OK I’m going to break my blogging silence and my aversion to blogging in my subject area to post about a really cool result that came out earlier this week.
Looking up at the night sky you see a hodge-podge collection of stars, perhaps a few thousand of the hundreds of billions of stars in the Galaxy. Some are extremely bright types of star that are really far away. Others like the Sun’s nearest neighbouring system Alpha Centauri are fairly run of the mill but appear bright because they are so close. But not all stars close to the Sun can be seen with the naked eye. Take Barnard’s Star, the second closest system to the Sun, it’s situated roughly twice as far away from us as Alpha Centauri but because it’s a red, faint type of star, it’s over 6,000 times fainter and 25 times too faint to see with the naked eye. This means that even though some stars are very close to us, they are so faint that we need to use a few tricks to pick them out from bright background stars.
One of the best tricks to use is to take a picture of the sky and look back a few years later and compare the positions of stars. Stars move around the Galaxy with different orbits and hence every star has a velocity with respect to the Sun. Due to their closeness, nearby stars appear to move more compared to background stars (their proper motion). This is simply a perspective effect, they aren’t actually moving through space faster. Hence if you look for stars moving quickly across the sky, chances are a lot of them will be near the Solar System. This isn’t simply a matter of cartography, if you want to pick out a population of faint objects, your best bet is to look close-by.
And that’s exactly what Kevin Luhman did. By taking the positions of objects observed by the WISE satellite, he found one which stuck out. It moved across the sky pretty fast and was very bright in infrared light. Looking back at images taken by other surveys he also found it detected there. This often happens in astronomy, sometimes you find an object nobody had noticed was interesting before but which may have been first detected 50 or even 100 years ago. Anyway, the object Luhman found was moving across the sky pretty fast. Well actually it wasn’t, nearby stars tend to have their motions measured in arcseconds per year. One arcsecond per year is the same angular speed as seeing the average tortoise walking at the distance of the Sun from Earth. The newly discovered high proper motion object was moving at about 2.8 Solar Tortoises, which is pretty big for stars. Well I say star, but it isn’t, it’s a brown dwarf, well actually not “a” brown dwarf.
Stars are fuelled by nuclear reactions in their core. These work because of the huge temperatures in their cores caused by all the mass above pushing down. It’s like the atomic nuclei in the core are caught at the bottom of a really big rugby ruck*. Anyway, they get so hot that they can sometimes overcome their mutual repulsion and fuse together. However some objects, with masses below about 8% of the Sun can’t reach the appropriate minimum temperature to begin stable fusion and hence are “failed” stars or brown dwarfs. When Luhman took a spectrum of his object, he found it was a brown dwarf, well actually while taking the observation he found that it was actually two brown dwarfs in orbit around one and other. Finally, using the data from the WISE satellite and other surveys he was able to work out its distance from an effect known as trigonometric parallax. This showed the two brown dwarfs are about 6.5 lightyears away, slightly more distant than Barnard’s Star.
My reaction to this was probably like others in my field, “how did we miss this?” Well the answer is simple, the system lies close to the Milky Way. The density of stars on the sky increases sharply as you go close to the plane of the Milky Way, meaning searches of nearby stars are often flooded with spurious candidates. Additionally the gas and dust in the plane make background stars appear redder and faint in the optical but still bright in the infrared. This can mimic the colour of brown dwarfs, again contaminating searches. Brown dwarf searches therefor often avoid the region around the Milky Way to make sure they can have clean samples without wading through a load of junk. Hence the extremely nearby, bright brown dwarf lay undiscovered for decades after it had first been detected.
And that brings me to my last point, this is a really cool discovery yet it hasn’t got the attention it deserves. The third closest system to the Sun was just found, that should at least be on the BBC News front page.
*There are no known instances of a rugby ruck leading to nuclear fusion
So you’ve probably heard the exciting news, that a small, earth-mass planet has been identified around one of the components of Alpha Centauri. It’s the nearest neighbouring system to our Sun. Despite that it isn’t the brightest star in the sky (that’s Sirius), also it’s a southern star so is not very familiar to those of us who live in the Northern hemisphere. In-fact I didn’t see it until I was 28 and had moved to Hawai`i. Here’s a picture I took of it rising above Kilauea.
