Spotty Star Reveals Magnetic Weirdness
A strange pattern of starspots has been mapped on a nearby massive star, revealing just how different other stars are to our own sun.
Our sun's magnetism waxes and wanes over cycles lasting approximately 11 years, producing a rash of dark sunspots that drift around the sun's equator at "solar maximum" and then all but disappear during "solar minimum." Astronomers have tracked sunspot numbers for centuries, but only now are we beginning to understand what drives them and how the underlying magnetic field is generated.
But as we get up-close and personal with our nearest star, solar physicists are curious as to whether our sun is typical - do other stars exhibit similar cycles? Do they have sunspots or, more accurately, starspots? If so, are they driven by the same mechanisms?
Of course this answer is hard to come by; zooming in on a distant point of light to decipher surface features on other stars, let alone resolve starspots, has been asking a little too much of astronomical techniques. But we are reaching a point where we can see small stars, particularly red dwarfs, exhibiting huge regions, covered with large spots, which cause the star's brightness to dim as they rotate.
Now astronomers have gone one better: they've mapped the photosphere of a large old star 180 light-years away and resolved individual spots as the star rotates. And it looks weird.
But "weird" is a very relative term. Just because other stars behave differently from our sun, it's just as likely that our sun is weird when compared with other stars in the universe. Or perhaps every star is unique in some way; countless physical variables all amounting to a rich and varied stellar family.
And in this particular case, zeta Andromedae, a star 15 times the size of the sun, has starspots dotted all over in an apparently random fashion, even at high latitudes (i.e. near the poles).
This is very different to our sun where the majority of sunspots tend to erupt around the equator and we have a pretty good idea as to why this is. The sun experiences differential rotation, in other words it rotates faster at the equator than it does at the poles. This has a dragging effect on the internal magnetic field, pulling it more around the equator. This is thought to "wind up" the magnetic field, making magnetic field lines pop through the photosphere as coronal loops, creating a rash of darkened regions - sunspots - in an equatorial pattern.
As zeta Andromedae doesn't appear to exhibit this pattern of starspots, astronomers can make an early conclusion: its internal dynamics are somehow different than our sun's differential rotation and solar cycle. This finding may not be such a surprise, however.
Our sun rotates at a leisurely pace of 2 kilometers per second. Zeta Andromedae is spinning at a rate of 40 kilometers per second and has a smaller binary partner. We're likely comparing oranges to apples; both are stars, but they have very different properties influencing the evolution of starspots. But one thing is pretty clear to researchers -- this star certainly doesn't have a sun-like magnetic dynamo, revealing just how different another star can be to our own.
The massive star zeta Andromedae has 15 times the radius of our sun. Therefore our sun could easily fit into one of these dark clusters of starspots as observed by the 6 telescopes in the Centre for High Angular Resolution Astronomy (CHARA) array in California.
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.