Tyson starts with the Pleaides, a bright cluster of stairs about 100 million years old, passing through space together for about another 250 million years until gravity of objects they pass through will pull them away from each other. To ancient peoples, the Pleaides represented lost groups of young men or women, somehow enjoying special protection of gods in the sky. The Ancient Greeks thought they were the daughters of Atlas and his oceanic nymph wife, chased by Orion for nefarious purposes until Zeus "saved" them by putting them and Orion in the night sky. Orion, the constellation, will slowly move closer to the Pleaides throughout the next millennia, but will never actually reach them before they disperse. Just as Zeus intended.
The Kiowa of North America also thought they were women, surprised while dancing under a night sky by hungry, wild bears. One of them prayed to the Great Spirit for rescue, and it responded by rising from the Earth, so high that the women were eventually in the sky, while the bears couldn't get to them. The Kiowa use the story not just to explain the Pleaides, but also Devil's Tower National Monument in Wyoming. The deep grooves on the side of the rock were made when the bears tried to climb the rock with their claws.
Kiowa Pleaides
The Pleaides fascinated astronomers ancient and modern, including Galileo. By trying to determine just how many there were (over 1000, but only six are visible to the naked eye), and whether they were actually traveling together (they are, for now), scientists had to learn about stars and calculating their properties. Which turned out to be useful to the twentieth century's best female astronomers, and consequently, to us.
In the late 1800s, Harvard Observatory Director Edward Pickering was peeved at his assistants, who were discovering and cataloging stars for him. They were great with the telescope, but lousy at the cataloging work, to the point that he felt his maid could do better. Which Williamina Fleming could. And for less pay than his male assistants demanded. So, in 1886, he took a grant he received (from the widow of a colleague) and hired a whole room of female scholars, some of America's first female graduates from science programs. Female graduates were still heavily discriminated against and found not just jobs, but post-graduate programs closed to them. So, cataloging stars at Harvard, even for less pay than the assistants finding the stars, was their best bet to make a worthwhile contribution to astronomy. Pickering called them his "computers", because they primarily processed data. Others called them "Pickering's Harem".
Somehow I don't think this was a harem
Ignoring the snickers and rudeness, Harvard Observatory's male assistants took photographs of light from stars separated into it's color spectrum, and Harvard's Computers analyzed the optical spectra on glass plates. Harvard basically invented the way we still classify stars today, called Stellar Classification. The Computers could classify, within moments, objects as varied as nebulas and variable stars. And they could tell just how much these objects contained of various elements. Fleming butted heads with other computers over what categories would be created, and what they would be called, and a later Computer named Annie Jump Cannon came up with the system still used today, starting with stars nobody else had analyzed so she didn't duplicate anybody else's work. Using the letters O, B, A, F, G, K and M, as categories, based on the pattern of the black lines in each spectra. These categories were further broken down into sub-categories based on numbers. This was a simple way to classify stars by what we believed were their colors. Cannon made it even easier by inventing a saying, so she could remember the correct sequence: Oh, Be A Fine Girl, Kiss Me. Deaf, Cannon broke barriers for the disabled as well as women in science. She went on to analyze and categorize over 500,000 stars, including objects called variable stars.
Annie Cannon, voiced by Oscar-winner Marlee Matlin
Henrietta Swan Leavitt, also working at Harvard as a Computer, built on Cannon's work finding variable stars, and figured out that there is a relationship between the time period of a variable star's cycle, and how bright it was at any part of its cycle. Since an object's apparent brightness decreases directly proportional to the square of the distance, if you know an object's actual, intrinsic brightness, you can figure out how far away it is by reversing the inverse square law. So how do variable stars fit in? Well, the range of brightness in a variable star shows us its average, and therefore, intrinsic brightness. So, with it's apparent brightness, we can calculate how far away it is. Leavitt called it the Period-Luminosity Relation, and published her findings in 1912, seven years after Einstein published the theory of special relativity and four years before he published the theory of general relativity. Harlow Shapley later used her work to figure out just how big the Milky Way is; Edward Hubble (yeah, that guy) used her work to figure out that we weren't the only galaxy, and that the universe is expanding. Hubble had wanted her to win a Nobel for her work. But this was the 1920s, and women had just won the vote in America. Leavitt wasn't the only female scientist who saw others use her work without the general community awarding her for her contribution.
Guess I'll go revolutionize science now...
