Orion is one of the more famous constellations, with its three belt stars, bright red-giant star Betelgeuse. When we observe Orion with the naked eye, we can see the bright Orion nebula (also known as M42) as a fuzzy patch within the sword of Orion. But the nebula we see is only the brightest region of a nebula that spans nearly the entire constellation, known as the Orion Molecular Cloud Complex.

The Orion Complex seen against a diagram of the constellation.

The Orion complex is about 240 light years across and only about 1,500 light years away, so it spans a fairly large region of sky. It is a large molecular cloud containing regions of reflection nebulae and emission nebulae, as well as dark nebulae such as the Horsehead nebula.

It is also a stellar nursery. Many of the stars seen in the constellation of Orion have their origins in the Orion complex. Most prominently, the three bright belt stars (Alnitak, Alnilam, and Mintaka) were formed within the cloud. The complex is one of the most active star production regions in the sky, and because of its proximity it gives us an excellent view of the process. When we view the region in infrared, we’ve found over 2,000 protoplanetary disks, where planets are likely forming around young stars.

There’s a lot going on in the region. But when we look at it with the naked eye, we simply see a bright, easy to find constellation. You could say the region is more complex than it seems.

The post Orion Complex appeared first on One Universe at a Time.

The Pillars of Creation is perhaps the most famous nebula in the night sky. One of the controversies regarding the pillars is whether they still exist. The nebula is about 7000 light years away from us, so what we see is the pillars as they were 7000 years ago. Observations of the Spitzer infrared telescope found evidence of a supernova shock wave, that by some calculations will destroy the pillars in about 6000 years. That would mean the pillars were destroyed about a thousand years ago. But we now know that isn’t the case, thanks to a bit of 3D imaging.

An infrared view of the pillars showing hot young stars in the nebula. Credit: NASA/ESA

The pillars were observed with an integral field spectrograph known as MUSE. This device allows us to measure the spectrum of an image at multiple points at the same time. From this we can determine not only where a particular pillar is in our field of view, but also how far away it is relative to other pillars. From this we can determine which parts of the pillars are behind the young O and B stars of the nebula, and which are in front. Using this data, a team of astronomers calculated the rate at which these bright stars are causing the pillars to evaporate. They found the pillars are deteriorating at a rate of about 70 solar masses per million years. Since the mass of the pillars is about 200 solar masses, it will be about 3 million years before they are completely destroyed.

That’s a short time on cosmic scales, but it also means that the Pillars of Creation are indeed still there, and will be for quite some time.

Paper: A. F. McLeod, et al. The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy. MNRAS 450 (1): 1057-1076 (2015).

The post Depth of Field appeared first on One Universe at a Time.

The wispy green clouds seen in these galaxies are unusual because they are so bright. They are emission nebula stimulated into glowing due to a burst of ultraviolet light. That by itself isn’t unusual, but the central quasars aren’t particularly bright. The brightness of these surrounding clouds indicates that the quasars were quite bright in the past. 

Given the scale of these clouds and their current brightness, the light stimulating the emission originated from the quasar tens of thousands of years ago. That might seem like a long time for a quasar to go quiet, but on a cosmic scale that’s pretty fast, and quasars don’t typically vary much in brightness. So what’s going on?

One idea is that the quasars are powered not by one supermassive black hole, but two. Two closely orbiting black holes could disrupt the flow of matter into each other, and this would cause the brightness of the quasar to vary. This would seem to be supported by the shape of the clouds themselves, which tend to have a twisted, spiral shape indicative of merged galaxies.

The post Green Goblins appeared first on One Universe at a Time.

The image above is a planetary nebula known as M2-9. It’s also known as the Butterfly nebula, but there are lots of other nebulae by that name. Planetary nebulae occur when red giant stars cast off their outer layers as they begin a transition toward becoming a white dwarf. The cast off material is caused to glow when the exposed interior of the star illuminates it with ultraviolet light, causing the material to ionize into a glowing plasma. We call them planetary nebulae because William Herschel first observed them as circular or “planetary” in shape, but we now know they come in a wide range of forms, including the layered, double-lobed shape of M2-9. Just how planetary nebulae produce such varied complexity is something we don’t fully understand. While we’re still trying to understand what the underlying mechanisms are, we do know very clearly what they aren’t.

