[/caption]
If you’ve been out at night, when the air is clear, the Moon is on the other side of the world, and city lights are far, far away, you’ve almost certainly seen a dark nebula or two. In the southern hemisphere, you’ll have seen the Coalsack; anywhere in the world, the Great Rift, that divides much of the Milky Way in two.
And that’s the distinguishing feature of a dark nebula – it’s only dark because it’s surrounded by brighter parts of the sky, whether a great swathe of the Milky Way (stars and emission nebulae), or just a part of an emission nebula (the Horsehead Nebula is perhaps the most famous of this kind), or something in between.
In fact, in some cultures, it’s patterns of dark nebulae which make up the memorable sky, like the Emu in the Sky of many of the Australian aboriginal tribes.
Dark nebulae are dark principally because they contain dust, which is interstellar grains a few microns across (actually, their sizes range from a few tens of nanometers to millimeters), mostly dirty graphite, various ices (or icy mixtures), various silicates, some carbon-based goo, and mixtures of these. Most dark nebulae are associated with, or part of, giant molecular clouds, which are perhaps the most distinct phase of the interstellar medium; they can have masses up to a million sols and measure up to a few parsecs across. In shape, dark nebulae come in a bewildering range, from amorphous blobs, to almost round disks, to sinuous snake-like things, to what look like negative clouds.
When we see a spiral galaxy on its side (or nearly so), it’s often split by a dust lane, or nearly so … which is just all the dark nebulae in the disk of that galaxy viewed (nearly) edge on; M64, M65, M104, and NGC 891 are good examples.
When we raise an apple up to a height of one meter, we perform approximately one joule of work. So what is a joule?
Joule is the unit of energy used by the International Standard of Units (SI). It is defined as the amount of work done on a body by a one Newton force that moves the body over a distance of one meter. Wait a minute … is it a unit of energy or a unit of work?
Actually, it is a unit of both because the two are interrelated. Energy is just the ability of a body to do work. Conversely, work done on a body changes the energy of the body. Let’s go back to the apple example mentioned earlier to elaborate.
An apple is a favorite example to illustrate a one joule of work when using the definition given earlier (i.e., the amount of work done ….) because an apple weighs approximately one Newton. Thus, you’d have to exert a one Newton upward force to counteract its one Newton weight. Once you’ve lifted it up to a height of one meter, you would have performed one joule of work on it.
Now, how does energy fit into the picture? As you perform work on the apple, the energy of the apple (in this case, its potential energy) changes. At the top, the apple would have gained about one joule of potential energy.
Also, when the apple is one meter above its original position, say the floor, gravity would have gained the ability to do work on it. This ability, when measured in joules, is equivalent to one joule.
Meaning, when you release the apple, the force of gravity, which is simply just the weight of the body and equivalent to one Newton, would be able to perform one joule of work on it when the apple drops down from a height of one meter.
Mathematically, 1 joule = 1 Newton ⋅ meter. However, writing it as Newton ⋅ meter is discouraged since it can be easily confused with the unit of torque.
Particle physics experiments deal with large amounts of energies. That is why it is also known as high energy physics. If you liked our answer to the question, “What is a Joule?”, you might want to read the following articles from Universe Today:
When a nucleus undergoes radioactive decay – or decays, radioactively – it changes its state to one of a lower energy, and emits a particle (sometimes more than one), a gamma ray, or both (and one type of radioactive decay involves the absorption, or capture, of an electron as well emission of a particle).
Among radioactive materials which occur naturally here on Earth, two kinds of radioactive decay are common: alpha (α) and beta (β). They get their names from the most obvious particles emitted, an alpha particle (which is the nucleus of the stable isotope of helium called helium-4) or a beta particle (which is either an electron or a positron; the positron is the antimatter counterpart to the electron). In either kind of decay a photon with gamma ray energy may be emitted too, and in beta decay a neutrino is nearly always emitted (antineutrino if it’s electron-type beta decay, neutrino if positron-type).
In the lab, and out in space, there are atomic nuclei which undergo radioactive decay in other ways – by emitting a proton, for example; these types of decay occur in isotopes which have very short lives.
You’ve heard of Schrödinger’s poor cat, right? Well, not so poor, because it’s a thought experiment (no real cat involved), but it’s a good device for understanding something rather quirky about radioactive decay. You see, if you have a few billion atoms of a radioactive isotope, potassium-40 say, you can say with great certainty how many will decay in the next year. However, you cannot say which particular nuclei will decay!
