Andromeda galaxy’s hidden history uncovered by Nasa’s Hubble

In the years since NASA’s Hubble Space Telescope began exploring the cosmos, astronomers have identified over a trillion galaxies. Yet, among these, one stands out as particularly significant for our Milky Way: the magnificent Andromeda galaxy, also known as Messier 31. On a clear autumn night, this neighboring stellar island is visible to the naked eye as a faint, cigar-shaped glow, its apparent size comparable to the Moon’s.
A century ago, Edwin Hubble revolutionized astronomy by establishing that Andromeda, then thought to be a “spiral nebula,” was actually located far beyond the Milky Way—approximately 2.5 million light-years away. This discovery shattered the long-held belief that the Milky Way encompassed the entire universe, revealing a cosmos vastly larger and more complex than previously imagined.
Today, the Hubble Space Telescope continues to unravel Andromeda’s mysteries, providing unprecedented insights into its evolution and history. By studying this galaxy in extraordinary detail, astronomers gain a deeper understanding of spiral galaxies, including the Milky Way. Unlike our vantage point within the Milky Way, which limits perspective, Andromeda offers a holistic view that reveals its intricate structure and past. Hubble’s sharp imaging has captured more than 200 million of Andromeda’s stars, though this is just a fraction of its estimated trillion-star population.
Photographing Andromeda was a monumental effort, requiring over 1,000 orbits of the telescope across more than a decade. The resulting mosaic, assembled from 600 fields of view, boasts a staggering 2.5 billion pixels. Complementary programs, such as the Panchromatic Hubble Andromeda Southern Treasury (PHAST), have expanded the survey, adding data from the galaxy’s southern disk, which offers critical clues about Andromeda’s history of mergers and interactions.
Andromeda’s evolution appears strikingly different from that of the Milky Way. Observations suggest it experienced a disruptive past, possibly involving a collision with the compact satellite galaxy Messier 32. Such events likely triggered bursts of star formation, followed by a decline as interstellar gas was depleted. Today, Andromeda shows characteristics of a transitional galaxy, with a mix of older stars in its bulge and diminished activity in its disk.
The detailed data from Hubble lays the groundwork for future studies by advanced telescopes like the James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope. Together, they promise to expand on Hubble’s legacy, deepening our understanding of Andromeda and its place in the cosmic tapestry.