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Introducing the Solar System

Introducing the Solar System. The solar system seen by Voyager 2 (NASA/JPL)‏. The Solar system: 1 star (the Sun)‏ 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune Many minor planets Many moons A belt of unaccreted rocky debris

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Introducing the Solar System

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  1. Introducing the Solar System The solar system seen by Voyager 2 (NASA/JPL)‏

  2. The Solar system: • 1 star (the Sun)‏ • 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune • Many minor planets • Many moons • A belt of unaccreted rocky debris • Many small, icy objects (Kuiper belt objects, etc..)‏ • Extensive comet cloud • Sun contains over 99.8% of the mass of the solar system • Of the remaining 0.2%: • Jupiter contains more than twice as much mass as all the other (known) planets combined – so 99.93% of the mass of the solar system is in either the Sun or Jupiter • Saturn is about 1/3 of the mass of Jupiter, so 99.98% of the mass of the solar system is in these three objects • Most of the rest is in the other two giant planets, Uranus and Neptune

  3. In brief, the solar system consists of: • The Sun • Small, rocky worlds near the Sun – the Terrestrial planets • Mercury • Venus • Earth • Mars • Only Earth has a large moon (so large that it is almost a double planet). Mars has two tiny moons (only a few km across)‏ • A belt of rocky debris • Large gaseous or icy worlds further from the Sun – Gas and Ice giants • Jupiter • Saturn • Uranus • Neptune • All of these have ring systems and large numbers of moons, some of which are very large • At larger distances from the Sun, small icy objects • Trans-Neputinian/ Kuiper Belt Objects eg. Pluto/Charon, Eris, Quaoar.. • Comet cloud at largest distances, may contain larger objects e.g. Sedna (status disputed – may be a KBO) • There is also a lot of dust, mainly concentrated in the plane of the planets • Apart from Comets, KBOs and assorted debris (“small stuff”), the solar system forms a flattened disc.. • Everything (bar a few anomalies) orbits and rotates in the same direction

  4. What is a planet, anyway? • ..and how many of them are in the solar system? • 2 definitions debated by the IAU in September 2006: 1. A planet is a celestial body that • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; • is in orbit around a star, and is neither a star nor a satellite of a planet. → Solar system contains: • 12 “planets” (Mercury, Venus, Earth, Mars, Ceres Jupiter, Saturn, Uranus, Nepture, Pluto, Charon, Eris – more likely to be added)‏ • numerous “Plutons” - icy, non-round outer solar system objects • many “minor solar system objects” Rejected 2. A "planet" is defined as a celestial body that • is in orbit around the Sun; • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; • has cleared the neighbourhood around its orbit. → Solar system contains: • 8 “planets” (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Nepture) • 3 “dwarf planets” (Ceres, Pluto, Eris – more likely to be added)‏ • many “minor solar system objects” Approved – but still much dispute

  5. 1 star (Sun): 1392000 km diameter 1.99×1030 kg mass Viewed in white light Temperature ~ 5800 K http://www.bbso.njit.edu/Images/daily/images/wfullb.jpg

  6. http://umbra.nascom.nasa.gov/images/latest.html Increasing temperature Sun shows more structure (more complexity) at higher temperatures Sun isn't as simple as it looks in white-light images

  7. Sun:a rather ordinary yellow-white Star (type G2), remarkable only because it is so close to us • Radius: 696000 km (0.005 AU, 109 Earth radii, 9.7 Jovian radii)‏ • Mass: 2.0x1030 kg (335000 Earth masses, 1053 Jovian masses)‏ • Density: 1400 kg/m3 (Earth density = 5517kg/m3, Jovian density = 1300 kg/m3)‏ • Rotation period: • Equator: 25.4 days • Poles: ~ 30 days • "Surface" temperature: ~5800 K • Luminosity: 3.83x1026 W • By constrast: • Sirius: • Mass = 2.1 MS • Radius = 1.8 Rs • Luminosity = 23 Ls • Betelgeuse: Mass = 12 MS, radius >750 Rs , luminosity = 100000 Ls

  8. Mean stellar mass is only 11% of mass of Sun. • Least massive stars ~ 0.23 Ms • Most massive stars ~ 60 Ms • Most stars are dwarfs • In spite of its higher-than-average mass, the Sun is a typical cool star Sun

