Any writer trying to describe the size of the universe faces a dilemma, because the numbers involved are so extreme that some don’t think it’s necessary to try. But is there any book about the universe that you don’t even try?
We need to start by understanding the numbers involved in the physics of space. For example, 1 million seconds is equivalent to 11 and a half days, 1 billion seconds is equivalent to 32 years, and 1 trillion seconds is equivalent to 32,000 years. Keep this comparison in mind as we struggle to think about what is to come.
We also need to understand a fact: the distance from the earth to the sun is 150 million kilometers. Suppose you decide to fly from the earth to the sun in what was once the fastest manned aircraft in the world, the SR-71 Blackbird, which can reach a speed of about 1 kilometer per second. It takes 2.5 hours.
Now imagine that you set off for the sun on your 11th birthday in an SR-71 Blackbird. You drive at a constant speed without slowing down for a second, and by the time you arrive at your destination, you’ve already graduated from high school. And that’s just one way.
The speed of the earth around the sun is 30 times that of the SR-71 Blackbird reconnaissance aircraft. So, if you want to have an intuitive feeling, you can recall what you were doing at this moment yesterday. No matter what you were doing, there was a difference of 2.5 million kilometers between where you were then and where you are now. You’re flying 50 times faster than a bullet, the equivalent of climbing Mount Everest 3 times per second. But even with the incredible speed of the Earth, it would take a full year to circle the sun.
The sun is not a simple ball of fire, it can hold 1 million earths, it is like filling a medicine ball with grains of rice, each grain of rice represents an earth. The mass of the sun is so large that it can attract planets within a certain distance, as far as Neptune, which is 4.5 billion kilometers away from the sun. It would take 142 years for the SR-71 Blackbird to cover this distance.
Continuing to fly outward, we come to the largest and most marginal structure in the solar system: the Oort cloud. This is a bubble of ice and rock that surrounds the Sun with a radius of 15 trillion kilometers. It takes a year and a half for a ray of light to go from the Earth to the Oort Cloud, and the SR-71 Blackbird spy plane takes 475,000 years—longer than Homo sapiens has existed.
Further out, we get to other “solar systems” than Wolf 359, which is the fifth closest, and Lalande 21185, which is the sixth. It takes about 8 years for light to travel from Earth to these two stars, and 2.4 million years for the SR-71 Blackbird. And the distance to other stars can continue to increase.
Clusters of stars form disk clouds. In the countryside without light pollution, when we look up at the night sky, we can see the edge of the disk cloud, like a band of light extending from one end of the horizon to the other—like an airplane cloud made of stardust.
The Greeks believed that this luminous ribbon was the milk splashed from the breasts of the goddess Hera, which was used to suckle the baby Hercules, who was abandoned on earth. This is also the origin of the word “galaxy” in English, and “galaxias” in Greek means “milk”. This story also gave birth to another name for the Milky Way: the Milky Way.
The size of the Milky Way has been one of the greatest mysteries in the history of science, but in March 2019, NASA’s Hubble Space Telescope and the European Space Agency’s Gaia satellite “collaborated” to carefully calculate the Milky Way’s mass by measuring the density of light 1.5 trillion times the size of the solar system, which contains about 200 billion stars. This number is equivalent to the number of water droplets in a cloud. We literally live in a nebula.
If you hold up a grain of sand in the night sky, the region it covers contains at least 10,000 galaxies, each containing billions of stars. The size of the universe is beyond hugeness, beyond hugeness, beyond broadness, beyond vastness, beyond grandeur, beyond vastness. We can only use one word to describe it: “astronomical”.

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