Andromeda Galaxy: Hubble's 2025 Deep Sky Mosaic and Its Turbulent Past

Short Summary

This Ashram video explores the 2025 image mosaic of the Andromeda Galaxy captured by the Hubble Space Telescope. Viewers are guided through a near edge-on portrait that resolves hundreds of millions of stars, revealing a blue ring of star formation at about 10 kiloparsecs from the center and a hidden inner dust ring. The narrative connects these features to Andromeda’s violent past, including a collision with its companion Messier 32 and the Giant Southern Stream, formed by past mergers. The program links the new image to historic observations from Abdul Rahman and Hubble, and explains how simulations and surveys like PHAT and FAST help illuminate Andromeda's future, including a possible Milky Way collision in about 4 billion years.

Overview

This article accompanies a video that examines the 2025 Hubble mosaic of the Andromeda Galaxy, focusing on how the largest photo mosaic of a galaxy has reshaped our understanding of our cosmic neighbor. The image presents Andromeda nearly edge-on, tilted about 77 degrees, spanning roughly 150,000 light years with a census of about 200 million resolvable stars. It demonstrates how high-resolution imaging across multiple wavelengths—across ultraviolet to near infrared—unveils intricate structures in Andromeda that were previously unresolved, setting the stage for deeper studies of galactic formation and evolution.

From PHAT to FAST: A Legacy of Detailed Imaging

The video traces the history of Andromeda observations, starting from early naked-eye recognition, through 1612 telescope observations, and into the 1764 classification as M31. It explains how the Panchromatic Hubble Andromeda Treasury (PHAT) program, created in 2015, spent years photographing a significant fraction of Andromeda’s star-forming disc across six spectral bands. The southern focus of the FAST survey expanded the mosaic to cover two-thirds of the disc, adding tens of millions of stars to the census and revealing regional differences between the north and south that hint at a violent past.

Key Structures: Rings, Streams, and Mergers

The outer ring of star formation around 10 kiloparsecs from the nucleus contains blue, hot stars rich in metals, while a second inner dust ring was identified using Spitzer data. These rings are interpreted as ripples left by past impacts, including a major collision with a dwarf galaxy, Messier 32, about 210 million years ago. The Giant Southern Stream, a vast stream of stars stretching up to 150 kiloparsecs into the halo, is another sign of Andromeda’s turbulent accretion history. The video also discusses the possibility that M32’s core survived as a stripped remnant known as M32P, which could explain several mid-age stellar populations and central black hole mass estimates for M32.