Hi! When I was only six (or so), I went out one clear but windy night with my uncle and peered through the eyepiece of his home-made 6" Newtonian reflector. The dazzling, shimmering, perfect globe-and-ring of Saturn entranced me, and I was hooked on astronomy, for life. Today I'm a freelance writer, and began writing for Universe Today in late 2009. Like Tammy, I do like my coffee, European strength please.
Contact me: [email protected]
Well, this week’s one should be a tad easier, though you will still need to cudgel your brains a bit and do some lateral thinking (five minutes spent googling likely won’t be enough). But, as with all Universe Puzzles, this is a puzzle on a “Universal” topic – astronomy and astronomers; space, satellites, missions, and astronauts; planets, moons, telescopes, and so on.
Which is the odd one (or two!) out?
Crawford Hill, Dover Heights, Kootwijk, Richmond Park, Seeberg Hill, Wheaton.
UPDATE: Answer has been posted below.
Seeberg Hill is where the Gotha Observatory (Seeberg Observatory, Sternwarte Gotha or Seeberg-Sternwarte) was located; it was an optical observatory. All the others are, or were, sites of radio telescopes, or observatories: Crawford Hill, in New Jersey, is where the CMB (cosmic microwave background) was first detected; Dover Heights is the site of Australia’s first radio telescope/observatory; Kootwijk is the site of the Netherlands’ first radio observatory; Richmond Park, in London, is where Hey, Parsons, and Phillips, in 1946, detected the first discrete extra-galactic astronomical source (Cygnus A); and Wheaton, in Illinois, is the site of Grote Reber’s first radio telescope.
Wheaton would also be a good answer; Grote Reber built his first radio telescope on his own (all the others are the result of efforts by institutions).
For ‘two out’, several answers are possible. For example, Seeberg Hill and Crawford Hill (the microwave region of the electromagnetic spectrum is not, necessarily, the same as the radio).
Mitch's Star; full caption below (Credit: Keel/PSS/SARA)
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“The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, ‘hmm… that’s funny…'” (Isaac Asimov)
A few short years ago, Zooite Hanny van Arkel discovered Hanny’s Voorwerp in an SDSS image of a galaxy (“What’s the blue stuff below? Anyone?”), and a new term entered astronomers’ lexicon (“voorwerpje”).
Very late last year, Zooite mitch too had a ‘that’s funny…’ moment, over a spectrum (yes, you read that right, a spectrum!).
Now neither Hanny nor mitch have PhDs in astronomy … Mitch's Mystery Star (SDSS, Galaxy Zoo)
But I digress; what, exactly, did mitch discover? Judge for yourself; here’s the spectrum of the star in question (it goes by the instantly recognizable name 587739406764540066): Spectrum of Mitch's Mysterious Star (SDSS)
“I asked a couple of white-dwarf aficionados, and neither recalls seeing any star with these features (nor does Jim Kaler, who wrote the book on stellar spectra),” Bill Keel, a Zooite Astronomer known as NGC3314 wrote, kicking off a flurry of forum posts, and a most interesting discussion!
“Can we rule out something along the line of sight, possibly a cold molecular cloud?” EigenState wrote; “If both stars are moving SE (towards the bottom left corner), could Mitch’s star (square) be affected by debris in the trail of the bright red star (triangle)? I am thinking of the trail left by Mira. So the spectrum would be white dwarf shining through cooled red star debris?” said Budgieye. NGC3314 continued “It can’t be like our current Oort Cloud since we don’t see local absorption from our own in front of lots of stars near the ecliptic plane. To show up this strongly, it would then have to be either much denser or physical much smaller. This just in – this may be the most extreme known example of a DZ white dwarf, which have surface metals. White dwarfs aren’t supposed to, because their intense surface gravity will generally sort atmospheric atoms by density, so this has been suggested (with some theoretical backing) to result from accretion either from circumstellar or interstellar material (so it could be at the star’s surface but representing material formerly in a surrounding disk). Watch this space…”
Then, two weeks after mitch’s discovery, Patrick Dufour, of the Université de Montréal, joined in “Hi everyone, I have known this objects for many years. I have done fits almost 5 years ago but just never took time to publish it. Will do it in the next few weeks. Meanwhile, enjoy this preliminary analysis… The abundances are very similar to G165-7, the magnetic DZ, but it’s a bit cooler (explaining the strength of the features).” Patrick, as you might have guessed from this, is an astronomer with specific expertise in white dwarfs; in fact the abstract to his PhD thesis begins with these words “The goal of this thesis is to accurately determine the atmospheric parameters of a large sample of cool helium-rich white dwarfs in order to improve our understanding of the spectral evolution of these objects. Specifically, we study stars showing traces of carbon (DQ spectral type) and metals (DZ spectral type) in their optical spectrum.”
