In the years after NASA introduced the Hubble Space Telescope, astronomers have identified over 1 trillion galaxies within the universe. However, only one galaxy is noted as the nearest and most significant stellar entity to our Milky Way: the stunning Andromeda galaxy (Messier 31). On a clear autumn night, this galaxy is visible with the naked eye as a faint, cigar-shaped figure, roughly the same angular size as the Moon.
After the launch of NASA’s Hubble Space Telescope, astronomers estimated there are over 1 trillion galaxies in the universe. Yet, one galaxy, the splendid Andromeda galaxy (Messier 31), is highlighted as the most crucial nearby stellar formation to our Milky Way. It can be spotted without a telescope on a clear autumn night, appearing as a subtle cigar-shaped object that has an apparent angular diameter similar to that of our Moon.
About a century ago, Edwin Hubble discovered that this so-called “spiral nebula” was located far beyond our Milky Way galaxy—approximately 2.5 million light-years away, which is about 25 times the diameter of our galaxy. Before this revelation, astronomers believed that the Milky Way contained the entirety of the universe. Hubble’s discovery revolutionized cosmology, expanding our understanding of the vast universe around us.
Now, a hundred years later, the Hubble Space Telescope, named in honor of Hubble, has conducted the most thorough survey of this fascinating star-filled region. It is uncovering new insights into the evolution of Andromeda, which appears substantially different from the history of the Milky Way.
Without Andromeda serving as a model for spiral galaxies throughout the universe, our knowledge of the structure and development of our own Milky Way would be significantly limited because we are located within it. It’s like trying to grasp the layout of New York City while standing in the center of Central Park.
“With Hubble, we can delve into immense detail about the entirety of the galaxy’s disk,” stated principal investigator Ben Williams from the University of Washington. Hubble’s exceptional imaging abilities can resolve over 200 million stars in the Andromeda galaxy, identifying only those stars that shine brighter than our Sun. They resemble grains of sand scattered across a beach, but that’s just the starting point. The total star count for Andromeda is estimated to be around 1 trillion, with many fainter stars below Hubble’s detection threshold.
Capturing images of Andromeda was a monumental endeavor since the galaxy occupies a much larger area in the sky compared to the more distant galaxies that Hubble typically observes, which are often billions of light-years away. The project was conducted through two Hubble programs and took over 1,000 Hubble orbits spanning more than ten years to complete.
The journey began with the Panchromatic Hubble Andromeda Treasury (PHAT) program around a decade ago. This initiative gathered images in near-ultraviolet, visible, and near-infrared wavelengths using the Advanced Camera for Surveys and the Wide Field Camera 3 onboard Hubble, focusing on the northern portion of Andromeda.
Following this, the Panchromatic Hubble Andromeda Southern Treasury (PHAST) was published in The Astrophysical Journal and led by Zhuo Chen from the University of Washington. This phase contributed images of about 100 million stars in the southern half of Andromeda, an area that is structurally unique and more sensitive to the galaxy’s merger history than the northern region explored by the PHAT survey.
The combined research efforts encompass the entire disk of Andromeda, which is observed nearly edge-on, tilted at 77 degrees from Earth’s perspective. The galaxy is so vast that the final mosaic was constructed from around 600 individual fields of view, amounting to an image with a minimum of 2.5 billion pixels.
These Hubble survey programs provide essential data regarding the ages, heavy-element content, and stellar masses within Andromeda, allowing astronomers to differentiate between various scenarios of Andromeda’s mergers with other galaxies. Hubble’s intricate measurements help refine models of Andromeda’s merger history and disk development.
A Galactic ‘Train Wreck’
Even though the Milky Way and Andromeda likely formed around the same time billions of years ago, observational data indicates their evolutionary paths have diverged significantly, despite their similar origins. Andromeda appears to have a greater number of younger stars and distinctive features like coherent streams of stars, suggesting a more vigorous recent star-formation and interaction history than the Milky Way.
“Andromeda is like a train wreck. It looks as if it has gone through some event that triggered a burst of star formation before slowing down,” remarked Daniel Weisz from the University of California, Berkeley. “This event was likely caused by a collision with another nearby galaxy.”
A potential offender could be the compact satellite galaxy Messier 32, which appears to be the stripped-down remnant of a former spiral galaxy that may have interacted with Andromeda in its past. Simulations indicate that when a close encounter occurs with another galaxy, the available interstellar gas may be depleted, resulting in a halt in star formation.
“Andromeda appears to be a transitioning type of galaxy, caught between being a star-forming spiral and a somewhat elliptical galaxy dominated by older red stars,” added Weisz. “We can observe a significant central bulge of older stars alongside a star-forming disk that doesn’t show as much activity as one might expect given the galaxy’s mass.”
“This meticulous exploration of resolved stars will be instrumental in piecing together the galaxy’s history of mergers and interactions,” echoed Williams.
Hubble’s latest discoveries will aid future investigations by NASA’s James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope. Essentially a wide-angle equivalent of Hubble (sharing the same mirror size), Roman is set to capture a hundred high-resolution equivalents of Hubble images in one shot. These findings will enhance and expand upon Hubble’s extensive dataset.
