Why is the Sun active?
Most of the time the Sun sits there quietly producing light. However, from time to time, parts on the surface of the Sun will suddenly increase, or flare, in brightness. A lot of light, usually in the form of ultraviolet (UV) and X-ray radiation is produced, but charged particles like electrons, protons, and other ions are also ejected in these events. Flares occur when charged particles are accelerated with the help of magnetic fields in the region. The amount of energy involved is huge, at least thousands of times the entire nuclear arsenal in our planet, and produce the high energy light that we see and accelerate particles that travel 150 million kilometers before reaching the Earth. These flares occur in sunspot groups on the surface of the Sun.
Sunspots are so called because they appear darker than the rest of the Sun. They are much cooler than the rest of the Sun with temperatures of order 3000 Kelvin (2727 Celsius, 4940 Farenheit). In comparison, the rest of the Sun is about 6000 Kelvin. So to say that they are 'cool' is only relative- the surface, or photosphere, of the Sun is quite hot whether or not you are in a sunspot. These spots are actually quite large, the smallest are about the size of the Earth, with many being many Earth-diameters across. The Sun is absolutely huge when compared to our planet: just over a hundred Earth's would be needed to span the Sun's diameter. These sunspot groups are the sites of strong magnetic activity and can lead to flare events or coronal mass ejections (similar explosions that eject a large amount of material out to space). Here is a satellite ultraviolet image of the solar flare event that took place January 27, 2012:
|Notice the solar flare on the upper right edge of the Sun. Photo: NASA / SDO / Helioviewer.org. |
See a video and discussion here.
One thing to bear in mind is that the Sun has an 11-year sunspot cycle. At some times we are at solar minimum with very few sunspots and associated activity (like solar flares). At others, we are at solar maximum and can expect lots of sunspots and flares. The peak in solar activity was supposed to occur sometime around 2011, but the cycle can be a bit irregular. It looks like now things are starting to warm up and we can expect a peak in activity around 2013 and 2014.
How does this affect the Earth?
During a flare event, UV and X-ray emission is produced. However, compared to other stars (see below) this isn't a huge amount. Also, the Earth's atmosphere is very good at blocking this high-energy radiation. You've probably heard of the ozone layer. This is a layer in the Earth's atmosphere which serves to stop most of the harmful UV light from reaching the surface. In fact, when astronomers want to study stars or galaxies at these wavelengths they have to use satellites in order to avoid the blocking effect of Earth's atmosphere.
However, in addition to the light there are also high-energy particles- protons, electrons, and a few small nuclei, that are produced. As these are particles, they cannot travel at the speed of light and take a bit longer, usually a few days, to reach the Earth. When they do, they encounter two things around the Earth- the magnetosphere and the atmosphere.
|The magnetosphere. The blue depicts magnetic field lines surrounding the Earth, in greenish yellow are the charged particles from the Sun. Credit: NASA|
The Earth is surrounded by a magnetic field, what we refer to as the magnetosphere, produced in the interior of the planet. Many of the solar system's planets, and even some of the moons, are known to possess these magnetic fields. These magnetic fields interact with charged particles (protons, electrons, etc) and deflect their paths. Some of these charged particles become trapped in the magnetic field and are channeled to the north and south magnetic poles. There they slam into the Earth's atmosphere.
The result, is the aurora. The impact of the charged particles on oxygen and nitrogen atoms cause these to emit light. Because of the magnetosphere's influence, though, these lights only are seen near the Earth's poles and so are generally called northern (or southern) lights. Or in more technical terms- the aurora borealis and aurora australis. During very strong (and rare) solar flares, these may occur much farther from the poles and be visible at latitudes closer to the equator.
|The aurora borealis above Bear Lake, Eielson Air Force Base, Alaska. Photo by Senior Airman Joshua Strang.|
Flares on Other Stars
Other stars also experience flares, particularly low-mass stars or young ones. As mentioned before, the mechanism behind producing such activity is tied to the magnetic fields at the surface of the star. The mechanism that controls the strength of such fields involves the rotation of the star and the convection going on in the outer layers. Well, it turns out that stars rotate faster when they are young and they continuously slow down as they age. Also, stars of lower mass have much larger convective envelopes. In fact, once you get below a certain mass, the entire star is fully convective. There is indeed a class of star, flare stars, that are known to undergo these events quite frequently. They can also be more energetic than our Sun- the energy released in a flaring event can be about a hundred times more than that released by a Sun's flare.
One particular project I've worked on is the search for young, low-mass stars. We make use of precisely this mechanism to search for candidates: young, low mass stars should be bright in UV and X-ray light. By looking at X-ray and UV source catalogs we can identify nearby stars that exhibit too much emission and can observe them in more detail to figure out what's going on. In many cases, these turn out to indeed be young, low-mass stars. Why is searching for young, low-mass stars important? You'll have to wait for a future blog post to find out...
What does this imply about life around flare stars?
This is actually a big issue in ongoing discussions. Lower mass stars are more common than higher mass ones. An active area of research is to figure out how often planets form in stars of different masses. Most initial studies focused on solar type stars, but we've since expanded to searching for planets among lower and higher mass stars. Even without a good handle on frequency of planets as a function of stellar mass, though, one would still expect that many planets could be around potentially active, low mass stars.
More telling, however, are the requirements for life around these worlds. In order to have life similar to that of Earth's, water is a key requirement. However, these lower mass stars do not produce the same amount of light as the Sun. Hence, a planet would have to be much closer to the star in order to be warm enough to have liquid water on it's surface. That certainly can happen, but we're considering stars that can potentially flare up and release X-ray and UV radiation much stronger than that of the Sun's. Being so close to the stars, these potentially habitable planets would be frequently bathed in this energetic radiation. The outcome is unclear: perhaps this will just drive the rate of mutations of any organism there or perhaps this will sterilize the world of any life. A further complication is that by being so close to the star, the planet may be tidally locked so that one side faces it all the time (similar to how the Moon always show the same side towards the Earth). It's not clear if life could develop and survive in such a scenario, but it's an interesting concept to think about.
For some continued reading on the Sun and some cool images, check out:
- Bad Astronomy; two recent solar activity articles here and here
- Helioviewer; with views of the Sun in many different wavelengths
- NASA's Solar Dynamics Observatory; with some news on the January 27 flare
- NASA's and ESA's Solar & Heliospheric Observatory (SOHO)
- Check out this YouTube video showing the aurora on Jan 24, 2012