The Veil Nebula

Today, instead of pictures of spiders and caves, I have a picture of the lovely celestial object known as the Veil Nebula. Now, I'll have to be more precise than that: you see, the Veil Nebula as a whole refers to a humongous supernova remnant about 1,470 light-years away in the constellation Cygnus, the Swan. This nebula is quite large: it can be traced for almost 3 degrees in the sky, making it 6 times wider than the full Moon (and about the width of the Andromeda galaxy), while covering 36 times the area. Because it is such a diffuse ball of gas spread over such a large area (nearly a hundred light-years), it is easiest to see around the edges.

There are a few places where those edges are thicker and easier to see, and it is one of those edges that I have for you to see today. This part is known as the Western Veil nebula, and is one of the two brightest portions of the nebula. Because the nebula is so tremendously huge on the sky compared to most objects, different parts of it have actually been assigned their own numbers in the New General Catalogue (NGC). This part is known as NGC 6960.

The Western Veil Nebula, NGC 6960, in Cynus. The bright star is an unrelated foreground star, 52 Cygni.

So in this picture, you can see part of the outer arc of the supernova remnant where it's a bit thicker than average. The original explosion site was to the right and slightly below this picture, and if you look closely to the right of the bright part of the nebula you can see very faint and wispy filaments of gas. The nebula looks sort of purple-ish to me; the blue light comes from doubly-ionized oxygen, and I suspect that the red light is your basic hydrogen-alpha.

Nebula, in Three Parts

Today I've got something a little different from the usual globular cluster pictures I've had a lot of recently. This is a picture of Messier 20, the Trifid Nebula, a fascinating object in Sagittarius, the Archer.

The Trifid Nebula, Messier 20, in Sagittarius.

This nebula gets its name from the way it appears divided into three parts by the dark nebula stretching across it. It got this name long before anyone knew what it was exactly, but the number three is also important to this object for another reason: it nicely illustrates all three types of nebulae.

The first type of nebulae, emission nebulae, are represented by the reddish region at the top. This red light comes primarily from hydrogen atoms in the gas being excited by copious amounts of ultraviolet light from the hot, young stars inside and around the nebula. The particular wavelength responsible is at 656.28 nanometers and is so important and wide-spread that it has its own name: hydrogen-alpha, or H-alpha for short.

The second type of nebulae is the blue reflection nebula seen below the emission nebula. These nebulae come about from starlight being reflected off of tiny dust grains in the gas cloud. The reason it appears blue is because the dust grains preferentially reflect blue light, the same way that the molecules in the Earth's atmosphere preferentially scatter blue light. Further from the young stars than the emission nebula is, the gas in the reflection nebula isn't being excited to emit in visible wavelengths very strongly.

Finally, the third type of nebulae, dark nebulae, also come about as result of dust and are represented by the dark clouds and bands of dust in front of the emission and reflection nebulae. Dark nebulae are full of tiny dust particles containing organic molecules that are extremely effective at absorbing visible light. Soot is actually fairly close in composition to these dust particles, so you have some idea of just how dark they are in visible light. Fortunately, they are much more transparent at other wavelengths, allowing us to probe their structure in infrared and radio wavelengths.

Also, to be comprehensive, there is a fourth type of nebula that typically gets its own name: planetary nebulae. These are really a subset of reflection nebulae, as they are the puffed-off atmospheres of old Sun-like stars that are being illuminated by the white dwarf core of the star, but they are different enough from typical emission nebulae to warrant their own designation.

Nebulous Comparisons

My post yesterday comparing the Lagoon and Orion Nebulae got me to thinking: how would they look compared to each other? I can say that the Lagoon Nebula (Messier 8) is about 110 light-years across while the Orion Nebula (Messier 42) is only 24, but those are just numbers. Being the visual person I am, I decided to see what they would actually look like if compared. So this afternoon I sat down and put together the two pictures seen below.

