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Planetary Systems

Solar Neighborhood

The boundary of the Solar system is marked by two features: the termination shock, which occurs where the supersonic plasma of the solar wind begins to slow down as it encounters the interstellar medium; and the heliopause, which is the true boundary where the merger occurs with a shock transition - a sonic boom at which the speed drops abruptly from supersonic to subsonic (see Figure 07-24). The giant magnetic bubble (produced by the stream of charged particles in the solar wind) inside the heliosphere partially protected the solar system from damaging cosmic rays. Fluctuation of its strength is said to have an effect on the earth's climate. Estimates of the distance to
Solar Boundary Solar Neighbor the termination shock from the Sun range from 85 to 120 AU ( ~ 0.0015 ly), which is well within the radius of the Oort cloud. The Voyager 1 spacecraft is now crossing the boundary at a speed of about 20 km/sec. Further out still, if the Solar system is itself moving supersonically relative to the interstellar medium, there may be a large bow shock as shown in the illustration. Figure 07-25 shows some celestial objects in the spiral arms of the Milky Way within 6000 ly of the Sun. Table 07-04 lists a few of the prominent objects just beyond the solar halo.

Figure 07-24 Solar Boundary [view large image]

Figure 07-25 Solar Neigh- borhood [view large image]

Name Type of Object Distance
from Sun (ly)		 Apparent
Magnitude		 Remarks
Proxima Centauri M5 red dwarf 4.22 10.7 Closest star to the Sun
Barnard's star M3.8 red dwarf 5.94 9.56 Largest proper (angular) motion ~ 10.29"/yr
Sirius A0 star 8.6 -1.46 Brightest star in the sky
Epsilon Eridani K star 10.5 3 Moon of an exoplanet may hold the seeds of life
Tau Ceti G8 Sun-like star 11.9 3.49 First object searched for ET radio signals
Vega A0 star 25 0 Zero point calibrateion. Legend of Weavermaid.
Aldebaran K5 red giant 65 0.80 Pioneers-10's destination, 2nd brightest
HD70642 Sun-like star 90 7.3 Harbouring Earth-like planet
Betelgeuse M2 red giant 200 0.92 Variable star, diameter measured
Pleiades Open star Cluster 380 1.6 Better known as the Seven Sisters
Polaris F7 star 600 2.0 Marking the North Celestial pole
Antares M1 Supergiant 600 0.94 Dying star in Scorpius
Deneb A2 supergiant 650 1.33 At the tail of Cygnus the Swan, Summer star
NGC7293 Planetary nebula 450-650 6.8 Large size ~ 2.5 ly (a.k.a. Helix Nebula)
Rigel B8 supergiant 770 1.63 Very hot star, luminosity ~ 55,000 Sun's
Orion Diffuse nebula 1000 4.0 Near the Orion Belt in Winter sky
M7 Open star cluster 1000 3.3 At the tail end of Scorpius
HH46/47 Young star 1140 Opaque Infrared object in Vela

Table 07-04 Solar Neighborhood

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