A satellite detects a bright burst of gamma-rays. Within minutes telescopes swing in to action expecting to see a massive star being torn apart by a cataclysmic explosion in a far-flung corner of the universe. But that wasn’t what they found……..
Gamma Ray Bursts (GRBs) are thought to be explosions of some of the most massive stars viewed at a fortuitous angle where we are blasted with a cone of high energy photons as the star itself implodes into a black hole. These happen in distant galaxies, billions of light years away.
There are huge observing campaigns to observe these blasts using quick reactions from a series of different telescopes. The sequence begins when a dedicated satellite detects a burst of gamma rays. It then locates the area where the burst is coming from and transmits the coordinates. These then trigger overrides where other telescopes stop what they are doing and swing to the position they are given, hoping to catch burst in other wavelengths of light before it fades away to nothing.
At 5:13 GMT on the 25th of April 2008 the Burst Alert Telescope on the Swift satellite detected a flood of gamma rays coming from the constellation of Lacertae. Within three minutes the other two instruments on Swift were taking data on the sources in X-rays and the ultraviolet. Twelve minutes after this the Liverpool Telescope on La Palma in the Canary Islands swung towards the target and took observations in visible light.
But quickly it became clear that this was no blast from a distant dying star, but something closer to home. The position of the source was one of a know nearby star EV Lac. Measurements showed it was six times brighter than its catalogued magnitude, so why was this star causing such an interstellar fuss?
EV Lac is a cool, red star 16 light years away, it’s also known to flare up every so often. Its a low mass star and has a very strong magnetic field. Higher mass stars like the Sun have weaker magnetic fields. The Sun can also have smaller outbursts of X-rays and gamma rays called solar flares (right). These are caused by the magnetic fields near the Sun’s surface rearranging themselves. This same thing happened in EV Lac, but because it has a much stronger magnetic field than the Sun the flare was much stronger. Even then this was a massive flare even for a star with a strong magnetic field. This flare was so bright that at one point 63% of the star’s energy output was in highly energetic X-rays. These factors combined with its relative proximity to the Earth lead to this stellar flare appearing as bright as the massive, distant bang the satellites were actually looking for.
Rachel A. Osten, Olivier Godet, Stephen Drake, Jack Tueller, Jay Cummings, Hans Krimm, John Pye, Valentin Pal’shin, Sergei Golenetskii, Fabio Reale, Samantha R. Oates, Mat J. Page, & Andrea Melandri (2010). The Mouse that Roared: A Superflare from the dMe Flare Star EV Lac
detected by Swift and Konus-Wind Accepted for publication in the Astrophysical Journal arXiv: 1007.5300v1
Cowardice is something that comes fairly naturally to me, this explains why I always played wing or full-back at schoolboy rugby. For non-rugby fans that means I was generally far away from the action and unlikely to end up at the bottom of a ruck with 16 huge forwards piling on top of me. One or two I wouldn’t mind, but two whole packs could have crushed a skinny little kid like me.
To use heavily technical, scientific terms, stars are big balls of stuff. That stuff is mostly Hydrogen, with some Helium and bits and bobs of other heavier elements. Hydrogen is a bit like I was in my rugger days, small, light and entirely unsafe for use in airships. Take a load of Hydrogen and pile a few rugby players on top of it and it gets compressed and hot. If you kept piling on more and more players it would get hotter. Eventually 16 million billion billion players piling on and forming a massive spherical ruck would lead to the poor Hydrogen getting as hot as three million degrees Celsius. This is a critical temperature as at this point the atoms of Hydrogen are so hot and whizzing about at such huge speeds that they can slam in to each other to form Helium. This releases energy and the Hydrogen has an energy source and can stay hot and continue to fuse into Helium. This is how stars work, the huge amount of material pressing down on the core causes it to get so hot these reactions happen and allow the star to support itself, produce energy and become stable. Luckily rugby union teams are limited to 15 players a side so there are no recorded incidents of skinny full-backs bursting into spontaneous fusion.
So imagine what happens if the star is slightly less massive than 16 million billion billion rugby players (8% of the mass of the Sun or 80 times the mass of Jupiter). The star doesn’t get hot enough to start the Hydrogen fusion reactions and it can’t produce energy or become stable. It radiates away heat and begins to cool. These stars that don’t work are called brown dwarfs.
