The Milky Way is just one of an infinite number of galaxies dotted throughout the expanding universe. To date, NASA’s Hubble Space Telescope has spotted 100 billion galaxies and the number is expected to rise. But the Milky Way holds a special place in the hearts of astronomers because it is the only place, as far as we can tell, where life has developed.
In the introduction to the Haynes Owner’s Workshop Manual for the Milky Way, astronomer Gemma Lavender outlined why the Milky Way is so dear to astronomers.
She wrote: “If you are lucky to live under a reasonably dark sky or get a chance to visit such a location, then you will see the beautiful shimmering spectre of the Milky Way arching overhead.
“It looks like a river of light but train a telescope on part of it and you will see that it is constructed from countless stars.
“Look even closer and you will see that among the stars are pockets of gas and dust, some colourful, others dark and hidden.”
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1. How big is the Milky Way galaxy? How many stars are in it?
From our vantage point on Earth, we cannot quite see the shape and size of the Milky Way.
But astronomers estimate the galaxy measures a mind-boggling 100,000 light-years across – a distance of 587,862,540,000,000,000 miles.
The Milky Way is a barred spiral galaxy and boasts four spindly arms coiling around a central supermassive black hole dubbed Sagittarius A* (read: a-star).
Miss Lavender said: “Our home galaxy is packed with around 200 billion stars, collaboratively tipping the scales at an impressive 1.5 trillion times the mass of our Sun and extending for up to 100,000 light-years in diameter with a thickness of some 1,000 light-years.
“Our first impression is that it’s home sweet home, but what would immediately become apparent could you zoo outwards is the Milky Way’s structure: a mixed-up pattern of haze and points of light warped into a winding spiral, with a blazing bright centre, stretched – or elongated – into the shape of a bar.”
2. How old is the Milky Way galaxy? When did it form?
Everything we can touch and see – including the things we cannot, like dark matter – can find its origin in the Big Bang about 13.8 billion years ago.
The Milky Way is no exception and the galaxy was kickstarted into existence many hundreds of millions of years after the Big Bang.
You will see the beautiful shimmering spectre of the Milky Way arching overhead
Gemma Lavender, author
Astronomers estimate the Milky Way is about 13.5 billion-years-old but it took a long time for the galaxy to form its current shape.
About 12 to 13 billion years ago, the proto-Milky Way was nothing more than a dwarf galaxy with a few million stars.
More than 10 billion years ago, the galaxy formed its thick disc – a long-dead region of space where new stars were born.
After that, less than 10 billion years ago, the thin disc was born and it holds about four-fifths of the visible matter in the galaxy.
During this time, the Milky Way cannibalised many smaller galaxies and added their masses to its own.
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3. Where is our solar system in the Milky Way galaxy?
The solar system, its eight planets and the Sun are located about 26,000 light-years from the galactic centre.
The system currently resides in the Orion arm – one of the galaxy’s two main arms – but it is always on the move.
The Sun is flying around the galactic centre at a speed of around 514,000mph (828,000km/h).
One lap around the galaxy lasts between 225 to 250 million years and our star has so far made about 20.4 laps around the Milky Way.
Miss Lavender said: “There could be something special about the Sun’s location in the galaxy.
“We are here after all; perhaps the chance that life can develop on a planet is all about location, location, location.
“It’s a concept known as the Galactic Habitable Zone and it can tell us a lot about the varying properties of the Milky Way’s disc.”
4. Will the Milky Way crash into the Andromeda galaxy?
Approximately four to five billion years from now, the Milky Way’s neighbouring Andromeda galaxy is expected to come crashing into us.
The resulting collision will create a new galaxy, sometimes referred to as the Milkomeda.
Miss Lavender said: “Our two galaxies will start ripping chunks out of each other, tidal streams trailing away their central bars destroyed and their spiral arms twisted this way and that.
“Perhaps there will be a second close encounter after the as they spiral towards each other, and then, in this deathly gravitational embrace, the two disrupted galaxies will ultimately spin back around and literally fall into each other’s arms, their stars and gas becoming mingled.
“Their molecular gas clouds will meet head-on and huge bursts of star formation will ignite.”
The Milky Way Owners’ Workshop Manual written by Gemma Lavender is available from www.haynes.com priced at £22.99