Because it was so far south, Alpha Cent. wasn’t well studied by early European astronomers. In the 1830s Thomas Henderson, the first Astronomer Royal for Scotland measured its distance using trigonometric parallax. This was the first distance measurement to a star other than the Sun. However he hesitated in publishing the result and Bessell scooped him to the first published distance to a star (61 Cygni).
The system itself actually consists of three stars. The two brightest components appear as a single source to the naked eye. The two stars orbit each other at a distance about 17-18 times the Earth-Sun distance. There’s third star in the system, Proxima Centauri. It’s 100 times too faint to see with the naked eye and while the two brighter components are of similar mass to the Sun, Proxima is less than an eighth of that. That said for people like me who study low mass stars, it’s still not that low mass. It was discovered by another Scot, Robert Innes who worked as a wine merchant in Australia before taking up astronomy fulltime. Simply, he found a star with a similar motion across the sky to Alpha Centauri and given their close positions (yet it’s still 17,000 times the Earth-Sun distance from Alpha Centauri AB), deduced they were a pair. This is what I do a lot in my research, but I use large catalogues produced by data pipelines, he used many painstaking measurements by hand. Sometimes I feel like modern astronomy is cheating. One interesting side-note, this double star with a wide companion set-up seems to be more common than a single star with a wide low mass companion. A rather nice recent paper by Peter Allen and others quantified this and indicated that this may say something about how these systems form.
A quick note about the planet. It appears to be too close to the star to sustain liquid water and hence life. This system has a fairly complicated Habitable Zone. The planet is orbiting the smaller of the two stars but it will also be heated by Alpha Centauri A. Duncan Forgan wrote a paper about this earlier this year saying the difference will be small but will induce oscillations of a few degrees. Also sometimes on the surface of the planet Alpha Centauri B will have set but the planet will still be lit by Alpha Centauri A. This will of course change as the two stars orbit each other. Duncan has a nice blog post about the paper where he also talks about sleeping rhythms on a hypothetical planet around Alpha Cent B. When Alpha Centauri A is on the other side of Alpha Centauri A from the planet I guess sunset will look a bit like that on Tatooine. Although with a different brightness ratio between the two stars.
So it’s Alpha Centauri has a planet. It isn’t able to support life, but maybe there’s another one in the system that can. What this discovery makes me think of is Civilisation. No, not the noted BBC TV series, but the classic Sid Meier strategy game. One of the victory conditions was to send a spacecraft to Alpha Centauri. I never got that far, on more difficult levels my civilisation would die in the Bronze Age and in the harder levels I’d get bored of nuking phalanxes in about 1900 and give up. However if I hadn’t given up perhaps I could have built a ship to head for what the Nature press release calls a “scorched barren rock”.
A week ago I posted about some of the videos that had been made at .astronomy4. The video I was involved in (Sh*t Astronomers Say) has so far got over 5,000 hits. I find that a little scary and it’s been odd having colleagues asking me questions on desk head-butting techinques. While this wasn’t a video with an agenda, I think there is a lesson to be learned from the post. In other words, I think we learned something today……
You remember the end of He-Man where in an earnest tone Ram-Man would explain that it’s bad to ram things with your head or Beast-Man would tell you not to put lit fireworks in your mouth*. Anyway, this is a bit like that. During the drafting of Sh*t Astronomers Say, we basically made a big list of things that we heard a lot. In many cases we added things because they are frequently done and they are annoying. Some things are annoying and are trivial, others get in the way of astronomers doing and communicating science.
The main bugbear for me was a lot of the stuff that went in to the talk section. I hate going to talks, sitting somewhere in the middle to back of the room and then not being able to see anything. The number one problem is with plots. About the worst thing you can do with a plot is jsut take the version from the paper and then slap it up there. Normally the axes will be unreadable for half the audience and often the datapoints will be a confusing jumble. Then there’s the issue of colour schemes, many are headache-inducing but often they just confuse the audience. Remember that about 8% of men and 0.4% of women are colourblind so avoid reds and greens that may be indistinguishable (I’m biased here as I’m red-green colourblind). Finally the last point, the organisors have been very nice to give you a slot to talk in, don’t outstay your welcome. There’s nothing worse than someone bringing 30 slides to a 10 minute talk and proceeding to go over each one in great detail. OK maybe regicide is worse but it’s borderline.