This picture shows the two nebulae side-by-side just as they appear on the sky:

Left: M8, the Lagoon Nebula. Right: M42, the Orion Nebula. North is up in both pictures.

These two nebulae are the biggest and brightest on the sky, and pretty much the only ones that can really be seen with the naked eye. The Orion Nebula looks a little bigger here due to its much closer distance (the Lagoon Nebula is about two-and-a-half times further away). Also note the difference in star density between the two pictures.

This next picture shows what they would look like if M42 was was at the distance of M8:

M8 and M42 as they would be if they were the same distance away.

Quite the difference, no? To be more realistic I should also have dimmed M42 by about 7 times to accurately reflect how it would look being ~2.5 times further away, but my first attempt at a rough approximation made M42 so dim it could barely be seen. I opted instead for the slightly-less-realistic but more visually interesting picture.

As I mentioned in my last post, despite their differences M8 and M42 are more similar than not. Both are star-forming regions, both are large cavities of gas (mostly hydrogen and helium) and dust being blown open from the inside by young stars, both would probably appear boring and dark from the other side. The main differences are their size, as seen, and their location: M8 is situated nearly directly towards the galactic core from us, while M42 is located almost directly away. That's why there are a lot more stars visible around M8 than around M42.

Celestial Lagoons

Today I have a picture of the Lagoon Nebula, a lovely star-forming region in Sagittarius. It is similar in nature to the famous Orion Nebula and is similarly visible, very faintly, to the unaided eye. It is over five times larger than the Orion Nebula (110 light-years across vs. 24), but appears slightly smaller on the sky due to its greater distance (3,000-4,000 light-years away, compared to ~1,300 for Orion).

Messier 8, the Lagoon Nebula in Sagittarius.

Like the Orion Nebula, the reddish color comes from hydrogen ionized by hot, massive young stars embedded in the nebula. The blue color comes from light scattering off dust in the cloud, similar to the way air molecules scattering light causes the sky to look blue.

The Lagoon Nebula is also similar to the Orion Nebula in that they both offer looks into the cavernous interiors of gigantic clouds of cool gas and dust. From the outside these clouds appear dark and boring, and you can see that slightly around the edges of the nebula. But when young stars inside them blow away the gas around them and offer a view inside, the sight is spectacular. Not unlike geodes, now that I think about it. (Geodes, for those who don't know, are rocks that look like any other rock on the outside to the untrained eye, but which contain beautiful crystal formations on the inside if broken apart.)

More Nebulae! Flames and Horseheads.

Today I have Yet Another Nebula Picture (YANP), this time of two nebulae close enough on the sky to share a field of view. One of the them, the Horsehead Nebula, is probably one of the more distinctive and well-known nebulae out there, while the lesser-known Flame Nebula is a visual treat.

Also, these nebulae are nice because they lie near a prominent feature in the night sky, one that many (if not most) people are familiar with. Take a look at the picture below:

The Flame and Horsehead Nebulae in Orion. Click for a larger version.

See that bright star there? That is Alnitak, and you probably know it better as the left (or eastern) star in Orion's belt. The large bright region to its left is the Flame Nebula. Its bright red glow comes from the hydrogen atoms in the cloud being ionized by Alnitak's intense ultraviolet radiation. Alnitak itself is an O-class star, the hottest and most luminous class of stars, and puts out nearly a million times more light across the electromagnetic spectrum than the Sun does.

Just below Alnitak you can see the prominent outline of the Horsehead Nebula silhouetted against a backdrop of glowing hydrogen. In fact, there's a very nice, nearly straight line where a large dusty cloud blocks the light of the nebula behind it that extends for over half a degree below Alnitak.

What's cool is that this cloud of dust is also responsible for the Flame Nebula's dramatic shape, because the same dust cloud that causes the Horsehead Nebula is also in view in front of the Flame Nebula's backdrop of glowing hydrogen. So in a way, these two nebulae are intimately linked.

