Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 approximately 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels. The complementary Hubble sur- vey programs provide information about the age, heavy-element abundance and stellar masses inside Andromeda. This will allow as- tronomers to distinguish between competing scenarios where An- dromeda merged with one or more galaxies. Hubble’s detailed measure- ments constrain models of Androm- eda’s merger history and disk evo- lution. Though the Milky Way and Androm- eda formed presumably around the same time many billions of years ago, observational evidence shows that they have very different evolu- tionary histories, despite growing up in the same cosmological neighbor- hood. Andromeda seems to be more highly populated with younger stars and unusual features like coherent streams of stars, say researchers. This implies it has a more active recent star-formation and interaction his- tory than the Milky Way. A possible culprit is the compact satellite galaxy Messier 32, which re- sembles the stripped-down core of a once-spiral galaxy that may have interacted with Andromeda in the past. Computer simulations suggest that when a close encounter with another galaxy uses up all the avail- able interstellar gas, star formation subsides. S ome interesting regions of the photomosaic include: a) Clusters of bright blue stars embedded within the galaxy, background galaxies seen much farther away, and photo-bombing by a couple bright foreground stars that are actually inside our Milky Way; b) NGC 206 the most conspicuous star cloud in Andromeda; c) A young cluster of blue newborn stars; d) The satellite galaxy M32, that may be the residual core of a galaxy that once collided with Andromeda; e) Dark dust lanes across myriad stars. [NASA, ESA, B. Williams (University of Washington)] it required over 1,000 Hubble orbits, spanning more than a decade. This panorama started with the Panchromatic Hubble Andromeda Treasury (PHAT) program about a decade ago. Images were obtained at near-ultraviolet, visible, and near- infrared wavelengths using the Ad- vanced Camera for Surveys and the Wide Field Camera aboard Hubble to photograph the northern half of Andromeda. This program was followed up by the Panchromatic Hubble Androm- eda Southern Treasury (PHAST), which added images of approxi- mately 100 million stars in the southern half of Andromeda. This region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped by the PHAT survey. The combined programs collectively cover the entire disk of Andromeda, which is seen almost edge-on — tilted by 77 degrees relative to Earth’s view. The galaxy is so large that the mosaic is assembled from ! T his the largest photomosaic ever assembled from NASA/ESA Hub- ble Space Telescope observations. It is a panoramic view of the neighboring Andromeda galaxy, located 2.5 million light-years away. It took over 10 years to make this vast and colorful portrait of the galaxy, requiring over 600 Hub- ble snapshots. The galaxy is so close to us, that in angular size it is six times the apparent diameter of the full Moon, and can be seen with the unaided eye. For Hubble’s pinpoint view, that’s a lot of celestial real es- tate to cover. This stunning, colorful mosaic captures the glow of 200 mil- lion stars. That’s still a fraction of An- dromeda’s population. And the stars are spread across about 2.5 billion pix- els. The detailed look at the resolved stars will help astronomers piece to- gether the galaxy’s past history that includes mergers with smaller satellite galaxies. [NASA, ESA, B. Williams (University of Washington)]