Was there a particular bugbear of your’s in the video? Feel free to comment on it below.
One thing before I outstay my welcome, we are going to have a go at a crowdsourced follow-up. Below are the details,
Did we leave out your personal favorite saying? Want to contribute to a new, crowdsourced version of Sh*t Astronomers Say? Send submissions to email@example.com by August 31! We’ll accept any video+audio formats compatible with iMovie – you can even record with PhotoBooth! (You have a Mac, right?!?!) Please keep videos under 30 seconds and leave enough space between sayings for editing. Include your name, institution, occupation (undergrad/grad/postdoc/faculty/etc) and filming location so we can give you proper credit and estimate the diversity of contributions. We can’t guarantee we’ll be able to use every submission, but we’ll do our best!
*OK I made up the last one, but the Ram-Man one is real
Last week I (along with 49 others) attended the 4th installment of the .astronomy conference. Part of the conference was a Hack Day where people went off and designed useful bits and bobs for astronomy. One example was the Zoonibot which is designed to monitor the Planet Hunters forum to help users with classifications have posted.
For the hack I was involved with we did a video. What better way to promote the science of astronomy than with in-jokes.
Also doing a video production were Amada and Nicole who made a response to the silly, shallow and demeaning “Science it’s a a Girl Thing” video. As you can see there’s is a lot slicker than ours which indicates they actually know how to use iMovie properly.
And yes, I know the sound is only on one side.
So I ave a confession to make, this time last week I’d never seen the Northern Lights. Growing up near Edinburgh it’s a bit too far south and often cloudy when there are solar storms. Nijmegen and Heidelberg are a bit too far south and as for Hawai`i forget it. So a week ago, along with a few other astronomers (and an accountant) from Heidelberg, I set off for the Arctic Circle. This sounds a lot more intrepid than it actually was, booking a flight to Tromsø and hiring a car.
Anyway, enough about my jaunt, here’s some nice aurora pictures. I don’t have a fancy DSLR so I was using my Sony DSC-H2 which is apparently a “bridge camera”. Additionally the tripod is only a small pocket one so there is a small amount of camera-shake on some of the star images.
And here’s a time-lapse of the aurora changing over about 5 minutes. Note the Pleiades star cluster setting in the background.
You know how astronomy works, you look up at something with a telescope and “oh look”, Jupiter has moons or there’s a 7th planet. But you can also find nothing. One of the great things about science is that a null result is still a result. Hence by looking at your measurements carefully enough, you can actually say something interesting about what you haven’t seen.
On the 3rd of November 2005 a gamma-ray burst (GRB) was detected in the constellation of Ursa Major. Further examination found that there was a well-known galaxy in the vicinity, M81- Bode’s Galaxy. Could this violent event have come from one of amateur astronomy’s favourite objects?
During the latter half of the 20th century, astronomy moved away from being purely based on optical light to a wider range of wavelengths across the electromagnetic spectrum. From radio to submillimetre, infrared, UV, X-ray and gamma-ray, astronomers now have a vast array of tools for studying the visible universe. There are however other sources of information that come from astronomical sources.
Gravitational waves were first predicted by Einstein. While they haven’t been directly observed, their emission has been inferred from the orbit of a pair of neutron stars. Gravitational waves subtly stretch and compress spacetime. Hence to detect them you have to very accurately measure stretches and compressions. This is done at labs like LIGO where they measure this stretching over long distances (several miles). Such long distances are needed as the effect of gravitational waves is fractional. Hence the bigger the distance over which you measure the stretching, the bigger the stretch.
The gamma-ray burst in the vicinity of M81 was what is known as a short duration burst. While long duration bursts are the product of exploding massive stars, most short bursts are though to be formed when two compact objects (neutron stars or black holes) slam together after spiralling in due to energy lost by gravitational wave emission. However there is also another possible cause, a massive flare from a magnetar, a neutron star with an extremely high magnetic field.
To investigate this, a team from LIGO searched through their data for a signal that could come from either a magentar or colliding compact objects. They found nothing.
