The aurora from Tromsø March 9, 2012

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.

Shot over towards the next island in the Tromsø archipelago, the aurora appears to erupt from the mountain.

The glow of the aurora behind the trees

Venus and Jupiter setting next to a plume of aurora

Orion seen through clouds and aurora

And here’s a time-lapse of the aurora changing over about 5 minutes. Note the Pleiades star cluster setting in the background.

The Earth has one Moon, but it’s not the only rocky thing orbiting us….. December 21, 2011

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

Things that go bang from below November 17, 2011

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 are associated with spectacular lightning events in the upper atmosphere similar to this red Sprite. Credit:NASA

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.

Source
AGILE Observations of Terrestrial Gamma-Ray Flashes , M. Marisaldi et al., 2011 Fermi Symposium proceedings

The Chicago skyline, the elevator and 20th century astronomy September 19, 2011

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,

http://exhibits.library.northwestern.edu/exhibits/elevator/history.html

http://uh.edu/engines/epi2272.htm

http://www.mtwilson.edu/his/art/g1a4.php

How much does the Moon cost astronomers? August 25, 2011

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.

The Moon, just how much does it cost astronomy every year? Credit:NASA/JPL

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.

Lahaina Noon July 17, 2011

One of the (clearly very few) benefits of living in Honolulu is that twice a year we get to witness an astronomical phenomenon that only happens in the tropics. The phenomenon is known as Lahaina Noon and in layman’s terms it’s when the Sun is directly overhead. Outside the tropics while the Sun gets to its highest point in the sky at local noon, this high point isn’t high enough for it to be directly overhead. Over the course of a year, any point in the tropics will have the Sun pass overhead twice every year on either side of the Summer Solstice.

The model boating pond in Ala Moana Park at Lahaina Noon

The most immediately cool thing about Lahaina Noon is that shadows are cast straight down. Hence some things have minimal shadows while others have none. It’s quite strange to be in bright sunlight and seeing a signpost casting no shadow. Does that make it a vampire signpost or something? Anyway I went to Ala Moana Park today and took some pictures of things with no or minimal shadows,

As he faced the Sun he cast no shadow, a sign at Lahaina Noon.

So is this a vampire lampost?

Some canoes, casting shadows straight down

It should be noted that the name Lanhaina Noon was chosen as part of a contest run by The Bishop Museum who have a very nice planetarium. Lahaina means “cruel Sun” and also a a town on Maui. Lahaina Noon in Lahaina is on the 18th of July at 12:32, a more comprehensive list of Lahaina Noon times for the islands is here.

AAS in Boston – wicked smaat May 24, 2011

So this is my first experience of the American Astronomical Society meetings. It’s being held in a rather posh area of Boston which feels like a merging of Piccadilly and the New Town but with suicidal pedestrians. The atmosphere of being in a large hotel in a city centre is somewhat different to the UK NAM (on a university campus) or the Dutch NAC (locked in a monastery in the rural Limburg* for three days).

So far we’ve had a very energetic talk on future Decadal Surveys by Malcolm Longair (do I even need to state that he was energetic and wonderfully enthusiastic, he always is). Also a very nice talk from Jeremy Drake of CfA about the weather on low mass stars. That had a lot of interesting detail on how X-rays can evaporate oceans and inhibit planet formation as well as touching on the controversial area of cosmic rays and climate. In the afternoon I was in the Pan-STARRS1 session, it was great to see what all fantastic work from all the key projects are doing as I only really see what the KP I work on does plus what I hear on the grapevine. There is a nice twitter stream describing the session here. I also popped in to the stars general session to see a nice but unfortunately too short talk on brown dwarf variability.

Waiting to speak on Wednesday and trying not to blow too much money on clothes you can’t get in Hawai`i and Irn Bru and Lucazade from Irish grocery shops.

*Limburg, where men are men and language is tonal.

Pluto gets hot(ter) April 18, 2011

You know Pluto, a Kuiper Belt object and a previous planet. It’s made of rock and ice, not just water ice but also nitrogen, methane and carbon monoxide ice. Like all planets (and Kuiper Belt objects) Pluto is on an elliptical orbit. However its orbit is much more elliptical than any of the eight planets in the Solar System. This means its distance from the Sun varies by up to 40% during its “year”. As the amount of energy received from the Sun is greater the closer you are to it, large changes in temperature occur over the course of one orbit. 

So what happens when Pluto gets closer to the Sun and heats up? It boils. Well actually it sublimates meaning the gases change from solid directly in to a gas. This means that when the ex-planet is near the Sun and hot, it is warmed up and gases are released from the surface forming an atmosphere. When it is far away it gets colder and those gases can freeze back on to the surface.

