We all live in a Galaxy – from the Greek γᾰ́λᾰ (“gala”), which means “milk” – the Milky Way.
We live on a tiny rocky planet — Earth — orbiting the center of mass of her host planetary system — the Solar System — every about 365 terrestrial days: what we call a terrestrial year.
The single star of our Solar System is the Sun, a middle-aged ordinary galactic star.
Our star the Sun is just one of the hundreds billions stars of our galaxy, the Milky Way.
When we have the chance to stare at the night sky away from big cities, after a few minutes, in addition to a myriad of stars scattered throughout the sky, our human eyes are able to capture the light of a large clear stripe — almost milk-colored — that crosses the whole sky: it is the Milky Way.
That milky-coloured stripe — the Milky Way — is the portion of our Galaxy that we can see from Earth. Our Galaxy is a cosmic structure made up of hundreds of billions of stars, and immense clouds of dust and interstellar gas; hundreds of billions of stars, huge clouds of gas and interstellar dust, globally arranged in a huge spiral structure, a hundred thousand light-years wide.
(How big is a galaxy a hundred thousand light-years wide? In comparison, the Earth-Sun distance is about 8 light minutes – or 150,000,000 kilometers … and the entire Earth’s diameter is about 13,000 km. So our Galaxy is almost a hundred thousand billion times larger than planet Earth. The Galaxy is our great Cosmic Home.)
Living on planet Earth, we cannot see our entire Galaxy – precisely because we live in it! But we can study its global structure and compare it to other galaxies with similar properties, and get an idea of what it would look like from the outside; something like this:
The Milky Way is a spiral galaxy of moderate mass, and we all live in one of its outer spiral arms at about twenty-seven thousand light years from the Galactic Center(*).
Together with the Sun and the other planets and bodies of the Solar System, we travel around the Milky Way’s center at 200 km/s, orbiting every about 200-220 million terrestrial years.
This means that when dinosaurs lived on Earth, Earth was roughly on the other side of the Galaxy — dinosaurs lived on Earth about half a galactic year ago.
Our Galaxy is a solitary one. Our closest neighbor — about 2.5 million light years from Earth — is Andromeda, a spiral galaxy very similar to the Milky Way.
The Milky Way — a galaxy of hundreds of billions of stars like our star Sun — travels in spacetime at more than 500 kilometers per second in the direction of the Virgo Cluster of galaxies, a (gravitationally-bound)family of thousands of galaxies, about fifty million light years from Earth.
Sometimes I forget that the planet that hosts us — the Earth — travels together with their host star — the Sun — around the center of their Galaxy — the Milky Way — and together with the entire Milky Way, the Earth and the Sun, you and I travel in intergalactic space towards the Virgo Cluster at more than 500 kilometers per second, every second.
Our cosmic address is: planet Earth, Solar System, Milky Way galaxy, Local Universe.
Again: we all live on a tiny rocky planet — planet Earth — host of the planetary system of an ordinary middle-aged star — the star Sun — an ordinary star among hundreds of billions of stars of an ordinary spiral galaxy, the spiral galaxy we call the Milky Way — but sometimes we forget about this.
Our little cosmic home: the Earth, a planet that rotates on itself in a time that we call terrestrial day, and that completes a revolution around its reference star — the Sun — in a time of about 365 terrestrial rotations, time which we call the terrestrial year.
The Sun: a star that rotates on itself in a time of about twenty-eight terrestrial days, and which together with its planetary system — the Solar System — makes a complete revolution around the center of its host Galaxy – the Milky Way – – in a time of about 220 million terrestrial years.
The Milky Way: the spiral galaxy that hosts us all, all of us in the Solar System and all the planetary systems of its other two hundred billion stars; the spiral galaxy that moves in the direction of the Virgo Cluster of galaxies with a speed of over 500 kilometers per second.
The Virgo Cluster of galaxies, gravitational union of a thousand interacting, spiral, and elliptical galaxies, all immersed in a very hot plasma that shines bright in X-rays; a cluster of galaxies whose center is approximately 50 million light years from our Galaxy — a cluster of galaxies that is our ultimate spacetime destiny.
Ours is a beautiful cosmic journey.
Galaxies can be thought of as the building blocks of the Universe.
No galaxy is identical to another: each is unique with its history of star formation, its central black hole activity, its halo of dark matter, its history of merging with other galaxies, and many other peculiar characteristics that make every galaxy in the Universe worthy of study. There is no boring galaxy out there.
Here are some optical images of spiral, elliptical, and interacting galaxies of our Local Universe. All the pictures have been taken from APOD, the ESO gallery, or the HubbleSite. Larger version of the images can be found in this Gallery.
(*) Oh, I almost forgot: at the center of the Milky Way galaxy there is an extremely compact cosmic object. A mass equivalent to three million times the mass of the Sun, is contained in a volume of a few cubic light-hours: it is a supermassive black hole. The life of our Galaxy and its central supermassive black hole appear to be deeply connected, and so it may be for many, or all, of the galaxies in our Universe.
Geometric collections that shine light from hundreds of thousands of millions of stars, and supermassive dark objects which evolve and travel together in intergalactic space …