International Space Station Solar Transit

The International Space Station Transits the Sun (© Martin Wagner)
The International Space Station Transits the Sun (© Martin Wagner)

Sometimes the NASA Astronomy Picture of the Day (APOD) is just too hard to pass up. Yesterday’s APOD features our sunspot-less Sun with a strange shape in the lower left-hand side of the image. On closer inspection suddenly it becomes clear as to what we are looking at. It’s the International Space Station transiting the solar disk. Stunning
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Breaking News: The Sun is Still Doing Nothing (Much)

The disk of the Sun as viewed by the Solar and Heliospheric Observatory (SOHO/MDI)
The disk of the Sun as viewed by the Solar and Heliospheric Observatory (SOHO/MDI)

As you can see, the Sun is keeping quiet, devoid of sunspots. As the world awaits an increase in solar activity to celebrate the onset of a new solar cycle, our closest star keeps a blank face and keeps us guessing. This most recent image was taken today by the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) instrument. MDI measures plasma velocity and magnetic field strength at the top of the convection zone, so it is an invaluable sunspot detector. Sunspots are a good indicator about how active the Sun is, as when the magnetic field becomes stressed and twisted, it is forced from the convection zone, through the photosphere, chromosphere and high into the corona. These protrusions then fill with plasma and glow as coronal loops. The more magnetic activity there is, more sunspots appear. But, it would seem, the Sun remains magnetically inactive seven months since the beginning of Solar Cycle 24…
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Auroral Substorm: THEMIS Pinpoints Location of Magnetic Reconnection in Magnetotail

The THEMIS satellites lined up inside the Earth's magnetotail. A reconnection event occurred between the 4th and 5th satellites (NASA)
The THEMIS satellites lined up inside the Earth's magnetotail. A reconnection event occurred between the 4th and 5th satellites (NASA)

Although magnetic reconnection is one of the bedrock theories within the field of space plasma physics, it has been very difficult to observe. We know that magnetic instabilities and electric currents operate within the plasma environment, but the triggering mechanism is difficult to understand. Reconnection occurs near the surface of the Sun and it occurs where the solar wind interacts with the sunward geomagnetic field. It also happens in the magnetotail (i.e. in the shadow of the Earth, where the magnetosphere is swept behind our planet by the pressure of the solar wind) and is most commonly linked with a terrestrial phenomenon: the aurora. Now, for the first time, scientists have located the point at which the magnetic field lines snap, blasting a huge amount of energy right at the Earth…
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What Type of Star is Our Sun?

High-spatial and temporal resolution view from the Hinode SOT G-band filter (NASA)

Our Sun is often called an “average” or “unremarkable” star. This is a little unfair, after all this unremarkable specimen is responsible for generating all the energy for all the planets in the Solar System and it has nurtured life on Earth for the past four billion years. We are also very lucky in that the Sun (or “Sol”) is comparatively stable with a periodic cycle. What’s more, it is alone, with no binary partner complicating matters. We live in a very privileged corner of the Milky Way, within the “Goldilocks Zone” (i.e. “just right” for life – as we know it – to thrive) from Sol, where there is a unique and delicate relationship between our star, the Earth and interplanetary space. This is all great, but in the star club, how does Sol measure up? Is it really just an average, boring star?

I noticed in the comments of my article Observing an Evaporating Extrasolar Planet that some readers were discussing the classification of our Sun. This was in response to the subject of the exoplanet called HD 209458b orbiting the yellow dwarf star HD 209458 in the constellation of Pegasus. I happened to point out that HD 209458 was “…not too dissimilar to our Sun (with 1.1 solar masses, 1.2 solar radii and a surface temperature of 6000 K),” but also highlighted that HD 209458 was a yellow dwarf star. To be honest, I didn’t think about the connection until Jerry Martin asked why our Sun is never referred to as a yellow dwarf star? Helpfully, Dave Finton posted a link to Wikipedia that discusses this topic. For the full wiki treatment, have a look at Wikipedia:G V star, otherwise read on…

The Hertsprung-Russell diagram – stars plotted by their absolute magnitude, luminosity, classification, and effective temperature.

In the Hertzsprung-Russell Diagram, all known stars fall into one of six broad classifications depending on their luminosity and surface temperature. Observed stars can either be classed as (from big to small) a super-giant, bright giant, giant, sub-giant, main sequence or white dwarf. Within those classifications are spectral sub-classes from “O” (surface temperature of 30,000K), “B”, “A”, “F”, “G”, “K” to “M” (at 3,000K). However, for the sake of keeping this article on-topic, we’ll focus on our star, Sol (which is Latin for Sun).

Granted, our Sun has a surface temperature of around 6,000K, giving it a spectral classification of “G”. On the luminocity scale, our Sun scores a “V”. So, the Earth orbits a “G V star” which is otherwise known as a Yellow Dwarf star (although their actual colour ranges from white to slightly-yellow). Why is Sol considered to be “average”? That’s because in the Hertzsprung-Russell Diagram, yellow dwarfs can be found right smack-bang in the centre of the chart, half-way down the Main Sequence. Using this chart gives an idea about where our star came from and where it is going. For the moment, it is a hydrogen-burning star, converting 600 million tonnes of hydrogen into helium per second. This “hydrogen burning phase” generally lasts for about 10 billion years (Sol is about half-way through this phase) until all the hydrogen fuel is exhausted. When this happens, a yellow dwarf will puff up into a Red Giant, eventually shedding its outer layer, producing a planetary nebula. Eventually, the core will cool and compress into a long-living white dwarf star.

