Star formation, formation of planets, and our solar system

Large outer planets formed in colder regions far from the Sun. Exam tip: understand outer planets as gaseous and giant.

A protostar is an early stage in star formation. Exam tip: remember it as a middle step before a star becomes stable.

In a protostar, the main energy source is gravitational contraction. As the cloud contracts, gravitational potential energy is converted into heat. Nuclear fusion begins only after the core reaches sufficiently high temperature and pressure, marking the main-sequence stage. Exam tip: associate protostars with contraction and mature stars with fusion.

Fusion of hydrogen gives energy in stars. Exam tip: remember hydrogen as the main fuel of stars.

Gravitational instability can gather matter into dense regions. Exam tip: link it with the beginning of star formation.

New stars can form from interstellar gas and dust. Exam tip: treat gas and dust as star forming material.

Planets formed from leftover material around the newly formed Sun. Exam tip: remember the Sun first and planets later.

Solar wind removed light gases from inner regions. Exam tip: this helped inner planets become rocky.

Near the Sun heat and solar wind reduced light gases. Exam tip: remember inner planets as small and rocky.

In distant cold regions gas and ice could remain in larger amounts. Exam tip: remember outer planets as huge and gaseous.

Rocky materials near the Sun formed inner terrestrial planets. Exam tip: place Mercury to Mars in this group.

In the early solar nebula, solid dust grains collided and began sticking together because of their mutual attraction and low-speed contacts. The immediate, simple result was the formation of larger small solid aggregates or clumps. With continued accretion, these aggregates could grow into planetesimals and, later, protoplanets. Thus option C describes the first step in the sequence rather than a later planetary outcome. Oceans formed only after suitable planets developed and cooled, and the atmosphere did not simply end. A galaxy breaking apart is unrelated to the local process of building planets in the solar nebula.

Planetesimals join to form protoplanets. Exam tip: remember the sequence dust particles planetesimals protoplanets planets.

Collisions and joining made planetesimals into larger bodies. Exam tip: connect collisions with formation not only destruction.

Angular momentum in the rotating cloud helped form a disk. Exam tip: link the disk structure with planet formation.

Matter in the disk collected mostly in one flat plane. Exam tip: connect it with planetary orbits lying nearly in one plane.

Matter collected at the center of the solar nebula and formed the Sun. Exam tip: link the center with Sun formation.

The outer region was colder so icy materials could remain. Exam tip: remember ice and gas in the composition of outer planets.

A galaxy is a vast gravitationally bound system of stars, gas, dust and other celestial objects, so it is the correct answer. A planet orbits a star, while a satellite orbits a planet; neither is a large group of stars. Exam tip: our solar system lies in the Milky Way galaxy.

The Milky Way is a galaxy and the solar system is part of it. Exam tip: treat the solar system as a smaller level than a galaxy.

Planets formed from a rotating protoplanetary disk so their orbits are nearly in one plane. Exam tip: remember the disk shape.

Near the Sun heavier materials like rock and metal could remain. Exam tip: write these for the composition of inner planets.

A star gives its own energy and light while a planet reflects sunlight. Exam tip: remember this basic difference.

The Sun contains almost all of the mass of the solar system, approximately 99.8 percent. It formed at the centre of the collapsing solar nebula, where most of the available gas and dust accumulated. Its enormous mass produces the gravitational force that keeps the planets, asteroids, and comets in their orbits. Option C is therefore correct. Jupiter is the most massive planet and contains much more mass than Earth, but it is still tiny compared with the Sun. Earth and the Moon contribute only very small fractions of the system’s total mass, so neither can be the answer.

Suitable distance from the Sun helped water and temperature on Earth. Exam tip: remember distance water and temperature for life.