JWST reveals spiral galaxies’ life cycles, from dust to stars | by Ethan Siegel | Starts With A Bang! | Feb, 2024

This composite of 19 separate nearby, face-on spiral galaxies all located within 100 million light-years of ourselves is a result of the PHANGS program, where multi-wavelength observatories including Hubble, the VLT, ALMA, and now JWST are all taking data of the same objects in an effort to map out the stars, star-forming regions, gas, dust, and even diffuse atoms that exist within these objects. These JWST views have revealed millions of new stars, 100,000 star clusters, and the most exquisite dust maps ever constructed within these galaxies. (Credit: NASA, ESA, CSA, STScI, J. Lee (STScI), T. Williams (Oxford), PHANGS Team, E. Wheatley (STScI))

Stars are born, live, and die within the spiral arms of galaxies like the Milky Way. These 19 JWST spirals deliver unprecedented riches.

If there’s one rule that’s practically universal in astronomy, it’s this: you can only observe the parts of the Universe that your instruments are sensitive to. As a result, when most of us think about the objects that are out there in the cosmos, we think about the components that are easiest to see: things that are bright, close by, and that emit radiation in the form of visible light. When we think of galaxies, we think of the stars within them; when we think of stars, we think primarily of the ones that appear brightest to our eyes and in our telescopes. But there’s a whole Universe out there to explore, and much of it doesn’t emit any visible light at all.

This is true even for large spiral galaxies like the Milky Way. We aren’t just a collection of stars, with the occasional “dust lane” of neutral matter that blocks the starlight from behind it, but rather a rich network of gas and dust, dotted with stars and with cavities blown in them by violent stellar cataclysms. By looking at 19 nearby, face-on spiral galaxies with JWST’s remarkable, unprecedentedly sharp and powerful eyes, the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) collaboration has at last uncovered the life cycles of stars within them. From breathtaking sights to breakthrough science, here’s what was revealed when we took our greatest look of all at these nearby Milky Way analogues.

The Andromeda galaxy, the closest large galaxy to Earth, displays a tremendous variety of details depending on which wavelength or set of wavelengths of light it’s viewed in. Even the optical view, at top left, is a composite of numerous different filters. Shown together, they reveal an incredible set of phenomena present in this spiral galaxy. Multiwavelength astronomy can shed unexpected views on almost any astronomical object or phenomenon, revealing details in one wavelength that are wholly invisible in another. (Credit: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendler)

For a long time, whenever we looked at a large, massive spiral galaxy like the Milky Way, we assumed that what we could see — the stars — lined up with where the mass was. After all, in our Solar System, the most luminous object of all, the Sun, makes up some 99.8% of the entire Solar System’s mass. But it turns out that’s not true for galaxies; stars represent only about 2% of a galaxy’s total mass. Most of a typical galaxy’s normal matter is in the form of gas, dust, and plasma, with stars only making up a…

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