Sagittarius arm

(redirected from Sagittarius-Carina Arm)

Sagittarius arm

See Galaxy.

Sagittarius arm

[‚saj·ə′ter·ē·əs ′ärm]
(astronomy)
A spiral arm of the Milky Way Galaxy that lies between the sun and the galactic center in the direction of the constellation Sagittarius.
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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.
First, the Sagittarius-Carina Arm here arcs out toward the galactic exterior, giving us a long view down its heart.
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.
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.
A window through the Sagittarius-Carina Arm dust allows us to see the 10,000- to 16,000-light-year-distant Small Sagittarius Star Cloud (M24), a star-rich stretch of the Norma Spiral Arm of our galaxy's deep interior.
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.