Stunning photo mosaic of neighbouring Andromeda Galaxy captures the glow of 200 million stars

Astronomers have released the most detailed image of the Andromeda galaxy with new clues about its evolutionary history to celebrate the 100th anniversary of Edwin Hubble's discovery that our Milky Way galaxy is not alone in the universe.

The Hubble Space Telescope is famous, but the astronomer it is named after is less well known. Yet, he is responsible for discovering the vast size of the universe, and the fascinating fact that it is getting bigger by the minute.

In 1925, astronomers were still debating whether the Milky Way galaxy encompassed the entire universe. It is incredibly huge, 100,000 light years from one side to the other. That means if you could travel at the speed of light, it would take you 100,000 years to cross the galaxy once. Even to go from where we live to the centre would take about 26,000 years.

Hubble introduced a new, larger perspective when he used the largest telescope in the world at the time, the 2.5-metre Hooker telescope at the Mount Wilson Observatory in California, to precisely measure the distance to a fuzzy object in the constellation of Andromeda.

He identified a star called V1, a Cepheid variable, which predictably pulsates in brightness over time depending on its size. Astronomers use these stars as beacons like lighthouses to measure distances in space.

To Hubble's astonishment, he found this star, and others like it, were tremendously distant — and thus belonged to an entirely different galaxy separated from ours by a vast gulf of space.

What's more, this implied there were still other tremendously distant galaxies as well. The universe suddenly became much larger.

This has echoes of the measurements made more than 2,000 years ago by ancient Greek mathematician Eratosthenes, who used shadows cast by the sun at two different locations to make the first accurate measurement of the circumference of the Earth, revealing that our planet was much larger than the known world at the time.

Today, we know the Andromeda galaxy, one of an estimated 6 to 20 trillion galaxies in our universe, is about 2.5 million light years away.

While Andromeda, the closest galaxy to our Milky Way, is moving toward us, further measurements of other galaxies by Hubble and Belgian astronomer Georges Lemaître subsequently found that more distant galaxies appear to be moving away from us. And the farther away they are, the faster they are moving. In other words, the universe is expanding. 

To honour the centenary of that discovery, the space telescope that bears his name was used to create a composite image of Andromeda revealing 200 million individual stars — still just a fraction of the trillion stars astronomers estimate it contains — something that was impossible in Hubble's time.

The image took the Hubble telescope over 1,000 orbits around Earth and combines about 600 separate fields of view. It is so detailed that astronomers can use it to piece together Andromeda's history and past mergers with other galaxies. 

Astronomers also took closeup images of V1, the star that Hubble saw, showing it at its maximum and minimum brightness.

Since its launch in 1990, the Hubble Space Telescope, which is about the same size as the Hooker telescope that Hubble used, has allowed scientists to precisely measure the age of the universe at 13.8 billion years old. 

In 1998, two astronomy teams used Hubble data to make the surprising discovery that the expansion of the universe is speeding up, leading to the proposal that a mysterious force dubbed dark energy is driving expansion.

In recent years, more precise measurements of distant objects in the universe — such as supernova explosions, red giant stars and Cepheid variable stars — suggest our universe is expanding faster than theoretical predictions, leading to a major contemporary problem for astronomy that could alter the age of the universe, dubbed "Hubble tension." 

Hubble's astounding revelations introduced us to a dynamic and gigantic universe full of galaxies, and to the realization that we are much smaller in the grand scheme of things than we thought.

Edwin Hubble would be amazed at the detail in the new image of Andromeda. And while we have learned a lot about the universe since his time, there is still much we don't know, such as the source of dark energy, the nature of invisible dark matter that surrounds entire galaxies, how the universe began and whether there was anything before that.

However, there is some comfort in the knowledge that even though Hubble showed that we are a tiny, insignificant speck in an unimaginably huge universe, we nonetheless figured all of that out. It shows us that when it comes to the human imagination, there is no limit to how large it can be.