But nothing can be interesting. After going back and looking at their measurement errors they were able to set upper limits on the flux of gravitational waves received from this gamma-ray burst. Consequently, by examining the flux they would expect to receive from merging compact objects they were able to set lower limits on the distance this burst was from Earth. Based on these limits they excluded a black hole – neutron star merger in M81 as the source of this GRB to at least 93% confidence. The constraint on a neutron star – neutron star merger was slightly weaker, but would require the event to have a very weakly beamed jet (and GRBs are known to almost always have tight, collimated jets). Based on a fairly generously unbeamed jet the LIGO results (seeing nothing) exclude an black hole – neutron star merger in M81 to greater than 99% confidence and a neutron star – neutron star merger to over 98% confidence. However the expected gravitational wave flux from an erupting magnetar is too low to be detected at the Earth – M81 distance so the results don’t rule that out.
So what was the cause of the bright flash of gamma-rays seen in Ursa Major seven years ago? Dunno, but seeing no gravitational wave signals tells us that it’s highly unlikely to be two massive compact objects slamming together in one of the sky’s prettiest galaxies.
The LIGO Scientific Collaboration, J. Abadie, B. P. Abbott, T. D. Abbott, R. et al. (2012). Implications For The Origin Of GRB 051103 From LIGO Observations Preprint arXiv: 1201.4413v1
I spend far too much time at pub quizzes. Perhaps it’s because I’m an irritating know-it-all or I just like a vaguely intellectual pretense for going to the pub. One of the more geeky parts of it is correcting the quiz-master when they are wrong (Reykjavik is north of Helsinki and Blazin Squad did not do the original of Crossroads etc.). One such wrong answer was a week or two back when it was claimed the Earth has four moons. Additional moons of the Earth have long been claimed and were popularised a few years back when QI claimed that a co-orbital body called Cruithne was a second moon. As far as the definition of stable, natural bodies orbiting the Earth goes there is only one, although it would be entertaining if schoolchildren were taught about the wonderfully named Wahrhafter Wetter-und Magnet Mond (or veritable weather and magnetic moon). However there are sometimes other bodies that briefly orbit the Earth.
The Solar System is a crowded place. Besides the eight planets and numerous dwarf planets there are millions of asteroids. Some of these have orbits that bring them close to the Earth. While most of these whizz by us, some are in orbits which mean that they can gravitationally interact with the Earth and the Moon and go in to orbit around it. These orbits are not stable and the objects will eventually be kicked out of the Earth-Moon system.
To date only one known object has been discovered to have undergone such a process. Known as 2006_RH120 it is a small body, only 3-5m across. In 2007-2008 it undertook four orbits of the Earth at a distance more than twice as far away as the Moon. But how often do objects like this perform their temporary dance with the Earth? Well a new paper of has been looking in to the rate of capture and when such events happen.
The authors use a simulation of the how asteroids will pass through the Earth-Moon System. They select a series of objects with orbital elements in the range where they could possibly be captured and then examine how they would be affected by coming close to the Earth and Moon. Previously it was thought that a close encounter with the Moon gave objects a gravitational tug allowing them to be captured by the Earth. However the new model finds that while the Moon does play a role in the capture, none of their simulated near-Earth objects came close enough to the Moon to get a sufficient enough tug for capture.
The model also found that capture most likely at aphelion and perihelion (when the Earth is furthest and closest to the Sun during its orbit). The same capture probability peaks were previously noted for temporary satellites of Jupiter. It’s also possible that the Moon itself could capture asteroids and get its own temporary satellites. However no objects in the simulation managed to complete an orbit of the Moon.
Objects in unstable orbits around the Earth will of course have the possibility entering the atmosphere and becoming meteors. About 1% of objects in the simulation impacted on the Earth, none on the moon. This means that a temporarily captured object is 3.5 times more likely to strike the Earth than an near-Earth object in a similar orbit. In total the authors estimate that a tenth of one percent of objects striking the Earth were in temporary orbit around us.
In all the authors estimate based on their model and the fact there aren’t a large population of observable temporary satellites that at any one time there is one object of approximately one metre in size temporarily orbiting the Earth along with potentially other smaller bodies. So the Earth only has one Moon, but it’s not the only natural object orbiting us.
Granvik, M., Vaubaillon, J., & Jedicke, R. (2011). The population of natural Earth satellites Icarus DOI: 10.1016/j.icarus.2011.12.003
In the 1960s Cold War paranoia lead to the discovery of the most violent explosions in the Universe. Now instruments intended to study these massive cataclysms have detected signals coming from the Earth. While not the covert nuclear tests the original satellites were originally built to identify, these signals raise a whole set of questions about the physics of some of the most violent events on our planet.
