# How big is nebula dust?

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Whenever I see the term dust for the particles of a nebula, I ask myself whether this is actually based on some reliable measure of particle size. Of course we can all agree it looks like dust from this distance. And we all know how such appearances often lead to assumptions which are incorrect and whose incorrectness we are often blind to.

Is there any reliable evidence by which this "dust" is known not to be pebbles, rocks, boulders etc.? Which in reality would seem vastly more likely.

How might we measure size from such distances?

The size of cosmic dust grains is in general given not by some size, but by a size distribution. The only direct measurements of such a distribution are made on dust collected on plates of satellites, which is of course a very local measurement. When we think that distributions look similar - though not exactly alike - in other locations of the Universe, so far away that we will never be able to go there, it is because we know how a given distribution and composition affects light traveling through an ensemble of dust grains.

A particle has a given probability of absorbing a photon with a given wavelength, and in general this probability peaks around wavelengths of the order of the size of the particle (for small particles). Thus, if we know how the spectrum of some light source looks if there were no dust around (and we do for many sources, e.g. stars), then the difference between the known intrinsic spectrum and the observed spectrum can be modeled assuming some distribution and composition. Often the composition needs not be assumed, but can be constrained from the emission of the dust at infrared wavelengths.

Usually, the model that fits best the observations is a steep power law of the form $P(r)propto r^{-a}$ with an index of roughly $asim3.5$; that is, the probability of finding a small grain is much larger than the probability of finding a large grain (more precisely, for $a=3.5$, grains of size $r=x$ is $10^{3.5}simeq3,000$ times more common than grains of size $r=10x$, and $10^7!$ more common than grains of size $r=100x$).

For very large grains, the probability of absorbing a photon becomes independent of the wavelength of the photon. Whereas the small grains as described above have a "color preference", large grains are said to be "gray". This is the case for boulders, rocks, pebbles, and even sand-grain-sized particles. Thus, if a cloud consisted of such particles, the spectrum of a background source would simply be diminished by a constant factor at all wavelengths. This is very rarely observed - rather the sources are diminished much more at the short wavelengths than at the long wavelengths, as expected if there are more small grains than large grains.

## How big is nebula dust? - Astronomy

Телескоп/объектив съёмки: Williams Optics Red Cat 51

Камеры для съемки: ZWO ASI1600MM-Cool

Монтировки: iOptron CEM40

Гиды телескоп/объектив: Williams Optics Red Cat 51

Камеры гида: ZWO ASI120mm-Mini

Программы: Astro Pixel Processor · ZWO ASIAIR OS APP · Adobe Photoshop CC 2020 · Mathworks MATLAB + Imaging Toolbox

Фильтры: Astronomik LRBG set Type 2C 1.25"

Аксессуар: ZWO EFW 1.25 8 Position · ZWO AsiAir Pro · zwo EAF · OAG · Deep Sky Dad Autofocuser 2 WO Red Cat

Кадры: 40x600" (6h 40')

Накопление: 6h 40'

Сред. возраст Луны: 21.60 дней

Средн. фаза Луны: 55.77%

Astrometry.net job: 4174546

Пиксельный масштаб: 2,523 угл. сек/пиксель

Направление: 90,568 градусов

Радиус поля: 1,804 градусов

Разрешение: 3905x3353

Источник данных: Путешественник

#### Описание

Acquisition details:
Imaging telescope: WO Red Cat 51 (Focal Length = 250 mm)
Camera: ASI ZWO 1600MM Pro

Dates: Dec. 07, 2020, and Dec. 09, 2020

Filters and exposure details:
Astronomik Red filter 1.25" : 10x600" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Red filter 1.25" : 30x10" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Green filter 1.25" : 10x600" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Green filter 1.25" : 30x10" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Blue filter 1.25" : 10x600" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Blue filter 1.25" : 30x10" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Luminance filter 1.25" : 10x600" (gain: 139.00) CMOS Chip temp: -10C
Astronomik Luminance filter 1.25" : 30x10" (gain: 139.00) CMOS Chip temp: -10C

## Detecting Dust

Figure 3. Visible and Infrared Images of the Horsehead Nebula in Orion. At left, (a) is a visible light image of the

The dark cloud seen in Figure 1 blocks the light of the many stars that lie behind it note how the regions in other parts of the photograph are crowded with stars. Barnard 68 is an example of a relatively dense cloud or dark nebula containing tiny, solid dust grains. Such opaque clouds are conspicuous on any photograph of the Milky Way, the galaxy in which the Sun is located (see the figures in The Milky Way Galaxy). The “dark rift,” which runs lengthwise down a long part of the Milky Way in our sky and appears to split it in two, is produced by a collection of such obscuring clouds.

