by Shane L. LarsonThe Cosmos is vast in ways that are difficult for humans to wrap their brains around. That doesn’t stop us from talking about it, of course, but it is vast, none-the-less. What do I mean by “vast?”
The Universe is 13.8 BILLION years old, 100 million times older than the oldest human. When you and I go out at night there are almost 10,000 individual stars we can see with the naked eye, but the Milky Way has some 400 BILLION individual stars, and there are some 500 BILLION individual galaxies in all the Cosmos. If you and I could somehow take a road trip, from one side of the Milky Way to the other, travelling at the fastest speed possible (the speed of light) it would take us 100,000 years to go from one side to the other —1000 times longer than any human has ever lived. And the entire Cosmos itself is far vaster.
These sorts of factoids are fun to know and think about. They melt your brain, and they can impress your friends and family at a dinner party. But what is always remarkable to me is even though you and I occupy only one small part of the Cosmos in space and time, we have still managed to piece together a story about the history of the Universe — its overall size and content, when it was born, how it has lived its long life to date, and what its ultimate future might be. As a species, we have only been cognizant of the science we call astronomy for a few centuries, though we have been looking outward into the Cosmos for far longer. But in those few centuries, in just a handful of human lifetimes, we have managed to piece the story together. Even though a human only lives through the merest flash of a moment in Cosmic time, less than a single heartbeat in the life of the Cosmos. This is a story about how we learn what we learn about the Universe around us and our place within it.I am a professional astronomer, but like most of us, my love-affair with the Cosmos began when I was young. I would spend long hours out in the backyard, laying on the ground trying to learn my constellations with an old beat-up paper star wheel. We didn’t have a telescope (they were expensive), but my mom is an avid birder and she had an old spotting scope. She used to let me take it out in the backyard and put it on an old card table, and I discovered there was far more to the Cosmos than my naked eye could see. I looked at the Moon and discovered craters, and mountains and sinuous canyons. I looked at the brightest points of light I could see, and found out they were Mars, and Jupiter, and Saturn — other worlds, tantalizingly close, but so very far away. There were other things to discover as well.
In the corner of the sky we call Andromeda, there is a smudge of light that looks like a wisp of cloud. It is the most distant object you can see with your naked eye, and we call it the Andromeda Galaxy. It is 2.1 million light years away, which means if you step outside tonight and look at the Andromeda Galaxy, the light that falls in your eye and makes its impression on your mind is ancient light. It left the Andromeda Galaxy 2 million years ago, at a time when the most advanced hominids on Earth were Australopithecus, and the world was dominated by mega-fauna like sabre-toothed cats (smilodons) and mastodons. This is one of the fundamental truths in astronomy: looking out is looking back in time, and the farther we can look, the more about the long history of the Cosmos we can discern. As astronomers we are always on an epic quest to build better tools to help us probe farther out into the Cosmos.Let’s look back to the time when I decided to become a professional astronomer, sometime during my early years in college. Thirty years ago, in 1988, I was already improving my backyard astronomy. I’d left my mom’s spotting scope at home, and after not too long had built my own telescope. It was bigger than my mom’s spotting scope, and could see much more of the Universe. At the same time, the largest telescope used by professional astronomers was the 5-meter (200-inch) Hale Telescope on Mount Palomar. At that time, it was the largest telescope in the world, a title it had held for 40 years since it was built in 1948. Astronomers are still using it today. So what do astronomers do with these great machines? On any given night, whether you are looking through a backyard telescope, or looking through a telescope like the Hale, the sky looks much like it did the night before. The stars are still where you remember them, living out their lives slowly, changing little. We find new and interesting things, of course, but what we are often most interested in are the unexpected events — energetic and dramatic events that appear in the sky and then are gone. Astronomers call such things “transients.” Consider a “supernova.” One of the things we have learned over the past century is that stars, like people, are born, they live long lives, and they ultimately perish. When the most massive stars reach the ends of their lives, they die in a titanic explosion that, for a few brief days or weeks, sheds enough light to be visible in the night sky. The last time an explosion like this was seen in the Milky Way was in 1604, before the first telescope was ever used to study the sky! Four hundred years ago, we didn’t know what supernovae were, but the events were momentous enough to note down. You can find written notations of the 1604 supernova in paper