Somehow yet another astronomer, Fergal Mullally heard about mitch’s mystery star and joined in too “Many other WDs with strong metal absorption lines are surrounded by a cloud of accretable material. This makes sense because the metals quickly sink below the surface (as mentioned by NGC3314). In some cases, metals are only visible for a few weeks before they are sink too deep to be seen. The disks are exciting, not only because they can be so young, but their composition suggests we might be looking at the remains of an asteroid belt (see http://arxiv.org/abs/0708.0198).” To which Patrick added “Mitch’s Mystery Star is a cool (~4000-5000 K) helium rich white dwarf with traces of metals (abundances similar to G165-7). The metals most probably originate from a tidally disrupted asteroid or minor planet that formed a disk around the star.”
So, mitch’s mystery star is just a rather weird kind of DZ star, and DZs are just unusual white dwarfs?
Yes … and no. “The asymmetrical line near 5000 is almost certainly MgH. As for the one at 6100, I am open to suggestion. I have never seen it anywhere else. For G165-7, the splitting is Zeeman. But the broadening is van der Waals by neutral helium. No splitting is observed in this star (and I took a really good spectra at MMT a few years ago to be sure).” Patrick again; so what is the mysterious asymmetrical line at 6100 Å?
Two more weeks passed, and a possible reason for Fergal’s interest emerges, in a post by NGC3314 “While we wait to see how Patrick’s new calculation shakes out, here’s an interesting new manuscript he was involved with, that points to likewise interesting things about the DZ stars. [] Wow. White-dwarf spectra as tombstones for planetary systems… wonder how the system stayed close enough to end up on the white-dwarf atmosphere all through the red-giant phases? The binary systems we can see look awfully far apart to have had helpful dynamical effects for this.” (in case you didn’t read up on Fergal, he’s very keen on exoplanets and ET). Curiouser and curiouser
Then, in February, a tweet: “At campus observatory, seeing whether we can measure orbital motion between Mitch’s star and its K-dwarf companion.” The tale is becoming curiouser and curiouser (exoplanets in binary star systems? If life had evolved on a planet in orbit around the star which later went red giant then white dwarf, could it have somehow survived and landed on a planet in orbit around the K-dwarf companion?)
I’ll let NGC3314 have the final word: “This furnishes one more example of how the wide interest in Galaxy Zoo allows things once unthinkable – during the SDSS, the whole analysis plan never conceived that every bright galaxy in the survey, and every one of the million or so spectra would actually be examined by a human being.”
Oh, and the Asimov quote seems to be an urban myth (if any reader knows when, and where, Asimov actually said, or wrote, those words …).
Source: Galaxy Zoo Forum thread Mitch’s Mystery Star
Full caption for image at the top of this article (Credit: Bill Keel): I had a look with the SARA 1m telescope in BVR filters last week, to check for obvious variability. Pending more exact measurements, it’s about as bright as it was in the SDSS images and the older Palomar plates. As SIMBAD shows, this is known as a star of fairly high proper motion (and that’s about all). You can see this when I register the original red-light Palomar photograph to the image from last week, a time span of almost 59 years. The attached picture compares red-light data from the original Palomar Schmidt sky survey in early 1951, the second-epoch Palomar survey around 1990, and SARA on Jan. 7, 2010. You can also see that the bright red star to the southeast has almost exactly the same (large) proper motion.
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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 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.
3Star-birth in SMM J2135-0102 (Credit: M. Swinbank et al./Nature, ESO, APEX; NASA, ESA, SMA)
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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.
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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).
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.
As this week’s puzzle may be a bit harder than most, I’ll be adding a HINT tomorrow, if it looks like no one is even close to being on the right track.
UPDATE: Answer has been posted below.
What do the following objects have in common?
NGC 6822, NGC 598, NGC 221, NGC 224, and NGC 5457.
Together with the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), these are the seven galaxies (“nebulae”) with the most reliable distances, used by Edwin Hubble to establish the distance-redshift relationship, in his landmark 1929 paper. Today we call this the Hubble relationship.
“The data are given in table 1. The first seven distances are the most reliable, depending, except for M 32 the companion of M 31, upon extensive investigations of many stars involved.”
Note that not all are in the Local Group, and they are not the five brightest galaxies in Table 1. Figure 1 from that paper is reproduced in the Universe Puzzle graphic; it’s at the top right.
