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,
clusters…but of all of them, you just can’t beat a nebula. Why? Because nebulae. “Nebula” is Latin for “cloud” and
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, they’re 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
it’ll emit in the ultraviolet, too. That’s true for many elements. Although most emission nebulae look substantial,
they’re 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 they’re
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 as… soot. Yup. When you light a match you’re 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 they’re 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; they’re 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
you’re 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. They’re called protoplanetary disks and they’re
so dense they absorb almost all the light from the stars forming inside them, so they’re
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. They’re still hot due to their
contraction, but in a few million years they’ll ignite fusion in their cores, and become real
stars. They’ll 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, they’ll 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 they’re
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? They’re 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.