While dust clouds are too cold to radiate a measurable amount of energy in the visible part of the spectrum, they glow brightly in the infrared (Figure 2). The reason is that small dust grains absorb visible light and ultraviolet radiation very efficiently. The grains are heated by the absorbed radiation, typically to temperatures from 10 to about 500 K, and re-radiate this heat at infrared wavelengths.

Thanks to their small sizes and low temperatures, interstellar grains radiate most of their energy at infrared to microwave frequencies, with wavelengths of tens to hundreds of microns. Earth’s atmosphere is opaque to radiation at these wavelengths, so emission by interstellar dust is best measured from space. Observations from above Earth’s atmosphere show that dust clouds are present throughout the plane of the Milky Way (Figure 4).

Figure 4. Infrared Emission from the Plane of the Milky Way: This infrared image taken by the Spitzer Space Telescope shows a field in the plane of the Milky Way Galaxy. (Our Galaxy is in the shape of a frisbee the plane of the Milky Way is the flat disk of that frisbee. Since the Sun, Earth, and solar system are located in the plane of the Milky Way and at a large distance from its center, we view the Galaxy edge on, much as we might look at a glass plate from its edge.) This emission is produced by tiny dust grains, which emit at 3.6 microns (blue in this image), 8.0 microns (green), and 24 microns (red). The densest regions of dust are so cold and opaque that they appear as dark clouds even at these infrared wavelengths. The red bubbles visible throughout indicate regions where the dust has been warmed up by young stars. This heating increases the emission at 24 microns, leading to the redder color in this image. (credit: modification of work by NASA/JPL-Caltech/University of Wisconsin) Figure 5. Pleiades Star Cluster: The bluish light surrounding the stars in this image is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the dust within it scatters light from a nearby bright source. The Pleiades cluster is currently passing through an interstellar cloud that contains dust grains, which scatter the light from the hot blue stars in the cluster. The Pleiades cluster is about 400 light-years from the Sun. (credit: NASA, ESA and AURA/Caltech)

Some dense clouds of dust are close to luminous stars and scatter enough starlight to become visible. Such a cloud of dust, illuminated by starlight, is called a reflection nebula, since the light we see is starlight reflected off the grains of dust. One of the best-known examples is the nebulosity around each of the brightest stars in the Pleiades cluster. The dust grains are small, and such small particles turn out to scatter light with blue wavelengths more efficiently than light at red wavelengths. A reflection nebula, therefore, usually appears bluer than its illuminating star (Figure 5).

Gas and dust are generally intermixed in space, although the proportions are not exactly the same everywhere. The presence of dust is apparent in many photographs of emission nebulae in the constellation of Sagittarius, where we see an H II region surrounded by a blue reflection nebula. Which type of nebula appears brighter depends on the kinds of stars that cause the gas and dust to glow. Stars cooler than about 25,000 K have so little ultraviolet radiation of wavelengths shorter than 91.2 nanometers—which is the wavelength required to ionize hydrogen—that the reflection nebulae around such stars outshine the emission nebulae. Stars hotter than 25,000 K emit enough ultraviolet energy that the emission nebulae produced around them generally outshine the reflection nebulae.

## Facts

The Butterfly Nebula has been known since at least 1888. The first known study of the object dates from 1907, when the American astronomer Edward Emerson Barnard drew and described the nebula.

