Unit, astronomical

Binzel et al. (1991) give an account of the origin and the development of the asteroids, while Gehrels (1996) discusses the possibility that they may pose a threat to the Earth. The giant planets, and in particular Jupiter, caused a great proportion of the asteroids to be catapulted out of the solar system these can be found in a region well outside the solar system, which is named the Oort cloud after its discoverer, Jan Hendrik Oort (1900-1992). Hie diameter of the cloud has been estimated as around 100,000 AU (astronomic units one AU equals the distance between the Earth and the sun, i.e., 150 million kilometres), and it contains up to 1012 comets. Their total mass has been estimated to be around 50 times that of the Earth (Unsold and Baschek, 2001). 

The extinction for dust particles is found to follow the empirical relation sx = 0.008 k 4/3 mag pc-1 (Equation 5.3 - a truly terrible astronomical unit for a scientist ). Calculate the extinction per metre at a wavelength of 501 nm. The light from a star at 501 nm is 0.25 of that expected for a star with that surface temperature and the extinction is attributed to dust in an interstellar cloud. Calculate the diameter of the cloud along the line of sight. 

Astronomical units 1AU = 1.496 x 10s km. 1 Maximum day-time high temperature at the equator,... 

Table 8.1 The orbital distances (astronomical units) at perihelion, mean and aphelion of the planets, compared to scaled intercepts, with divergence angle of 108°(3/10 x 360), on a golden spiral...

The age of the sun is 4.6 x 109 years. Its composition is 75% H, 24% He, and at least 70 other elements, up to 1% of the total. Its distance from the earth is minimum (= perihelion) 147.1 x 109m on about 3 January, maximum (= aphelion) 152.1 x 109 m on about 4 July, mean 149.6 x 109 m (this mean value is defined as the astronomical unit of length, AU). Summers are warmer in July in the Northern Hemisphere because the solid angle subtended by the earth is bigger in July than in January (even though the sun is farther). The surface temperature of the sun is 5500° C the temperature at the center of the sun is estimated at 1.5 x 107oC. 

SEDs) demonstrate that massive disks often extend to hundreds of astronomical units. A lower estimate for the initial mass distribution of our Solar System is provided by the minimum mass solar nebula (MMSN) model, which is the minimum mass required to produce the observed distribution of solids from a disk with solar composition. This analysis predicts a disk mass between 0.01 and 0.07 M extending out to 40 AU. Mass estimates for circumstellar disks derived from submillimeter and longer-wavelength observations are consistent with the range estimated for the MMSN (e.g. Beckwith et al. 1990 Williams etal. 2005). 

Figure 5.5 Winds in the solar nebula might be one of the possible processes responsible for the mixing of hot and cold components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions (CAIs, formed at about 2000 K) and chondrules (formed at about 1650K), which may have been created near the proto-Sun and then blown (gray arrows) several astronomical units away, into the region of the asteroids between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive, carbon-based cold components. The hot component in comets, tiny grains of annealed silicate dust (olivine) is vaporized at about 1600 K, suggesting that it never reached the innermost region of the disk before it was transported (white arrows) out beyond the orbit of Pluto, where it was mixed with ices and some unheated silicate dust ( cold components). Vigorous convection in the accretion disk may have contributed to the transport of many materials and has been dramatically confirmed by the Stardust mission (Nuth 2001).

Further, many of the astronomical data do not probe dust in the disk mid-planes, but rather dust in the surface layers of the disks, whereas chondrites sample material from the mid-plane. Comparisons therefore rely on assumptions of communication between the surface layer and mid-plane at a few astronomical units, such as vertical mixing. 

In the absence of planetary migration, oligarchic growth forms lunar to Marssized bodies in the inner few astronomical units of a disk in 106 years (Weidenschilling el al. 1997 Kokubo Ida 2002 Chambers 2006 see Fig. 10.4). 

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