The 59 ‘Milky Way Year’-Old Astronomer’s Story

Part 7: The universe is not only ‘big in space’, it’s ‘big in time’ as well. Consequently, studying the universe makes us travel both space and time.

The age of the universe appears to be about 13,7 billion years old. To better understand this very long time scale, we can compress the age of the universe into a shorter period, such as a human lifetime.

If someone lives a life of 80 years, the Earth will orbit the Sun eighty times during his lifetime. Now, let’s consider an imaginary astronomer whose life is stretched out to fill the history of the universe. Let’s suppose our astronomer has a lifespan of 80 Galactic, Cosmic or Milky Way years, instead of 80 normal years.

Approximate orbit of the Sun (yellow circle) around the Galactic Centre

The Sun takes 230 million years to complete an orbit around the center of the Milky Way Galaxy, with an average speed of 828,000 km/h (230 km/s) or 514,000 mph (143 mi/s); that means that a ‘Milky Way year’ will be 230 million times longer than an ordinary year. That way, our astronomer will live long enough to see events that happen on cosmological time scales rather than human time scales.

This is his story.

“I am born together with the Big Bang. The universe was very small, hot and dense when it began. Since then, it has expanded and cooled. At first, the universe contained almost entirely hydrogen and helium gas. It was very uniform, with no galaxies, stars or planets. I was only two days old when atoms formed. The universe had cooled enough for structure to begin forming in the gas that uniformly filled all of space.

Remember, what I see as two days amounts to a million years of time on your normal clock. While a million years seems like an extremely long time for you, that’s only a tiny part of the history of the universe and me.

When I was around five ‘Milky Way years’ old, the first stars and galaxies began to form. Galaxies continued forming and developing well into my teenage years, when the universe was billions of years old. Today, galaxies still continue to evolve and change.

There are two schools of thought on how the universe formed: “top down” and “bottom up”. Top down theorists think that large clusters formed after the Big Bang, which later broke down into stars and galaxies. “Bottom up” theorists instead pose the theory that matter was originally dispersed fairly evenly by the Big Bang, and accumulated later into stars and galaxies. Recent data from Hubble Deep Field photographs appear to support the “bottom up” theories. The photos show young galaxies, from up to 11 billion light-years away. These young, small galaxies, from early in the universe’s history, support the theory that large structures formed out of smaller ones.

This amazing shot of 10,000 Galaxies may be the Hubble Telescope’s Most Revealing Photo Ever

An important necessity for the appearance of life are heavy elements. Since only hydrogen and helium gas were formed during the Big Bang, everything heavier than that had to be made in stars later. These other elements – all of the elements on the periodic table – were made in stars.

Stars use light elements like hydrogen and helium as their fuel. Like nuclear bombs, they use the power of the atom for energy. Stars are unlike nuclear bombs however, in that most nuclear bombs get their energy from heavy atoms like plutonium and uranium. In a nuclear bomb, the energy comes from converting heavy elements into lighter elements. This process is called ‘fission’. In a star, the energy comes from converting the light elements into heavier elements. This process is called ‘fusion’.

All of the material in the universe heavier than hydrogen and helium is made by fusion in a star. U2!

An ordinary star can make many of the elements, especially the most important ones, for life. It can’t make all of the elements, though; it can’t make anything heavier than iron. The heavier elements were made in supernovae. A supernova occurs when a massive star reaches the end of its life. Supernovae are extremely bright and extremely hot. This is where the heaviest elements in the universe are made.

All of the material needed to make rocky planets (and us!) was made in stars and some of it was made in supernovae. This means that planets couldn’t exist until there had been enough time for a star to completely go through its life cycle and become a supernova. The material would then be ejected back into space and would form a new star, possibly with planets.

It also means that everything in the world, including all life forms, came from a star!

As the astronomer Carl Sagan said, “We are star stuff.”

Illustration by Antoine de Saint-Exupéry. Public Domain.