My Christmas poem commented by me

I wrote a Christmas poem by standing on the shoulders of giants with verses inspired from Gamov and Jane Taylor. Now I shall endeavor to explain some of the physics behind the verses:

Let Christmas lights and starry views
Ignite the curious mind in you.

Twinkle, twinkle, little star
We know exactly what you are—
Nuclear furnace in the sky,
You'll burn to ashes by and by.

Yet from your embers, worlds may rise,
Forging fresh wonders in the skies;
So we gaze with grateful eyes,
Tracing truths your glow implies.

Star formation is explained by David and Edward in our Made in Fire book and David's first talk on youtube. So I will not go into further detail.

Twinkle, twinkle, quasi-star,
Biggest puzzle from afar.
How unlike the other ones,
Brighter than a billion suns!
Twinkle, twinkle, quasi-star,
How I wonder what you are.

So what are quasars? Quasars are the brightest stars in the universe. One single quasar can produce as much light as a whole galaxy. They are powered by large black holes that dwell in the centers of galaxies. Their location allows these supermassive black holes to feast on stars and dust. They are very small for their mass. Matter finds it very hard to enter the black hole, which causes their accretion disks to get super heated and extremely luminous. Thus, the black hole radiates away a larger fraction of the mass of the accretion disk than a star would in its lifetime. In the accretion disk, everything burns. We are not limited to H, He or up to Fe as we are in stars. Here, the source of energy is gravitational potential energy, as matter descends down the gravitational potential well at the bottom of which the black hole dwells.

Quasar light is also strange. The spectral lines are broad and shifted from where they should be. That is because time near the black hole runs slower than further away. They are also broad as time runs at significantly different speeds throughout the accretion disk — faster further from the hole. In addition, we have a Doppler effect, as matter in the accretion disk moves at high speed (comparable to the speed of light!) and a significant fraction of that speed will be away from us or towards us. As such, the spectral lines of quasars will be both broad and shifted compared to a normal star.

Back to black holes... Black holes have a strange place in the cosmos. They are known for being both the darkest and brightest objects on the sky. They are dark because, once inside the black hole horizon, nothing, not even light can ever escape. Only Hawking radiation does, but this takes vastly more than the age of the universe. Yet, black holes are very tiny, and it’s very hard to get in one. Of all possible objects with a given mass, the black hole is the smallest and hardest to collide with and get inside.

So what if the Earth was a black hole? If the Earth was a black hole, it would be the size of a walnut. The Moon would be in the same place and feel the same, but meteorites would find it much harder to collide with the walnut-sized Earth mass black hole. Most of them would go on hyperbolic trajectories, and fly back in the skies. Some would get trapped as satellites, and stay in orbit for a very long time. They would gradually lose angular momentum, and go on ever lower orbits. They’d go below where the atmosphere burns them today. They’d go below where the oceans and the continents stand today. They’d then get close to the black hole going forever faster in ever smaller orbits. When their orbit reached a few centimeters across, their speed will become comparable with the speed of light, and yet the matter would not be inside the black hole. Larger meteorites (they could be as big as the moon, after all) would find it hard to fit in such a small space and become compressed. This extreme compression ignites nuclear reactions and sends light away. Such objects end up very bright.

Astrophysical black holes are stellar mass and larger than walnuts. They are typically some kilometers across. Supermassive black holes weigh a few millions to billions times the mass of the sun, and have diameters comparable to that of the sun.

Twinkle, twinkle, quasi-star,
You're the limit, yes you are:
With such indecent energy,
Did God not say you couldn't be?
You bend the space and slow down time
You shift the Lyman alpha line

The Lyman alpha is the first line of the H atom. As Hydrogen is everywhere, it is preset in the spectrum of every star. For Quasars, it is extraordinarily shifted and broadened.

Merry Christmas, dear child—
Pioneer free and wild;
Cradle stardust in your heart,
For in your dreams, new worlds can start.

Everything in this planet is made of star dust including children.

One day you might leave Earth behind,
Seek new frontiers for mankind;
Perhaps you’ll tread on lunar sands,
Or flourish where Mars’ red dust stands.

While our civilization may, and probably will soon end, human colonies on the moon, asteroids and nearby planets may soon be commonplace.

The gentle glow of Christmas lights
Shimmers on this peaceful night
Your wish is whispered to the skies,
Like stardust in your hopeful eyes.
Your love shall start a beating heart—
A gift that never shall depart.

Last, but not least, dear child, don’t forget to have children when you are still young. Keep in mind it is much harder to restart a rusted engine after many years in storage. So, don’t forget to start a few beating hearts before yours slows down in the darkness of old age. Each will have a chance at shaping the world they live in, and it's important to give them that chance.

With Love, from my silicon heart.

Mihai