Many of us tend to ponder about the universe when looking at the night sky on a beautiful evening. We all know the type of questions that emerge during these occasions. Interestingly enough, even seemingly straightforward ones like 'why is the night sky black?' and 'how large is the universe?' can sparkle a deep conversation. These questions come naturally to us humans, and we have probably contemplated them since ancient history.
The most commonly heard questions often relate to distance: 'How far away is the sun?' 'how far away is the moon?' how far away is that star? Etc. It is challenging to convey these types of astronomical distances as they compare to nothing else we are used to on a human scale.
In this article, we will attempt to put these vast distances into perspective and provide you with some entertaining insights along the way.
By Stef Verhagen
The distance to the Moon in perspective
Perhaps the distance to our Moon is easiest to describe; after all, it is the closest celestial body to Earth. The actual distance between Earth and the Moon varies throughout the Moon's orbit, but the Moon's average distance is approximately 384,402 kilometers.
A distance of 380,000 kilometers is on the edge of being conceivable on a human scale. If you own a car, have a look at your odometer for some perspective. We estimate that it would take the average person about 10 to 20 years to achieve a 'mileage' comparable to the Moon and Earth's average distance.
Quite some way to reach the Moon, one would think, but let's immediately put this in perspective by comparing it to the distance to Mars when it makes its closest approach to Earth at an approximate distance of 54,000,000 kilometers (142 times the distance of the moon).
Dr. James O'Dongoghue, a familiar guest on our Twitter feed, has put this neatly into context in the image below. Keep in mind that the distances on display here are to scale, but the planetary bodies have been enlarged 20 times (otherwise, they wouldn't be visible)
Looking at this 'best-case scenario' image, it immediately becomes clear why going to mars is a tad more challenging than visiting the Moon. In fact, the distance to Mars is so vast that communication works with a delay of several minutes, while the lag between Earth and the Moon is expressed in mere seconds, not minutes.
The distance to the Sun in perspective
Let's take another metaphorical step back and have a look at the distance between the Sun and Earth. On average, you will have to travel approximately 149,597,870 kilometers to reach the Sun. At these distances, we start encountering numbers that are hard to comprehend on their own accord.
Conveniently, astronomers defined a unit of length specifically usable at this scale, called the Astronomical Unit or AU. One AU is 149,597,870 kilometers which is roughly equal to the distance from Earth to the Sun. Described in this unit of length, the distance to Mars is 0.37 AU at the closest approach.
The astronomical unit is particularly serviceable in describing the distance to objects outside the asteroid belt. Saturn, for example, is 1,201,336,738 kilometers away from Earth at its closest approach, which is 8.03 AU. Pluto, a more distant dwarf planet, is over 4 billion kilometers away at its closest approach, more neatly defined as 28.68 AU. 4 billion kilometers sounds a lot more abstract than a more comprehensible 28 times the distance of the Sun.
You could also use another unit of length to articulate the Sun's distance, namely the light-second. Light travels at a speed of 299,792,458 meters per second through a vacuum (roughly 300,000 km/s or 186,000 mi/s). As such, light originating from the Sun takes about 499 seconds to reach our planet. Stated differently, the distance to the Sun is 499 light-seconds.
Interesting Fact: Although the 'speed of gravity' is assumed to be infinite in Newtonian physics, it is actually the same as the speed for light. Meaning that if the Sun were to disappear suddenly, we would only notice it on Earth after about 8 minutes.
The distance to the nearest stars in perspective
Let's zoom out further to a tiny star called Proxima Centauri, our closest (known) neighbor star. Although it is located relatively nearby, it has only been discovered quite recently (1915) due to its small size.
Proxima Centauri is part of a so-called 'triple star system' called Alpha Centauri. It is located at an approximate distance of 40,208,000,000,000 kilometers (40.2 trillion km or 268,770 AU). A distance so vast that even using Astronomical Units to get perspective doesn't make a whole lot of sense anymore.
At this scale, the most logical unit of length to use to describe distance would be the light-year. A light-year is defined as the distance light travels in a vacuum in one year, which is 9.46 trillion kilometers. Proxima Centauri is about 4.2 light-years away from the Sun.
One additional unit of length that makes sense at these distances is the Parsec. The Parsec was famously misused in Star Wars: A New Hope, where Han Solo claimed that his ship made the Kessel Run in less than 12 parsecs, mistaking the Parsec as a unit of time, not a unit of distance. A parsec is about 3.26 light-years or 206,000 AU. Proxima Centauri is located roughly 1.3 Parsec away from the Sun.
The main reason why Parsec, a word derived from the parallax of one arcsecond, exists is to make calculations of astronomical distances from raw observational data more convenient for astronomers. However, it can also be helpful to put large distances in perspective. You could think of parsecs as distances between our Sun and the nearest neighboring star, given their close similarity.
The distance to the nearest galaxies in perspective
Jumping from our stellar neighborhood to our galactic neighborhood is a colossal leap. The closest galaxy to us is the Canis Major Dwarf Galaxy, at 236,000,000,000,000,000 kilometers (25,000 light-years) from the Sun. The supposed small galaxy contains a relatively high percentage of red giants and is thought to have an estimated one billion stars overall. Some researchers dispute the galaxy claim that it is actually part of our own Milky Way Galaxy.