At the height of the Cold War the US, concerned about the possibility of covert Soviet nuclear tests sent up a series of satellites to look for tell-tale flashes of radiation. These satellites began seeing something strange, they saw flashes. The Soviet military scientists hadn’t been working overtime, these were signals of astronomical origin. After decades of debate, it was shown that these were caused by the explosion of the most massive stars. Since then a succession of satellites have been flown to study light at the extreme end of the electromagnetic spectrum. One such instrument is the AGILE satellite, an Italian space observatory capable of surveying large chunks of the sky at once. As well as detecting distant Gamma Ray Bursts, it has also mapped events in our own Galaxy, such as the sudden brightening of the Crab Nebula in gamma rays last year. However in a strange completion of the historical circle it has also been detecting signals from the Earth.
Terrestrial Gamma-ray Flashes (TGFs) are short bursts of gamma-ray radiation. These were first noticed by the Compton satellite and are associated with thunderstorms. Strong updrafts in clouds cause the formation of layers of positive and negative charge. There are typically eased by lightning strikes removing the net charge from one or more layers. However the strong electric fields in the clouds can accelerate electrons to velocities close to the speed of light. When a fast moving electron such as this (or from a source such as a cosmic ray) interacts with another electron it can accelerate it too leading to a run-away growth of fast-moving electrons. These are then diverted by interactions with atomic nuclei in the cloud releasing “braking radiation”. As the electrons are moving so fast, this radiation takes the form of extremely energetic gamma-rays.
As the AGILE satellite passes along its orbit it is capable of detecting TGFs from below. However an orbital inclination of 2.5 degrees limits the area where the satellite can detect these flashes to close to the equator. Happily this is where some of the Earth’s largest thunderstorms happen. From June 2008 to January 2010, AGILE scientists isolated over a hundred TGFs detections. The largest number of detected events came from Africa with others being found over Indonesia and a few over South America. However when it came to associating these events with lightning events they found something striking (excuse the pun). At first glance it appeared that the distributions of Terrestrial Gamma-Ray Flashes and lightning matched rather well. But when the distributions were studied in more detail, it was found that while in South America there was an 87% chance that that TGFs came from a random sub-sampling of lightning, this probability dropped to 3% in Africa. The authors don’t go on to explain this discrepancy, but it is clear there is still something unknown about the mechanisms driving some of the most violent events on the planet.
AGILE Observations of Terrestrial Gamma-Ray Flashes , M. Marisaldi et al., 2011 Fermi Symposium proceedings
I’m currently in Chicago visiting Stuart on my way back to Europe. Chicago is known for a few things, that the Cubs will never win anything again, that some bloke from Honolulu lived here for a while before moving to D.C. and the skyscrapers. Anyway there is a fascinating connection between Chicago’s skyline and some of the most important astronomical research of the 20th century.
In the mid-19th century, Chicago was the main trading hub for commodities being produced by the vast new farming lands of the American mid-west. A booming town crammed up against Lake Michigan on one side, it grew rapidly in the years up until 1870. Then the fire started. The exact cause is unknown although many local legends exist. The end result was that most of the city was burnt to the ground and needed to be rebuilt.
The constrained geography and high demand led to a need to build up. While proto-skyscrapers had existed in medieval cities such as Edinburgh, important new technological advances meant that buildings could become taller than ever before. The key invention was the elevator (yes I’m using the American English word since this is about Chicago). While crude elevators had existed since antiquity, new mechanical technology allowed safe, steam-powered contraptions to be commercially available in the 1850s. By 1870, two Chigacoans, C.W. Baldwin and W.E. Hale had developed sooth-running, hydraulic elevators. This invention was perfectly timed for the post-fire construction boom.
William Hale and his “Hale Water Counterbalance Elevator” became rich on the huge skyscrapers being thrown up in Chicago in the late-19th century. This money provided his son George Hale with both capital to pursue his passion for astronomy and with access to the wealthiest industrialists of the time.
George Hale’s talent for astronomy became apparent when he invented the spectroheliograph (a device for looking at particular wavelengths of light in the Sun) while still an undergrad at MIT. He himself made significant discoveries in the fields of solar research, particularly while studying sunspots. However perhaps his greatest impact was as a fundraiser for some of the great American observatories.
