Earth Sciences 1086F/G Lecture Notes - Lecture 4: Milky Way, Solar Wind, Photosphere
Course CodeEARTHSCI 1086F/G
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Chapter 4: The Sun
•brighter than about 85% of the other stars in our Milky Way Galaxy.
•There is not really a ‘surface’ to the Sun; made of gas that becomes denser as you move toward the centre
The!photosphere!(visible outer layer of a star) represents the depth within that gas at which we can see no deeper
•The Sun's atmosphere changes from being transparent to being opaque over just a few hundred kilometres,
•Surrounding the photosphere (2,500 km thick) chromosphere; irregular layer of gases extends up/away
•Further out, the!corona; extends for millions of km into space above the photosphere.
•Diameter about 1.4 million km (about 109 times the diameter of Earth).
•Volume about 1.3 million times that of Earth.
•Mass is about 330,000 times that of Earth (the!Sun contains 99.8 % of the Solar System’s mass).
•Age is just over 4.6 billion years (Earth is about 4.6 billion and the Sun = little older)
•zero for Kelvin is the point where all motion within an atom ceases (0K = -273 °C)
•Interior: 15,000,000 K, Surface: 5800 K
•The distance between Earth and the Sun is about 150 million kilometres.
•One!astronomical unit!(1 AU) = the mean average distance between the Sun and Earth.
•Spectroscopy: the measurement and analysis of energy spectra to determine the composition of matter.
•Just as a telescope collects light, so a spectroscope splits it up
•Hydrogen (H) accounts for 71% of the total mass, Helium 27%
•difference between elements on Earth and in the Sun is Sun is so hot that all the elements are in gaseous state.
•NASA's!Genesis!mission set out to capture particles of the solar wind and return them for study on Earth.
•prominences!– arcs of gas that begin on the bright surface and soar up to 10,000 kilometres into the corona.
•ﬂares! short-lived gas eruptions that generally last for no more than 20 min
•the magnetic ‘generators’ sometimes become ‘confused’ and become unstable – and suddenly the poles of the
magnet switch! In just a matter of days, what was north magnetic pole suddenly becomes south magnetic pole.
•On Earth that switch occurs irregularly every few thousand years to maybe 50 millions of years
•Sun the switch happens almost like clockwork every 11 years or so (the complete 22 year cycle = solar cycle)
•Sunspots!- not black spots, but they appear so because they are cooler than the surrounding; spots may be
4000 K, or about 1000 degrees cooler than their surroundings
•during intense ﬂares, corresponding intense magnetic storms occur on Earth. There are two obvious effects on
Earth: interference to complete breakdown of power grids and systems carrying telemetric signals and ‘northern
lights’. Both phenomena are the result of matter carried into Earth’s upper atmosphere by the!solar wind.
•made up of charged particles sent out from the Sun at all times and in all directions.
•made of a!plasma!- an ionized gas made up of a mixture of electrons and protons
•Despite the Sun’s awesome gravity, these little particles escape because of their high kinetic energy (they leave the
Sun at average speeds of 400 km/s, although the rate can vary from 300 km/s to 800 km/s). During a period of
solar ﬂares, the stream of solar wind is devastating – even as far away as Earth.
•The magnetic ﬁeld volume around a body like Earth, (which generates a magnetic ﬁeld) = magnetosphere.
•the reaction essentially protects most of Earth’s surface from devastating bombardment by the solar wind
•Energy from the Sun, in the form of heat and light, supports almost all life on Earth (via photosynthesis)
•bad thing is the ultraviolet radiation from the Sun: while it is great to sterilize things like tools and water, it also is a
primary cause of skin cancer. Fortunately, the ozone in our atmosphere greatly attenuates that radiation
•At about 4.6 or so billion years along, the Sun is about halfway through its life cycle
•During this long period, the Sun has grown about 30% brighter than when it was ﬁrst born.
•As it grew brighter, its diameter grew proportionately, and as the last of the inner hydrogen fuel is consumed (in
about 5 billion years from now), the Sun will expand into a!red giant.
•That expansion will bring the Sun’s radius past the present position of Earth
•In really big stars, there is a subsequent phase: when all the helium is consumed, it is replaced by carbon. That
requires a star to reach an internal temperature of 600 million Kelvin, which is not likely to happen to the Sun.
Instead, it will blow off its outer envelope into space, leaving its core to cool and become a!white dwarf,
•and ultimately, when it stops glowing altogether, end up as a!black dwarf.
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