The ground state of hydrogen has two closely-spaced energy levels, where oneelectron is spinning up ...

Answer to q. 1

a

Answer to q. 2

1 . radiation pressure: low-mass specks of dust and individual atoms and molecules were pushed outward and eventually driven from the system

2 . solar wind: strong surging wind from the young Sun may have helped push dust and gas out of the nebula

3 . sweeping up of space debris by the planets: All of the old, solid surfaces in the Solar System are heavily cratered by meteorite impacts during the period of heavy bombardment 4 billion years ago

4 . ejection of material from the Solar System by close encounters with planets: If a small object such as a planetesimal passes close to a planet, the small object's path will be affected by the planet's gravitational field. In some cases, the small object can gain energy from the planet's motion and be thrown out of the Solar System. Ejection is most probable in encounters with massive planets, so the Jovian planets were probably very efficient at ejecting the icy planetesimals that formed in their region of the nebula.

Answer to q. 3

e

Answer to q. 4

First, stars are very far apart in relation to their sizes and their relative velocities, so they collide very infrequently. Only a tiny fraction of stars in our Galaxy have ever suffered a collision or close encounter with another star. Also, the gas pulled from the Sun and the star would be much too hot to condense to make planets, and it would have dispersed instead. Furthermore, even if planets did form from that gas, they would not have gone into stable orbits around the Sun.

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