NGC 6302 lies within our Milky Way galaxy, roughly 3,800 light-years away in the constellation Scorpius. The glowing gas is the star’s outer layers, expelled over about 2,200 years. The “butterfly” stretches for more than two light-years, which is about half the distance from the Sun to the nearest star, Alpha Centauri. The central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust, which appears as a dark band pinching the nebula in the center. The thick dust belt constricts the star’s outflow, creating the classic “bipolar” or hourglass shape displayed by some planetary nebulae. The star’s surface temperature is estimated to be about 400,000 degrees Fahrenheit, making it one of the hottest known stars in our galaxy. Spectroscopic observations made with ground-based telescopes show that the gas is roughly 36,000 degrees Fahrenheit, which is unusually hot compared to a typical planetary nebula. The WFC3 image reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated “wings” of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape. Image: NASA, ESA and the Hubble SM4 ERO Team

The Butterfly Nebula has a bipolar structure with two primary lobes and possibly another pair of lobes from an earlier phase of mass loss. The nebula’s central star is obscured by a dark lane that runs through the nebula’s waist.

NGC 6302 has a prominent northwestern lobe which is believed to have formed about 1,900 years ago.

The central star has not been detected because of the dusty torus obscuring it and absorbing a large amount of the light coming from the nebula’s central region, and because of the star’s bright background. The star has a mass approximately 0.64 times that of the Sun. It was originally much more massive, with a mass about five times solar, but ejected most of its mass in the event that resulted in the formation of the nebula. The star is currently evolving into a white dwarf. It is about 34 times as hot as the Sun and one of the hottest known stars. The ultraviolet radiation from the star is making the nebula glow.

The Butterfly Nebula was one of the bipolar planetary nebulae lying near the galactic core that were discovered to be preferentially aligned to the galactic plane of the Milky Way. The discovery, announced on September 4, 2013, suggests that there is an external force that is shaping their orientation, possibly a strong magnetic field emitted by the galaxy’s bulge.

Researchers studied over a hundred planetary nebulae in the central region of the galaxy using the Hubble Space Telescope and European Southern Observatory’s New Technology Telescope (NNT) when they found that the bipolar nebulae were in a surpring alignment with each other, with their long axes aligned along the plane of the Milky Way. The nebulae are in different locations, they have different compositions and histories, and don’t interact with each other, yet they are mysteriously aligned with one another. This is not the case with all planetary nebulae, only the bipolar ones.

The shape of planetary nebulae is believed to be determined by the rotation of the central star or star system. The shape of bipolar nebulae is thought to be a result of jets blowing mass outwards from the central star perpendicular to its orbit. While the shape of planetary nebulae is determined by the characteristics of the progenitor stars, the new finding suggests that the central bulge of the Milky Way with its magnetic fields has a stronger influence over the entire galaxy than previously thought.

The Bug Nebula, NGC 6302, is one of the brightest and most extreme planetary nebulae known. It is located about 4,000 light-years away, towards the Scorpius constellation (the Scorpion). The nebula is the swansong of a dying solar-like star lying at its centre. At about 250,000 degrees Celsius and smothered in a blanket of hailstones, the star itself has never been observed as it is surrounded by a dense disc of gas and dust, opaque to light. This dense disc may be the origin of the hourglass structure of the nebula. This colour image, which nicely highlights the complex structure of the nebula, is a composite of three exposures through blue, green and red filters. It was made using the 1.5-metre Danish telescope at the ESO La Silla Observatory, Chile. Image: ESO/IDA/Danish 1.5 m/R. Gendler, A. Hornstrup and J.-E. Ovaldsen

Butterfly Nebula – NGC 6302

Butterfly Nebula location, image: Roberto Mura

Constellation: Scorpius
Coordinates: 17h 13m 44.211s (right ascension), -37°06󈧓.94” (declination)
Visual magnitude: 7.1B
Absolute magnitude: -3.0B
Distance: 3,800 light years
Mass: 5 solar masses
Apparent dimensions: >3′
Designations: Butterfly Nebula, NGC 6302, Bug Nebula, PK 349+01 1, Sharpless 6, RCW124, Gum 60, Caldwell 69

## How big is the solar nebula?

Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula&mdasha spinning, swirling disk of material.

Subsequently, question is, what is the size of the solar system? Our solar system's largest planet is an average distance of 484 million miles (778 million kilometers) from the Sun. That's 5.2 AU.