Radioactive decay is very important for a wide range of human activities, from medicine to electricity production and beyond, and also to astronomers. For example, the characteristic light curve of Type Ia supernovae – which are used to estimate the age of the universe (among other things) – comes from the decay of a radioactive isotope of nickel (nickel-56, and its daughter isotope, cobalt-56), produced in copious quantities by the suicidal star.
There’s a lot of material, out there on the web, on radioactive decay; here are some good links for you to click on: Radioactive Decay in Supernova Remnants (NASA), Radioactive Decay (Carlton College), and Decay (an applet, Michigan State University).
“Dark matter”, in astronomy, usually means “cold, non-baryonic dark matter”. This is a form of mass which reacts with other matter via only gravity – and, possibly, the weak force – and which comprises approximately 80% of all matter in the universe. There is also “baryonic dark matter”, which is just ordinary matter, like dust, gas, rocks, and even stars that does not emit radiation yet detected by our telescopes (or absorb it, from more distant sources). And there is also “hot, non-baryonic dark matter”, which is just neutrinos.
The first hints of the existence of dark matter came from an analysis of the line-of-sight velocities of galaxies in the Coma cluster, by Fritz Zwicky, in the early 1930s. Zwicky found that the galaxies are moving much too fast for them to be held together in a cluster, by gravity, if the only mass in the cluster is that in the galaxies themselves (it’s pretty obvious that the galaxies form a bound system). Since Zwicky could find no evidence of mass in the Coma cluster, from the light detected by the telescopes he used, other than in the galaxies, he postulated that there is a lot of matter that is ‘dark’ – does not emit light.
Fast forward to the early 1970s, and the discovery of diffuse x-ray emission from the Perseus and Coma clusters.
Zwicky was right, the Coma cluster contains a great deal of mass outside the galaxies, and that matter does not emit light (it emits x-rays), because it is very hot. But this thin plasma is still not enough, mass-wise, to explain why the galaxies are gravitationally bound to the cluster (and the Coma cluster is nothing special; today we know of thousands of clusters just like it). Further, the plasma is also gravitationally bound to the cluster, but does not have enough mass itself to keep it there. Some more mass is needed, and that mass is dark matter.
Around the same time, Kent Ford and Vera Rubin made a similar discovery, concerning spiral galaxies; namely that they must contain a lot more matter than could be inferred from the stars, gas, and dust observed by various telescopes, in order for the galaxies to be rotating as fast as they are. Dark matter had been discovered in galaxies.
How many oceans are there in the world? This question may not be as easy to answer as you may think. First we need to see the origins of the word ocean. The Ancient Greeks gave us the word ocean and it described what was to them the outer sea that surrounded the known world. Even then the ancients later believed that there were only 7 seas, the Mediterranean, the Caspian, the Adriatic, the Red Sea, the Black Sea, the Persian Gulf and the Indian Ocean.
The number of oceans in the world varies on how you look at it. From the scientific point of view there is only one major ocean called the World Ocean and if you include inland seas such as the Black Sea and Caspian Sea there are 3. The scientific method of counting oceans looks at saline bodies of water that have oceanic crust.
Another way to look at it is to divide the world ocean by the different continents and other major geographic regions it touches. Using this method there are 5 oceans. There is the Atlantic Ocean which separates the American Continents from Europe and Africa. Then there is the Pacific which separates Asia and the Americas. The Southern Ocean is tricky but is named as such because it encircles Antarctica touches Australia and the southern end of South America. The Indian Ocean is named after Indian subcontinent. The Arctic Ocean is named for its location north of all the continents and being the North Pole. Originally only the Southern Ocean was not officially recognized so this only demonstrates how the designation can easily change.
The way you count the oceans can vary depending on your profession or understanding of the Ocean. Either way you look at the large bodies of salt water are very important. They are a major source of food, regulate the Earth’s climate and are the major source water for all life.
So in the end it becomes not so important to know how many oceans there are but what the ocean is and how important it is to life on this planet.
If you enjoyed this article there are several other articles on Universe Today that you will like and find interesting. There is a great article on sea floor spreading and another interesting piece on ancient oceans.
You can also find some great resources on oceans online. You can learn more about oceans currents and how they affect climate. You can also learn about Ocean Biomes on University of Richmond website.
You should also check out Astronomy Cast. Episode 143 talks about astrobiology.
This Hubble Space Telescope image of young brown dwarf 2M J044144 show it has a companion object at the 8 o'clock position that is estimated to be 5-10 times the mass of Jupiter.Credit: NASA, ESA, and K. Todorov and K. Luhman (Penn State University)
[/caption]
Big planet or companion brown dwarf? Using the Hubble Space Telescope and the Gemini Observatory, astronomers have discovered an unusual object orbiting a brown dwarf, and its discovery could fuel additional debate about what exactly constitutes a planet. The object circles a nearby brown dwarf in the Taurus star-forming region with an orbit approximately 3.6 billion kilometers (2.25 billion miles) out, about the same as Saturn from our sun. The astronomers say it is the right size for a planet, but they believe the object formed in less than 1 million years — the approximate age of the brown dwarf — and much faster than the predicted time it takes to build planets according to conventional theories.