Another way to illustrate just how dark and dusty the molecular cloud that's producing these nebulae is, is to note that there is no inherent difference in the density of stars across the picture. The left side of the picture has just as many stars as the right side (you're looking through the plane of the Galaxy, after all); it's only the presence of the dust cloud that's blocking out the light from the ones behind it. (The ones you see on the left are either stars in front of the cloud, or ones seen where it is thinner.) All in all, a very beautiful section of the sky, one with a lot of very young, hot, and luminous stars.

Eta Carinae Et Nebulae

Today I have a picture of the huge expanse of nebulosity found in the southern constellation Carina, the Keel. The Carina Nebula, as it's known, lies roughly 6,500 to 10,000 light-years away in the Carina-Sagittarius Arm of the Galaxy. It is actually about four times larger than the Orion Nebula, but is much less well-known due to its southerly location.

The Carina Nebula in the constellation Carina, the Keel.

This picture is unfortunately not quite up to my usual standards due to losing tracking between taking the luminance data and the color data, because someone walked in front of the telescope which caused it to lose guiding (the nebula is pretty low on the horizon from Hawaiʻi. Normally my targets are much higher in the sky, so it isn't a problem, which is why I was unprepared for it). Thankfully, I was able to recover fairly well, though it's not quite as focused as usual.

Like the Orion Nebula, the Carina Nebula is a star-forming region, a stellar hatchery if you will. It contains some of the largest and most massive stars known to exist in our galaxy. One of these stars is known as Eta Carinae, and it is actually visible in this image. Below, I have an crop of the center of the image, with a few prominent objects marked in it.

The large circle on the right is a star cluster known as Trumpler 14, the structure in the middle is a dark dusty nebula called the Keyhole Nebula (silhouetted against the glowing hydrogen behind), and the star in the small circle on the left is Eta Carina.

Eta Carina is pretty mysterious as stars go, and there is much we still have yet to learn about it. For starters, it is probably more than one star. Currently it is thought to be two stars, one with a mass of ~100 times that of the Sun, and one with about 30 solar masses. It's hard to say, because back in 1841 it put out a huge cloud of gas and dust that makes it pretty much impossible to see the star itself. This corresponded with a major increase in its luminosity (the amount of energy it puts out). In fact, just two years later, in 1843, Eta Carinae underwent what is known as a supernova impostor event. Basically, it put out as much light as a normal supernova would, but didn't blow up.

How is this possible, you ask? Well, remember how I said Eta Carinae was one of the most massive stars in the Milky Way. Normally, a star only about 8 times more massive than the Sun can go supernova. Eta Carinae, of course, is much, much more massive than that, so it can easily fling out huge amounts of material and still survive. Eta Carinae (or at least the larger component of it) is so large, in fact, that it is in serious danger of blowing itself apart just from its normal energy generation. In a normal star, energy generated by fusing hydrogen to helium is just sufficient to counteract the force of gravity trying to collapse the star. As more mass is added, the force of gravity increases, which increases the rate of energy generation, keeping the star in hydrostatic equilibrium. However, there comes a point where the energy output of the core becomes so great that it can actually start to drive off the outer layers of the star.

This point is known as the Eddington Limit, after Sir Arthur Eddington who first proposed it. It is roughly 32,000 \(\times\) the mass of the star in solar masses, so our Sun would have to be putting out about 32,000 times more energy than it currently does before it would begin to disintegrate under its own power. It doesn't do this because its mass is not sufficient to crush its core hard enough to generate that amount of power. Eta Carina, however, may be big enough to do so (or at least get really, really, close). It's hard to tell, because we don't know for sure just how luminous it is. This theory may help explain why it put out such a huge burst of material back in the 19th century. Or it may be some other, completely unrelated mechanism. The bottom line is, we just don't know yet. That's all just part of the wonder and excitement of studying the universe!

