An astronomer used wartime blackouts to peer toward the Milky Way's heart
In the early 1940s, Los Angeles went dark under wartime blackout rules, and Walter Baade seized the chance. Using the 100-inch Hooker Telescope on Mount Wilson, he found a gap about one degree wide in the dust near the star Alnasl, a rare clear sightline to the crowded stars around our galaxy's core. Astronomers still call it Baade's Window.
The centre lies roughly 26,000 light years away toward Sagittarius, where the Milky Way looks brightest, yet thick dust hides it from visible, ultraviolet and soft X-ray telescopes. Immanuel Kant guessed in 1755 that a great star sat at the middle, perhaps Sirius. In 1918 Harlow Shapley noticed that the halo of globular clusters seemed centred on Sagittarius. Radio astronomers near Sydney, at Dover Heights, built an 80-foot dish by 1954 and pinpointed an intense source they named Sagittarius A, some 32 degrees from where the centre had been assumed, and in 1958 it became the official zero point for galactic coordinates.
Inside that source sits Sagittarius A star, a black hole weighing roughly four million Suns. A 2008 study linking radio dishes in Hawaii, Arizona and California found the radio glow to be about 44 million kilometres across, slightly less than Mercury's closest distance from the Sun. In January 2015 NASA recorded an X-ray flare from it 400 times brighter than normal, possibly an asteroid being torn apart as it fell in. Towering above and below the core are the Fermi bubbles, lobes of energetic plasma stretching about 25,000 light years, which simulations by 2022 tied to the black hole.
The neighbourhood is crowded and puzzling. Around 10 million stars pack the central parsec, and perhaps 25,000 stellar-mass black holes lurk nearby. More than 100 massive young stars were a shock, since the black hole's tides were expected to prevent stars forming there; the leading idea is that they formed within a gas disk around it. Old stars pose the opposite riddle: rather than crowding toward the black hole, their density peaks about half a parsec out and then drops, leaving an unexplained hollow.
A ring of gas several million times the Sun's mass is building up nearby. Researchers predict it will ignite a starburst in roughly 200 million years, part of a cycle thought to recur about every 500 million years.
Source: Galactic Center
