How Big Is Our Galaxy?

Ever wondered just how vast our Milky  Way is? To grasp its immense size, we first need to understand the  staggering distances between the planets in our own cosmic backyard, the  solar system. This is just the beginning; beyond lies a scale so vast, it dwarfs  everything within our solar neighborhood.

Our first stop is surprisingly close, yet  profoundly significant: our very own Moon. Positioned a mere 384,400 kilometers from  Earth, this distance may seem vast to us, yet it's but a tiny step on the cosmic scale.  To put this into perspective: imagine boarding an airplane cruising at a steady speed of 1,000  kilometers per hour.

Embarking on a direct flight to the Moon, it would take approximately  16 days of continuous travel to reach the lunar surface. In terms of communication, light,  which zips through space at unimaginable speeds, takes about 1. 28 seconds to bridge this gap  between us and the Moon.

This results in a slight but noticeable delay in conversations  of about 2. 5 seconds for a message to travel back and forth, enough to disrupt the flow of a  chat, making timely communication a bit tricky. But this delay becomes even more  relevant as we shift our gaze further, setting our sights on Mars.

The red planet, at  its closest approach to Earth, stands about 56 million kilometers away. That same airplane, if  it could somehow sustain a flight path to Mars, would take about 6. 2 years to reach its  destination.

It takes light about 3 minutes to traverse this distance. This translates into  a communication delay of at least 6 minutes for a round trip message. In the context of space  exploration, where precision is paramount, such delays aren't merely inconvenient,  they could have critical implications.

Now, it's essential to clarify a  point from our previous video. We discussed the Sun-to-Earth distance,  approximately 150 million kilometers, a measure known as one Astronomical Unit.  There was, however, some confusion about why this distance isn't a fixed number.

The reason  lies in the definition of an Astronomical Unit, which represents the average distance  between Earth and the Sun, not a constant. Earth's orbit around the Sun is not a perfect  circle but an ellipse. This means that the distance between Earth and the Sun varies  throughout the year.

At its perihelion, or closest approach, Earth is about 147. 1  million kilometers from the Sun. This occurs roughly two weeks after the December solstice. 

Conversely, at its aphelion, or farthest point, the distance stretches to 152. 1 million  kilometers, occurring about two weeks after the June solstice. Interestingly enough, Earth  is farthest from the Sun during the summer in the Northern Hemisphere, a fact that surprises many  given the warmth of the season.

But the change of seasons occurs because of Earth's axial  tilt, not the varying distance from the Sun. It takes light, the fastest  entity in the universe, about 8 minutes and 20 seconds to traverse the  vast expanse between the Earth and the Sun. Venturing further into our  solar system, we reach Neptune, the most distant planet from the Sun, situated  an average of 4.

5 billion kilometers away, or 30 Astronomical Units. At this vast distance,  sunlight takes 4 hours to journey from the Sun to Neptune. When Neptune and Earth align on  the same side of the Sun, at their closest, the distance narrows to 4.

3 billion kilometers.  Imagining a flight to Neptune at the speed of an airplane cruising at 1,000 km/h, it would  take a staggering 490 years to complete this interplanetary journey, showcasing  the immense scale of our solar system. Beyond the orbit of Neptune lies a frontier  populated by dwarf planets, with Pluto, located about 39 Astronomical Units from the Sun,  being the most renowned among them.

Despite its remarkable distance, this region does not  mark the threshold to interstellar space; that boundary is defined by the heliopause.  It is the final known boundary between the heliosphere and interstellar space that is  filled with material, especially plasma, not from our own Sun, but from other stars. Crossing the heliopause, we transition from the familiarity of our solar system into  the vastness of interstellar space.

Here, distances stretch to truly staggering  scales. For instance, Proxima Centauri, the star closest to our Sun, is located  approximately 40,208,000,000,000 kilometers away. This equates to nearly 270,000 Astronomical Units  (268,770 AU).

Within this interstellar realm, the scales and distances involved dwarf  everything within our solar system, challenging our understanding and  imagination of the scale of our galaxy. Within the Milky Way, the average distance  between stars is about 316,205 AUs, illustrating the vastness of our galaxy.  To comprehend such astronomical distances, we consider stars like Betelgeuse.

One of the  largest known, with a diameter surpassing the orbits of Mars or even Jupiter around the  Sun—nearly 700 times the size of our Sun, or about one billion kilometers in diameter. Betelgeuse is so bright that it can be easily found in the night sky. Even though it’s  at about 650 light years away from us.

To put this colossal distance into  perspective, consider the daily routine of an average moderately active person, who  takes around 7,500 steps a day, equating to roughly 6 km of walking every day. For most of  the world, throughout most of human history, the average life expectancy from birth was  around 24. This translates to a lifetime walking distance of approximately over 50,000  km (52,560) km for the average human.

If we extrapolate this to the estimated 117 billion  members of our species that have ever been born over the course of approximately 200,000  to 300,000 years, the cumulative distance humanity has walked spans over six quadrillion  kilometers (6,149,520,000,000,000 km). Imagine if the entire human species could pass  a baton from one member to the next, each walking their lifetime average  distance. Collectively, we would reach Betelgeuse, yet this staggering distance still  represents less than 1% of our entire galaxy.

In this thought experiment, it would take our  species about 2. 8 trillion years to journey across the space between the Sun and Betelgeuse.  These numbers are way beyond what we're used to in everyday life, making it hard if not impossible  for us to wrap our minds around them.

For example, the Milky Way, our galactic home, spans  approximately 100,000 light-years across. While we can articulate this number, and even  employ it in mathematical calculations, truly understanding the scale it represents is another  matter entirely. Our minds struggle to visualize the enormity of our galaxy.

Therefore, to better  comprehend such immensity, we will compress and scale down the entire Milky Way to something  more relatable, akin to the size of the Earth. If we were to compress the Milky Way, a galaxy  that spans an astonishing 100,000 light-years across, down to the size of Earth, approximately  12,742 km in diameter, we would be scaling the galaxy down by about 74 trillion times  (74,248,394,856,229 times, to be precise). In this drastically scaled-down galaxy, the  Sun, which in reality measures about 1.

4 million kilometers (1,392,000 km) in diameter,  would shrink to less than the width of a human hair, approximately 18 micrometers in  diameter (18. 7 micrometers, to be exact). In this miniature Milky Way, with  its diameter equivalent to Earth's, Proxima Centauri, our nearest stellar  neighbor, would be located just about half a kilometer away (0.

54 km to be precise)  from our minuscule Sun. Meanwhile, Betelgeuse, one of the largest known stars and easily visible  in the night sky from Earth, would find its place in this scaled-down version of our galaxy just  over 80 kilometers away (82. 82 km, to be exact).

The Milky Way, our galactic home stretching  100,000 light-years from one end to the other, is a vast expanse that challenges human  comprehension. The last time light completed its journey across our galaxy, our ancestors,  mere hunters and gatherers, roamed the Earth, unaware of the cosmic ballet unfolding across the  heavens. And yet, as immense as our galaxy is, it's but a single speck in a universe filled  with an estimated 2 trillion galaxies.

Each galaxy, with its own stars, planets,  and potential life, contributes to the rich story of our universe, making it a  place of infinite worlds beyond our own.