Supernova Remnants, and the Pulsars that Light Them

Last week I showed you a picture of what happens to small stars (those less than ten times more massive than our Sun) when they run out of fusible hydrogen in their core. Today, let's take a look at what happens to a big star.

Upon running out of hydrogen to fuse, large stars initially proceed very similarly to their less weighty brethren. They begin to fuse the helium produced by hydrogen fusion into heavier elements such as carbon, oxygen, and neon, but unlike smaller stars they have enough mass and gravitational force in their cores to continue. Neon gets fused into heavier elements such as silicon, magnesium, and calcium, slowly working up the periodic table until the star starts making iron in its core. Iron is an interesting element because its nucleus has the highest binding energy per nucleon of any element. (Though certain isotopes of nickel have virtually the same binding-energy that iron does.) What this means, practically, is that you cannot get any energy from iron by either fusion or fission, either by fusing it into more massive elements or by splitting it apart.

Once a star arrives at iron in its core, it's done. It can't squeeze any more energy out of its core, so all that is left supporting the star against its own great weight is electron degeneracy pressure in a slowly-growing iron-nickel core. Electron degeneracy pressure is a quantum mechanical effect with no real analog in classical mechanics. Simply put, electrons exert pressure because two fermions (a class of particle of which electrons, protons, and neutrons are a member) cannot be in the same place and the same quantum mechanical state at the same time. Squeezing them together causes all the low-energy states to be taken, so the electrons vigorously resist any further compression, which would require large amounts of energy to raise electrons into high-energy states. However, there is a limit to how much pressure electrons can exert; if the mass of the core exceeds the Chandrasekhar limit of about 1.38 solar masses, electron degeneracy pressure catastrophically fails and the core collapses in on itself.

At this point, if the star is less than about 20 solar masses there is only one mechanism that can save the star from collapsing into a black hole: neutron degeneracy pressure. Similar to electron degeneracy pressure but involving neutrons, the star's core collapses into a neutron star, a sphere of neutrons packed as tightly as an atomic nucleus and about the size of a city.

As the core collapses into a neutron star, the star's outer layers fall inward tremendously fast, at speeds up to 23% of the speed of light. The neutron star at the center, however, is already packed as tightly as it possibly can be, so the infalling material rebounds off the core in a colossal shockwave to produce what we see as a supernova.

(There is a lot more going on at the same time, of course – supernovae are incredibly fascinating events where relativity and quantum mechanics are both in play, and I'm giving you merely the barest overview of all the processes happening.)

If the star in question started off heavier than about 20 solar masses, even neutron degeneracy pressure will be unable to support the core and nothing in the universe can prevent it from continuing to collapse into a black hole. However, I'm going to focus on neutron stars in this post because the picture I have for you today contains one. The story behind this particular supernova is ancient and varied, so settle in...

The story starts about a thousand years ago, in Anno Domini 1054, when a new star appeared in the constellation Taurus. As was customary, Chinese and Japanese astronomers noted the appearance of a "guest star" and recorded its location. It was also apparently observed by as least one person in the Arabic world. This star was apparently bright enough to be seen in the daytime for a period of several weeks, after which it slowly faded over a period of about two years and finally disappeared, whereupon it dropped out of history.

In 1731 a mysterious nebula (one of the first discovered telescopically) was discovered just off the tip of one of the horns of Taurus by one John Bevis. In 1758, while searching for the return of Halley's comet, Charles Messier stumbled upon this nebula and initially mistook it for his quarry. After watching it for a few weeks he realized that it wasn't moving, and came up with the brilliant idea to publish a catalog of objects that looked like comets but weren't, so that other amateur comet hunters wouldn't be fooled as he had. Thus, this nebula became the first object on what would become his now-famous list of not-comets: Messier 1.