star atlases of the day, but one of my favorites is shown in the astrolabe above, which is part of the Adler Planetarium’s historical collection. An astrolabe is a mechanical device used to visually measure the positions of stars in the sky by eye. They are elaborate and intricate machines, but also stunning and artistic in their elegance and form.You’ll see on the upper right ring of this astrolabe that Supernova 1604 is marked, preserved forever in the solid copper record of the day. You’ll notice there are other transients on this astrolabe, including the previous supernova observed in the Milky Way (Supernova 1572), as well as the great comet of 1618. My colleague Pedro Raposo, an astronomy historian at the Adler Planetarium, points out that depicting supernova and comets on an astrolabe is an indicator of how our understanding of the Universe was evolving. At that time, we didn’t know what supernovae and comets were. Their nature was widely debated, with many believing they were atmospheric phenomena. The fact that they were recorded on a mechanical starmap is an indicator that we were slowly coming to the understanding that these events were in the deep, cosmic sky. Our views about the Cosmos were changing. Now spool ahead to the 1980s. In 1988 we understood much more about supernovae than we did when that astrolabe was built, but we had never been given the opportunity to study one up close. In the entire 400 year history of telescopic astronomy, there has not been a supernova here in our own galaxy, close enough for us to see all the fine details and study how stars reach the end of their lives. But on 23 February 1987, there was a supernova not too far away, in a small galaxy next door to the Milky Way, called the Large Magellanic Cloud. We called it Supernova 1987A, and it was visible to the eye for several months. Astronomers could see it in their telescopes, and still today the most powerful telescopes can detect the faint echoes of light coming from the explosion.
But SN1987A was special for another reason. When a star dies in a supernova, it not only sheds light, it also releases a cosmic rain of particles called neutrinos. When this supernova exploded, 1057 neutrinos were released (that’s 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 neutrinos!), bursting out in every direction in the Universe. Neutrinos are notoriously hard to detect because they tend to go blasting right through matter as if it isn’t even there. When the neutrino burst from SN 1987A reached Earth, 30 trillion of them went through me and you and every other person on planet Earth, and we didn’t even know it! But astronomers had been thinking about this for a long time, and had constructed a special observatory — a neutrino telescope.A neutrino telescope is not like an ordinary telescope, because neutrinos are very difficult to capture. Instead neutrino telescopes watch for neutrinos interacting with other things. In 1987, the worlds largest neutrino telescopes were enormous tanks of very pure water underground. Sometimes when a neutrino goes through these tanks, their interaction with the atoms in the water generates bursts of light that can be detected with very sensitive cameras lining the inside walls of the tank. In the hours right before telescopes detected the light from the supernova, 25 neutrinos were seen in detectors around the world. Only 25 neutrinos from all the 1057 that were released in the supernova, but it was enough to convince astronomers they had seen neutrinos from the supernova. This was the first time in history that astronomers had detected an astrophysical event with light AND particles; this was the beginning of what we now call “multi-messenger astronomy.” It was a watershed moment in our quest to probe the Cosmos — we had, for the first time, used two machines to probe the Cosmos using different pieces of information together to make one story. It was the beginning of a new way of thinking and learning about the Universe, and it is a story that is still going on today.
This was the frontier of astronomy 30 years ago. In our next post, we’ll fast-forward to today and ponder how we plumb the deep sky with all our modern technology and combine it with the meager knowledge that we’ve gained over the past few decades.
This post is the first of three based on a talk I have given many times over the last few years, updating it each time to reflect the latest coolest things. The next two posts in the series are:
The Cosmos in a Heartbeat 2:
The Cosmos in a Heartbeat 3:
This post was enabled by a new version of the talk done as a Kavli Fulldome Lecture at the Adler Planetarium in Chicago. I would like to thank all my colleagues at Adler who worked so hard to translate what was in my brain into a story told in the immersive cradle of the Grangier Sky Theater. The talk was given on 9 Nov and 10 Nov 2018, and captured in 3D. You can watch it on YouTube using GoogleCardboard. When I have the link, I’ll post it here.
I also put this post up today to celebrate the occasion of Carl Sagan’s birthday. I, like many around me, was inspired at the right moment by exposure to Sagan’s “Cosmos: A Personal Voyage”. Friday (9 Nov 2018) would have been Carl’s 84th birthday. He left us more than 20 years ago now, but I still hear his voice when I think about and ponder the deep mysteries of the Cosmos around us. Happy birthday, Carl.