Considering this, what is solar nebula?

solar nebula. solar nebula. noun. A large cloud of gas and dust from which the sun, planets, and other solar system bodies formed. MLA Style.

How important is solar nebula?

Planetary nebulae will often orbit the new star and the leftover gas and dust will likely for planets. Just like the way our solar system was born. This nebula is known as the "Pillars of Creation". Incredible in size and potential to create many brand new stars.

## Types of Nebulae

HII regions and dark nebulae are where stars can form. They are made mostly of hydrogen and helium, with traces of other gases and infusions of dust grains. hey are found largely in the spiral arms of our galaxy. Our own solar system was born in such a region more than 4.5 billion years ago. The best-known molecular clouds are the Orion Nebula, the Eta Carinae Nebula, The Eagle Nebula (also, known as the Pillars of Creation), the Tarantula Nebula in the Large Magellanic Cloud, the Horsehead Nebula, the Coal Sack, and the Lagoon Nebula. Most of them, except for the Coal Sack, are bathed in the light of the stars that formed within them. The Coal Sack is an example of a dark nebula that obscures nearby stars, and may be forming stars within.

Supernova remnants are the final remains of massive stars that have blown themselves apart at the ends of their lives. These are expanding clouds of gas and dust with neutron stars or even black holes marking the final resting place of the star. The most famous supernova remnant is the Crab Nebula in Taurus. Its explosion appeared in our skies in the year 1054 AD. It contains a pulsar — a spinning neutron star — surrounded by filamentary clouds of material blasted out when its progenitor star exploded.

Planetary nebulae are the leftovers of stars like the Sun. They consist of a cloud of gas and dust surrounding a slowly cooling white dwarf star. The best-known planetary nebula is the Ring Nebula in the constellation Lyra. It was once a sun-like star that gently blew its outer atmosphere to space as it aged. What’s left of that atmosphere is a ring-shaped cloud that glows from the radiation of the dwindling white dwarf star.

## How big is nebula dust? - Astronomy

Tarantula Nebula and the region of space surrounding it [Image: European Southern Observatory]

Even if spiders make you nervous, don't worry about this tarantula. It's not a big spider. It's a big nebula that looks a bit like a spider in some photographs. It's also so far away that even its light takes about 170,000 years to get to us. Stars are born there, stars die there, and it's one of the most spectacular nebulae in the Galactic neighborhood.

1. A nebula is a giant cloud of gas and dust in the spaces between the stars.
Nebulae have the stuff to make new stars, and many of them are stellar nurseries. There are also nebulae made from the outer layers of massive stars that ended their lives in an enormous explosion called a supernova. This kind of nebula is called a supernova remnant. The Tarantula has both types of nebula.

2. The Tarantula Nebula isn't in our Milky Way Galaxy – it's in a nearby galaxy called the Large Magellanic Cloud (LMC).
The LMC is a dwarf galaxy that can easily be seen in the southern hemisphere in the constellation Dorado (the Mahi Mahi fish).

3. Although it's 170,000 light years away, the Tarantula Nebula is so bright that you can see it without a telescope. At first, people thought it was a star.
An early 17th-century star atlas showed it as a star. Later the nebula was listed in star atlases as 30 Doradus, which is a star number. Telescopes finally showed that the object was a nebula, not a star, but it's still often called 30 Doradus. In the big telescopes of the early 20th century, the nebula reminded some astronomers of a big spider. That's how it got its name.

4. The Tarantula Nebula is much bigger than the Orion Nebula, but it's over a hundred times farther away.
Let's compare the Tarantula to one of the brightest and best known nebulae in our Galaxy, the Orion Nebula. The Orion Nebula is nearly 25 light years across and 1350 light years away. You can see it without a telescope in the constellation Orion as a faint fuzzy patch. The Tarantula Nebula is over 600 light years across, but it's also over 100 times farther away. Try to imagine the Tarantula Nebula in the Orion Nebula's place. If you went outside to look at the night sky, it would be bright enough for you to see your shadow.

