The confirmation by astronomer Edwin Hubble that the giant spiral of stars known as Andromeda was a separate galaxy well beyond our own Milky Way shed light for the first time on the unimaginable scale of the universe.

The confirmation by astronomer Edwin Hubble that the giant spiral of stars known as Andromeda was a separate galaxy well beyond our own Milky Way shed light for the first time on the unimaginable scale of the universe.

When Galileo Galilei (1564–1642) looked through his telescope at the Milky Way – a broad, fuzzy band stretching across the night sky – he could see it was made of countless distant stars.

We live near the edge; the fuzzy band across the sky is a result of looking towards the centre where there is a greater density of distant stars.

There are also smaller fuzzy patches distributed throughout the sky.

Astronomers initially called them ‘nebulae’ (nebulas) from the Latin word for cloud.

Some are visible to the naked eye, but many more can be seen through a telescope. Many are clouds of gas and dust inside our galaxy from which new stars form – but not all.

In 1781, French astronomer Charles Messier (1730–1817) published a catalogue of 103 nebulae.

Throughout the next century astronomers wondered what they might be.

The most puzzling was Andromeda, also known as ‘Messier 31’ (M31).

It had been observed for hundreds of years – it is just bright enough to see with the naked eye.

The first photographs of M31, taken in 1887, revealed it as a spiral.

In 1885, astronomers saw what they thought was a bright ‘new star’ appear in M31.

Stars like this had been seen in our own galaxy – they are called novae (novas), and they are actually old stars flaring up near the ends of their lives.

The nova in M31 appeared as bright as those they had seen before, so most astronomers concluded M31 must be part of the Milky Way.

But others were convinced M31 was a separate star system outside our galaxy, which would make the nova much further away and especially bright.

The puzzle would be solved if someone could work out the size of our galaxy and the distance to M31.

In 1912, American astronomer Henrietta Leavitt (1868–1921) discovered a way of working out distances to one kind of star known as a ‘Cepheid variable’.

These stars ‘pulse’, regularly expanding and contracting over periods of a few days to a few weeks.

Leavitt discovered a link between the duration of each pulse and a star’s inherent brightness.

Its distance could then be calculated from the relation between how bright that star appears on Earth and how bright we know it to be from the length of its pulse.

In 1918, another American astronomer, Harlow Shapley (1885-1972), used Leavitt’s method to estimate the size of our galaxy – it was larger than anyone had imagined.

Edwin Hubble (1889–1953), another American, finally settled the Andromeda question.

In 1924 he was using what was then the world’s largest telescope – at Mount Wilson observatory in California – to examine M31.

Hubble saw individual stars in its spiral arms and identified Cepheid variables.

He estimated their distance away, and the results showed M31 to be about two million light years away (11,756,964,328,322,000,000 miles), well beyond the limits of our galaxy, and thus proved Andromeda is a huge, separate star system like our own galaxy.

We now know there are hundreds of billions of galaxies in the universe. Each contains billions of stars.