plate 2 · the milky way
a barred spiral, roughly 85,000 light-years of stellar disk, somewhere between one and four hundred billion stars. the map is a density model — logarithmic spiral arms at a 12.5 degree pitch, an exponential disk, a boxy bar — tuned to the geometry mapped by maser parallaxes (reid et al. 2019). nobody has ever seen this view. we are inside it, and every picture of the whole galaxy is a reconstruction like this one.
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the two dominant arms are scutum-centaurus and perseus, anchored to the ends of the central bar. the sagittarius and norma arms are thinner; the sun sits on a minor spur (the local arm) 8.2 kpc — 27,000 light-years — from the center, orbiting at 230 km/s. one lap takes about 230 million years. the last time we were on this side of the galaxy, the dinosaurs were new.
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the sun, 8.2 kpc from the middle (disk to scale!)
seen edge-on the galaxy is absurdly flat: the thin disk is about 700 light-years thick against 85,000 across — proportionally about as flat as a vinyl record. the dots above and below are globular clusters, some of the oldest objects known. at the center sits sagittarius A*, a black hole of 4.15 million solar masses, photographed by the event horizon telescope in 2022. all of it — disk, bulge, halo — floats inside a dark-matter halo of about a trillion solar masses that no telescope can see at all.