A distance of 1,000 parsecs (3,262 ly) is denoted by the kiloparsec. Astronomers generally use kiloparsecs to express space between parts of a galaxy or within groups of galaxies. It makes more considerable distances manageable. Using this unit of length, the Canis Major Dwarf Galaxy 'stands' at a distance of 7.7 kiloparsecs. Some perspective: our Milky Way has an estimated (visible) diameter of 150,000 light-years or 46 kiloparsecs. These figures make it understandable why it might be hard to distinguish the Canis Majord Dwarf Galaxy as a separate entity.
Let's skip ahead to the largest galaxy in our local group, the Andromeda Galaxy, a gorgeous barred spiral galaxy containing an estimated 1 trillion stars (2x to 10x as many as the Milky Way). It is located approximately 2,500,000 light-years or 770 kiloparsecs from Earth.
Interesting Fact: The Andromeda Galaxy is approaching the Milky-Way Galaxy on a collision course with a mind-bending speed of 396,000 kilometers per hour, as indicated by blueshift measurements. At this speed, the galaxies are predicted to clash in about 4.5 billion years.
Let's have a look at an overview of our galactic neighborhood akin to that of our stellar neighborhood earlier. Our galactic neighborhood consists of the so-called 'Local Group' and has a diameter of roughly 10,000,000 light-years (or 3,000 kiloparsecs).
The 'Local Group' consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites construct one lobe while the aforementioned Andromeda Galaxy and its satellites compose the other.
At this scale, even the kiloparsec becomes a bit messy to use. That is why astronomers commonly express the distances between neighboring galaxies and galaxy clusters in megaparsecs (Mpc). A megaparsec is one million parsecs or approximately 3,260,000 light-years. Using this unit of length, the diameter of the Local Group is about 3 Mpc.
The scale of galaxy superclusters in perspective
Let's zoom out even further and have a look at galaxy superclusters. A galaxy supercluster is a gigantic structure that commonly contains thousands of galaxies. (Each containing billions to trillions of stars).
Our local group is part of the Virgo Supercluster, a galactic supercluster that is estimated to contain more than 47,000 galaxies! This is not where it ends though, in 2014, astronomers determined that the Virgo Supercluster is actually a component of an even larger supercluster called Laniakea.
Laniakea is Hawaiian for open skies or immense heaven a befitting name for this stupendously large structure. The Laniakea supercluster is estimated to contain 100,000 to 150,000 galaxies. Research indicates that the Laniakea Supercluster is not gravitationally connected; it will probably scatter rather than continue to sustain itself.
With an estimated diameter of 500,000,000 light-years or 153 megaparsecs, it is almost impossible to comprehend—Nonetheless, let's attempt to draw some form of perspective. If we were to discover intelligent life in the outer reaches of our own supercluster, an attempt to connect them would take millions of years (at least with contemporary communication methods, bound by the laws of physics). When our signal would finally reach its destination, we ourselves might not even exist anymore.
Naturally, 'lightspeed lag' works both ways. For example, light from our solar system would take nearly 70 million years to reach Galaxy NGC 2525, another stunning barred spiral galaxy located in the constellation Puppis. If intelligent life exists in NGC 2525, it would consequently see Earth as it was 70 million years ago. There would be no sign of human life; instead, they would observe the latter stages of the cretaceous epoch with dinosaurs still roaming the planet.
Cosmic supermassive structures
Although the Laniakea supercluster is an exceptionally enormous structure, even larger 'cosmic supermassive structures' have been identified largest, among which is the so-called Hercules–Corona Borealis Great Wall. Discovered in 2013, it is (currently) the largest known structure in the observable universe.
The wall has an estimated mean size of 10 billion (10,000,000,000 light-years). If we're honest, at this scale, it is no longer possible to offer a meaningful perspective other than that the gigaparsec is often used by astronomers to convey these mind-melting distances. One gigaparsec (Gpc) is 1 billion parsecs / 3.26 billion light-years. Expressed in this unit of length, the wall's mean size is about 3 Gpc. The Hercules-Corona Borealis Great Wall is so vast that it covers one 5th of the distance to the horizon of the observable universe.
The observable universe
Taking a metaphorical step back once more and we have finally reached the limits of what we can see. The observable universe is a globular section of the universe containing all matter that can be observed from Earth for this moment. This region is defined by the speed of light and the time light has had to reach us since the big bang.
The observable universe has a diameter of 93,000,000,000 light-years or 28,5 gigaparsecs. Most recent assessments based on data from NASA's interplanetary space probe new horizons predict that the total number of galaxies in the visible universe reaches several hundreds of billions.
Due to unknown reasons (commonly referred to as dark energy), the universe is expanding at an increasing rate. In due course, all presently observable objects will seem to freeze in time and ultimately disappear from our horizon. That is, of course, unless the properties of dark energy will change over time.
The un-observable universe?
Is there something outside our observable universe? There is no good scientific reason to think that the universe ends at our visible horizon. The size of the entire universe is uncharted, and it might be infinite in extent; according to many of his astrophysicists, the universe might, in fact, be infinite...
Sources and suggested further reading:
The speed for gravity (New Journal of Physics)
The Cosmic Distance Scale (NASA)
The M31 Velocity Vector. I. Hubble Space Telescope Proper-motion Measurements (The Astrophysical Journal)
Galaxy NGC 2525 (ESA Hubble)
New Horizons Observations of the Cosmic Optical Background (The Astrophysical Journal)
An Introduction to Modern Cosmology (Wiley via Google Books)
The richest superclusters (Astronomy and Astrophysics, Journal)
Nearest Superclusters (NASA)
Metric Paper & Everything in the Universe (CGP Grey)
Observable universe (Wikipedia)
Virgo Supercluster (Astronomy.com)
The Nearest Neighbor Star (NASA)