So, to answer the question, the Sun is a yellow dwarf star… and it certainly is not unremarkable

Is the Sun a Dark Matter Factory?

Hinode X-ray observation of a solar sigmoid (David McKenzie/Montana State University)
The hypothetical axion is a particle that might help scientists work out where the bulk of dark matter may be held in the Universe. So far, there has been much talk about the search for another type of hypothetical particle, the weakly interacting massive particle (WIMP), and little attention has been paid to the lowly axion. WIMPs are very appealing to scientists as proving they exist will help patch some holes in quantum theory. What’s more, WIMP detectors need to be huge, large volumes of underground caverns filled with hi-tech sensors and cleaning fluid – this makes for a cool funding proposal; think up and grand idea, explain that it will prove our understanding of the Universe and then receive a multi-billion $/£/€ cheque (it’s not quite as easy as that, but there are socioeconomic and political reasons for building such an awesome structure).

So how do you go about finding an axion? Surely this exotic particle will need an even bigger detector, especially as it has zero charge, very low mass and cannot interact via the strong and weak nuclear forces? Actually, a large WIMP-type detector would be useless for axion detection. Fortunately axions have a neat interaction with magnetic fields that can be detected with existing instrumentation. What produces the strongest magnetic field in the Solar System? This is where the Sun can help out…
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Dynamic Coronal Loops as seen by STEREO (Video)

The STEREO (Behind) view of three loop systems moving into view (NASA)
What with all the surprise activity of the Sun at solar minimum of late, I’ve found myself looking around the solar observation sites an awful lot more than I used to. During all the commotion back in 2003 when the Sun was blasting out record breaking X-ray flares one after another, I really didn’t think I could be surprised with anything else the Sun would do. That was until, very much unannounced, three sunspots rotated into view, blasting another X-ray flare into space… at solar minimum. The strange thing was, that these sunspots weren’t even from this solar cycle, they were from the previous one that ended some time around December 2007! And now we get a stunning, detailed view of more unexpected solar activity from the Solar Terrestrial Relations Observatory (STEREO), a hi-res video of dynamic coronal loops…
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Solar Flare, CME and Tsunami Generated by a “Blank Sun”

The empty solar disk on April 27th - still generated an X-ray flare (SOHO)

Even during solar minimum, the Sun can be surprisingly dynamic. We are currently observing a sunspot-less solar disk, but on Saturday the Solar and Heliospheric Observatory (SOHO) observed a noteworthy X-ray flare. It was a B3.8 flare, producing a coronal mass ejection (CME), sending vast quantities of hot plasma into interplanetary space. Admittedly, it is strange to witness CMEs of this size at this time in the solar cycle, but what is even weirder is that the flare was produced by a region devoid of sunspot activity (see image). SOHO captured the CME event with its LASCO instrument and the two-probe Solar Terrestrial Relations Observatory (STEREO) captured an incredible “solar tsunami” (or Sun Quake) as the flare caused the Sun’s surface to ripple. And all this without an intense magnetic field and sunspot pair…
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What’s Going on with These Sunspots? Are they from Solar Cycle 23 or 24?

Magnetic view of the new sunspots appearing at the Suns equator (credit: SOHO)

On posting the story “The Sun Bursts to Life: Sunspots, Flares and CMEs” on the Universe Today, something was strange about my source material. Although the Sun had started Solar Cycle 24 back in January of this year, the new sunspots recently observed were the “leftovers” from the previous cycle and not new ones from this cycle. Something is strange. Surely one cycle ends and another begins? Think again…
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Snippet: Will the Earth be Safe From Solar Expansion? The Outlook Isn’t Great…

As the Sun runs out of fuel, it will swell… but will it swell enough to swallow the Earth? Image credit: Mark Garlick/HELAS. Source: http://www.sciam.com/article.cfm?id=exoplanet-red-giant-space-astronomy-stars

In 7 billion years time, the Sun will run out of fuel. As it dies, it will swell so big that many predict that it will reach as far as Earth’s orbit. Naturally, the likelihood of the Earth still harbouring life may be debatable (after all 7 billion years is a long, long time), but should the human race still be around, and evolved into something totally unrecognizable, what will we see?
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The Chaotic Nature of Magnetic Reconnection and Coronal Dynamics

TRACE image of a hot coronal loop. Image credit: NASA

The solar corona is a strange place. For the last few decades solar physicists have been trying to understand why it is so hot. Yes, it’s the Sun, and yes, it’s hot, but the corona is too hot. There are many possible solutions to the “coronal heating phenomenon”, but physicists are generally in agreement that this extreme heating is down to waves propagating along magnetic fields, interacting with coronal plasma, or by reconnection events (small explosions). In a study published earlier this year, scientists suggest that to account for the temperatures and densities observed in the corona, chaotic forces may be at work, regulating the scales of reconnection in the coronal plasma.

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