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  • Astronomers have it pretty easy when we talk to the public. I may be biased, but I think

  • astronomy is the most beautiful of all sciences. Sure, other fields of science have lots of

  • eye candy, but all I have to do is pull out a shot of Saturn, and I win. Because Saturn.

  • It’s ALL gorgeous. Planets, moons, stars, clustersbut of all of them, you just can’t

  • beat a nebula. Why?

  • Because nebulae.

  • Nebulais Latin forcloudand for once in astronomy we have a name that

  • actually describes the object accurately. Nebulae are clouds of gas and dust in space.

  • I’ve already talked about them a bit. For example, stars form from nebulae; our Sun

  • did 4.6 billion years or so ago. When a medium-sized star dies, it blows off winds of gas, then

  • lights them up as the white dwarf core of the star is revealed, creating a planetary

  • nebula. Also, when a high-mass star explodes it catastrophically vaporizes itself, becoming

  • a violently expanding cloud of gas. Nebulae are literally part of the births, lives, and

  • deaths of stars.

  • So, besides being beautiful, theyre also pretty versatile.

  • There are a lot of ways of categorizing nebulae. One way is by how we see them. For example,

  • if a cloud of gas is blasted by light from a nearby massive star, the gas in it becomes

  • excited; the electrons in its atoms jump to a higher energy level. When the electrons

  • drop back down, they emit light. The gas glows, and we call this an emission nebula.

  • The color of an emission nebula depends on the gas in it and how hot it is. Hydrogen,

  • for example, glows most strongly in the red, and we see that color in most emission nebulae.

  • Oxygen tends to glow green, but to a lesser extent it gives off blue light too.

  • Other elements span the spectrum in colors they give off.

  • And these colors aren’t limited to visible light. Hydrogen can emit infrared and even

  • radio light, and if it’s energized enough itll emit in the ultraviolet, too. That’s

  • true for many elements.

  • Although most emission nebulae look substantial, theyre actually incredibly tenuous. A typical

  • density in a gas nebula is only a few thousand atoms per cubic centimeter. Mind you, in the

  • air you breathe there are about 10^19 atoms per cubic centimeter, a thousand trillion

  • times denser than a typical nebula! Really, a nebula is barely more than a vacuum.

  • The reason they look so cloudy is that theyre big. Really, really big. A decent sized nebula is

  • several light years in diameter, and that’s a LOT of centimeters. That much gas adds up,

  • and so some nebulae can be pretty bright.

  • While EMISSION nebulae glow due to their own light, REFLECTION nebulae are bright because

  • can you guess? — they reflect the light of nearby bright massive stars. In this case

  • though the nebula isn’t made of gas, but is instead mostly dust.

  • I don’t mean like the hair and skin flake dust bunnies you find under your couch either.

  • When astronomers talk about dust, they mean tiny grains a micron across. Just so you know,

  • a human hair is 100 times wider than that! These tiny grains contain things like things

  • like silicates, aluminum oxide, and calcium. And in many cases this dust is laced with

  • complex molecules called polycyclic aromatic hydrocarbons.

  • While I love that fancy name for them, you might know them better assoot. Yup. When

  • you light a match youre pretty much making some of the same stuff that lurks between the stars.

  • Dust doesn’t emit visible light. But it can affect the visible light from stars if

  • theyre inside the dust cloud or nearby. Turns out,

  • dust is very good at scattering light. That means that when light hits it, the light gets

  • sent off in some other direction. This scattering is highly wavelength dependent, so blue light

  • is scattered very strongly, while red light can go right through.

  • We saw this in the last episode; the dust surrounding the Pleiades star cluster is a

  • reflection nebula. The light from the stars in the Pleiades is scattered by the nearby

  • dust, and the blue light light gets sent in every direction, including toward us. The

  • red light doesn’t scatter nearly as well, so we don’t see it; it never gets sent toward us.