In 1844, almost a hundred years later, the nebula was sketched for the first time by William Parsons, 3rd Earl of Rosse, whose love of astronomy and independently wealthy nature led him to build the largest telescope in the world at that time ("the Leviathan of Parsontown", 6 feet in diameter). His sketch reminded him of a crab, and so he gave our nebula the whimsical name the Crab Nebula. It remained a popular object of observation with astronomers, both amateur and professional.

In 1921 the American astronomer Carl Lampland noted changes in the Crab Nebula which implied a small size for it. In the same year, another astronomer demonstrated that the nebula was expanding. Several astronomers noticed its proximity to the "guest star" of 1054, but nothing was made of it until 1928 when the venerable Edwin Hubble definitively proposed that the nebula be associated with the star. However, it wasn't until later, when the theory behind supernovae had been worked out, that Nicholas Mayall showed that the Crab Nebula was nothing less than the remains of the supernova that exploded into the sky nearly 900 years earlier.

Although the association of the nebula with a supernova was now clear, it wasn't until the 1960's that neutron stars were first predicted by Franco Pacini. A few short years later, in 1968, a neutron star was detected in the center of the Crab Nebula, which made it both the first neutron star ever known and a shining confirmation of Pacini's hypothesis. It also explained why the nebula was so much brighter than a 900-year-old supernova remnant was expected to be.

The neutron star that lurks at the center of the Crab Nebula – known as the Crab Pulsar – is a fascinating beast by terrestrial standards. It is about 25 kilometers (about 15.5 miles) across, and makes a complete rotation every 33.08471603 milliseconds – 30 times a second! As the neutron star spins, it sends out a constant stream of electromagnetic radiation all across the electromagnetic spectrum (including visible light) from both poles of its extremely powerful magnetic field. The axis of its magnetic field is not the same as its rotational axis (much like the Earth, though a bigger offset), and as it spins around it sends off a powerful beam that appears to "blink" on and off as seen from Earth, much like the beam from a lighthouse. The neutron star is thus known as a pulsar, a portmanteau of pulsating star.

This electromagnetic energy being given off comes from the rotational energy of the Crab Pulsar, which is slowly slowing down by 38 nanoseconds per day. The energy being given off along with the star's powerful magnetic field (thousands of times more powerful than the Earth's) being spun through the Crab Nebula 30 times a second causes it to light up. Electrons are accelerated to nearly half the speed of light and spiral along the magnetic field lines of the pulsar, giving off synchrotron radiation as they do, which create a blueish glow visible in the center of the Crab Nebula in long exposures.

Now, after all this background, I suppose I should show you the picture you no doubt read this post for. I hope you can now better appreciate just how amazing this object is, even if my picture cannot do it justice. Here it is, the Crab Nebula:

The Crab Nebula, Messier 1, in Taurus.

While the Crab Pulsar itself is quite invisible in a small telescope such as the imaging telescope, you can easily see the remnant of that titanic explosion 958 years ago. Within that shell of gas exist fantastic filaments and mysterious structures, brought about by electrons powerfully accelerated to relativistic speeds by the pulsar's magnetic field. Despite being around 6,000 light-years away it is persistently the strongest source of X-rays and gamma rays from outside the solar system. The central neutron star itself is a sphere of ultra-dense matter more massive than the Sun compressed into an area smaller than New York city and spinning over 30 times a second. This matter (commonly called "neutronium") is so dense that a single thimble-full would weigh over 100 million tons. I could go on and on at length about how fascinating this single member of the group of objects know as pulsars are, but you get the idea.

I hope this post has given you a sense of the wonder and excitement I get when I study astronomy and physics. Learning more about the incredibly varied denizens of our universe never fails to amaze and astound me, and I enjoy nothing more than bringing that feeling to others. If this post made you stop and think at all, then I will feel I have succeeded. A hui hou!

The Great Nebula in Orion.

Last Wednesday I was able to take out the imager at the Vis under good conditions for all of the second time this year.

