Particles From Space
The video below shows a summary of cosmic rays, their history and their importance within Astrophysics.
Cosmic Rays are not actually "rays" at all but a stream of high Energy particles and rays from outside of our Solar System.
Cosmic rays come in a variety of forms, as show in the table below :-
The Energy associated with cosmic rays can be huge, upwards of 1 x 1018 eV.
Note - As a comparison, the greatest Energy collisions capable within the LHC are only in the 1x1012eV range.
The largest Energy particle detected was the "Oh my God" Particle, detected over Utah in 1991. This Ultra High Energy Cosmic Ray ( UHECRs )had an Energy of 3 x 1020 eV and was most likely a Proton. At present, astrophysicists are unable to explain the process that would generate such a high Energy event, but at least 15 more have been detected with similar Energy since, so it is not simply a data error. All that is understood about UHECRs is that they must occur quite close ( within ~100 million light-years ) as if they originated any further away, the UHECRs would have interacted with the CMBr and decayed into Pions.
The video below gives a more detailed look at the OMG Particle event ( and UHECRs in general )
Historical Proof of Origin
In the early 20th Century, an Austrian scientist called Victor Hess was working on measuring radiation. He found that even without an obvious radioactive source, radiation was detected - Background Radiation. At this point, all background radiation was thought to be from the Earth, so in order to prove this, Hess flew detectors from weather balloons.
His assumption was that the radiation level would drop the further from the Earth the balloon reached, proving the Earth as the source of background radiation. However, as the balloon got higher, even more radiation was detected.
Hess' conclusion that the increase in radiation was caused particles from space penetrating the atmosphere won him the Nobel Prize for Physics in 1936 for the discovery of Cosmic Rays.
However, the source of these cosmic rays was still up for debate; did they originate from the Sun or from further away ?
In order to prove one way or the other, detectors were flown on weather balloons during a total Solar eclipse. If the cosmic rays were coming from the Sun, then there should be a drop in detected radiation as the Moon would absorb some of the cosmic rays. However, if there was no change in the radiation levels detected, then the cosmic rays must have come from beyond our Solar System.
The results of this experiment showed that there was no change in the radiation levels detected - the Sun is not the source of cosmic rays.
Interactions with the Upper Atmosphere
When these high Energy cosmic rays enter the Earth's atmosphere, they interact with the gases that make up the atmosphere, producing a large chain of interactions called an Air Shower.
The diagram below shows an example of an air shower caused by a cosmic ray entering the upper atmosphere :-
The exact composition of the air shower depends upon three factors :-
1. Cosmic Ray type and Energy
2. Initial Atmospheric Gas particle interaction
3. Subsequent decay stages
Detecting Cosmic Rays
Due to the effect of the cosmic ray interactions within the upper atmosphere, there are two main areas to detect cosmic rays :-
Direct detection - above the atmosphere, by high altitude balloons or satellites.
Indirect detection - At ground level, by detecting the resulting air shower particles that reach the Earth's surface.
The nearest star to Earth is, of course, the Sun. Being "only" 150 Million km from the Earth, Scientists have been able to study the Sun in great detail, making it the most well-understood star. The Sun itself, however, is a fairly unremarkable star; placed within an H-R Diagram, the Sun appears in the middle of the Main Sequence having approximately average Brightness, Temperature and Mass.
These observations have allowed scientists to understand the Physics of stars, to understand their composition, their Chemical and Nuclear process, as well as their full life cycle. By observing the Sun, an understanding of stars in general can be derived.
Note - The following information on the Sun's composition is non-examinable knowledge, however questions can be set in exam relating to this material as applications of other sections - e.g. Prominences are an example of charged particles moving in a Magnetic Field. Examinable material begins at Solar wind .
The Sun is a near-perfect sphere of hot Plasma ( Surface T ~ 5,800 K ) comprising mostly of Hydrogen. The structure of the Sun is split into six separate layers, as shown in the diagram below :-
In the above diagram, three layers combine together to form the Solar Atmosphere :-
1. The Photosphere
2. The Chromosphere
3. The Corona
Within this extended region (the Corona has been shown to extend to ~20 Solar Radii), there are many complex processes occurring as heat and light Energy are transferred from the Convention Zone below.
The Photosphere is classed as the effective surface of the Sun. This surface from Earth appears quite featureless. However, if the Photosphere is imaged at high resolution, this region of the Sun is massively active, with the entire Photosphere showing a roiling surface covered in granules, pockmarked by Sunspots and Prominences as well as areas of massive eruptions called Solar Flares.
The image below shows a high resolution view of part of the Photosphere :-
In the above image areas of granulation can be seen across the entire limb of the Sun. Also a Solar Prominence can clearly be seen over the surface at the center of the image.
The "mottled" patterning on the surface of the Sun is caused by a process called Granulation. This granulation is the result of convection currents within the Convection Zone below transferring warmer gas up to the Surface, whilst cooler gas sinks back down. As the Luminosity ( L ) is proportional to the 4th power of Temperature ( T 4 ), the small temperature difference gives a large contrast in brightness between the rising and falling gas.
Note - The "cooler" gas above is still at a Temperature and Luminosity greater than that of a Thermite reaction. It is darker only in comparison to the surrounding areas. This also applies to Sunspots (see below).
Prominences and Sunspots
As stated earlier, the large structure in the center of the image is known as a Prominence. A Prominence is a huge structure ( 1000s to 100,000s of km in length ) arching upwards from the Photosphere and curving back down to it again.
A Prominence forms when a region of the Magnetic field of the Sun loops out of the Sun's surface as shown in the diagram below :-
High Temperature Plasma ( consisting of ionised gases ) follows the Magnetic field lines like Iron filings around a Bar Magnet, creating the visible Prominence. The individual particles making up the Plasma are charged, and as such move in helical motion around the Field lines, as seen in Unit 3 : motion in a Magnetic Field.
As can be seen in the diagram above, Prominences and Sunspots occur together. A Sunspot is a region on the surface of the Sun which is cooler than that of the surrounding area. The Sunspot is caused by the Magnetic field preventing the hot gas rising to the surface, giving a relatively cool patch (again hotter than Thermite). These Sunspots can be huge, covering an area greater than 150,000 km in diameter.
The diagram below shows a large Sunspot, with the Earth also shown for scale :-