  • Thick dust is also very good at absorbing visible light. If a star is embedded in enough

  • dust, the light from it is dimmed considerably. If the cloud is dense enough or big enough

  • the dust can completely extinguish the light seen from a star.

  • At the same time, if the dust is at the right density, the blue light from a star inside

  • a dust cloud gets scattered, while the red light can get through. This effect reddens

  • starlight, and in some dust clouds it provides a striking view: Stars outside the cloud look

  • normal enough, but closer in they get redder and redder, and then fade out entirely. The

  • result is a fuzzy, red-edged hole in space. Pretty cool.

  • You can see that effect in Barnard 68, a small dust cloud, just half a light year in size.

  • These are also sometimes called molecular clouds; theyre cold enough that atoms can

  • stick together to form molecules. Their cores can be hundreds of degrees below 0 Celsius.

  • Some dust clouds like this are relatively small, but others get downright huge. We call

  • these giant molecular clouds, because why not. These can be incredibly massive, with

  • thousands or hundreds of thousands of times the mass of the Sun, and stretch for hundreds

  • of light years.

  • And that brings us to one of the most glorious objects in the sky: the Orion Nebula. This

  • is an emission nebula located just below Orion’s belt. It’s actually a naked-eye object,

  • visible in modestly dark skies. It looks like a star by eye, but even binoculars reveal

  • it to be fuzzy, and through a telescope, or with long exposure images, you get unmitigated majesty.

  • The Orion Nebula is a star-forming factory; a bunch of stars have been born in it. Some

  • of them are very massive and incredibly luminous. The entire nebula is lit by four stars located

  • in its heart, collectively called the Trapezium. These are four brutes; huge, brilliant stars

  • that are each far more massive than the Sun. Their light is so fierce it illuminates the

  • entire nebula, which is about 20 light years across.

  • And here’s a funny thing about Orion: What youre seeing is not just a gas cloud in

  • space. It’s actually just a bubble sitting on the edge of a much, MUCH larger molecular

  • cloud, hundreds of light years across. That cloud is cold and dark, and so we don't see

  • it by eye. The Trapezium stars formed inside that cloud, very near the edge. When they

  • turned on, fusing hydrogen into helium, they started blasting out a mind-numbing amount

  • of ultraviolet light, which began eating away at the gas and dust. Eventually, they blew

  • a hole in the side of the cloud, like a weak spot in a bicycle tire blowing out.

  • What we see as the magnificent Orion Nebula is just a dimple, a cavity, in the side of

  • the cloud, filled with gas heated to glowing by the stars.

  • There are still stars forming there today, too. I mean literally, right now. We can see it happening!

  • In Episode 9, I talked about the solar system, and how it starts off as a flattened disk

  • of gas and dust. When we look at the Orion Nebula with Hubble, WE SEE THOSE DISKS. Theyre

  • called protoplanetary disks and theyre so dense they absorb almost all the light

  • from the stars forming inside them, so theyre dark, and we see them in silhouette against

  • the brighter gas of the nebula.

  • Unless you look in the infrared. That kind of light can pierce the dark disk, and when

  • we use infrared telescopes we can see the protostars forming in the centers of those

  • disks. Take a good look: THOSE ARE BABY STARS, literally stars that are forming right this

  • very minute. Theyre still hot due to their contraction, but in a few million years theyll

  • ignite fusion in their cores, and become real stars. Theyll blow away the remaining material

  • around them, revealing themselves, and perhaps any planets orbiting them as well.

  • In fact, once stars start forming inside a nebula, its days are numbered. The Eagle Nebula

  • is another star factory, with active star birth going on inside of it. Some of these

  • are massive, luminous stars, and give off so much ultraviolet light it erodes away at

  • the surrounding nebula, in a process called photoevaporation. However, dense knots of

  • material forming new stars can resist that erosion better, and protect the material behind

  • them, in essence shadowing it. This results in long fingers of material we

  • see in silhouette against the hotter gas, like sandbars in a stream. Observing their

  • infrared light, we can also see the stars embedded inside them.