When stars are burning Hydrogen, the more massive they are, the hotter they are. The surface temperature of the star determines the colour of the star, hot stars blue, cool stars red. It also determines what sort of elements and molecules dominate the star’s spectrum.
While the interior of a star is a hot plasma with atoms split into nuclei and electrons, in the atmospheres of stars atoms can stay together. Lights flies through the atmosphere and comes into contact with the atoms. In this case the atoms can gobble (absorb) the light up. Atoms are picky eaters so they can only gobble up light of particular wavelength and only when the atmosphere is in a narrow temperature range. Each chemical element has a different series of wavelengths it can absorb and each has a separate temperature range where it absorbs most. Hence you can tell how hot a star is from what wavelengths of light the atoms have taken a tiny nibble out of.
Really cool stars can also have molecules in their atmosphere. Unlike lone atoms, molecules are rather gluttonous, taking huge bites out of stellar spectra. The coolest of the original spectral class determined in the early 20th century was “M”, which (like its namesake played by Judy Dench) has huge chunks out its visible spectrum from Titanium Oxide. In the last few decades of the last century cooler objects were found so new spectral classes of L and T have since been added. While L has features from metal hydrides, the main spectral feature of the very cool T spectral class (below 1100C) is huge mouthfuls taken out the spectrum by methane and water. Generally hot stars are blue and cool stars red. While T type objects follow these rules for their spectra in visible light in the infrared the spectral gobbling of methane and water cause them to look blue. Jupiter, while not a brown dwarf and cooler than all known T type objects, also shows methane absorption leading to this stunning infrared image (above). Now to make life difficult, there are stars that are spectral class M or L and brown dwarfs can be anything from M, L, T to even the still theoretical Y class. These spectral classes are an extension of the standard spectral classification system (hot to cool) O,B,A,F,G,K,M (now with L,T). The traditional way of remembering the old ones is “Oh Be A Fine Girl Kiss Me” (trust me that only works for remembering stellar spectral types). I don’t think there is an “official” extension to the mnemonic, but suggestions are welcome in the comments section.
So I’ve told you what a brown dwarf is and what one looks like. However finding them and distinguishing them from stars can be pretty tricky. I’ll tell you how this can be done in the next post.
Kirkpatrick, J., Reid, I., Liebert, J., Cutri, R., Nelson, B., Beichman, C., Dahn, C., Monet, D., Gizis, J., & Skrutskie, M. (1999). Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2 Micron All‐Sky Survey (2MASS) The Astrophysical Journal, 519 (2), 802-833 DOI: 10.1086/307414
Burgasser, A., Kirkpatrick, J., Brown, M., Reid, I., Burrows, A., Liebert, J., Matthews, K., Gizis, J., Dahn, C., Monet, D., Cutri, R., & Skrutskie, M. (2002). The Spectra of T Dwarfs. I. Near‐Infrared Data and Spectral Classification The Astrophysical Journal, 564 (1), 421-451 DOI: 10.1086/324033
Following on the footsteps of giants, it was my turn this month to spend an evening looking through entries for the Astronomy Photographer of the Year competition run by the Royal Observatory Greenwhich and creating my own gallery.
It was a joy, thanks to all who made me smile! Endlessly.
I’m in need of some cheering up today, as the fun observations I wanted to make with the Herschel Space Telescope have turned out to be impossible. Luckily, this observation planning also involved a lot of procrastination, which led me to this: the Dumb Or Overly Forced Astronomical Acronyms Site (DOOFAAS). On this site astronomer Glen Petitpas has been gathering together the best of the bizarre, forced, peculiar or just plain geeky astro-acronyms (and they’re all real projects). Perhaps the most famous (i.e. even my mum’s heard of it) of these is OWL – the Overwhelmingly Large Telescope – though this is probably because its lack of imagination is extremely easy for people to mock (and since it follows on from the Very Large Telescope and the Very Large Array, I kind of see their point)!
A bit of googling shows that some astronomers are really committed to their chosen name. For instance, the SAURON (Spectrographic Areal Unit for Research on Optical Nebulae) instrument uses data reduction software called GANDALF and published a paper called “The One Eye that Sees All: Integral Field Spectroscopy with SAURON on the WHT”. Others call themselves things like POLARBEAR (POLARization of the Background millimEter bAckground Radiation) but then don’t even use the animal as their logo. At least the Wavelength Oriented Mircowave Background Analysis Team have an appropriate mascot.
