Tom was up observing all last night, so he’s asked his good buddy, the Amazing Captain Astro, to answer some of the many questions about astronomy that have been pouring in.
Hey Captain Astro,
What are the spectral characteristics of Seyfert galaxies?
Timmy Johnson, age 9
That’s an excellent question, Timmy. At a time when people are saying that youngsters like yourself aren’t interested in science, it’s a pleasure to see that you know so much about astronomy. Er, ah, next question, please.
Hey Captain Astro,
How big is the universe?
Timmy Johnson
It’s big, Timmy. Really, really big. It’s so big that it’s hard for mere mortals like us to comprehend how big.
Hey Captain Astro,
Aren’t you going to answer any of these questions?
Timmy Johnson
Okay, okay, I’ll give it a try. But fair warning. The answer might boggle your mind. Old Captain Astro’s mind exists in a perpetual state of cosmic bogglement.
The universe is so big that our usual distance measures are too small to make much sense. So astronomers use measures like the “light-year” to talk about the distance from one astronomical object to another. A light-year is the distance it takes light to travel in one year — about 6 trillion miles.
Let’s start close to the Earth and work our way out. Earth is the third planet from the uun of the eight major planets revolving around it. The light from the sun takes over eight minutes to get to our eyes. That makes Earth over eight “light minutes” from the sun.
The planets in our solar system that are farthest from us are in orbits that are hours away by light beam. Neptune, the farthest planet from the sun, is more than four light-hours away from us.
But the stars are much farther apart than the planets are from our star, the sun.
The nearest star to the sun is Alpha Centauri, over four years away by light beam. That’s over 24 trillion miles.
The sun and Alpha Centauri are two of about 300 billion stars that make up our home galaxy, the Milky Way. The Milky Way is a pinwheel-shaped collection of stars, dust, and hydrogen gas.
If you turned on a flashlight at one end of the Milky Way, the light would take about 100,000 years to get to the other end, making our galaxy about 100,000 light-years across.
That’s big, but it’s nothing compared to the distances between galaxies.
The closest major galaxy to ours is the Andromeda Galaxy. It’s much farther away than the stars in our galaxy.
It took over two million years for the dim light of that galaxy to get here. And it’s a close galaxy.
Trillions of galaxies make up the Universe. Using space-based telescopes, astronomers have observed light from extremely distant galaxies.
The farthest galaxies from us are also the oldest. The light from them takes as long as 13.2 billion years to get to the Earth.
The farthest light we can see is the Cosmic Microwave Background radiation, which happened 385,000 years after the Universe began.
Before that, there was no light to see. Astronomers estimate that the universe is 13.8 billion years old. It began with a rapid expansion that astronomers call the Big Bang.
Consequently, from our perspective, the universe looks like a ball of galaxies and microwave radiation with a radius of 13.8 billion light-years or a diameter of 27.6 billion light-years. And we are at the center of the ball.
But that’s not the answer, I am sorry to say. The Big Bang is still happening. During the 13.8 billion years since the Big Bang, the universe has continued to get bigger at nearly the speed of light.
Furthermore, during the first billionth of a second after the Big Bang, the universe expanded much faster than the speed of light, a period called cosmic inflation.
If it got bigger at a constant rate after cosmic inflation, the universe would have a radius of over 46 billion light-years and a diameter of nearly 93 billion light-years.
But that’s not the answer either.
You’d expect that the expansion of the universe is slowing down after all this time. The gravity of all the galaxies interacting should drag down the speed.
If that’s the case, the Universe might be smaller than 93 billion light-years wide.
However, the expansion Universe seems to be getting faster than expected. A mysterious force astronomers call dark energy appears to be responsible for the increase.
Taking together all the stuff we know about the universe, astronomers at the University of Oxford calculated that the universe might be as much as seven trillion miles wide, 250 times the part of the universe that we can see.
But what if dark energy doesn’t exert its influence uniformly across the universe? Then, all bets are off about the universe’s size.
Something else to consider is the curvature of space. The matter in a star or galaxy bends the surrounding space. The more matter an object has, the more gravity it has, and the more the surrounding space is curved. Gravity, said Albert Einstein, is just the curvature of spacetime.
Now think of the whole universe as a thing, the whole thing. If the universe has enough stuff in it, it might have enough gravity for the universe to curve in on itself.
Earlier, I compared the universe to a solid ball. If the universe has enough stuff, it might be more like the ball’s surface. Everywhere you look — up, down, or sideways — you’re looking along the ball’s surface.
In that case, the Universe might be infinite or have the appearance of being infinite. No matter how long you traveled, you would never come to its edge.
Is your mind boggled yet? If not, consider this:
Astronomers estimate that our solar system began to form about 4.5 billion years ago. That means that the most distant galaxies are so far away that the light we observe from them began its trek toward the Earth long before there was an Earth to travel to.
In other words, I don’t have a clue about how big the universe is. Nobody does. Yet.
I hope that answers your question, Timmy. It leaves me filled with awe and incomprehension at the immense vastness of our universe.
Hey Captain Astro,
Yeah. Er. Okay. What about the spectral characteristics of Seyfert galaxies?
Timmy Johnson
Go home, kid. I think your mother is calling you.