  • There are several of these giant towers in the Eagle Nebula, three of which have been

  • called the Pillars of Creation. Stars are forming at their tips. Eventually, though,

  • the light from the massive stars will win, zapping away at the structures, dissolving

  • them. There’s also some very hot gas in the nebula that might be the result of a star

  • that has already exploded; if so, then the pillars REALLY don’t have long to live.

  • In a few thousands years they won’t be eroded away, theyll be BLASTED away.

  • In a lot of nebulae there’s no sharp edge; they just kinda fade away. Sometimes that’s

  • because the gas thins out, so there’s not enough stuff there to get lit and see. Other

  • times it’s because there’s just one or maybe a few stars lighting up the whole cloud,

  • and at some distance from them the starlight fades and can’t illuminate the gas anymore.

  • But sometimes nebulae do have sharp edges. That usually happens when a gas cloud is expanding,

  • like in a planetary nebula or supernova. The gas slams into the much thinner gas that is

  • strewn between the stars, what we call the interstellar medium. The expanding gas piles

  • up like snow in a snowplow, getting denser and glowing more brightly.

  • Gas inside a nebula can be in turmoil, too. Winds from stars compress the gas, shock waves

  • form when stars explode and when theyre born. These can create lovely sheets, tendrils,

  • and filaments in nebulae as well.

  • All of these factors can come together to create great beauty. Not too far from the

  • Orion Nebula in the sky is another dark nebula, superposed on a bright emission nebula. By

  • coincidence, the dense dark material is shaped like a gigantic chess piece, and it’s called

  • the Horsehead Nebula. It’s being eroded by a star called Sigma Orionis, off the top

  • of the frame here, and that’s also making the gas behind it glow in that sharp ridge.

  • One of my favorite nebulae in the sky is Barnard’s Loop, a huge arc of material that’s formed

  • either by the expanding gas from supernovae or the winds of all the massive stars being

  • born in the Orion complex. It’s also the outer edge of a huge bubble surrounding a

  • substantial amount of real estate in the constellation Orion. In this image you can see both the

  • Orion and the Horsehead nebulae; the Loop is so big you could fit 25 full Moons across it!

  • One more thing: I’ve been talking about bright and dark nebulae, but that’s an old

  • fashioned way of thinking of them. I’ve also mentioned that infrared light can get

  • through them, but remember from Episode 24 that the kind of light an object gives off

  • depends on its temperature. Clouds of dust that look dark to the human eye are actually

  • glowing if you observe them in the far infrared, well outside the colors our eyes can detect.

  • But we have telescopes that can see at these much longer wavelengths.

  • In Orion, there’s a reflection nebula called M 78. Between M78 and Earth are long filaments

  • of very cold and dark dust, blocking the light from the reflection nebula behind and looking

  • like dark rivers running through it. But when you use a telescope that can see light with

  • a wavelength of a millimeter or so, that dust glows brightly, threading through M78 like ribbons of fire.

  • Like so much else in life, what you see really depends on how you see it. If there’s a

  • life lesson there, feel free to take it.

  • Today you learned that nebulae are clouds of gas and dust in space. They can glow on

  • their own or reflect light from nearby stars. When they glow it’s usually predominantly

  • red from hydrogen and green from oxygen, and when they reflect and scatter light it’s

  • from massive hot stars, so they look blue. Stars are born in some nebulae, and create

  • new ones as they die. Some nebulae are small and dense, others can be dozens or hundreds

  • of light years across.

  • Also? Theyre incredibly beautiful.

  • Crash Course Astronomy is produced in association with PBS Digital Studios. Head over to their

  • YouTube channel to catch even more awesome videos. This episode was written by me, Phil

  • Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.

  • It was directed by Nicholas Jenkins, edited by Nicole Sweeney, the sound designer is Michael

  • Aranda, and the graphics team is Thought Café.

Astronomers have it pretty easy when we talk to the public. I may be biased, but I think

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