Kamen Todorov of Penn State University and his team conducted a survey of 32 young brown dwarfs in the Taurus region.
The object orbits the brown dwarf 2M J044144 and is about 5-10 times the mass of Jupiter. Brown dwarfs are objects that typically are tens of times the mass of Jupiter and are too small to sustain nuclear fusion to shine as stars do. Artist's conception of the binary system 2M J044144. Science Credit: NASA, ESA, and K. Todorov and K. Luman (Penn State University) Artwork Credit: Gemini Observatory, courtesy of L. Cook
While there has been a lot of discussion in the context of the Pluto debate over how small an object can be and still be called a planet, this new observation addresses the question at the other end of the size spectrum: How small can an object be and still be a brown dwarf rather than a planet? This new companion is within the range of masses observed for planets around stars, but again, the astronomers aren’t sure if it is a planet or a companion brown dwarf star.
The answer is strongly connected to the mechanism by which the companion most likely formed.
The Hubble new release offers these three possible scenarios for how the object may have formed:
Dust in a circumstellar disk slowly agglomerates to form a rocky planet 10 times larger than Earth, which then accumulates a large gaseous envelope; a lump of gas in the disk quickly collapses to form an object the size of a gas giant planet; or, rather than forming in a disk, a companion forms directly from the collapse of the vast cloud of gas and dust in the same manner as a star (or brown dwarf).
If the last scenario is correct, then this discovery demonstrates that planetary-mass bodies can be made through the same mechanism that builds stars. This is the likely solution because the companion is too young to have formed by the first scenario, which is very slow. The second mechanism occurs rapidly, but the disk around the central brown dwarf probably did not contain enough material to make an object with a mass of 5-10 Jupiter masses.
“The most interesting implication of this result is that it shows that the process that makes binary stars extends all the way down to planetary masses. So it appears that nature is able to make planetary-mass companions through two very different mechanisms,” said team member Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University.
If the mystery companion formed through cloud collapse and fragmentation, as stellar binary systems do, then it is not a planet by definition because planets build up inside disks.
The mass of the companion is estimated by comparing its brightness to the luminosities predicted by theoretical evolutionary models for objects at various masses for an age of 1 million years.
Further supporting evidence comes from the presence of a very nearby binary system that contains a small red star and a brown dwarf. Luhman thinks that all four objects may have formed in the same cloud collapse, making this in actuality a quadruple system.
“The configuration closely resembles quadruple star systems, suggesting that all of its components formed like stars,” he said.
The team’s research is being published in an upcoming issue of The Astrophysical Journal.
3Star-birth in SMM J2135-0102 (Credit: M. Swinbank et al./Nature, ESO, APEX; NASA, ESA, SMA)
[/caption]
Einstein started it all, back in 1915.
Eddington picked up the ball and ran with it, in 1919.
And in the last decade or so astronomers have used a MACHO to OLGECASTLES … yes, I’m talking about gravitational lensing.
Now LABOCA and SABOCA are getting into the act, using Einstein’s theory of general relativity to cast a beady eye upon star birth most fecund, in a galaxy far, far away (and long, long ago).
APEX at Chajnantor (Andreas Lundgren)
How galaxies evolved is one of the most perplexing, challenging, and fascinating topics in astrophysics today. And among the central questions – as yet unanswered – are how quickly stars formed in galaxies far, far away (and so long, long ago), and how such star formation differed from that which we can study, up close and personal, in our own galaxy (and our neighbors). There are lots of clues to suggest that star formation happened very much faster long ago, but because far-away galaxies are both dim and small, and because Nature drapes veils of opaque dust over star birth, there’s not much hard data to put the numerous hypotheses to the test.
Until last year that is.
“One of the brightest sub-mm galaxies discovered so far,” say a multi-national, multi-institution team of astronomers, was “first identified with the LABOCA instrument on APEX in May 2009” (you’d think they’d give it a name like, I don’t know, “LABOCA’s Stunner” or “APEX 1”, but no, dubbed “the Cosmic Eyelash”; formally it’s called SMMJ2135-0102). “This galaxy lies at [a redshift of] 2.32 and its brightness of 106 mJy at 870 μm is due to the gravitational magnification caused by a massive intervening galaxy cluster,” and “high resolution follow-up with the sub-mm array resolves the star-forming regions on scales of just 100 parsecs. These results allow study of galaxy formation and evolution at a level of detail never before possible and provide a glimpse of the exciting possibilities for future studies of galaxies at these early times, particularly with ALMA.” Nature’s telescope giving astronomers ALMA-like abilities, for free.
