Most Detailed Image from NASA’s Hubble Yet of Andromeda Galaxy

The largest photomosaic of the Andromeda galaxy, created from observations by the NASA/ESA Hubble Space Telescope, reveals hundreds of millions of stars. This mosaic, the largest and most intricate of its type, was constructed from over 600 individual images and contains an astonishing 2.5 billion pixels of cosmic information. It offers a unique view into the galaxy’s complex structure, its evolving history, and the various processes that have influenced its development over billions of years.

Hubble’s Photomosaic Achievement

Since the deployment of NASA’s Hubble Space Telescope, astronomers have estimated that the universe contains over one trillion galaxies. Among these, the Andromeda galaxy (Messier 31) holds particular significance as the most prominent neighboring galaxy to our Milky Way. On a clear autumn evening, it is visible to the naked eye as a faint, elongated object with an angular size comparable to the diameter of the Moon.

Andromeda’s Historical Significance

Around a hundred years ago, Edwin Hubble made a groundbreaking discovery, revealing that the “spiral nebula” was not within our Milky Way galaxy as previously believed but rather located about 2.5 million light-years away—a distance equivalent to roughly 25 times the diameter of the Milky Way.

Before this revelation, astronomers assumed that the Milky Way constituted the entirety of the universe. Hubble’s groundbreaking discovery revolutionized cosmology, revealing a universe vastly greater than previously imagined.

Now, a century later, the Hubble Space Telescope, named after Edwin Hubble, has performed the most extensive examination of this fascinating star system. Its observations are providing fresh insights into Andromeda’s evolutionary past, which appears to differ significantly from the historical development of the Milky Way.

Without Andromeda serving as a representative example of spiral galaxies in the broader universe, astronomers would have a significantly limited understanding of the structure and evolution of our own Milky Way. This limitation arises from our position within the Milky Way itself. It is like trying to grasp the layout of New York City while standing in the center of Central Park.

Ben Williams, principal investigator of the University of Washington said, “With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,”. Hubble’s exceptional imaging precision allows it to identify over 200 million stars in the Andromeda galaxy, focusing exclusively on those that outshine our Sun.

These stars resemble tiny grains of sand scattered across a vast beach. However, this observation barely scratches the surface. Andromeda is believed to harbor approximately 1 trillion stars in total, many of which are smaller and dimmer than what Hubble can detect.

Stellar Population Insights

Capturing images of Andromeda was an immense challenge, primarily because the galaxy appears significantly larger in the sky compared to the distant galaxies typically studied by Hubble, which are often located billions of light-years away. Creating the complete mosaic required contributions from two separate Hubble observation programs and involved over 1,000 orbits of the space telescope spread across more than ten years.

This sweeping view began approximately a decade ago with the Panchromatic Hubble Andromeda Treasury (PHAT) project. Observations were conducted in near-ultraviolet, visible, and near-infrared wavelengths using Hubble’s Advanced Camera for Surveys and Wide Field Camera 3 to capture detailed images of Andromeda’s northern half.

This program was followed up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), recently published in The Astrophysical Journal and led by Zhuo Chen at the University of Washington, which added images of approximately 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 observational programs comprehensively cover the entire disk of the Andromeda galaxy, which is viewed nearly edge-on, at a tilt of 77 degrees from Earth’s perspective. Due to the galaxy’s vast size, the mosaic image is constructed from around 600 individual fields of view. This mosaic consists of at least 2.5 billion pixels.

The additional Hubble survey programs offer insights into the age, abundance of heavy elements, and stellar masses within Andromeda. This information will enable astronomers to differentiate between various scenarios regarding Andromeda’s merger with one or more galaxies. Hubble’s precise measurements help refine models concerning the galaxy’s merger history and the evolution of its disk.

A Galactic ‘Train Wreck’

The Milky Way and Andromeda galaxies are believed to have formed around the same time, billions of years ago. However, observations have revealed that they have undergone significantly different evolutionary processes, despite being located in the same cosmic region. Notably, Andromeda appears to have a higher population of younger stars and exhibits unusual features, such as coherent streams of stars.

According to researchers, this suggests that Andromeda has experienced more recent and intense star formation, as well as interactions with other galaxies, compared to the Milky Way.

“Andromeda’s a train wreck. It looks like it has been through some kind of event that caused it to form a lot of stars and then just shut down,” said Daniel Weisz at the University of California, Berkeley. “This was probably due to a collision with another galaxy in the neighborhood.”

A potential suspect is the compact satellite galaxy Messier 32, which bears resemblance to the stripped core of a spiral galaxy that may have collided with Andromeda in the distant past. Computational models indicate that when a galaxy’s interstellar gas is depleted due to a close encounter with another galaxy, star formation gradually diminishes.

“Andromeda looks like a transitional type of galaxy that’s between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars,” said Weisz. “We can tell it’s got this big central bulge of older stars and a star-forming disk that’s not as active as you might expect given the galaxy’s mass.”

“This detailed look at the resolved stars will help us to piece together the galaxy’s past merger and interaction history,” added Williams.

A New Way for Future Research

The latest discoveries from Hubble will pave the way for future observations by NASA’s James Webb Space Telescope and the forthcoming Nancy Grace Roman Space Telescope. As a wide-field counterpart to Hubble, boasting the same mirror size, Roman will be capable of capturing an enormous field of view, equivalent to at least 100 high-resolution Hubble images, in a single exposure. These observations will seamlessly complement and expand upon Hubble’s vast and extensive dataset.

For over 30 years, the Hubble Space Telescope has been a cornerstone of space exploration, yielding revolutionary discoveries that have transformed our understanding of the universe. A collaborative effort between NASA and the European Space Agency (ESA), Hubble’s operations are overseen by NASA’s Goddard Space Flight Center in Maryland, with additional support from Lockheed Martin Space in Denver.  The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, plays a crucial role in conducting Hubble’s scientific operations on behalf of NASA.