5. The Tarantula has been a very active starbirth region for several million years.
How can a cloud of gas and dust be bright enough to see from so far away? The answer is “stars”. There are star clusters of different ages in the Tarantula. A star cluster is a group of stars that formed together in the same part of a giant nebula and stayed together because of their gravitational pull on each other. Half of the Tarantula's light comes from R136, the central part of one star cluster.

6. R136 contains over half a million fairly young stars.
Not only does R136 have a lot of stars, it also has some amazingly big ones. There are at least nine stars that are over a hundred times more massive than the Sun. One of these stars R136a1 is over 250 times more massive than the Sun.

7. The biggest stars are also the hottest. A gigantic star burns brightly, but has a short life that ends in a supernova explosion.
A supernova is a massive explosion that gives out as much light and energy as a whole galaxy of stars. It makes a shock wave that can start off new star formation in a nebula. Heavy elements like copper, gold and titanium are made in a supernova, so they are added to the nebula and recycled into the new stars. The stars of R136 are still too young to have ended in supernovae, but the Tarantula has seen many supernovae in the past.

8. Hodge 301 is a star cluster older than R136, and about 150 light years away from it.
Astronomers have studied Hodge 301 and estimate that at least forty supernovae have happened there. They also think that these supernovae may have set off the star formation in R136.

9. The first supernova for nearly four hundred years to be visible without a telescope was in the Tarantula Nebula.
The first supernova of 1987 (SN 1987A) was also the first one since 1604 to be visible without binoculars or telescope. Unfortunately for those living in the northern hemisphere, it was a southern hemisphere event. It was certainly a big event for astronomers who were at last able to study a nearby supernova with modern telescopes and other detectors.

10. The Tarantula Nebula has several known superbubbles.
A superbubble is a very large cavity blasted out of nebulae by supernovae. They're hundreds of light years across. Our Solar System formed in the middle of an an ancient superbubble.

This content was written by Mona Evans. If you wish to use this content in any manner, you need written permission. Contact Mona Evans for details.

## Crab Nebula

Herschel has produced an intricate view of the remains of a star that died in a stellar explosion a millennium ago. It has provided further proof that the interstellar dust which lies throughout our Galaxy is created when massive stars reach the end of their lives.

The Crab Nebula lies about six and a half thousand light years away from Earth and is the remnant of a dramatic explosion, called a supernova, originally seen by Chinese Astronomers in 1054 AD. Starting out at 12-15 times more massive than the Sun, all that was left after the dramatic death of the star is a tiny, rapidly rotating neutron star and a complex network of ejected stellar material.

The Crab Nebula is well known for its intricate nature, with beautiful filamentary structures seen at visible wavelengths. Now, for the first time, thanks to Herschel’s exquisite resolution, we can see these filaments of dust in the far-infrared region of the electromagnetic spectrum. After ruling out other sources, astronomers using Herschel showed that these filaments are made of cosmic dust, lying in exactly the same place that we see the densest clumps of supernova ejecta. This provides definitive evidence that the Crab Nebula is an efficient dust factory, containing enough dust to make around 30,000-40,000 planet Earths. The dust is made of a combination of carbon and silicate materials, which are crucial for the formation of planetary systems like our own Solar System.

The Crab Nebula and surrounding area, showing the lack of contaminating dust in the foreground and background.

Previous infrared images of the Crab Nebula, using the Spitzer Space Telescope, used much shorter wavelengths and so only showed the warmer dust. Spitzer found only a tiny amount of dust, simply because it missed the massive reservoir of colder dust now known to exist. Herschel, observing at longer wavelengths, is able to detect both warm dust (shown in green/blue in the image) and also cool dust (shown as yellow/orange), some as cold as -260 Celsius. This has allowed astronomers to measure the total mass of dust for the first time.

Large amounts of dust have been seen in supernova remnants before, but the Crab Nebula is particularly exciting as it provides the the cleanest view of what is going on. Unlike many other remnants there is almost no dusty Galactic material in front of or behind the Crab Nebula, so the image is uncontaminated by material in between it and the Earth. This also allows astronomers to rule out the possibility that the dust was swept up as the shockwave expanded throughout the surrounding region.

