Kepler's 'Superflare' Stars Sport Huge, Angry Starspots
Astronomers studying stars like our sun that are known to generate powerful 'superflares' have also discovered that these superflares are likely associated with monster 'starspots.'
Astronomers studying stars like our sun that are known to generate powerful "superflares" have also discovered that these superflares are likely associated with monster "starspots."
When our sun sees an uptick in magnetic activity, sunspots erupt all over the solar surface, acting as a warning sign that a storm is brewing. Every 11 years or so (a period known as the solar cycle), the sun will wax and wane in magnetic activity and the number of sunspots will also rise and fall. This relationship between the innermost magnetic dynamo of the sun and sunspot number is used by space weather forecasters to predict when and where the next solar flare may erupt.
Solar flares can have dramatic impacts on Earth - they bathe our upper atmosphere in powerful radiation, blocking global communications and they can irradiate in-space assets like satellites and even give harmful doses of radiation to unprotected astronauts. Flares are often triggered in regions of intense magnetic activity and it just so happens that sunspots are also created by intense regions of magnetic activity forcing through the sun's photosphere.
Although solar flares may be dramatic, like the X2-class solar flare that caused a radio blackout on May 5, they are nothing when compared with the superflares other stars in our galaxy have been seen to kick out.
In 2012, using Kepler Space Telescope data - which is usually associated with the detection of exoplanets as they drift (or transit) in front of their host stars - astronomers were able to identify several hundred superflare events on a number of sun-like stars. These gargantuan events kicked out flares 10-10,000 times more energy than our sun is able to muster.
Keeping in mind that these stars are sun-like stars, what makes them such superflare powerhouses? Why is our sun such a featherweight in comparison?
In an effort to understand the dynamics of superflare stars and perhaps answer these questions, astronomers from Kyoto University, University of Hyogo, the National Astronomical Observatory of Japan (NAOJ) and Nagoya University turned to the High Dispersion Spectrograph on the Subaru Telescope, located atop Mauna Kea in Hawaii, to carry out spectroscopic measurements of 50 of Kepler's superflare targets.
Through these spectroscopic observations, the researchers found that all of the stars selected for study possessed periodic changes in brightness. In all cases, these variations were caused by starspots (not exoplanets) rotating across the stars' surfaces. Some of the superflare stars even rotated as slowly as our sun, which has an equatorial rotation period of nearly 24.5 days.
Also, through analysis of the Ca II 854.2 nm (ionized Calcium) absorption line, the researchers realized that the periodic dimming of the stars' brightness was being caused by vast starspots that would dwarf even the biggest sunspot our sun can produce.
Therefore, the researchers conclude that sun-like stars can trigger superflares if they possess large sunspots, but further work is needed to understand the intricacies of this relationship.
But this research poses a quandary. If our sun is so similar to these sun-like superflare stars, why are their magnetic dynamics so radically different? We're pretty sure our sun isn't capable of generating superflares (as it seems unlikely that life would have been able to evolve in such a radiation-drenched solar system), so these oddities are unlikely to be "just a phase" in the life of a sun-like star.
It just goes to prove that just because a star may look like the sun from afar, up close, the reality can be radically different.
Source: Subaru Telescope
Artist's impression of a superflare star in visible light (left) and the Ca II line (right) where the areas around the huge starspots are bright.
The sun may be an average star when compared to the menagerie of stars that exist in our galaxy, but to Earth and all life on our planet, the sun is the most important object in the Universe. However, regardless of its importance and close proximity, our nearest star holds many mysteries that continue to fox solar physicists after decades of modern studies with cutting-edge observatories. One of the biggest mysteries surrounding the sun is the underlying mechanisms that drive solar flares and coronal mass ejections (CMEs). Monday evening (EST), the sun reminded us that it hasn't quite finished with the current solar maximum (of solar cycle 24), unleashing a powerful X4.9 solar flare -- the biggest of 2014. An armada of space telescopes witnessed the event, including NASA's Solar Dynamics Observatory that can spy the sun's temper tantrums in astounding high definition. Shown here, 5 of the 10 filters from the SDO's Atmospheric Imaging Assembly (AIA) instrument are featured, showing the sun's lower corona (the solar multimillion degree atmosphere) through 5 wavelengths; each wavelength of extreme ultraviolet light representing a different plasma temperature and key coronal features -- such as coronal loops (highlighted here in the 'yellow' 171A filter) and ejected plasma that formed a CME.
At 7:13 p.m. EST (00:13 UT, Feb. 25) -- pictured here on the far left -- the active region (AR) 1990 was crackling with activity. Then, as magnetic field lines from the sun's interior forced together and through the solar photosphere, large-scale reconnection events occurred. Reconnection is a magnetic phenomena where field lines "snap" and reconnect, releasing huge quantities of energy in the process. At 7:44 p.m. EST (00:44 UT) -- second frame from the right -- a kinked coronal loop can be seen rising into the corona. At 7:59 p.m. EST (00:59 UT) -- far right -- solar plasma contained within the magnetic flux is accelerated to high energy, generating powerful x-rays and extreme ultraviolet radiation, creating the X-class flare.
The X4.9 flare was caught through the range of SDO fliters, including this dramatic view as seen through the 131A filter. The flare was so bright that photons from the flare overloaded the SDO's CCD inside the AIA instrument, causing the signal to "bleed" across the pixels. This bleeding effect is common for any optical instrument observing powerful solar flares.
Intense coronal activity is often associated with active regions -- the active lower corona is pictured here, left. In this case, the flare erupted from AR1990, at the limb of the sun. Also associated with active regions are sunspots, dark patches observed in the sun's photosphere (colloquially known as the sun's "surface") -- pictured right. The sun's cooler photosphere has been imaged by a different SDO instrument called Helioseismic and Magnetic Imager (HMI), which detects the intensity of magnetic fields threading though the sun's lower corona and photosphere.
In the case of AR1990, a large sunspot can be seen at the base of the coronal loops that erupted to generate the powerful flare. This is a prime example of how sunspots can be used to gauge solar activity and how they are often found at the base of intense coronal activity and flares.
The HMI monitors magnetic activity across the disk of the sun and can also generate a picture on the direction of the magnetic field polarity. In this observation of the sun's magnetic field around the time of the recent X-class flare, other active regions can be easily seen -- intense white and black regions highlighting where magnetic field lines emerge and sink back into the sun's interior in active regions.
The joint NASA/ESA Solar and Heliospheric Observatory (SOHO), which has been watching the sun since 1996, also spotted the flare, tracking a CME that was generated shortly after. Seen here by SOHO's LASCO C2 instrument, that monitors the interplanetary environment surrounding the sun for CMEs and comets, a growing bubble of solar plasma races away from the sun.
Approximately an hour after the flare, the CME grew and continued to barrel into interplanetary space. Space weather forecasters don't expect that this CME will interact with the Earth's atmosphere as it is not Earth-directed. This observation was captured by SOHO's LASCO C3 instrument -- an occulting disk covers the sun to block out any glaring effect. By combining observations by the SDO, SOHO and other solar observatories, the connection between the sun's internal magnetic "dynamo", the solar cycle, flares and CMEs, solar physicists are slowly piecing together what makes our nearest star tick, hopefully solving some of the most persistent mysteries along the way.