The Visual Milky Way
The Inner Galaxy (l = 280° to 55°)
The central Galaxy, from Aquila to Carina, is the brightest and most spectacularly figured potion of the naked eye Milky Way panorama. Across this entire width we look into the Galaxy across the visual extent of the Sagittarius-Carina spiral arm and centered on the nearest point of the arm. All the matter visible in this section of the Milky Way is inside the Solar circle, closer to the galactic nucleus than is the Sun.
The toy spiral arms of the Vallée consensus model have been used to estimate the visual profile of the Galaxy inner spiral arms Sagittarius-Carina and Scutum-Crux assuming a thin disk scale height of 350 parsecs and a gas disk thickness of 50 parsecs. The curving blue lines indicate the nominal extent of the Sagittarius-Carina thin disk and the narrower cyan contours its thin disk of gas and dust. The arm curves toward us from the Sagittarius tangent, about 4700 parsecs distant at l = 50°, into its nearest point about 975 parsecs away at l = 350°, then curves away into the Carina tangent, about 3900 parsecs distant at l = 285°. The high profile of the Sagittarius-Carina arm indicates that much of the Milky Way visible in this direction lies within that arm, while its narrower gas disk limits indicate that most of the dark clouds silhouetted against the background stars are closer to us than the arm. All naked eye stars with an absolute magnitude of M ≥ -3.5 (that is, all main sequence stars of type B1 or later) are closer to us than this arm.
Only the thin disk of the Scutum-Crux arm is shown (yellow contours), stretching between tangents at l = 30° and 310°, both more than 6600 parsecs away. While much of the Scutum-Crux arm is obscured by intervening dark clouds, as the nearest and densest concentration of Solar type stars in this direction it contributes to much of the central brightness of the Milky Way, and imparts a distinctly yellow light due to its late type stellar population.
From our viewpoint, the near end of the "boxy" central bar is centered at around 14°, extends roughly 30° in width and in height slightly above the Sagittarius arm. Its profile forms an ellipse (not shown) that encircles most of the Greater Sagittarius star cloud, located at the spout of the Sagittarius "teapot". This 5° wide area is too dense to be within the Sagittarius arm and too far below the galactic equator to lie in the Scutum arm: it is a direct view of the near end of the central bar through a fortuitous tunnel of low intersteller extinction known as Baade's Window. A similar but smaller window creates the Sagittarius star cloud (catalogued as M 24 or IC 4715), which provides a view as far as 3000 pc into the interval between the Sagittarius and Scutum spiral arms containing mostly young or "blue" stars, in contrast to the Greater star cloud which reveals the aggregate light from older, "yellow" or Solar type stars within the Scutum arm or the near side of the galactic nucleus.
A remarkable texture of shredded and intertwined dark clouds of gas and dust is silhouetted against the Sagittarius arm, receding in perspective from the nearby Aquila Rift, Lupus, Pipe Nebula and Coal Sack dark clouds to farther, fainter concentrations much closer to the Sagittarius-Carina arm. This texture is visible on a galaxy wide scale in the dramatic disk photographs by the Hubble Telescope of the Sculptor Galaxy (NGC 253, below), a barred spiral 3.5 million parsecs distant that comprises about 100 billion solar masses. Clouds above the disk are not as dense as the clouds near or within the thin disk, a contrast visible in the Galaxy as the thinning density of the Aquila Rift as it rises above the galactic plane into the southern part of Ophiuchus.
The dark clouds nearest to the Sun have been outlined for contrast. Many of these comprise sites of active star formation or recently condensed star clusters. The Pipe Nebula (LDN 42 and 1773) is roughly 145 pc distant and is part of the Ophiuchus star forming region. Many small and very dense gobules of condensed gas and dust are visible nearby, including Barnard 68, the famous "hole in the Heavens" first observed by William Herschel. The Lupus star forming region is about 170 pc away, the same distance as the Scorpius-Centaurus OB association of some 300 stars, including many of the brightest stars in all three constellations. The Coal Sack is about 180 pc distant; the Aquila Rift, 225 pc distant and 80 pc thick, is the large end of the Great Rift that extends from Deneb through Aquila and upward into the star forming regions in Serpens and around rho Ophiuchi; it appears to divide the Milky Way in Cygnus into two bands. All these areas of dense gas and dust form a ring around the Sun associated with the Gould belt of bright southern hemisphere stars and OB associations.
The key chart shows only a handful of the more than 400 galactic star clusters cataloged within this panorama. Because of the vast extent of the Aquila Rift the galactic longitudes l = 15° to 55° display the fewest open star clusters of any part of the Milky Way. The area from l = 315° to 0° is also sparse of clusters, due to the obscuring effect of clouds connected to the Lupus star formation region. Despite these impediments, the Sagittarius-Carina arm contains many of the largest star forming regions known, including Westerlund 51 (at the same galactic longitude as the Sagittarius tangent), NGC 6530 (M8 or the Lagoon Nebula), NGC 6611 (M20 or the Eagle Nebula), and NGC 3372 (the Carina Nebula). These include very young clusters such as NGC 6611 or 6618, each just 1 million years old, and a few "middle aged" clusters such as IC 4651 (an estimated 1.1 billion years old). Overall, the clusters here, most of them associated with the Sagittarius-Carina arm, are relatively young and compact, with a median age of 153 million years and a median diameter of 3.3 parsecs.
Because there is relatively little obscuring dust behind us in the galactic rotation we can see great distances along the SC arm, from its nearest point at l = 350° to the Carina Nebula at l = 290°, an estimated 2300 parsecs away. This creates a remarkable visual transition in the stellar texture from the central to southern Milky Way as the background stars gradually become fainter and more closely packed.
Ther ESO image also displays a noticeable color transition from the creamy yellow color of stars in the Sagittarius star cloud to the blue stars in Carina. Because spiral arms consist mostly of empty space we can readily see through them, which means we glimpse the relatively dense population of older, yellow solar type stars in the Scutum-Crux arm through the content of the Sagittarius arm, which is skewed toward star forming regions and the young, bluish stars associated with them.
A final point of contrast is the large number of globular clusters concentrated in this part of the Milky Way. The key diagram shows only a portion of the total number within this area, but enough to illustrate how Harlow Shapley was able to use them to deduce the approximate location of the galactic barycenter.
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