What is the Milky Way?

Discover our Galaxy and learn how exploration of the Milky Way has led to some big discoveries, both close to home and further afield.

Milky Way above the Chilean Atacama Desert. It is here, that ESO'c Very Large Telescope (VLT) is located

The Milky Way can be seen from Earth with the naked eye, but only under dark skies away from light pollution. This image was captured by John Colismo from Chile’s Atacama Desert. Credit: John Colosimo (colosimophotography.com)/ESO

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The Milky Way; our galactic home. A barred galaxy made up of stars, gas and dust bound together by gravity.

It formed around 13.6 billion years ago and contains approximately 100 billion stars.

At the centre lies a supermassive black hole around which the celestial ensemble revolves.

Just as Earth orbits the Sun, the Sun and our Solar System orbit the centre of the Milky Way, taking approximately 200-250 million years to complete a single rotation.

In terms of our location within the Milky Way, we reside in the suburbs, about 25,000 lightyears away from the centre in one of its spiral arms.

A Hubble Space Telescope image of a Barred spiral galaxy NGC 1300; similar to our Milky Way
A Hubble Space Telescope image of barred spiral galaxy NGC 1300, which may be similar to our Milky Way. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Despite living in galactic suburbia, we are no strangers to the bright lights of the city. On a dark night away from light pollution we can catch a glimpse of the bulge of the Milky Way with our own eyes.

This stellar sight right on our doorstep is our window into the Universe.

The milky band of light sprinkled across the night sky is what inspired our Galaxy’s very name, but this band is just a snippet of the Milky Way.

If you wanted to view the entire Milky Way with your own eyes (appropriate eye and whole-body protection aside), you would need to travel about 36,000 lightyears out from Earth.

Current technology is a long way off allowing someone to undergo such a monumental journey, but luckily we have a trick up our sleeve.

Operated by the European Space Agency (ESA), the Gaia mission seeks to survey the sky from orbit and produce the largest and most precise three-dimensional map of the Milky Way.

An artist's impression. Gaia spacecraft operated by the European Space Agency (ESA) seeks to map the Milky Way.
An artist’s impression. Gaia spacecraft operated by the European Space Agency (ESA) seeks to map the Milky Way. Credit: ESA/ATG medialab;background image:ESO/S.Brunier
ESA/ATG medialab;background image:ESO/S.Brunier

This mission is a collaboration between ESA’s Gaia spacecraft and the European Southern Observatory (ESO) Very Large Telescope (VLT) observations.

This partnership involves Gaia surveying the sky and the VLT Survey Telescope keeping a close eye on Gaia, so that reliable movement and distance measurements can be achieved.

Gaia's all-sky view of the Milky Way based on the measurements of almost 1.7 billion stars.
Gaia’s all-sky view of the Milky Way based on the measurements of almost 1.7 billion stars. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO
ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Since December 2013, Gaia has accurately measured over one billion stars, and it doesn’t look like stopping anytime soon.

Recent findings from the Gaia mission are believed to be evidence of a ‘stellar baby boom’ within our Galaxy.

When a galaxy is formed, the fusion of gas and dust into stars is anticipated to continue at a constant rate (more or less), until the gas is depleted.

However, according to data from the Gaia mission, around five billion years ago this expected trend began to reverse.

Within a gush of cosmic activity, star formation peaked between two and three billion years ago.

The star-forming region of NGC 3603. Situated in the Milky Way, this is one of the most impressive massive young star clusters in the Galaxy
The star-forming region of NGC 3603. Situated in the Milky Way, this is one of the most impressive massive young star clusters in the Galaxy. Credit: NASA, ESA, and the Hubble Heritage
NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

This stellar baby boom was no small feat: it is thought that over half the stars in the Milky Way formed during this period.

The Gaia mission is fundamentally important for our desire to understand the cosmos.

It not only helps us build a three-dimensional representation of our Galaxy but the data provided also helps to further our knowledge of the structure, origin and ultimately the evolution of the Milky Way.

However, we are not alone. Our Galaxy is only one of billions of others in the Universe.

Despite this vast amount, we are unlikely to be bothered by our galactic neighbours anytime soon. Down the road is the Andromeda Galaxy, our closest spiral-galaxy, around four billion lightyears away.

Despite the distance, on a very dark night in the northern hemisphere the Andromeda Galaxy is visible to the naked eye (under ideal conditions and with good eyesight!)

The sharpest ever view of part of the Andromeda Galaxy.
The sharpest ever view of part of the Andromeda Galaxy., captured using the Hubble Space Telescope. Credit: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler
NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler

Though keeping an eye on our closest neighbour may get easier over the next few billion years, as Andromeda is moving closer.

Travelling at an estimate rate of 400,000 km per hour, Andromeda is expected to collide with the Milky Way in about four billion years, although the outcome is difficult to predict.

Some closer galactic neighbours are the Large and Small Magellanic clouds, satellite galaxies of our own Milky Way.

Lying around 163,000 and 197,000 lightyears away respectively, the Large and Small Magellanic Clouds can be seen from the Southern hemisphere.

Part of the Large Magellanic Cloud (LMC),image showing the Tarantula Nebula's outskirts. A famously beautiful nebula located within the LMC.
Part of the Large Magellanic Cloud (LMC), showing the Tarantula Nebula’s outskirts. A famously beautiful nebula located within the LMC, as seen here by the Hubble Space Telescope. Credit: ESA/Hubble & NASA, Acknowledgement: Josh Barrington.

These galaxies are actually so close that the Milky Way absorbs gas and dust from the clouds.

Similar to the Andromeda Galaxy, the Large and Small Magellanic Clouds will eventually merge with the Milky Way.

The closer of the two Galaxies – the Large Magellanic Cloud – is predicted to collide with the Milky Way in 2.5 billion years.

Though the collision is expected to lead to a surge in star formation; like with the Andromeda collision, the exact outcome of such an impact is unknown.

What we do know is there is still so much to discover in our own cosmic neighbourhood.

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With missions like Gaia, astronomers hope to shed light on the mysteries of the Milky Way and to use this knowledge to aid our understanding of the cosmos, and the galaxies yet to be discovered.