The idea for this post came about because of a discussion with someone whose scientific interests lean toward the electric universe model. Regular readers will know I don’t find the electric universe model remotely compelling for several reasons. One of the EU ideas that has gained popularity is the idea that lobed planetary nebulae such as M2-9 are caused by plasma z-pinches in the great currents of cosmic plasma. The idea is ridiculous to anyone who’s studied astrophysics, but how do we know it’s wrong? After all, the EU folks have clearly studied this in detail, what makes me so arrogant as to tacitly dismiss the idea?

The pinch effect can crush beer cans too. Credit: Bert Hickman, Stoneridge Engineering

For those who aren’t familiar with plasma physics, a z-pinch is an effect which can occur when a plasma is constrained by a surrounding magnetic field. The magnetic force on a current is always perpendicular to the direction of the current, so when a current of plasma flows into a cylindrical magnetic field, the magnetic forces squeeze the plasma inward, causing a pinch. The pinch effect has been studied for nearly a century, and has been used in everything from crunching beer cans to work on harnessing fusion. If you look at M2-9, it’s easy to imagine a z-pinch. The plasma current flows in, pinches in the center, and flows out the other side. This is what Donald Scott and other EU folks claim, and I would agree there’s certainly a resemblance. However Scott goes further to argue that because it looks like a z-pinch, that’s what it must be. This is a classic “quacks like a duck” fallacy, which most scientists are pretty wary of. Doubly so if what it looks like would turn a hundred years of astrophysics on its head.

But this is one of those cases where there is a clear prediction from the EU model. That is, the flow of plasma is through the pinch. This means if you look at the Doppler shift of light coming from the nebula you should clearly see that the material flows in on one side and out on the other. The standard red giant model makes a very different prediction. It says the Doppler measurements should show material flowing out from the center on both sides. It turns out we’ve made Doppler observations of M2-9, and sure enough it agrees with the standard red giant model.

While this might seem like a pretty clear refutation of the z-pinch idea, but the counter argument seems to be that you could have two layers of current flow so that it goes in both directions (also contradicted by Doppler observations) or all manner of complex plasma phenomena that might explain away the Doppler data. But that isn’t very compelling because tweak theories are weak theories.

The other counter argument raised in the discussion was that astrophysicists don’t entirely understand the nebula either. If you read through the cited paper you’ll see a discussion of several models for the behavior of the lobes, peppered with phrases such as “possibly” and “might be.” It’s clear there’s a lot we don’t understand about M2-9. That’s because good scientists are cautious about making unsupported claims. We tend focus on the known unknowns.

But that’s how science pushes forward toward better models, instead of clinging to ones that clearly don’t work.

Paper: Doyle, et al. The Evolving Morphology of the Bipolar Nebula M2-9. AJ, 119:1339-1344 (2000)

The post Known Unknowns appeared first on One Universe at a Time.

Imagine you’re an astronomer interested in comets. You scan the sky with a small telescope, looking for a faint fuzzy patch in the sky. Soon enough you find one. But as you watch it over the next few nights you notice it isn’t moving against the background stars. So it’s not a comet, but rather a nebula. Looking through the night sky you again find a faint fuzzy object. Again, it’s a nebula. The problem is that nebula and comets often look similar, and only after observing the motion of a comet for a while can you be sure it’s not a nebula. Nowadays, you can simply take the coordinates of your object and compare it to a catalogue of known objects. But in the 1700s this similarity was a big frustration for astronomers, and it drove Charles Messier to create the first catalog of nebulae.

Messier’s sketch of the Orion nebula (M42).

Charles Messier’s early astronomical work focused on the Sun, and he observed the transit of Mercury in 1753. As his interests moved to comets, he ran into the difficulty of confusing comets and nebulae. So with his assistant Pierre Méchain he began creating a catalog of nebulae in 1770. By 1774 the first version of his catalog was published, and contained 45 objects. Not all of the objects were discovered by Messier, in fact all but 17 of them were previously known. But it was the first detailed catalog of such objects, and so they came to be known as Messier objects.

Messier continued to add to the catalog until 1781, and by then the number had grown to 103. A century after his death, seven more objects were added to the list since they were discovered by Messier or Méchain, so now the official list numbers 110. The objects are popular with amateur astronomers, and a personal observation of all 110 objects is a popular goal. There are even Messier marathons, where one tries to observe as many Messier objects as possible in one night.

Messier also discovered 13 comets, including Lexell’s comet and the great comet of 1769. But he will always be known for the nebula he cataloged so he could get on with real work. Sometimes an act of frustration turns out to have the biggest impact.

The post Act of Frustration appeared first on One Universe at a Time.

