The Andromeda galaxy is a colossal marvel in our sky, hosting over 1 trillion stars.

Now, astronomers have used the Hubble Space Telescope to capture hundreds of detailed images of our vast galactic neighbor, creating the largest-ever photomosaic of the galaxy. It took over 10 years to create.

“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,” NASA, which operates Hubble with the European Space Agency, explained. “The full mosaic was carried out under two Hubble programs. In total, it required over 1,000 Hubble orbits, spanning more than a decade.”

The image below shows the complete mosaic. Beneath that is a selection of intriguing sights within the mosaic, labeled A through E:

• A: Star clusters in Andromeda, foreground stars in our Milky Way galaxy, and two background galaxies far beyond Andromeda (shown in yellowish-orange).

• B: A bright star cloud in Andromeda called NGC 206.

• C: A star-forming region in Andromeda with young blue stars.

• D: A satellite galaxy known as M32. It could be the leftover core of a galaxy that once collided with Andromeda, the European Space Agency explains.

• E: Dust lanes amid an enormous number of stars, which brilliantly light up the Andromeda galaxy.

Andromeda, at some 2.5 million light-years away, is relatively close in space terms (though that’s still an enormous distance; a light-year is nearly 6 trillion miles). This cosmic proximity allows us to understand our own spiral galaxy, on which we occupy a spot on one of the spiral arms.

“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,” NASA said. “That’s because we are embedded inside the Milky Way. This is like trying to understand the layout of New York City by standing in the middle of Central Park.”

Our galaxy, while not small, isn’t nearly as big as Andromeda. We harbor some 100 to 400 billion stars. One day, however, the two galaxies may collide, forming a giant, egg-shaped elliptical galaxy. But that great merger won’t happen for billions of years.