, we find ourselves looking at a confusing clutter of dust clouds, young open clusters, and stellar associations.
Beyond that, 5,000 to 7,000 light-years distant, lies the Sagittarius-Carina Arm, the next spiral arm inward from our own.
This is either the Sagittarius-Carina Arm (the next arm inward from ours), the Norma Arm (the arm inside that), or possibly a place where both arms intersect.
That's why the bright nebulae of the Sagittarius-Carina arm such as M16 and M17 stretch only to longitude 17[degrees] in this direction, whereas the Eta Carinae Nebula, also in the Sagittarius-Carina arm, lies 72[degrees] on the opposite side of the galactic center, at longitude 288[degrees].
Moving outward from the Norma Arm toward the Sun, the next spiral feature is the Sagittarius-Carina Arm -- so named because many major bright emission nebulae and open clusters are distributed along it from Sagittarius to Carina.
Although it's rather far south for observers in the northern United States and Europe, NGC 6231 in the Tail of Scorpius, a major tracer of the Sagittarius-Carina Arm, is well worth viewing in binoculars.
The Sagittarius-Carina Arm clusters and nebulae in Sagittarius and Scorpius are between 4,500 and 7,000 light-years distant, suggesting that in this direction the arm is centered about 5,500 light-years from us.