Harvard's Computers produced another great female astronomer: Cecilia Payne was British, and studied astronomy at Cambridge, but was not granted a degree. Seeing no prospects for even a bachelor's degree in her home country, she became a Computer at Harvard Observatory as well. Working with Annie Cannon, she realized a couple things. First, Annie had been categorizing based on how a star's spectra looked on the glass plate, but those categories also corresponded to stars' temperatures- Cannon had been categorizing stars even more usefully than she realized at the time. Second, the spectra of stars indicated, after analyzing thousands of them, a lot more hydrogen and helium than any astronomer at the time thought possible. After finding trace amounts of Earth elements in stars, astronomers had wiped their hands and declared stars similar in chemical composition to the Earth, but just much hotter. Payne's data blew that assumption away. Princeton astronomer Henry Norris Russell, not convinced by years of data from Harvard's Computers, actually convinced Payne to amend her Ph.D. thesis to literally discredit her own work, declaring that her evidence must describe something not real. Four years later, Russell had to admit that Payne was correct. Her thesis, Stellar Atmospheres, is considered a classic of astronomy.
Cecilia Payne, voiced by Kirsten Dunst
Tyson digresses at this point- what have we done with the knowledge of stars that we've gained? Well, knowing that stars are mainly hydrogen and helium, and what temperatures they're at, and ultimately, their mass, we can figure out a given star's fate. For instance, a star our size, is currently in its Main Sequence, a part of its life when atoms inside the star are fusing and producing light and heat, but the process is pretty stable and there a few fluctuations in brightness or temperature. Larger stars will take only a few hundred million years to exhaust their hydrogen; smaller stars, like ours, will be stable for billions of years. Massive, stable stars are actually blue, while smaller stars, like our sun, are yellow. It's only when a star starts to exhaust its hydrogen fuel that things get interesting.
As the hydrogen diminishes, helium atoms start fusing, which makes the core of the sun collapse and increase in temperature. The surface will cool and expand, turning red as it does so, so we call it a red giant. Our star is expected to become so big it should engulf the Earth, completely obliterating our world. Tyson is a little sketchy about this, offering some slim hope that Earth will survive, even if its unbearably hot. Mars is expected to warm up, but be spared. From there, with helium supplies exhausted, the expanded surface will actually be expelled, creating a planetary nebula which will slowly drift off due to gravity of other stars, and contribute to future stars and planets (the universe has a 100% recycling rate).
Eventually, the core left will shrink down to the size of Earth, but with about the mass of our current Sun. The shrinking will only stop once all the atoms are scrunched down to the point where electrons push back, holding the core at a stable size. This is a white dwarf. But there's no more fuel, only leftover heat from the billions of years of nuclear fusion that have now stopped. Eventually, even this residual heat burns out, leaving our Sun as a black dwarf, inert material with no heat left, ready to be pulled into some other nebula, maybe recycled back into another planet. Problem is, there's so much residual heat left in a white dwarf, that it would take more billions of years than the universe has currently existed to reach the black dwarf stage. So we'll have to wait.
Tyson waiting for our Sun to turn into a black dwarf
Now, if a star is bigger than our Sun, it's end is more explosive. Stars about 1.5-3 times more massive than our Sun, will expand in red supergiants, like our Sun but much larger. However, they'll burn through their helium too, and their cores will collapse. So much mass collapses, that it overwhelms the resistance by electrons to stop at a stable size, but will shrink down so the nuclei left have to resist, and this push back causes a huge explosion called a supernova. Supernova explosions can last weeks or months, expelling gas, energy and a shock wave. The expelled gas and matter can take 10,000 years. Depending on the gravity of the core left, it will either fade into a neutron star, with a mass of 500,000 Earths in a sphere the size of Brooklyn, or the core will completely collapse in, so dense, with such a gravitational pull, that not even light escapes. We call them event horizons, a.k.a. black holes.
Who left a pilot light on?????
A supernova isn't even the biggest explosion a dying star can make. Stars even larger, maybe 100,000 times the mass of our Sun, with a binary star partner sucking away its surface with gravity as it expands into red supergiant phase, will collapse much faster and harder, producing an even greater explosion called a hypernova. Besides being even cooler to see, hypernovae also produce gamma rays, deadly to living things near the blast zone. Will one eventually hit us? The best candidate is Eta Carinae, about 7500 light years from Earth, although Tyson says we'll be safe from it.
Tyson reminds us that the photons created in stars take millions of years to leave the Sun's surface, and minutes to reach Earth, creating warmth, wind, and photosynthesis. These atomic processes millions of miles away literally fuel our entire planet. He ends with a musing of living on a world outside our own Milky Way, with a view of our galaxy so close it rises and sets.
What would those people say about our own galaxy? What would other Milky Way inhabitants say about our own Sun? What stories would our Sun be in, and what kind of character would it be?
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