Since it's still winter in the northern hemisphere, and the Earth's night side is still pointed more or less away from the galactic core, there aren't many of the objects I typically like to image (globular clusters) out at night. This led me to look around for another object to image, and I settled on the Great Nebula in Orion, AKA Messier 42, AKA the Orion Nebula. This nebula is, without a doubt, the most impressive nebula in the sky. It's also the brightest, and one of the few that can be seen with the naked eye (it's the middle "star" in the three stars that make up Orion's sword. No, not his belt, but his sword. I should post a picture...). I decided to devote an entire night to it and ended up getting almost two hours' worth of exposures. (Normally I like to image as many things as possible in a night, and for things like globular clusters that works alright, but the picture I got has taught me to reconsider that position when imaging nebulae...)

But enough words! You want to see the picture, and I want to show it to you:

The Great Nebula in Orion, Messier 42.

Gorgeous, no?

The Orion nebula is the closest star-forming region (or "stellar nursery") to Earth, at 1,344 \(\pm\) 20 light-years distant. It is probably the most photographed celestial object in history \(-\) in fact, it was the very first nebula ever to be imaged, on September 30, 1880 by Henry Draper (who was a pretty amazing character, one of the pioneers of astrophotography, and the person in charge of the U.S. expedition to observe the transit of Venus in 1874).

The Orion nebula is a large ball or bubble of gas (which fluoresces red) and dust (which silhouettes as black, or reflects light as blue) about 24 light-years across. Several extremely young and massive stars much hotter and more luminous than our Sun have formed near the center and are steadily blowing a hole in the side of the bubble of gas and dust in which they are embedded. The four brightest are known as the Trapezium and are a little too bright to see in this picture (they're located in the center where it's brightest). This allows us to see into the central cavity.

Interestingly, it's quite possible that the Orion nebula doesn't look anywhere near as impressive from the other side. From that perspective it might appear as a mostly featureless dark nebula, with perhaps some small emission nebulae around the edges. This is because the stars in the Trapezium appear to be blowing open the side of the bubble asymmetrically, and we happen to be on the side where it's open. It does make you wonder about the other dark nebulae we can see in the sky, and whether they might not be visions of such cosmic grandeur from the other side...

One last note: on the left side of the image, you might be able to make out two faint straight lines. Those are satellite tracks that appeared in the luminance images while I was imaging. They're pretty faint, so you may or may not be able to see them.

Atmospheric Aquiline

Today I have a rather stunning image (if I say so myself) that I apparently took back in October 2009. I say that because I don't remember reducing the data, and yet I discovered it today sitting in a folder about 90% of the way to being finished. So I finished it, and it turned out to be quite a beauty.

(Edit 3/31/18: In retrospect, while I started learning to use the imager fairly quickly I wonder if this wasn't data given to me by the guy who taught me how to use it as October 2009 was when I first came up to the VIS. That would fit with me not remembering having reduced it. So if that is the case, sorry for taking your credit all these years Nathan.)

Messier 16, the famous Eagle Nebula in Sagittarius.

This is the famous Eagle Nebula, entry #16 in Charles Messier's famous list of not-comets. It is a place of stellar birth, where clouds of gas and dust are contracting into new stars. The faint red glow you see throughout the region comes from hydrogen excited by the many young, hot, massive stars in the area which are emitting copious quantities of ultraviolet light. The red light is called hydrogen-alpha by astronomers and has a wavelength of 656.28 nanometers. It comes about when the electron of a previously excited hydrogen atom jumps down from the third shell of orbitals surrounding the nucleus to the second, releasing a photon of red light in the process.

The Eagle Nebula achieved stardom (ha!) mainly after 1995 when an iconic picture of it was taken with the Hubble space telescope that focused on the dark dust lanes seen near the center. These became known as the Pillars of Creation, and are believed to be locations where stars are actively being born.

If you're wondering why it's called the Eagle Nebula, as with most such names it works better if you're looking at it through a moderately-sized telescope. You can kind of see it in deeper images that reveal more of the outlying details, as well.