OK, so what did Mark Swinbank and his colleagues find? “The star-forming regions within SMMJ2135-0102 are ~100 parsecs across, which is 100 times larger than dense giant molecular cloud (GMC) cores, but their luminosities are approximately 100 times higher than expected for typical star-forming regions. Indeed, the luminosity densities of the star-forming regions within SMMJ2135-0102 are comparable to dense GMC cores, but with luminosities ten million times larger. Thus, it is likely that each of the star-forming regions in SMMJ2135-0102 comprises ~ten million dense GMC cores.” That’s pretty mind-blowing; imagine the Orion Nebula (M42, approximately 400 parsecs distant) as one of these star-forming regions!
James Dunlop of the University of Edinburgh suggests that such galaxies as SMMJ2135-0102 formed stars so abundantly because the galaxies still had plenty of gas – the raw material for making stars – and the gravity of the galaxies had had enough time to pull the gas together into cold, compact regions. Before about 10 billion years ago, gravity hadn’t yet drawn enough clumps of gas together, while at later times most galaxies had already run out of gas, he suggests.
But I’m saving the best for last: “the energetics of the star-forming regions within SMMJ2135-0102 are unlike anything found in the present day Universe,” Swinbank et al. write (now there’s an understatement if ever I’ve heard one!), “yet the relations between size and luminosity are similar to local, dense GMC cores, suggesting that the underlying physics of the star-forming processes is similar. Overall, these results suggest that the recipes developed to understand star-forming processes in the Milky Way and local galaxies can be used to model the star formation processes in these high-redshift galaxies.” It’s always good to get confirmation that our understanding of the physics at work so long ago is consistent and sound.
Einstein would have been delighted, and Eddington too.
Sources: “Intense star formation within resolved compact regions in a galaxy at z = 2.3” (Nature), “The Properties of Star-forming Regions within a Galaxy at Redshift 2” (ESO Messenger No. 139), Science News, SciTech, ESO. My thanks to debreuck (ESO’s Carlos De Breuck?) for setting the record straight re the name.
As with last week’s Universe Puzzle, something that cannot be answered by five minutes spent googling, a puzzle that requires you to cudgel your brains a bit, and do some lateral thinking. This is a puzzle on a “Universal” topic – astronomy and astronomers; space, satellites, missions, and astronauts; planets, moons, telescopes, and so on.
This puzzle is actually from Universe Today reader, Vino; thanks Vino!
What comes next in the sequence?
0.789, 0.854, 0.941
UPDATE: Answer has been posted below.
1.091
These are the periods, in days, of the transiting extrasolar planets so far discovered, in ascending order (source): WASP-19b, CoRoT-7b, WASP-18b, and WASP-12b.
And now there’s a paper which seems, at last, to explain the observations; the cause is, apparently, a complex interplay of gravity, angular motion, and star formation.
[/caption]
It is now reasonably well-understood how supermassive black holes (SMBHs), found in the nuclei of all normal galaxies, can snack on stars, gas, and dust which comes within about a third of a light-year (magnetic fields do a great job of shedding the angular momentum of this ordinary, baryonic matter).
Also, disturbances from collisions with other galaxies and the gravitational interactions of matter within the galaxy can easily bring gas to distances of about 10 to 100 parsecs (30 to 300 light years) from a SMBH.
However, how does the SMBH snare baryonic matter that’s between a tenth of a parsec and ~10 parsecs away? Why doesn’t matter just form more-or-less stable orbits at these distances? After all, the local magnetic fields are too weak to make changes (except over very long timescales), and collisions and close encounters too rare (these certainly work over timescales of ~billions of years, as evidenced by the distributions of stars in globular clusters).
That’s where new simulations by Philip Hopkins and Eliot Quataert, both of the University of California, Berkeley, come into play. Their computer models show that at these intermediate distances, gas and stars form separate, lopsided disks that are off-center with respect to the black hole. The two disks are tilted with respect to one another, allowing the stars to exert a drag on the gas that slows its swirling motion and brings it closer to the black hole.
The new work is theoretical; however, Hopkins and Quataert note that several galaxies seem to have lopsided disks of elderly stars, lopsided with respect to the SMBH. And the best-studied of these is in M31.
Hopkins and Quataert now suggest that these old, off-center disks are the fossils of the stellar disks generated by their models. In their youth, such disks helped drive gas into black holes, they say.
