Step away from the streetlights on a clear night and a pale band spreads across the sky. NASA’s plain description is that the band is what you see when you look edge-on through the flat disk of stars that makes up our own galaxy, from the inside.
That last part matters. Nobody has ever photographed the Milky Way as a whole, because nobody can get outside it. Space.com compares the old struggle to describing a forest while lost in the middle of it.
How astronomers know its shape
NASA lists a few clues. The band itself says the galaxy is basically flat. Had we lived inside an elliptical galaxy, stars would be scattered all around the sky instead of crowded into one strip. Astronomers also map young, hot stars and clouds of ionized hydrogen, which mark the spiral arms in other galaxies, and the pattern in ours points the same way. The latest data, NASA says, shows four arms. The result is a barred spiral: a spinning disk with a straight bar of stars through its middle.
The Gaia mission, launched by the European Space Agency in 2013, has done much of the recent work. Space.com reports that it measures the positions and distances of more than 1 billion stars.
Size, and a few numbers that disagree
The disk is about 100,000 light-years across and only 1,000 light-years thick, according to Space.com. Star counts vary. NASA’s estimate is approximately 100 billion stars. Space.com says about 200 billion. Counting from the inside is hard. Space.com adds that the galaxy’s mass, dark matter included, comes to 1.5 trillion solar masses by recent NASA estimates, and that dark matter may make up as much as 90 percent of it.
Our own neighborhood is quiet. NASA puts the Sun about 25,000 light years from the center and calls it the suburbs. Space.com gives about 26,000 light-years. Either way, one trip around the galactic center takes roughly 250 million years. Space.com notes that the last time Earth sat at this spot in its orbit, dinosaurs were only just emerging.
The thing at the center
Sagittarius A*, a supermassive black hole, sits at the heart of the Milky Way. Space.com puts its mass at about 4 million times that of the sun in one passage and 4.3 million in another. The Event Horizon Telescope produced the first image of it on May 12, 2022. What the picture shows is a shadow, ringed by faint light from heated matter racing toward the hole.
Once, it was thought to be everything
Not long ago astronomers believed every star in the sky belonged to one galaxy. In 1920 Harlow Shapley and Curtis argued over the scale of the universe in “The Great Debate”. Shapley said the Milky Way was far larger than assumed and that we were not at its center. Curtis accepted that, but held that objects like Andromeda were separate “island universes”. Edwin Hubble settled it with measurements of Cepheid variable stars, which showed Andromeda lies far outside our galaxy.
Andromeda is now about 2.5 million light years away, NASA says, and it is closing in. The two galaxies are expected to collide in about 4 billion years. Space.com adds that individual stars should mostly pass each other untouched, because the gaps between them are so wide.
Sources
- Milky Way galaxy: Facts about our cosmic neighborhood | Space.com
- The Milky Way Galaxy | NASA Imagine the Universe