It’s not just projects either – who wouldn’t want to go to a conference called TANGOinPARIS (maybe only people interested in Testing Astroparticle with the New GeV/TeV Observations)? There’s also a disappointing lack of information about whether this is the last of these meetings!
My favorite acronym of the bunch has to be GADZOOKS! – Gadolinium Antineutrino Detector Zealously Outperforming Old Kamiokande, Super! (and the best part is that the exclamation mark is part of it!) Turns out this is actually a pretty interesting project which aims to improve the performance of the Super-Kamiokande neutrino detector to a level where it’ll be able to detect faint neutrinos from supernovae from outside our galaxy.
Neutrinos are elementary particles with a very small mass, and a speed generally close to the speed of light. They rarely interact with other particles which makes them very hard to detect – there are trillions of neutrinos from the Sun passing through each of us every second! Super-Kamiokande is essentially a giant underground tank containing 50,000 tonnes of water, surrounding by sensitive cameras (that’s it in the photo above [credit: STFC], with some scientists in what looks to be a rubber dinghy, cleaning the detectors). If a neutrino happens to interact with one of the particles in the tank it produces a flash and the cameras record a detection. At the moment it can only do this for neutrinos that originate within our galaxy; ones from outside are weaker so their flashes are lost in the noise. The GADZOOKS! team think they can improve this by dissolving the metal gadolinium into the water – this will ‘tag’ each extragalactic neutrino interaction and make them easier to distinguish. There’s lots more details on this in their paper and, if (like me) you need it in simpler language, here. It looks like they’re testing the technology now, and aiming to fully implement it in the next couple of years. I really hope they’re successful, not just because of the science, but because it would be a shame to see such a brilliant acronym go to waste!
John F. Beacom, & Mark R. Vagins (2003). GADZOOKS! Antineutrino Spectroscopy with Large Water Cerenkov Detectors Phys.Rev.Lett. 93 (2004) 171101 arXiv: hep-ph/0309300v1
**Now UPDATED with the first flyby images!**
I found out earlier today that the Rosetta spacecraft is due to flyby the asteroid 21 Luetia today (that’s an artist’s impression of it at the top of this post via the ESA website). There’ll be a live webcast from the main control room as the event unfolds, starting at 18:00 CEST (17:00 BST) on the ESA webpage, followed by the presentation of the first images at 23:00 CEST. The full timeline of the event, along with more information can be found here. Rosetta’s main mission is to explore comets (it even aims to land a probe on one); this encounter is part of its 10 year journey to reach its main objective. This is the largest asteroid ever visited by satellite so fingers crossed for some interesting pictures!
UPDATE: First (pre flyby) images are back from Rosetta:
More at the Rosetta blog
And now the first images of the flyby itself have been released by ESA. Here’s Lutetia with Saturn in the background from 36,000 km away:
the sequence leading up to closest approach:
and a final moody farewell shot from the departing satellite:
All images are from the Rosetta blog.
Ok, ok, I know I’m a day late with this, but this first all-sky image from the Planck Telescope is so beautiful it deserves a mention. Planck was launched in May 2009, on the same rocket as the Herschel Telescope. It’s designed to measure the leftover radiation from the Big Bang – the Cosmic Microwave Background (CMB), which Rita has talked about here before. The picture above is the result of its first year of observing – it takes that long to cover the whole sky, as this neat video (halfway down) shows – and it will repeat this every year until the mission comes to an end.
The most striking feature of this image are the billowing clouds of dust (blue) and gas (red) in our Galaxy. These extend far above and below the main galactic disc (the bright horizontal line in the centre), where we live. Unfortunately, it’s the yellow mottled patches coming from the first light in the Universe (the CMB) that the scientists that built Planck are mainly interested in, and, as you can see, these foreground clouds get in the way, except at the top and bottom of the map. Luckily, with a lot of careful analysis, these can be removed, and then lots of science can be done!
Here’s a version of the image with various features in and outside our Galaxy labelled:
Finally, if you want to play more with this Planck image it’s been added to the excellent Chromoscope. Get over there and zoom around!