A 10,000 Kilometer Galactic Bridge.
Image Credit & Copyright: Maxime Oudoux, Jean-Francois GELY
It is interesting to contemplate how different the southern Milky Way is compared with the northern Milky Way. It is obvious from the APOD that there is a lot more scattered red (or pink-looking) hydrogen alpha in the northern Milky Way than in the southern Milky Way. On the other hand, the southern Milky Way is a lot brighter than its northern counterpart, because the center of the Milky Way is located in the southern constellation of Sagittarius.
It is really true that there is more red hydrogen alpha in the northern hemisphere than in the southern:
Hydrogen Alpha in the Milky Way from Orion to Scorpius. Credit and copyright:
Alistair Symon. Note all the red stuff to the left, and the yellow stuff to the right.
In the southern Milky Way, the "swollen" all-yellow background bulge is an imposing presence, and pink emission nebulas are confined to the central dust lane (apart from the Rho Ophiuchi region).
Look at NGC 7814 to see how a yellow bulge can dominate, and look at NGC 1448 to see how a galaxy's outer disk can "flare out" and be full of star formation:
Let's conclude with the Gaia space telescope's portrait of the Milky Way. Please note that Gaia "suppresses" bright blue stars, because all stars are seen as individual dots of the same size (and saturation?), unless I am very much mistaken. But the overall shape of the edge-on Milky Way, as well as the distribution of dust lanes, is seen very clearly:
Edit: All right, I admit it. The most likely explanation as to why we see so many red nebulas in the northern Milky Way is because these nebulas are quite literally closer to us, whereas the bright nebulas of the south, such as the Lagoon Nebula, are further away from us and located in a more distant spiral arm.
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