Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 tronomers had long thought that the Milky Way encompassed the en- tire universe. Overnight, Hubble’s discovery turned cosmology upside down by unveiling an infinitely grander universe. Now, a century later, the space tele- scope named for Hubble has accom- plished the most comprehensive survey of this enticing empire of stars. The Hubble telescope is yield- ing new clues to the evolutionary history of Andromeda, and it looks markedly different from the Milky Way’s history. Without Andromeda as a proxy for spiral galaxies in the universe at large, astronomers would know much less about the structure and evolution of our own Milky Way. That’s because we are embedded in- side the Milky Way. Hubble’s sharp imaging capabilities can resolve more than 200 million stars in the Andromeda galaxy, de- tecting only stars brighter than our Sun. They look like grains of sand across the beach. But that’s just the tip of the iceberg. Andromeda’s to- tal population is estimated to be 1 trillion stars, with many less massive stars falling below Hubble’s sensitiv- ity limit. Photographing Andromeda was a herculean task because the galaxy is a much bigger target on the sky than the galaxies Hubble routinely observes, which are often billions of light-years away. The full mosaic was carried out under two Hubble observing programs. In total The largest photomosaic of the Andromeda galaxy by NASA/ESA Bethany Downer I n the years following the launch of the NASA/ESA Hubble Space Telescope, astronomers have tal- lied over 1 trillion galaxies in the universe. But only one galaxy stands out as the most important nearby stellar island to our Milky Way: the magnificent Andromeda galaxy (Messier 31). It can be seen with the naked eye on a very clear autumn night as a faint cigar-shaped object roughly the apparent angular diam- eter of our Moon. A century ago, Edwin Hubble first established that this so-called “spi- ral nebula” was actually far outside our own Milky Way galaxy — at a distance of approximately 2.5 mil- lion light-years, or roughly 25 Milky Way diameters. Prior to that, as-