In most supernova remnants, much of the dust is destroyed as it ploughs into the surrounding interstellar gas and dust, crushed by violent shockwaves. A final treat is that the Crab Nebula is a much kinder environment for dust grains, so the dust does not seem to be destroyed. This may be the first observed case of dust being freshly-cooked in a supernova and surviving its outward journey carried along by the shock wave. We now have definitive evidence that supernovae created the raw materials for the first solid particles, the building blocks of rocky planets and life itself, in a blink of an eye.

The Crab Nebula as seen in visible (left), showing the glow from hot, energised gas, and far-infrared (right), showing warm dust (green/blue) and cooler dust (yellow/orange) shining in the remnant. Image credit: ESA/Herschel/SPIRE/PACS/MESS (Far-IR) NASA/ESA/STScI (Visible)

## The Tarantula Nebula is very, very big

If you leave our Milky Way galaxy and back away from it — and even at the speed of light this will take, oh, two or three hundred thousand years, so pack a lunch — you’ll see we’re surrounded by dozens of much smaller galaxies. Two stand out, though: the Large and Small Magellanic Clouds, the biggest of the bunch, though still tiny compared to the mighty Milky Way.

The LMC (as those of us in the know call the bigger one) lies about 160,000 light-years from Earth, and is a smeared-out splotch of stars that nearly, but not quite, has a discernible shape. It’s classified as an irregular galaxy, though some astronomers argue it may be a nascent spiral.

Despite its relatively small stature, the LMC hosts what may be the single biggest star-forming nebula in the entire Local Group of 50 or so galaxies. It’s so big and has so many substructures that it goes by a host of names, but most astronomers know it as the Tarantula Nebula.

It’s so big and sprawling that words fall short of describing it in any way it deserves. The main nebula is something like 300 light-years across — compare that to the famous Orion Nebula, which is “only” a dozen or so light-years in size. At its heart is a cluster so massive — it contains stars that add up to half a million times the mass of the Sun — that they may actually be forming a globular cluster!

I could go on and on, but of course a picture is worth 1 kilowords. So here, gaze upon this, and have your sense of scale crushed into dust:

The hugely sprawling Tarantula Nebula, a vast star-forming complex in a nearby satellite galaxy of the Milky Way. Credit: ESO

OK, now let’s talk about this for a sec. This image was taken by the Omegacam on the VLT Survey Telescope (or VST the VLT stands for Very Large Telescope, making for a confusingly redundant nested acronym). OmegaCam is a 256 megapixel detector (!!), and the telescope is a 2.6 meter beast, the largest telescope currently existing on Earth dedicated to doing surveys of the sky (as opposed to pointing at specific targets).

What you see here is nowhere near the full resolution image available I had to download a smaller version (a mere 4,000 x 4,000 10 Mb version), and even then I had to crop it and save it at lower res to make sure the servers didn’t gag on it (I also rotated it 90° clockwise for aesthetic reasons).

If you want the full res version, you can get it here. Mind you, it’s 16,655 x 16,719 pixels and tips the scale at 157 Mb!

The Tarantula part of the nebula is the roundish region to the right. Here it is at higher resolution (though I still had to shrink it by 30% or so to get everything in it to fit here):

Detail of the Tarantula Nebula showing newly formed massive stars blasting away. Credit: ESO

The bright clutch of stars to the right is R136, that phenomenal cluster I mentioned above. You can see how chaotic this entire region is. Any civilization evolving on a planet anywhere near there will have a substantially different view of the cosmos than we do.

I wonder what their mythology would be like?

You can read more about this nebula at the European Southern Observatory site, or in any of the numerous articles I’ve written about the Tarantula. But there’s something else I want to point out in this image.

Let your eyes just wander over it, looking at medium scale features, not the smaller stuff. If you do this, you’ll notice there are a lot of loops and arcs of gas in it. And wherever you see these, you’ll notice a cluster of stars at its center. Here’s one for you:

Detail of the Tarantula Nebula of a cluster of stars plowing up gas around them into a thin shell. Credit: ESO

This is a small part of the big image (taken from just above and to the right of center). It’s pretty obvious here! The gas forms a complete circle around a decent collection of bright stars.

