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Telescopes, page 3 Astronomy - The Solar System, 1 of 2 Solar System, page 2

Our Solar System consists of the Sun and its members that orbit around it: the eight planets (including our Earth), asteroids and comets. It also includes satellites that orbit round the planets and also small bits and pieces such as meteoroids and the interplanetary medium. Note that, before August 2006, there were officially nine planets in the solar System. Since then, Pluto was demoted to a 'dwarf planet', leaving only eight major planets.

The whole Solar System orbits the center of the Milky Way (our galaxy), which is a spiral galaxy. The Milky Way Diagram on the left shows the image of our galaxy from an edge-on perspective. The image was produced at several near-infrared wavelengths with NASA's Cosmic Background Explorer (COBE), a spacecraft used to measure infrared and microwave background radiations, believe to be the leftovers from the Big Bang. The view looks distant because most of the lights come from stars near to the galactic center. In fact, the Solar System is located towards the end of one of the spiral arms of the galaxy, about 26 000 light years from the galactic center. The Milky Way has two small galaxies orbiting it. They are called the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). On the other hand, the Milky Way, together with billions other galaxy, is traveling through intergalactic space. There are about 200 billion (200 000 000 000) stars in our galaxy and most of the stars we can see with our naked eyes belong to the Milky Way.

The planets and most of the satellites and asteroids revolve round the Sun in the nearly circular orbits, in a counter-clockwise directions. The planets orbit the Sun in or near the same plane to that of Earth, called the ecliptic. The exception being the planet Pluto which orbit inclines at an angle of 18 to the ecliptic. In addition to its small size, this is one of the reason why Pluto was being disqualified as a planet.

The axis of rotation of planets is mostly perpendicular to ecliptic. The exceptions are Uranus and Pluto, which are tipped nearly in the plane of ecliptic.

The Sun is continously ejecting charge particles and and electromagnatic radiation called the solar wind that extends far into space and well beyond the planet Pluto. The region within which the stellar medium is dominated by the Sun's magnetic field (except immediate environment of planets which have their own fields) and solar wind is called the heliosphere (Sun's magnetosphere). No one exactly sure what is the actual size and shape of this Sun's sphere of influence. It is believed to be semi-circular or teardrop-shaped, with the tear front facing the direction of the Sun's motion.
interaction of instellar winds with heliosphere
Diagram above shows the interactions between interstellar winds ('winds' from other stars) and the heliosphere. Only two outermost planets (Neptune and Pluto) are shown schematically (not to scale) the Solar System. At some distance from the Sun, the supersonic solar wind will be held back from further expansion by the interstellar winds. The boundary where the solar and interstellar winds interact is called the termination shock. This is the region where the solar wind begins to slow and large changes to the wind directions and magnetic field orientation begins to occur. Termination shock is believed to occur at around 85 5 AU from the Sun. It is yet remain to be seen when will the Voyager I, one of the most distance spacecraft, will arrive this region.

After crossing termination phase enters the region called heliosheath. This region is still dominated by the Sun's magnetic field and particles. No one (yet) knows the thickness of this region. It is believed to be in the order of tens of AU. At the outer edge of heliosheath is the heliopause. This is the boundary that defines the end of the solar influence (heliosphere) and the begining of the interstellar space where the interstellar winds dominate.

Telescopes, page 3 Solar System, page 2

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