Hale’s first telescope was built with money from his father, however in 1897 he solicited a donation from Charles Yerkes to build an observatory bearing his name in Wisconsin. At the time this boasted the world’s largest telescope and still has the world’s largest refracting telescope. During this time Hale’s father donated a 60 inch mirror which was eventually used for the Mount Wilson Observatory in California. Hale then used his contacts with the wealthy again to persuade California businessman John D. Hooker to donate $45,000 to build a 100 inch reflector at Mount Wilson. It was using this telescope that Edwin Hubble made his pioneering studies of galaxies and the expansion of the Universe.
It is Hale’s last telescope, finished in 1948 after his death in 1938 that bears his own name. Built using money from the Rockefeller Foundation, this 200 inch telescope on Mount Palomar in California was the largest telescope in the world for 28 years and is still a productive observatory today. Hale was also instrumental in founding the institution that runs this telescope, the California Institute of Technology.
Anyway, I hope the link from pretty buildings in Chicago to major observatories of the 20th century has not been too tenuous. Below are a few links I used for writing this if you want more information on the subject,
The moon, bright , entrancing, intensely irritating for astronomers. Scattered moonlight makes observing more inefficient meaning astronomers have to stick on a target for longer. Observatories are expensive, high-tech facilities so the question is, how much does the Moon cost astronomy?
Astronomers are in essence counters. Telescopes collect photons from a source and astronomers count the number in their detectors. The telescopes also collect photons from the night sky. This is never 100% dark so there are also some additional background photons which must be subtracted off. As with all counting there are uncertainties. The fainter the object the higher the uncertainty, the brighter the sky the higher the uncertainty. The Moon is basically a big mirror reflecting sunlight into telescopes. As the amount of sunlight the Moon reflects towards the Earth varies over a lunar cycle, the sky background is much higher at Full Moon than at the New Moon. This means there is a much higher uncertainty in astronomical measurements at the Full Moon so astronomers have to observe for longer and get more signal to have a measurement as certain as one from a shorter observation taken at New Moon.
So how much observing time does this cost and how much is that in terms of money? I’m going to use some very rough estimates based on publicly available numbers. If these are off by a bit, feel free to correct them in the comments.
So firstly how much time does this cost? Astronomers typically describe nights as dark, grey or bright depending on the phase of the Moon. Let’s assume that for 50% of the time when the telescope is open it is integrating in the optical*. This takes into account overheads such as slewing and that some of the time the telescope will be observing in the infrared. The wavelength is important as the amount of reflected sunlight varies with the colour of the filter you are observing through. Bluer, shorter wavelengths are typically more seriously affected by the Moon. Hence let’s ignore the effect on infrared observing. Picking a typical optical observation band (the R band) which is not really badly affected by scattered moonlight I had a look at some Integration Time Calculators. These are tools which allow astronomers to work out how long they have to observe a source for. To reach a particular uncertainty of observation you need to integrate for 60% longer in bright time than dark time and 5% longer in grey time than dark time. So assuming 1 week of bright time per lunar cycle plus one week of dark time and two of grey time. That means that astronomers lose about 6% of telescope time due to having to observe for longer in bright and grey time taking into account our 50% overheads/IR observing factor.
So how much money is this? Well the Keck Observatories (two 10m telescopes) have annual budget of $11m. But that is only the running cost, what about the construction cost? The VLT in Chile (four 8m telecopes) cost €330m in 1999 to construct. Converting to 2011 dollars that’s $650m. Assuming a 40 year lifetime for the telescopes about $16m per year is spent on construction. So $4m per annum for one 8m telescope and $5.5m running cost for one 10m telescope. Let’s assume these numbers are typical for one 8m class telescope. There are 16 telescopes of 6.5m diameter of larger, assuming all these have a $9.5m annual cost and ignoring other telescopes, that comes to $150m spent annually on large telescopes. Six percent of that time is taken away by the Moon at a total annual cost of $9m.
So that’s a very rough number for the annual cost to astronomy of the Moon. This was just for fun so I don’t expect it to be correct to the last cent but hopefully to an order of magnitude. Any better estimates are welcome in the comments section.
* Yes I know infrared observations are more likely to be scheduled during bright time.