If you look at a photo of a face-on spiral galaxy, you’ll notice spiral arms filled with glowing gas clouds, sparkling star clusters, and opaque dust lanes curving gently away from the galaxy’s core. An image of an edge-on spiral gives a completely different picture. The gas, dust, and stars now occupy a thin disk that shows just a hint of the gorgeous detail seen in their face-on cousins.
The same structures define the spiral galaxy we call home. The Milky Way may seem amorphous at first glance, but that’s only because we see it from the inside. The big picture reveals that most of the young star clusters and the emission nebulae that give birth to them reside in a narrow strip that coincides with our galaxy’s equator.
A cosmic crustacean
One of the finest examples of a newborn cluster emerging from its natal shroud lies less than 1° from the galactic equator in the constellation Scorpius the Scorpion. The Lobster Nebula (NGC 6357) spans nearly 300 light-years and holds enough raw material to produce tens of thousands of stars. Many of its first generation of stars reside in the massive open cluster Pismis 24 at the nebula’s center.
The James Webb Space Telescope (JWST) recently captured this dazzling cluster in glorious detail. Astronomers targeted Pismis 24 because it lies only 5,500 light-years from Earth, a mere stone’s throw away in galactic terms and thus a prime location for studying massive stars and the impact they have on their environment.
JWST’s infrared eye penetrates much of the gas and dust that still envelops the cluster. The faint bluish haze that permeates the image shows hydrogen gas ionized by the hottest and most massive stars, while grains of dust glow with an orange hue and cooler, denser molecular hydrogen appears a deeper red.
Astronomers once thought the cluster’s brightest star, Pismis 24–1 (seen at the image’s center), might be the most massive star in our galaxy. Estimates of its heft ranged up to 300 solar masses, which would place it near the theoretical limit of what a star can weigh in today’s universe. But more recent observations show it to be a binary system comprising stars of “only” 74 and 66 solar masses. At least a half-dozen other stars in Pismis 24 join these stellar heavyweights in spectral class O, the hottest class of normal stars.
The surface temperatures of these behemoths reach nearly eight times that of the Sun, so much of the radiation they pump out is energetic ultraviolet light. This ionizes hydrogen spread throughout the Lobster Nebula and causes it to glow with the characteristic reddish color seen in visible-light images.
A sculpting wind
These high-mass stars also generate fierce stellar winds, which clear out much of the surrounding gas and dust and create the cavity that allows us to see Pismis 24. The densest regions of gas and dust can resist the onslaught for a time, creating colorful spires reminiscent of the famous “Pillars of Creation” seen in the Eagle Nebula.
The tallest tower appears slightly below center and points upward directly toward Pismis 24–1. The spire’s tip measures 0.14 light-year across, which means more than 100 solar systems (out to the orbit of Neptune) could span its width. But don’t expect lots of planetary systems to develop in this region. Astronomers estimate the frequency of protoplanetary disks near Pismis 24 to be lower than in similar clusters, perhaps due to its exceptionally strong wind and radiation environment.