That’s no coincidence. When stars form en masse in a nebula, especially by the thousands, a few of them are going to be massive. When they turn on after they form, becoming true stars, they blast out a ridiculously fierce wave of both light and wind (like the solar wind, composed of subatomic particles, but far, far stronger). These carve out a cavity in the gas around them, and also push the gas like a snowplow driving through snow. The effect is roughly symmetric around the cluster, so you get a spherical shell of gas around it. When we view it off to the side, we see it as a thin circle of light, like a soap bubble.

And they’re all over this image! That’s a clear sign of ongoing star birth in the nebula. And that’s why it’s so bright those hot, luminous stars light up the nebula, zapping it with ultraviolet light and causing it to glow. It’s a cosmic birth announcement, written across a card more than three quadrillion kilometers end to end.

Sometimes astronomy is subtle, with faint, small, distant objects barely detectable against the darkness of eternal night, a puzzle that hints at the science and grandeur behind it.

And sometimes it’s a sledgehammer pounding your eyes and brain with ostentationess and zest. I’m good with that too.

## Nebulae

Messier 27, a.k.a. The Dumbbell Nebula is a showpiece planetary nebula 1,300 light-years distant. The distance was measured in 2009 by astronomers using the Fine Guidance Sensors on the Hubble Space Telescope. Knowing its distance and its apparent diameter on the sky provides the real diameter of the brightest parts of the nebula: 2.3 light-years. Very deep images of M27 show that the central star responsible for the nebula, has episodically puffed off layers of gas in the past.

The red light is produced by ionized hydrogen the green light is produced by oxygen that has lost two of its electrons. This star is returning heavy elements back to the interstellar medium, making them available for new stars and planets.

The Ring Nebula: Faintly visible during summer months in modest telescopes from dark sites, it is a sun-like star nearing the end of life. Hydrogen fuels the stars, and that fuel must someday run out.

Having run out of hydrogen in its core, the star at the center of this colorful and expanding cloud of gas is fusing the nuclear ashes from its last few billion years (helium) into carbon. Fusing helium offers less energy than hydrogen burning, so it can do this for a time that is only 10% of the time that it previously fused hydrogen into helium.

This is the fate of the Sun in another 5,000 million years.

These nebulae are called planetary nebulae. Because of their generally round appearance they reminded astronomers of faint copies of planets. The term is misleading, but is well established in the literature after 200 years of use.

M97, The Owl Nebula was discovered by Méchain in 1781, just below the bowl of the Big Dipper. Another example of a planetary nebula, there is an obvious central star that provides the high energy radiation that makes the gas glow.

Interestingly, when the 3 rd Earl of Rosse at Birr Castle, Ireland, pointed the largest telescope in the world at the Owl, hesaw a star in both of the “eye sockets.” Did he simply misplace one of the stars we see today in his drawing? William Parsons was a careful observer and the first to see the spiral shapes of some brighter galaxies. We may never know.

There is a faint halo of expanding gas (not seen in this image) that was ejected by the star more than 40,000 years ago. The shell visible in this image is less than 10,000 years old. At the adopted distance of the Owl, the visible shell is a little more than one light-year in diameter. Alas, the distances to planetary nebulae are generally known to a poor accuracy.

NGC 2024, The Flame Nebula is part of the same gas and dust complex that hosts the Horsehead Nebula. The bright flare in this image comes from the easternmost star in Orion’s Belt.

Behind the dark silhouette of gas and dust, new stars are forming. These young, hot stars cause the gas to glow. At the same time these new stars heat the cloud, causing it to expand, turning off the star formation process. Star formation is an inefficient process only a few percent of the mass of the cloud will go into star formation, insuring the life of the Milky Way Galaxy will extend into the distant future.

The Horsehead Nebula: Named for its obvious resemblance to a Bluegrass Thoroughbred, The Horsehead is a dark cloud of gas and dust seen in silhouette against a fluorescent cloud of hydrogen. It is notoriously difficult to see visually but is much easier to photograph.

Though hard to see, it’s not hard to find: 30 arc-minutes (one full-moon-width) south of the easternmost star in Orion’s belt, Zeta Orionis or Alnitak.