The Milky Way has four spiral arms
The core idea stands, but key numbers or details have changed.
TaughtOur galaxy is a four-armed spiral, and there is a diagram in the back of the book showing exactly where we sit in one of them.
NowData from NASA's Spitzer Space Telescope showed the Milky Way has two dominant spiral arms of stars, Scutum-Centaurus and Perseus, plus minor arms and spurs. It is also a barred spiral.
What actually happened
Mapping the galaxy you live inside is like mapping a forest while standing in it. Every diagram in your textbook was an artist's reconstruction stitched from indirect measurements, and the four-arm model came largely from surveys of gas rather than stars.
Spitzer's infrared survey counted stars through the dust that blocks visible light, and found that two of the four proposed arms, Sagittarius and Norma, do not have the stellar density to qualify as major arms. They are minor arms: plenty of gas and young stars, not much of the old stellar population that defines the galaxy's structure. Separately, the evidence that our galaxy has a central bar solidified over the 1990s and 2000s, which older diagrams omit entirely.
The Sun sits in the Orion Spur, a smaller structure between the two major arms. If your classroom poster confidently placed you in a grand four-armed pinwheel, it was a good-faith reconstruction that the data has since revised.
Newsletter
Get new weekly revisions
Sources
- NASA/JPL: Spitzer maps the Milky Way's armsjpl.nasa.gov
- NASA Science: The Milky Way galaxyscience.nasa.gov
Who was taught this
Still standard through 2009, so anyone who finished school between 1950 and 2008 learned the earlier version.