WATCH: Unit 3 Overview - Stars and Elements

Okay! On this episode of The Rachel  Hansen Show we have a special guest star. Yeah Bob. Bob's my producer. uh-huh uh-huh okay, got it, thanks So it's not called The Rachel Hansen show, it's actually the overview for Unit 3,  stars and elements.  And in this context guest star  is not a famous person but it is-- wait for it-- an ancient Chinese science term! You're welcome. [instrumental music] Our night sky seems so permanent. You look up every night and the constellations are right there where you expect them. Always in the same shapes. The north star and southern cross   have helped sailors navigate for thousands of years. Ancient astronomers thought of the sky in much the same way. They believed that stars were fixed on a sphere surrounding us. Sometimes called the firmament. But is our sky really so stable? Humans have been observing and cataloging the stars for over 3,500 years.  and we've noticed some dramatic changes in that time. Stars are born, stars die, and some very lucky humans were in  the right place and the right time to witness it. For example, a supernova is what happens whenever a really big star dies in a massive explosion of incredible size and brightness. You're going to learn more about them in this unit. The very first supernova recorded by humans was observed by Chinese astronomers working for the Han Dynasty in the year 185 CE. The event suddenly lit up the night sky and shone for about eight months. The astronomical observations were recorded in a later history of the Han dynasty called the book of the later Han.   The astronomers called it a guest star, which explains my confusion earlier. They recorded its size and its position  in the night sky that it was multi-colored   and that it dimmed over time. Astronomers of the past might have thought about the heavens as fixed and unchanging, but even 18 centuries ago they were uncovering the first hints that our universe is vast and in a constant state of change. [instrumental music] There I go again, mixing science into your history class. But, this supernova story is what Big History is all about! To understand supernovae, astrophysicists sometimes have to rely on the work and tools of historians. You might be asking, "Why would today's scientists, with all their space age technology, need to worry about stories written  down thousands of years ago by Chinese astronomers?" Well, for one thing, stars don't blow up all the  time. The last supernova recorded in our galaxy was observed 400 years ago in 1604 CE. So, historians and astronomers turn to ancient texts for clues. And Ancient China has some of the best texts for that task. That's because Han dynasty astronomers  were government appointed, full-time professionals, who regularly observed and made detailed records of changes in the night sky.   No other society in the ancient world kept such detailed records. In 185 CE Chinese astronomers recorded a  bright guest star in the sky and warned   that it predicted rebellion. That's the main reason the Chinese emperors kept so many professional astronomers on the payroll.   They believed astronomy was a tool for guiding government policies and predicting the future.  When guest stars are recorded in Chinese texts historians look for a few clues about what  astronomical phenomenon was being observed. Clues such as the color, position, movement shape, and  brightness can help tell us what was a supernova,   and what was just a comet. Astronomers then combine historical evidence with modern astronomical data. For example, in order to confirm that this  was actually a supernova, astronomers today searched the area of the sky mentioned in the  Chinese text and found the remnants of a supernova: clouds of expanding gas.   Our universe is littered with ghosts, and this is one of them. RCW 86.   This dusty cloud is all that remains  from the supernova observed 18 centuries ago. Okay, it's not that scary, but it is impressive  how astronomers created this image by combining x-ray and infrared data from three separate  telescopes. By studying the knowledge and records passed down through history astronomers gain a better understanding of our universe. The ancient Chinese astronomers might not have  understood how stars are born and die, but their   observations helped astronomers today form a  more complete picture of our early universe.   [instrumental music] In the last unit, we learned about the  origin of the universe, which began as   an incredibly dense, incredibly hot speck that  exploded to create all of space-time, gravity   simple elements like hydrogen and helium, and  nuclear forces that helped hold it all together. We also discovered how our understanding of  this origin story changed over time thanks to collective learning. From the geocentric model of  Ptolemy to the heliocentric model of Copernicus to the discovery of gravity in a rapidly expanding universe.   We've come a long way in the last 500 years. A tiny moment in time compared to the  13.8 billion year history of the universe. Then, we learned about how using  interdisciplinary thinking, for   example by combining history and astronomy, helps  us form a more complete understanding of the past. Interdisciplinary thinking is a core concept  in Big History, just like collective learning. It helps us understand how our knowledge  has changed over time. You'll continue your exploration of collective learning and  interdisciplinarity at the end of unit 3.   [instrumental music] Unit 3 begins with the cosmic dawn, which started  13.7 billion years ago as the first stars lit up. In this unit, you're going to learn about the  birth and death of the first stars and how they shaped the fabric of the early universe. In this unit, we'll cross two thresholds the first, threshold two, is the formation of stars, which added a lot of complexity to a cold, dark universe. Then, threshold three, the death of stars,  adds even more complexity. Stars are cool, right? Actually, I'm wrong, because they're super  hot masses of hydrogen and helium. Thanks to the pressure of gravity, these masses get hotter  and hotter until the hydrogen atoms break apart   and then fuse back together as helium. That's called nuclear fusion. And that's what happened at the end of the dark ages of our universe to birth the first stars and begin our cosmic dawn. But stars aren't immortal, and eventually all stars,  including our sun, will run out of fuel and die. For really massive, super gassy stars, that  usually means they'll explode as a supernova,   like the one Chinese astronomers witnessed in  185 CE. The extreme heat and gravity involved in star death produces all the naturally  occurring elements of our periodic table. In their, death stars create the building blocks for all life. Hydrogen, oxygen, nitrogen, carbon, calcium, and phosphorus. These six elements make up  99% of our human bodies.   And they were all created in supernovae. That's why the astrophysicist Carl Sagan once said that we're all made of star stuff. How do we know what happens inside a dying star? Well, you're going to use interdisciplinary  thinking to find out. You'll practice thinking like an astrophysicist to solve the mystery  of the first star births.   And then, you'll think like a chemist to determine exactly how star  deaths generated all those new elements. But, like any historian or astronomer reading a 2000  year old text, you'll need your claim testers ready to evaluate the narratives you encounter. Are you ready to watch some stars blow up? [explosion noises] All right, Bob, that's a wrap on unit three. Hey, for unit four I'd like to get some real guest stars in here. Can we do that? You know, like somebody  really famous like from the Yankees, or a movie   star or something? Or--   You don't need to stay for this, I'll see you in the next unit. What do you mean we blew our entire budget on paperclips and  time machines? I thought those were returnable?