Andromeda galaxy survey unveiled

On a clear autumn night, the farthest object visible to the naked eye can be seen without a telescope or binoculars. Located just northeast of the Great Square of Pegasus, it appears as a spindle-shaped haze with a bright center. This object is the Andromeda galaxy, the nearest galaxy to the Milky Way. The faint light from Andromeda left the galaxy 2.5 million years ago, journeying across intergalactic space to Earth. At that time, Homo habilis, one of the earliest known humans, appeared, known as "handy man" for creating the first stone tools.

Here are some fascinating quick facts about the Andromeda Galaxy:

1. Nearest Major Galaxy: At about 2.5 million light-years away, Andromeda (M31) is the closest spiral galaxy to the Milky Way.

2. Massive Size: It spans roughly 220,000 light-years, making it larger than the Milky Way, though its mass is comparable due to differences in density.

3. Future Collision: Andromeda and the Milky Way are on a collision course. In about 4.5 billion years, they will merge to form a new galaxy, often called "Milkomeda."

4. Home to a Trillion Stars: Andromeda contains an estimated 1 trillion stars—more than twice as many as the Milky Way.

5. Bright Companion: Visible to the naked eye from dark skies, it's one of the farthest objects you can see without a telescope.

6. Rich in Satellites: Andromeda has over 35 known satellite galaxies, including M32 and M110, which are easily observable with small telescopes.

7. Black Hole at Its Core: The galaxy has a supermassive black hole at its center, estimated to be 100-200 million times the mass of our Sun.

8. High Star Formation Rate: Andromeda's disk is rich in star-forming regions, making it a hotbed for new stellar activity.

9. Historical Importance: It was the first galaxy proven to lie beyond the Milky Way, thanks to Edwin Hubble's 1920s observations, which revolutionized our understanding of the universe.

10. Part of the Local Group: Andromeda, the Milky Way, and the Triangulum Galaxy (M33) dominate a group of about 80 galaxies known as the Local Group.

11. Fast Speed Toward Us: It's moving toward the Milky Way at a speed of about 110 kilometers per second (68 miles per second).

12. Rich Stellar Population: Andromeda contains stars of all ages, from young, hot blue stars to ancient red giants.

13. Halo of Mystery: Its halo is immense, stretching over a million light-years, and contains clues about the galaxy's past interactions and mergers.

NASA's Hubble traces hidden Andromeda galaxy history: Hubble M31 PHAT+PHAST Mosaic

Image credit: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern)

Fast forward to the early 21st century, when NASA's Hubble Space Telescope spent over a decade creating a detailed photomosaic portrait of Andromeda, capturing over 600 images. The reason for this monumental task is that the galaxy, though close, spans six times the apparent diameter of the full Moon in angular size, requiring Hubble's precise view to cover such a vast celestial area. The resulting mosaic reveals the glow of 200 million stars, a fraction of Andromeda's total population, composed into at least 2.5 billion pixels. Hubble's high-resolution images provide astronomers with new insights into the galaxy's history, including past mergers with smaller satellite galaxies.

Compass and Scale Image of M31 PHAT PHAST Mosaic

Image credit: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern)

Hubble's Andromeda galaxy survey sheds light on Andromeda's evolutionary history

Since the launch of Hubble, astronomers have identified over 1 trillion galaxies in the universe, but only one, Andromeda, stands out as the most significant nearby galaxy to the Milky Way. Visible to the naked eye on a clear autumn night, Andromeda appears as a faint cigar-shaped object roughly the size of the Moon.

A century ago, Edwin Hubble established that Andromeda was not part of the Milky Way but was instead located 2.5 million light-years away. This discovery expanded the scope of the universe, overturning previous beliefs that the Milky Way encompassed all of space.

Now, a century later, the Hubble Space Telescope has completed the most extensive survey of Andromeda, yielding new clues to its evolutionary history, which is distinct from the Milky Way's. Studying Andromeda helps astronomers understand the structure and evolution of spiral galaxies, providing a unique perspective unattainable from within the Milky Way.

"With Hubble, we can achieve enormous detail across the entire disk of the galaxy," said Ben Williams, principal investigator from the University of Washington. Hubble's imaging can resolve over 200 million stars in Andromeda, focusing on those brighter than our Sun. However, these are just a fraction of Andromeda's estimated 1 trillion stars, many of which are too faint to be detected by Hubble.

The challenge of photographing Andromeda stemmed from its enormous size in the sky. The full mosaic, assembled from approximately 600 separate images, was completed under two major Hubble programs. The effort required over 1,000 orbits of Hubble over more than a decade.

The Panchromatic Hubble Andromeda Treasury (PHAT) program, which began about a decade ago, captured images in near-ultraviolet, visible, and near-infrared wavelengths, focusing on the northern half of Andromeda. This was followed by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), which added data on 100 million stars from the southern half of the galaxy. This region is crucial for understanding Andromeda’s merger history, which differs from that of the northern disk.

The completed mosaic, covering the entire disk of Andromeda, reveals the galaxy as seen almost edge-on, tilted 77 degrees from Earth's view. The mosaic’s 2.5 billion pixels provide essential data on the galaxy's stellar age, chemical composition, and mass, offering insights into Andromeda's complex merger history.

Andromeda M31 PHAST mosaic video

Andromeda M31 PHAST mosaic video

A Galactic 'Train Wreck'

Though the Milky Way and Andromeda likely formed around the same time, their evolutionary histories differ. Andromeda appears more populated with younger stars and exhibits unusual features such as coherent streams of stars, suggesting a more active recent history of star formation and galaxy interactions.

"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 from the University of California, Berkeley. This event may have been a collision with a nearby galaxy, such as Messier 32, which may have interacted with Andromeda in the past. Computer simulations suggest that such an encounter can exhaust interstellar gas, halting further star formation.

"Andromeda looks like a transitional galaxy, caught between being a star-forming spiral and an elliptical galaxy dominated by aging red stars," Weisz added. "It has a large central bulge of older stars and a star-forming disk that is not as active as expected for its mass."

Hubble’s detailed observations will help researchers further unravel Andromeda’s past mergers and interactions. These findings will also support future observations from NASA's James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope, which will extend Hubble’s data.

NASA’s Hubble Space Telescope, which has been operational for over three decades, continues to shape our understanding of the universe. The telescope is a joint project between NASA and the European Space Agency (ESA), with operations managed by NASA’s Goddard Space Flight Center, and Lockheed Martin Space in support. The Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, conducts the science operations.

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