The chemical interaction between the gas and dust in interstellar space can be quite complex. Given the low temperature and pressure of space, one might think that chemical reactions are simple and rare, but as I’ve written about before, the surface of dust grains can act as a kind of catalytic converter that allows complex chemistry to occur.

Most of the evidence of this complex chemistry comes from the detection of molecules in interstellar gas clouds. These molecules can be detected by their emission and absorption spectra, so we know they are there. The details of how they form are largely based on Earth-based experiments and computer simulations of chemical reactions. But now a recent paper in the Astrophysical Journal presents evidence of these reactions in space.

The authors looked at molecular hydrogen (two hydrogen atoms bonded together) in a nebula known as IC 63. Molecular hydrogen is very common in interstellar space, but it cannot form between hydrogen atoms alone. Instead they need some other molecule or body to take away a bit of their energy so they can chemically bond.

It has long been thought that dust grains serve this role. A single hydrogen atom adheres to the surface of a grain of interstellar dust, then another hydrogen atom adheres to the surface. The two hydrogen atoms shift along the surface until they meet each other. They then bond to form molecular hydrogen and release from the dust grain.

When the hydrogen molecule pushes off from the dust grain, it’s like a burst of gas from a rocket engine. As a result, the dust grain is caused to spin rapidly. This process is known as a Purcell rocket, after Edwin Purcell who first proposed the idea. As the dust grain spins rapidly, any static charge on grain (which is common) will create a magnetic field like a small magnet. This will cause the dust grain to align with the magnetic fields of interstellar space, similar to the way a magnetized needle will try to point north.

At least that has been the theory. Now if this is right, then regions where molecular hydrogen is being produced should also be a region where dust grains are aligned with the magnetic field of interstellar space. This is exactly what the authors found. They first looked at the distribution of molecular hydrogen in IC 63, but then they looked at light coming from more distant stars and passing through IC 63. Specifically, they looked at the polarization of the starlight. Polarization is an orientation that light can have. If the dust grains of IC 63 are all aligned in a similar direction, then the starlight passing through the nebula should be polarized in the same orientation.

You can see the results in the figure above. The green and red regions are where molecular hydrogen is being produced, and the white lines show the polarization of starlight passing through the nebula. As you can see, the lines are aligned in the region where molecular hydrogen is most prominent. This is exactly what the Purcell rocket model predicts.

So with a little rocket science we can see astrochemistry in action.

Paper: B-G Andersson et al. EVIDENCE FOR H2 FORMATION DRIVEN DUST GRAIN ALIGNMENT IN IC 63.  ApJ 775 84 (2013)

The post Purcell Rockets appeared first on One Universe at a Time.

Yes, there is a giant cloud of alcohol in outer space. It’s in a region known as W3(OH), only about 6500 light years away. Unfortunately it is methyl alcohol (commonly known as wood alcohol, though this stuff is not derived from wood), so it isn’t suitable for drinking. There is some ethyl alcohol (the drinkable kind) there as well, but it’s not nearly as common.

It might seem strange that there are alcohol clouds in space, but as noted in an earlier post, a great deal of complex molecular chemistry goes on between the molecular clouds and dust in outer space, and all sorts of chemical compounds exist there. Alcohol is a relatively simple molecule, made of relatively abundant elements (hydrogen, carbon, oxygen), so it shouldn’t be surprising that it exists in large quantities in space.

While mention of an alcohol space cloud usually leads to snickers, there is actually some interesting astrophysics going on. Because of the abundance of similar simple molecules, adding a bit of energy to the mix can lead to a stimulated emission of light, known as an astrophysical maser. The term maser stands for Microwave Amplification by Stimulated Emission of Radiation. When the same effect occurs with visible light, it is called a laser. In fact the early lasers were known as optical masers. It’s the stimulated part that makes lasers and masers particularly interesting.

Normally when an atom or molecule emits light, it happens randomly. The electrons of an atom have a bit of extra energy (are in an excited state), and they drop to a lower energy level by emitting a photon. This is known as spontaneous emission. Because of the quantum behavior of electrons, they can only move between specific discrete energy levels. This means the photons an atom or molecule emits have specific energies, and thus specific colors. This is also why emission spectra have specific patterns.

But if an electron is in an excited state, and the molecule is struck by a photon, then the electron can be triggered to drop to a lower energy level and emit a photon. This is known as stimulated emission. The catch is that stimulated emission can’t be triggered by just any photon, it has to be triggered by a photon of the same energy the electron will emit. So what happens is a photon of just the right energy strikes the molecule, triggering a stimulated emission, and then the first photon and a new photon of the same energy go along their way.

With stimulated emission, a photon can trigger a molecule to release a photon, and they can trigger even more molecules to release photons, causing a cascade of stimulated emission. Because the photons all move together (are coherent) the emission is very bright. Because they all have the same energy or color (are monochromatic), they are bright at that particular wavelength.