  9. Unlike any other star, we can observe the Sun indetail The outer layers of the Sun viewed by the TRACE spacecraft in extreme ultra-violet light (~1.6 MK)‏ All of these structures are moving http://trace.lmsal.com/

  10. Fine-scale prominences above the solar limb – Hinode optical imager

  11. Outflow above a solar active region (Hinode EUV imaging spectrograph)

  12. NASA SDO http://sdo.gsfc.nasa.gov

  13. Why is the Sun hot? Early ideas: Anaxagoras (434 BCe): the Sun is a mass of hot iron (which doesn't explain why it stays hot)‏ By the mid-19th Century the Sun was known (from measurements of its spectrum) to be: 74 % Hydrogen, 25 % Helium, 1% other elements (by mass)‏ William Thompson (Lord Kelvin) and Hermann von Helmholtz (1880s): the Sun is a ball of hydrogen and helium, contracting under gravitational pull and heated by compression "Gravitational Collapse" Need ~20m/year shrinkage for observed solar luminosity  Solar lifetime of ~2.5107 years The Kelvin/Helmholtz lifetime for the Sun caused a lot of debate in the last century, as it didn't really give enough time for the evolution of life... Now know that Earth is ~4.5109 years old - what could keep the Sun shining for so long?

  14. Proton-proton fusion • Need temperatures of ~1107 K for fusion - fusion cannot be occuring throughout the Sun ("surface" temperature ~ 5800 K)‏ • Pressure of gas increases with depth •  temperature increases with depth • Core of Sun is hotter than "surface" • Energy source (fusion region) is in core of Sun

  15. Solar material is hot – hot enough to split off electrons from atoms to form a plasma (a gas of ions and electrons)‏ Plasmas are electrically conductive In the convection region, there are flows of plasma => The Sun contains a layer which acts as a dynamo, generating a strong magnetic field

  16. Looking inside the Sun • “The singing Sun” • 1970: Roger Ulrich suggested that sound waves could travel around the inside of the Sun, being reflected at the surface and refracted by the changing density and temperature inside the Sun • These waves can interfere to produce standing waves in the Sun, moving areas of the surface in and out – and this produces small (but measurable!) Doppler-shifts in the wavelengths of the Sun's light All modes 1 mode 3 modes

  17. Sun doesn't rotate as one mass Breakdown in rigid rotation near base of convection zone (tachocline)‏ Different variation in rotation rate with depth at different latitudes Bands of high- and low-speed flow at different latitudes and depths in the Sun These bands move over time All pictures from: http://soi.stanford.edu/press/GONG_MDI_03-00/

  18. The Sun has a magnetic field, generated in the convection zone The convection zone contains bands of flow which vary with time => The Sun's magnetic field varies with time Solar activity and the solar cycle Photosphere of Sun often shows small dark features which rotate with the Sun - Sunspots Sunspots come and go (the largest may last 2-3 months) and their average numbers vary with time Sunspot

  19. Sunspots are regions where the Sun's magnetic field is so strong that it slows down convection • => Produces a cooler (and lower-lying) area which appears dark on the disc • Also get regions of weaker field where convection is stronger and the photosphere is hotter – faculae • Magnetic field breaks through photosphere in sunspots • Sunspots often occur in pairs or multiples (N & S magnetic points)‏ • Sunspot pairs (or multiples) linked by loops of magnetic field above the photosphere Image courtesy of Dr. Dan Brown

  20. Sunspots first emerge at ~30°N/S after solar minimum • Number of sunspots increases towards solar maximum and location moved down in latitude • In declining phase of cycle sunspot location moves towards equator – fields cancel • Good match between sunspot locations and boundaries of faster- and slower- rotating streams below the solar “surface” • Helicity - “twisting” in solar magnetic fields increases towards solar maximum

  21. The 11-year solar cycle http://science.nasa.gov/ssl/pad/solar/sunspots.htm

  22. Sunspot numbers since last maximum: http://www.swpc.noaa.gov/SolarCycle/index.html

  23. Sunspot magnetic fields vs. time (Livingstone and Penn, 2009)

  24. Magnetic fields on the Sun change over short periods, too

  25. .. and this can also effect the Earth

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