The Boomerang Nebula is the coldest natural location in the universe. It has a temperature of 1 K, or just one degree above absolute zero. This is particularly interesting because the cosmic microwave background is about 3 K. That makes the nebula colder than empty space.

But how is it possible for this nebula to be colder than the universe? It turns out this nebula is transitioning from a red giant to a planetary nebula. A planetary nebula doesn’t have anything to do with planets, but rather is a gaseous nebula caused by a dying red giant. The term was first coined by William Herschel in the 1700s because he thought they looked like planetary systems starting to form, and the name stuck despite our improved understanding.

A planetary nebula forms when a sun-like star begins to run out of hydrogen to fuse for energy. The star begins fusing helium, which creates energy at a greater rate causing it to swell into a red giant. After its core can no longer fuse helium the star enters what is known as the asymptotic giant branch. While it can’t fuse hydrogen or helium in a continuous fashion, it is able to fuse them in bursts. This is similar to a gas engine running out of fuel, where it throttles up, dies down and throttles up again. These bursts of energy create tremendous stellar winds, which causes outer layers of gas to flow out from the star a high speed. In the case of the Boomerang Nebula, the outward flow is at about 160 km/s.

As gas expands it gets colder. You may have experienced this if you’ve used an aerosol can and felt the can get cold as you spray. In the same way, the outward flow of gas in the Boomerang Nebula causes the temperature to drop, which is how it can be colder than deep space. Such a low temperature is a product of the nebula’s dynamic evolution.

Recently in the Astrophysical Journal new observations of the nebula were presented. We’ve known about the nebula’s cold temperature for a while, and we’ve had good images of the nebula in the visible spectrum, such as the Hubble image above, but these new observations from the ALMA radio telescope array give us a high resolution radio image.

One of the interesting things the ALMA team found was that the nebula isn’t quite the shape seen in visible images. Many planetary nebula have a dual-lobed shape as seen in the visual images, but the Boomerang Nebula appears to have a more spherical shape. The reason the nebula appears lobed is because there is a thick dust region around the equatorial region of the star, which blocks light from the central star. As a result, only the polar regions are strongly illuminated from the central star, hence the lobed appearance.

While the Boomerang Nebula is currently the coldest known region in space, it won’t stay that cold forever. Already the outer envelope is starting to warm a bit due to its exposure to the (relatively) warmer surrounding space.

The post Cold Embers appeared first on One Universe at a Time.

E. E. Barnard is an astronomer perhaps best known for measuring the proper motion of a faint red dwarf about six light years away, now known as Barnard’s star. But Barnard was also a pioneer of astrophotography, and he did a great deal of work studying dark nebulae.

Dark nebulae are clouds of dust that are so dark they absorb most of the light in the visible spectrum. It takes astrophotography to study them, because they are only seen as shadows against background light from emission or reflection nebulae. Capturing their image takes long exposures, and Barnard was able to capture many of them in the early 1900s.

By 1919, Barnard had cataloged 182 dark nebulae, and published this in the Astrophysical Journal. He went on to catalog a total of 370 dark nebulae, now known as Barnard objects. Perhaps the most famous is B33, also known as the Horsehead nebula, seen above.

We now know that dark nebula contain dust grains that are nanometers to millimeters in size. This is why they scatter or absorb visible light. Their dark interiors can be quite cold (10s of Kelvin), which can allow for the formation of complex molecules. We can also see what is behind them in the radio spectrum, since they are generally transparent to radio waves.

But it was during the dawn of astrophotography that we began to understand these dark shadows in the night sky.

The post Dark Shadows appeared first on One Universe at a Time.

With the discussion of light echoes today, here’s another interesting one. In 2002 the star v838 Monocerotis swiftly brightened to about a million times the brightness of the Sun before dimming down again. This burst of light then traveled outward from the star, illuminating the gas and dust surrounding the star. Because the light burst was relatively short, this meant that successive layers of the gas and dust were illuminated as the sphere of light expanded.

In the image below, a collection of images taken between May 2002 and October 2004 have been put together. It should be emphasized that the gas and dust are not expanding outward, but the burst of light is expanding. Sometimes light echoes can be quite impressive.

The post More Echoes appeared first on One Universe at a Time.

The Pillars of Creation (seen above) is an image of a portion of the Eagle nebula (M16) taken by Hubble Space Telescope in 1995. It soon became one of the most iconic space images of all time. The Eagle nebula is a stellar nursery, with several regions of gas and dust where stars are actively forming, including the pillars.