With common lasers, the excited molecules are contained within an oscillation cavity, so the stimulated emission photons pass back and forth within the cavity (with each pass triggering more stimulated emission) before finally leaving as a very bright, monochromatic beam of light. This is why tiny laser pointers can make such a bright dot on your wall. Astrophysical masers don’t have an oscillation cavity, so the stimulated photons only make a single pass through the excited molecules. So the microwaves emitted aren’t quite as intense as the term “microwave laser” suggests. Still, astrophysical masers produce bright light at a narrow range of wavelengths.

So what does this have to do with cosmic alcohol clouds? In order to trigger an astrophysical maser you need to have the right conditions. For one, you need a type of molecule with strong emission lines, such as methyl alcohol molecules. You also need them to be fairly concentrated, so that stimulated photons can hit other molecules to cause a cascade. An interstellar cloud of methyl alcohol will do nicely. Finally you need an energy source, such as protostars as they begin to heat up.

Those exact conditions exist in W3(OH), because the alcohol cloud surrounds a stellar nursery. You can see this region in the image above. Here the alcohol cloud is seen as a false-color region, while the white dots indicate the locations of astrophysical masers.

When masers were first invented in the 1950s, they were thought to be an entirely human creation. We now know that masers are a natural occurrence.

All we needed was a bit of proof.

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

Between the vast expanse of stars in our galaxy there is diffuse gas, dust and plasma known as the interstellar medium. It has been known for quite some time through its effects on radio waves and other light sources. But making a detailed map of this medium has been difficult.

When x-ray telescopes were developed in the 1970s, one of the things they began to observe was a cosmic x-ray background. Over the years there has been some debate as to the source of these soft x-rays. One idea was that they are due to the solar wind. As ionized particles streaming from the Sun collide with neutral hydrogen in the interstellar medium they can create x-rays. Another idea is that they come directly from the interstellar medium via ionized plasma. Now a new paper in Nature shows that it is the latter more than the former.

The team analyzed x-ray data from the ROSAT x-ray sky survey and other observations to demonstrate that only about 40% of the x-ray background can come from solar wind interactions. The rest must come from interstellar plasma.  What’s interesting about this is that these soft x-rays tend to be absorbed by interstellar gas, so there must be less interstellar gas in our corner of the galaxy than previously suspected. In other words, our Sun exists in a local bubble of low density.

These new results confirm other observations that point to the existence of a local bubble. It’s estimated that the density of the interstellar medium within this bubble is less than a tenth that of other regions in our galaxy. The bubble doesn’t just surround our Sun, but a local region encompassing several stars, as seen in the image above. Of course this raises the question as to the cause of such a bubble. It is thought that such bubbles are created by supernovae, which clear out regions of space. For our own local bubble, the likely candidate is a neutron star known as Geminga. Geminga was likely formed about 300,000 years ago when its progenitor star exploded.

Wider surveys of our region of the Milky Way hint at similar bubbles likely caused by other supernovae and interstellar wind. This paints a picture of our galaxy filled with low-density bubbles between which are higher density regions. At the intersection of two bubbles, shock waves of interstellar dust can occur, which may lead to stellar nurseries and star production.

Paper: Galeazzi et al. “The origin of the local 1/4-keV X-ray flux in both charge exchange and a hot bubble.” Nature online, 27 July 2014.

The post Bubble Pop appeared first on One Universe at a Time.

Earth gets bombarded from space all the time. With all the micrometeorites, not to mention the occasional meteor or comet, almost 50 tons of material falls to Earth every day. Since all this material from space reaches Earth, you might think that interstellar material falls to our planet all the time. But it turns out that isn’t the case. The vast majority of material comes from within our solar system. Even cometary material comes from the Oort cloud at best, and that’s still on the outer edge of our solar system. There is plenty of interstellar gas and dust beyond our solar system, but even when some of it heads our way it tends to be pushed away from the inner solar system by the solar wind. So we haven’t had any samples of interstellar dust to study.

But now that may have changed. A new paper published in Science has announced the discovery of dust grains that appear interstellar in origin. The samples were gathered by NASA’s Stardust spacecraft, which made a flyby of the comet 89P/Wild. The spacecraft was equipped with aerogel blocks to collect samples of cometary dust.  By analyzing the tracks made by dust fragments as they are caught by the aerogel, the team could determine their trajectory of origin. Part of the reason for doing this is to distinguish cometary fragments from tiny fragments of material that came from the spacecraft itself.

An interstellar dust grain caught by aerogel.
Credit: Rhonda Stroud, Naval Research Laboratory

When analyzing the tracks, the team found seven dust grains with tracks that seem to be of interstellar origin. Initial analysis of the particles finds that they don’t match any single model for interstellar dust formation. So while these models might be on the right track, they don’t provide the whole picture.