Luckily for astronomers wanting to study it, Pluto made its closest approach to the Sun in 1989. This means it is still pretty warm and so it has an atmosphere which can be studied. However this is not a simple thing to do.

Pluto and Charon, its largest moon. Credit: NASA

Pluto is a long way away and pretty small. Hence observing the tiny silver of atmosphere around it is difficult. The simple trick is to wait for Pluto to pass in front of a background star who’s brightness can be monitored. Subtle changes in the star’s brightness provides valuable information about the icy body’s atmosphere. This doesn’t happen often but luckily it is passing through the plane of the Galaxy which means the number of background stars is very high. So this combined with the fortunate timing of its closest approach means the rather jammy folk who study Pluto can look at its atmosphere when it is warm in detail every few years when it passes in front of a star. One such event happened in June 2006. A study based on this found that the atmosphere had warmed by 1.2-1.7 degrees Celsius compared to a previous close encounter with a star in 1988 (although it’s still about 170 below zero). It also detected the atmosphere up to about 135km.

So to a paper which came out today. Astronomers based in St Andrews, Scotland and on the Big Island of Hawai`i used the James Clerk Maxwell Telescope on Mauna Kea to probe the upper atmosphere of Pluto. It’s been thought for a while that there could be part of Pluto’s atmosphere which extends to a few thousand kilometres. This study looks for the telltale traces of carbon monoxide. They have a clear detection of carbon monoxide and use the shape of the spectral line to estimate the temperature and hence the height above the surface this gas would lie at. It turns out to be over 3000km above the surface of Pluto (compared to Pluto’s solid radius of 1153km), much higher than the denser lower atmosphere detected when it passed in front of a star in 2006. The researchers also find a very slight shift in the wavelength of the spectral line, opening the possibility that Pluto may have a comet-like tail.

All this provides an interesting preview of the sort of results NASA’s New Horizons missing which will arrive at the frigid sphere in 2015. While Pluto isn’t significant enough to be considered a planet, it’s certainly still fascinating astronomers.

J. S. Greaves, Ch. Helling, & P. Friberg (2011). Discovery of carbon monoxide in the upper atmosphere of Pluto MNRAS arXiv: 1104.3014v1

Young, E., French, R., Young, L., Ruhland, C., Buie, M., Olkin, C., Regester, J., Shoemaker, K., Blow, G., Broughton, J., Christie, G., Gault, D., Lade, B., & Natusch, T. (2008). VERTICAL STRUCTURE IN PLUTO’S ATMOSPHERE FROM THE 2006 JUNE 12 STELLAR OCCULTATION The Astronomical Journal, 136 (5), 1757-1769 DOI: 10.1088/0004-6256/136/5/1757

Live from the top of the world March 29, 2011

Over the last few months I’ve had a lot of observing time on Mauna Kea. Unfortunately the weather this year has been dreadful, frequently we lose entire nights due to cloud, humidity or freezing fog. The two telescopes I use most are now run remotely with no observer at the summit so I’m often reduced to staring at the weather information pages begging for the clouds to clear and the humidity to drop.

One of the interesting side effects of so many telescopes on the mountain going remote is that they often install webcams so they can monitor the conditions more accurately. A nice example is the all sky camera on the University of Hawai`i 88 inch telescope. This provides a live view of the sky over Mauna Kea. Not only can you see constellations and planets, you can see the laser guide stars from Keck, Subaru and Gemini as lines on the sky and also a green cross which marks where the UH88 is pointing (hint: in bad weather it’s parked facing straight up). There are also more webcams showing the views near various telescopes. However these look a bit dull in the dark. Still if you are ever giving an outreach talk during the day in Europe you can show a live view of the sky from one of the world’s top observatories.

However the award for the best webcam on the mountain has to go to the CFHT cloudcam. This was recently installed facing from the mountain towards Hilo. It’s really useful as it gives early warning of fog coming over the plateau towards the ridge. While it also gives a lovely view of the sky the best part are the timelapse movies. In a typical move you can see the shadow of the moutain rise as the sun sets, stars and planets rising and aircraft flying into Hilo airport. There’s also a loop marking individual constellations.

A sample view from the CFHT cloudcam showing Orion and Sirius rising. Credit: CFHT

Going through the CFHT cloudcam movies looking for cool stuff is great. Take the spectacular moonless night on the 4th of February where you can see the plane of the Galaxy rise late in the night. Or on the other side, how about supermoon from the 19th of March. These sort of movies look great for talks, live outreach during the day or just to show a class of schoolkids something to stare in awe at.

I’ve just found an ESO webcam from La Silla which gives a lovely view of the Galactic centre. If you know of other good astronomy webcams then put them in the comments thread. Or if you can find a good CFHT cloudcam movie put it there too.