The new study “is interesting in that it may explain such oddball [stellar disks] by a common mechanism which has larger implications, such as fueling supermassive black holes,” says Tod Lauer of the National Optical Astronomy Observatory in Tucson. “The fun part of their work,” he adds, is that it unifies “the very large-scale black hole energetics and fueling with the small scale.” Off-center stellar disks are difficult to observe because they lie relatively close to the brilliant fireworks generated by supermassive black holes. But searching for such disks could become a new strategy for hunting supermassive black holes in galaxies not known to house them, Hopkins says.
Sources: ScienceNews, “The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth”, Hopkins, Quataert, to be published in MNRAS (arXiv preprint), AGN Fueling: Movies.
But how did these magnetic fields come to have the characteristics we observe them to have? And how do they persist?
A recent paper by UK astronomers Stas Shabala, James Mead, and Paul Alexander may contain answers to these questions, with four physical processes playing a key role: infall of cool gas onto the disk, supernova feedback (these two increase the magnetohydrodynamical turbulence), star formation (this removes gas and hence turbulent energy from the cold gas), and differential galactic rotation (this continuously transfers field energy from the incoherent random field into an ordered field). However, at least one other key process is needed, because the astronomers’ models are inconsistent with the observed fields of massive spiral galaxies.
“Radio synchrotron emission of high energy electrons in the interstellar medium (ISM) indicates the presence of magnetic fields in galaxies. Rotation measures (RM) of background polarized sources indicate two varieties of field: a random field, which is not coherent on scales larger than the turbulence of the ISM; and a spiral ordered field which exhibits large-scale coherence,” the authors write. “For a typical galaxy these fields have strengths of a few μG. In a galaxy such as M51, the coherent magnetic field is observed to be associated with the optical spiral arms. Such fields are important in star formation and the physics of cosmic rays, and could also have an effect on galaxy evolution, yet, despite their importance, questions about their origin, evolution and structure remain largely unsolved.”
This field in astrophysics is making rapid progress, with understanding of how the random field is generated having become reasonably well-established only in the last decade or so (it’s generated by turbulence in the ISM, modeled as a single-phase magnetohydrodynamic (MHD) fluid, within which magnetic field lines are frozen). On the other hand, the production of the large-scale field by the winding of the random fields into a spiral, by differential rotation (a dynamo), has been known for much longer.
The details of how the ordered field in spirals formed as those galaxies themselves formed – within a few hundred million years of the decoupling of baryonic matter and radiation (that gave rise to the cosmic microwave background we see today) – are becoming clear, though testing these hypotheses is not yet possible, observationally (very few high-redshift galaxies have been studied in the optical and NIR, period, let alone have had their magnetic fields mapped in detail).
“We present the first (to our knowledge) attempt to include magnetic fields in a self-consistent galaxy formation and evolution model. A number of galaxy properties are predicted, and we compare these with available data,” Shabala, Mead, and Alexander say. They begin with an analytical galaxy formation and evolution model, which “traces gas cooling, star formation, and various feedback processes in a cosmological context. The model simultaneously reproduces the local galaxy properties, star formation history of the Universe, the evolution of the stellar mass function to z ~1.5, and the early build-up of massive galaxies.” Central to the model is the ISM’s turbulent kinetic energy and the random magnetic field energy: the two become equal on timescales that are instantaneous on cosmological timescales.
The drivers are thus the physical processes which inject energy into the ISM, and which remove energy from it.
“One of the most important sources of energy injection into the ISM are supernovae,” the authors write. “Star formation removes turbulent energy,” as you’d expect, and gas “accreting from the dark matter halo deposits its potential energy in turbulence.” In their model there are only four free parameters – three describe the efficiency of the processes which add or remove turbulence from the ISM, and one how fast ordered magnetic fields arise from random ones.
Are Shabala, Mead, and Alexander excited about their results? You be the judge: “Two local samples are used to test the models. The model reproduces magnetic field strengths and radio luminosities well across a wide range of low and intermediate-mass galaxies.”
And what do they think is needed to account for the detailed astronomical observations of high-mass spiral galaxies? “Inclusion of gas ejection by powerful AGNs is necessary in order to quench gas cooling.” SKA central region with separate core stations for the two aperture arrays for low and mid frequencies and for the dish array. Graphics: Xilostudios and SKA Project Development Office
It goes without saying that the next generation of radio telescopes – EVLA, SKA, and LOFAR – will subject all models of magnetic fields in galaxies (not just spirals) to much more stringent tests (and even enable hypotheses on the formation of those fields, over 10 billion years ago, to be tested).
