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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 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 • 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
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
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
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
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
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
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
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/
Fine-scale prominences above the solar limb – Hinode optical imager
Outflow above a solar active region (Hinode EUV imaging spectrograph)
NASA SDO http://sdo.gsfc.nasa.gov
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 ~2.5107 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 ~4.5109 years old - what could keep the Sun shining for so long?
Proton-proton fusion • Need temperatures of ~1107 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
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
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
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/
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
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
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
The 11-year solar cycle http://science.nasa.gov/ssl/pad/solar/sunspots.htm
Sunspot numbers since last maximum: http://www.swpc.noaa.gov/SolarCycle/index.html
Sunspot magnetic fields vs. time (Livingstone and Penn, 2009)