It’s hard to fathom the true size of the pillars. What looks like an image of dark clouds is actually about 10 light years across. If you look for the reddish star in the right pillar, and move left until you reach a reddish star in the middle pillar, that is roughly the distance from the Sun to Alpha Centauri (about 4 light years). So these pillars would span a large region of our local neighborhood. Of course the Pillars of Creation is just one small section of the much larger Eagle nebula, which spans more than 100 light years.

The Eagle nebula is only about 7000 light years away, so we are able to get a good view of this region. We can see regions where stars are being formed, and regions where gas and dust are being pushed away by intense radiation. Because of this, the pillars are shifting, though on a very slow time scale. For example, images from the Spitzer infrared telescope hint at a high velocity shock wave (possibly from a supernova) heading toward the pillars. Given the speed shock wave, it will begin to disperse the pillars in another millennia or so.

Of course, since the pillars are 7000 light years away, that would mean the shock wave dispersed the pillars about 6000 years ago. So depending on your definition of “now” it’s likely that the pillars no longer exist.

When we look at images such as the Pillars of Creation, they seem to be constant and unchanging, but in fact nebulae change incredibly fast on the cosmic scale. Within a few million years, stars will have formed, the remaining dust and gas will have been dispersed, and the Eagle nebula will be no more.

We’ve simply captured a moment of stellar creation in a very big universe.

The post Pillars of Creation appeared first on One Universe at a Time.

There’s a lot of gas and dust in the universe. Some of it has coalesced into dark nebulae, such as bok globules that almost look like holes in the starry night. We can observe these by the background light they absorb. Some clouds of dust are close enough to a star that light reflects off them, creating reflection nebulae such as the one near T Tauri. But sometimes a cloud of gas and dust is near a hot star, but too diffuse to scatter light much. In this case it can produce a faint nebula known as an emission nebula.

A typical emission nebula spectrum. Credit: Les Tomley

Since hydrogen is by far the most common element in the universe, these nebulae are mostly made up of hydrogen. When ultraviolet light from a nearby star strikes the hydrogen, the gas becomes partially ionized. When the hydrogen recombines, it emits light. Because the light emitted is mainly at red wavelengths, these emission nebulae typically appear as faint red clouds.

In astronomy these nebulae are often known as H II regions due to their quantities of ionized hydrogen. Because of their spectrum they can be identified in other galaxies, and since they only appear near very bright (and thus short lived) stars, they can be used to measure stellar production rates in galaxies. When both the H II region and bright nearby stars can be observed, we can also get an idea of a galaxy’s distance.

A few of the brightest nearby H II regions can be seen with the naked eye, but they weren’t noticed until after the introduction of telescopes. With modern astrophotography we can get wonderful images of these faint hydrogen clouds. Most of the popular images of nebulae have brilliant reflection regions and sharply contrasting absorption regions. But the faint emission nebulae have a beauty all their own, and they help us better understand both our own galaxy and others.

The post Red Light District appeared first on One Universe at a Time.

The visible universe is vast. It is 93 billion light years across, and contains more than 100 billion galaxies. The average galaxy contains about 100 billion stars, and untold numbers of planets. Yet a century ago there was serious doubt among many astronomers that the universe was much more than 100,000 light years across. Arguments about whether the universe was small or large became known as the Great Debate.

It is often known as the Shapley-Curtis debate, so named after Harlow Shapley and Heber Curtis, and a public debate they had in 1920. Shapley, you may remember, used observations of globular clusters to correctly show that the Sun is not the center of our galaxy. Curtis was an astronomer who studied nebulae, as well as solar eclipses.

The debate centered on the distance to certain nebulae. At the time, “nebula” referred to anything (excluding comets) that appeared “fuzzy” rather than distinct like a star or planet. So things like the Orion nebula (a stellar nursery), the Crab nebula (a supernova remnant) were considered nebulae just as they are today, but what we now call galaxies were also known as nebulae. The Andromeda galaxy, for example, was known as the Great Andromeda Nebula.

Curtis argued that Andromeda and other spiral nebulae were in fact “island universes”, similar in size to our own Milky Way “universe”. This would mean that not only were these nebulae 100,000 light years across or more, they must be millions of light years away. He based this argument on the fact that more novae were observed in Andromeda alone than were observed in the entire Milky Way. Why would that be the case if Andromeda were small and close. He also noted that some spiral nebulae had rather large redshifts, meaning that they were moving much faster than other objects in the universe.

