From the Orion Arm
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From the Orion Arm

Our Address in the Universe

Image: ‘The Pale Blue Dot’ captured by Voyager 1— updated by NASA (Credits: NASA/JPL-Caltech)

This famous image was captured by the Voyager 1 spacecraft on February 14, 1990. See that tiny dot in the middle left?

That’s the Earth. A speck of sharp color amidst an empty space. Every single thing that has happened to us — or ever will be (at least, in the near future) — has taken place on this thing — a dot disturbing not even a complete pixel of its background. And yet, this image is still a wide exaggeration of this rock’s size, shot from our very neighborhood.

And just one out of the many million species on this lone rock, called Homo sapiens, calls itself to be advanced? Irony. The power of this image is such that it can shatter the worst of pride. Anyday, we can be thrust into desolation, and yet, nothing in the Universe will change. All our daily drivel is just too insignificant to be even called ‘negligible’ for the Universe.

But there is not just despondency in this image but also hope — such insignificance against the Universe’s face has not stopped humans from questioning. It is our curiosity that has let us out into the Universe. It is curiosity itself that has enabled us to capture this photograph. It is this quest to discover who we are that has prompted us to search the cosmos. Just see the sky on an evening, from a dark spot, and you would immediately feel the same curiosity flow through you that once characterized minds like Galileo and Kepler.

But the above image, shot from our neighborhood tells us little about our Universal address. Let’s span out more to discover where we are situated in the Universe:

Solar System and Beyond

Image: ‘Family Portrait’ by Voyager 1 showing 6 of the 8 planets in the Solar System in a series of photographs over about 5 months (Credits: NASA/JPL-Caltech)

The above picture(s) captured by Voyager 1 spacecraft shows 6 of the 8 planets of the Solar System. Unfortunately, Mercury and Mars could not make it to the family photograph, but still, this photograph remains the only such photograph till date. The inner planets comprise Mercury, Venus, our Earth, and Mars, surrounded by a not-so-dense-as-Star-Wars-shows-it ring of Asteroid Belt. The outer planets comprise Jupiter, Saturn, Uranus, and Neptune. (Each of these planets requires lengthy articles of their own and hence, won’t be much disturbed in this one. Keep an eye out for their articles very shortly.)

A rendition of Kuiper Belt objects
Image: An artistic rendition of the Kuiper Belt (from Wikimedia Commons)

Spanning out of the planetary orbits, we find a ring of icy rocks called the Kuiper Belt in the outer regions of the Solar System, home to trans-Neptunian bodies, including Pluto, its controversy–causing moon Charon, Xena, Sedna, the farthest Solar System object ever visited by a man-made object Arrokoth, et cetera.

An artistic rendition of the Oort Cloud
Image: An Artistic Rendition of the Oort Cloud (from Wikimedia Commons)

The solar system’s outer edges are formed by a region of icy rock formations that we don’t understand completely, known as the Oort Cloud. It consists of an inner Hills Cloud (inner Oort Cloud) and an outer Oort Cloud and is the source of a large number of comets that light up our night skies. A few hypotheses suggest it to be accretion material of the Sun pushed out due to the gravitational interactions of the outer planets. The end of the Oort Cloud marks the end of the Sun’s Hill Sphere (the region where gravitational effects of the Sun are strong enough to capture a body in orbit), marking the boundary of the Solar System and the start of interstellar space. (More on the Oort Cloud in a later article.)

Is that all that comprises the solar system? Yes, with occasional intrusions from the interstellar space. And the most confounding one of these was Oumuamua. Discovered in 2017, it resembles an asteroid and a comet. But is it an asteroid? No. A comet? Not quite. An extraterrestrial spaceship? Can’t say.

Interstellar Space

A Rendition of the Milyway and its arms.
Image: A Rendition of the Milyway and its arms.

This is the Milkyway, our galaxy. It comprises a large number of spiral arms centered around a singularity — a supermassive black hole, known as Sagittarius A*, recently photographed by the Event Horizon Telescope Collaboration. The Solar System rotates in one of its minor arms, called the Orion Arm (that gives this publication its name), situated between two major arms: the Sagittarius–Cygnus Arm, and the Perseus Arm. The Orion Arm also contains some of the most recognized night sky objects like Betelgeuse, Rigel, and Bellatrix.

Andromeda Galaxy — The Largest member of our Local Cluster of Galaxies
Image: Andromeda Galaxy — The Largest member of our Local Cluster of Galaxies

The galaxies do not move freely about the Universe. Bound by the force of gravity, they move together in galactic clusters. The Milkyway itself moves in a galaxy cluster called the Local Cluster (with its satellite galaxies that include the two Magellanic Clouds visible in the Southern sky), formed by a few other galaxies and their satellite galaxies, that include the largest member, Andromeda (seen in the image above) and the third-largest, Triangulum.

The Local Cluster itself is a part of a gravitationally bound galactic cluster known as the Virgo Supercluster that is itself found to be moving as a part of another galactic cluster called the Laniakea Supercluster, a relatively recent addition to our astronomical jargon. Research suggests that the Laniakea Supercluster is not gravitationally bound yet it does not disperse as the Universe expands, as it is thought to be held together by a gravitational anomaly, a focal point known as the ‘Great Attractor’.

A rendition of the Laniakea Supercluster
Image: A rendition of the Laniakea Supercluster by Nature. The red dot depicts the position of the Milkyway in the galactic supercluster. The yellow shade depicts the supercluster with its ill-defined boundaries. The white lines depict the concentration of galaxies in the supercluster. For more information on the Laniakea Supercluster, see this video by Nature (Credits: Nature video)

The Laniakea Supercluster is still not the farthest location yet. It is itself located in Pisces-Cetus Supercluster Complex, the first ‘galaxy filament’ to be discovered. These filaments are guided by large ‘arrays’ of dark matter called the ‘Cosmic Web’. The cosmic web is what dictates the location and formation of galaxies in the Universe.

This is what constitutes the observable universe, the spherical region, observing beyond the boundary of which (known as particle horizon) is physically impossible because the light from beyond is yet to reach the Solar System after the Big Bang. This is the Universe on its grandest scale — the edge of the universe as we know it — the limit on how much we can know (till date).

From this grandest scale of the cosmos, let us return back to our lonely rock in the Orion Arm of the Milkyway, the most specific field in our address block. Look again at the image at the start of this article.

“Look again at that dot. That’s here. That’s home. That’s us.

This is our origin —also of our advancements, breakthroughs, and discoveries — “a mote of dust suspended in a sunbeam”, inhabited by a curios species made of stardust. This is our ‘Pale Blue Dot’, our only abode in a valley of loneliness.

Quoted lines are from the writings of Carl Sagan. Images in this article are in Public Domain unless otherwise credited. Image credits are given alongside the image captions.

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We are a community of astronomy and physics enthusiasts spreading knowledge and igniting curiosity about humanity’s indeclinable asset — the night sky — and collaborating in efforts to preserve it.

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Agrim Arsh

Agrim Arsh

Physics Fanatic. Night Sky Enthusiast. Amateur Android Developer. High-Schooler

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