I have got chance to knoe some but some are un answered still..
Star birth:
Stars form from dense interstellar clouds of gas and dust. These clouds are typically ~30 light years in dimension and ~10,000 times larger than our Sun. When mutual gravitational attraction dominates, regions of the cloud start to condense into stars. Fusion reactions start when energy provided by further gravitational collapse is large enough to heat the core to ~10,000,000 K (1E7K).
How were the Planets formed?
Hei you know ,we dont get all the planets and sun in all same time it tokk many years..really..!!(millions of years )
The Nebula hypothesis. Probable sequence of steps in the formation of the solar system.
** Gravitational contraction of a rotating gas cloud leads to a dense central region (eventually forming the Sun) and a more diffuse, flattened nebula.
** Dust particles from the nebula settle onto a disc.
** Accretion of dust into numerous small planetesimals, each a few kilometers in diameter. Collisions between planetesimals lead to capture, disintegration, or deflection of their orbits.
** Eventually larger bodies capture the smaller ones. Uncondensed gas is blown away by the "solar wind"; this process may begin in earlier stages.
The Nebula Hypothesis
The planets of our Solar System formed due to two properties of interstellar clouds: rotation and turbulence.
The role of dust grains
Planets such as our own could not have formed from gas alone, but need matter in the solid phase, such as dust grains. To This process is known as accretion. The dust grains continue to accrete slowly, eventually forming clumpy "protoplanets" or "planetesimals" of a few kilometers in dimension, like the asteroid shown. Collisions between the planetesimals eventually lead to a few larger bodies that capture smaller ones. This process is chaotic, with collisions sometimes leading to break-up of the planetesimals, changes in orbits, and often forming craters on the larger bodies.
Summary of Planetary Evolution
- Spin forms a fragmented disk about the protosun (core) of the shrinking nebula.
- Chemistry, acting on elements in interstellar Nebula, allows the formation of dust grains, up to a few mm in size.
- Gravity allows the dust grains to collide and coalesce (starts "accretion").
- Accretion, over tens of millions of years, builds planets.
Chemical Composition of the planets.
The outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto) have compositions different from the Earth and more consistent with the composition of the solar system - lots of hydrogen and helium. The table below compares the properties of the inner and outer planets and the planet contain inks to images.
Some Properties of the Planets
Planet | Diameter (km) | Distance from Sun (x106 km) | Surface temperature (°C) | Density (g/cm3) | Main atmospheric constituents |
Sun | 1,392,000 | - | 5,800 | - | |
Mercury | 4,880 | 58 | 260 | 5.4 (rocky) | - |
Venus | 12,100 | 108 | 480 | 5.3 (rocky) | CO2 |
Earth | 12,750 | 150 | 15 | 5.5 (rocky) | N2, O2 |
Mars | 6,800 | 228 | -60 | 3.9 (rocky) | CO2 |
Jupiter | 143,000 | 778 | -150 | 1.3 (icy) | H2, He |
121,000 | 1,427 | -170 | 0.7 (icy) | H2, He | |
Uranus | 52,800 | 2,869 | -200 | 1.3 (icy) | H2, CH4 |
Neptune | 49,500 | 4,498 | -210 | 1.7 (icy) | H2, CH4 |
Pluto | 2,300 | 5,900 | -220 | 2.0 | CH4 |
The reason for the difference between the rocky dense inner planets and the icy/gaseous outer planets is : the composition of each planet is determined by the type of material that can survive in the solid form given the temperature of the particular part of the Nebula: Condensation theory.
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| Sequence of condensation of minerals in the nebula as a function of temperature. At temperatures above about 1300K, metals and silicates can condense and become solid dust grains. At lower temperatures more volatile minerals become solids, and at temperatures of less than ~400K, hydrogen-bearing gases such as methane and ammonium become solids. Hydrogen and helium remain gases. For the inner planets, at high temperatures, the planet-building dust grains were made up of rocky materials (silicates, iron, etc.). The hydrogen and helium could have been blown away by the solar wind. For the outer planets, the hydrogen and helium was retained by a combination of the larger gravity for these massive bodies and the formation of ice. |