Shapley argued that what we now call the Milky Way galaxy was the bulk of the universe. Spiral galaxies such as Andromeda must be relatively close and small. He based this view on several points. In 1917 Shapley and others observed a nova in the Andromeda nebula. For a brief time the nova outshined the central region of Andromeda. If Andromeda were a million light years away, as Curtis contended, then this nova (we now know it was a supernova) would need to be far brighter than any known mechanism could produce. There were also observations of the Pinwheel galaxy, seen above, by Adriaan van Maanen. He claimed that the Pinwheel had visibly rotated over the span of years. If the Pinwheel was rotating as van Maanen observed, then it couldn’t possibly be 100,000 light years across. For an object that large to rotate in a matter of years the stars would need to move faster than light.

After the debate the general opinion was that Shapley had won. His own observations of the shape of the Milky Way and the 1917 supernova, and van Maanen’s observations gave the small universe model solid footing. Besides, the idea that objects could be millions of light years away seemed patently absurd.

But Shapley was wrong in this case.

In 1912 Henrietta Leavitt discovered that Cepheid variable stars vary at a rate proportional to their brightness. Her work wasn’t widely accepted initially, but after more prominent astronomers such as Edwin Hubble supported her results it became accepted as an observational tool.

In 1925 Edwin Hubble used Leavitt’s period-luminosity relation to precisely determine the distance to the Andromeda galaxy. He demonstrated conclusively that Andromeda was about 2 million light years away. He went on to show that other “nebulae” were in fact distant galaxies. By 1929 Hubble demonstrated a relation between the distance of a galaxy and its redshift, and thus that the universe was expanding.

Thus we came to know that our Milky Way is an island galaxy in a much larger universe.

The post Island Universe appeared first on One Universe at a Time.

A rogue planet is a planet-sized object that doesn’t orbit a star. Instead these objects move through the galaxy just as stars do. They’ve long been thought to exist, but their small size and low temperatures made them difficult to observe.

Then in 2011, a team looked at data from the Optical Gravitational Lensing Experiment (OGLE), which observed 50 million stars in our galaxy looking for momentary brightenings of a star due to an effect known as microlensing. If a massive object passes in front of our line of sight to a star, the light from the star is gravitationally lensed around the object, and this microlensing effect can make the star appear brighter.

The team observed over 400 microlensing events, out of which 10 were due to Jupiter-mass planets not orbiting a star. From these observations, the team estimated that there are likely two rogue planets for every star in the galaxy. That would mean there are 200 to 800 billion rogue planets in our galaxy alone.

Since these rogue objects don’t orbit a star, you might wonder why they would be called “planets” in the first place. After all, asteroids are moon-sized objects that orbit the Sun like planets, and we don’t call them “rogue moons”. The reason they are often referred to as planets is that they are generally thought to have once been planets orbiting a star, but were then ejected from the stellar system due to a close encounter with either another planet in the system or another star. The asteroids, in contrast, formed in orbit around the Sun, and were not once moons around a particular planet, which is why they would not be considered rogue moons.

But new evidence suggests that not all rogue planets begin their lives orbiting a star. In a recent paper in Astronomy and Astrophysics, observations of the Rosette Nebula found dust clouds smaller than the orbit of Neptune, with masses of about 10 Jupiters. These can be seen in the image above.

Small clouds such as these, known as globulettes, have been observed before. But these particular globulettes have two properties which make them very interesting. Observations of their radio emissions find that they are unusually dense. This means that despite their small size, they are likely to gravitationally collapse into planet-mass objects. The second is that they are moving exceptionally fast away from the nebular region, about 80,000 kilometers per hour (50,000 mph). At this speed they will escape the gravitational pull of the nebula to become freely roaming objects. This is the first evidence that rogue planets can be born free.

It would seem then that referring to these rogue objects as “planets” might be a bit unfounded. Some have proposed calling them Interstellar Planetary-Mass Objects, while the International Astronomical Union (IAU) has proposed calling them sub-brown dwarfs. But the term “rogue planet” will likely be used for quite a while.

Of course the term rogue planet also raises images of a planet racing through our solar system, either colliding with Earth or sending it into a dangerous orbit. This idea was popularized by the 1951 movie When Worlds Collide, based on the book of the same name. In reality such an event is extraordinarily unlikely. The odds of a dangerous close approach is essentially zero, and even if a rogue planet passed close to our solar system the planets would be hardly affected.

So while some planets go rogue, and others are born rogue, they aren’t anything to be feared.

And none of them are named Nibiru…

The post Going Rogue appeared first on One Universe at a Time.