This project is also an example of how citizen science can be successful. To analyze the trajectories of particles in the aerogel, the team used a project called stardust@home, where volunteers can help determine particle trajectories. A significant portion of the trajectories were determined through this project. Which just proves the point that you don’t have to be a scientist to do real science.

Paper: Andrew J. Westphal, et al. Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Vol. 345 no. 6198 pp. 786-791 (2014)

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

When light passes through gas and dust in the interstellar medium, some of the light is absorbed. Since the gas and dust only absorb certain wavelengths or colors of light, by by looking at these absorption bands we can determine the type of material that makes up the interstellar media.  Well, most of the time. It turns out there are a range of absorption bands that we haven’t been able to identify. They are known as diffuse interstellar bands.

One of the difficulties with these diffuse interstellar bands is that they don’t seem to match any known atoms or molecules. We know there are molecules that could form within the interstellar medium, but many of them haven’t been analyzed in the labs. Analyzing the line spectra of different compounds in a range of conditions such as vacuum and low temperatures is time consuming. Since it isn’t very glamorous, it doesn’t tend to get much funding, and that limits our ability to analyze the bands.

Another challenge is that different bands can be stronger or weaker relative to each other. This means they are likely due to a range of processes.  There is some evidence that the strength of the bands correlates with the amount of dust in the region, so they are likely related to some kind of dust feature.  Right now one of the favored ideas is that they are due to some kind of larger hydrocarbon molecule. But as for the details, we just don’t know.

The post Battle of the Bands appeared first on One Universe at a Time.

Usually in astronomy we study objects by the amount of light they emit.  Most regular matter gives of light in some form or another.  Even the cold interstellar medium will emit some light at infrared or radio wavelengths. But one downside of this is that the light generally comes from the surface regions of an object.  To study the interior of an object we generally have to use aspects of emitted light from the surface to determine properties of the interior.  For bright objects like stars this works pretty well, but for dim objects like dark interstellar clouds this is more of a challenge.  

Recently a team with the Spitzer space telescope has used a different method.  Spitzer is a sensitive infrared telescope, and the team has been using it to observe cold, dense interstellar clouds by the infrared light that passes through them.  There is a great deal of infrared light in the universe, and when that ambient background light passes through a dark cloud we can determine things like its density and composition from the light they absorb.  Basically it is a way to study dark clouds by the shadows they cast.

Some of the team’s results were recently published in the Astrophysical Journal Letters.  One of the things they’ve announced is the darkest and densest interstellar cloud ever discovered.  It has a mass of about 70,000 Suns, and is only 50 light years across.  This cloud is probably in the earliest stages of collapsing into a cluster of large and bright stars (O-type stars).  Gaining a better understanding of dense clouds like this one will help us understand just how such large stars form.

Paper: Michael J. Butler et al. The Darkest Shadows: Deep Mid-infrared Extinction Mapping of a Massive Protocluster. ApJ 782 L30. (2014) doi:10.1088/2041-8205/782/2/L30

The post Shadow Facts appeared first on One Universe at a Time.

Stars form within large clouds of gas and dust known as stellar nurseries. Of course, when a star forms, that leaves less gas and dust to form other stars. So you can do a bit of simple math concerning star formation. Take the rate at which new stars form in a galaxy (and their typical mass), compare that to the amount of gas and dust a galaxy has, and you can estimate the time over which stars can form.

For a spiral galaxy such as the Milky Way, this calculation tells us that, at the given rate of star formation, it can only keep producing stars for about a billion years. This raises a bit of a puzzle, because the Milky Way is far older than a billion years, so how does it have enough gas and dust to keep producing stars?

Part of it comes from the gas and dust given off by supernovas, but that isn’t sufficient to keep things going over billions of years. It has been assumed that another source comes from outside the galaxy itself. We know that galaxies tend to cluster together. Our local group, for example consists of the Milky Way, the Andromeda and Triangulum spiral galaxies, and lots of other smaller galaxies. Since galaxies cluster, it is reasonable to assume that intergalactic hydrogen also clusters. This intergalactic gas would gradually be attracted to nearby galaxies, where it could become part of stellar nurseries.

Credit: Wolfe, et al

But mapping this intergalactic hydrogen is difficult, because it is generally cold and neutral. So while we’ve suspected it as a source of gas, its been hard to prove. This month in Nature researchers presented a map of neutral hydrogen between the Andromeda and Triangulum galaxies, and the results support the idea of intergalactic gas as a source for stellar formation.

You can see an image of this gas in the figure. The upper figure is a high resolution image, while the lower figure is an averaged image. What it shows is that about half the gas is clumped into discrete regions, while the other half is relatively diffuse. The total mass of this gas is about 2.6 million solar masses, which is plenty of material for star formation.

It seems even galaxies have a gas station.

The post It’s a Gas appeared first on One Universe at a Time.