Question

According to observations, the overall chemical composition of our solar system and other similar star systems is approximately (a) \(98 \%\) hydrogen and helium, \(2 \%\) all other elements combined; (b) \(98 \%\) ice, \(2 \%\) metal and rock; (c) \(100 \%\) hydrogen and helium.

Short answer

Option (a) is correct: 98% hydrogen and helium, 2% other elements.

Step-by-step solution

Understand the Question

The question provides three options for the chemical composition of star systems like our solar system. We need to identify which of these options most accurately represents the generally accepted scientific understanding.

Analyze Option (a)

Option (a) states that the composition is 98% hydrogen and helium, and 2% of all other elements combined. This reflects current scientific models which suggest that the universe is primarily composed of hydrogen (roughly 75%) and helium (about 23%), since these are the lightest and most abundant elements.

Evaluate Option (b)

Option (b) claims the composition is 98% ice, 2% metal, and rock. Scientifically, this does not represent star systems because ice, metal, and rock are less abundant than cosmic elements like hydrogen and helium. This option is more relevant to celestial bodies like comets or certain types of planets in specific circumstances.

Consider Option (c)

Option (c) states the composition is 100% hydrogen and helium. While hydrogen and helium are indeed the most abundant, saying 100% excludes the known presence of other elements, even if in small amounts, which are necessary for forming planets and other structures.

Conclude with the Correct Answer

Option (a) is the most accurate and realistic representation of the chemical composition of the universe, including star systems, since it acknowledges the dominance of hydrogen and helium while allowing for minor contributions from other elements.

Key concepts

Hydrogen and Helium Abundance

The vast majority of our solar system, as well as most other star systems in the universe, is chiefly made up of two elements: hydrogen and helium. These are the first two elements on the periodic table and were among the earliest to form in the universe. When we say that approximately 98% of the universe consists of hydrogen and helium, it mirrors the cosmic proportions observed in various celestial bodies.
Hydrogen is the simplest chemical element, consisting of just one proton and one electron. It accounts for about 75% of the total elemental mass in the universe. On the other hand, helium, with two protons, two neutrons, and two electrons per atom, makes up about 23%. Together, these elements dominate the universe's composition because they were the main products of the Big Bang nucleosynthesis process.
These elements are crucial in star formation. Stars, including our Sun, are predominantly composed of hydrogen, which they fuse into helium through nuclear reactions, releasing energy that we perceive as light and heat. This abundance underscores not only the chemical simplicity of the universe but also the ongoing process of stellar evolution.

Star System Composition

The chemical make-up of star systems is largely consistent across the cosmos. They mainly consist of hydrogen and helium, with trace amounts of other elements such as carbon, oxygen, and iron. These heavier elements are often referred to as "metals" in astronomical terms, even though they can be non-metallic chemically.

When a star system forms, it begins in a molecular cloud where gravity pulls these elements together. The cloud primarily consists of hydrogen and helium gas. As the gas contracts under gravity, it forms a protostar, eventually igniting hydrogen fusion. This fusion forms a star and might trigger the formation of planets, moons, and other celestial objects from the remaining "metallic" elements.
Given that hydrogen and helium are the premise of star formation, it's accurate to state that most star systems share this commonality, reflecting option (a) in the exercise, bearing 98% hydrogen and helium and about 2% other elements that serve crucial roles in forming diverse planetary systems and life-supporting conditions.

Elemental Abundance in the Universe

The elemental abundance in the universe reveals a fascinating hierarchy of elements, where hydrogen and helium vastly outnumber all others. After hydrogen and helium, oxygen is the most abundant, closely followed by carbon, neon, iron, nitrogen, silicon, magnesium, sulfur, and so on, in much smaller proportions.

These heavier elements (everything other than hydrogen and helium) are products of stellar processes. They are generally formed during a star's lifecycle, especially in supernovae, where the intense energy and neutrons available lead to nucleosynthesis of elements heavier than iron. This elemental distribution helps to enrich the interstellar medium, leading to the formation of new stars, planets, and ultimately giving rise to the complexity required for life as we know it.
In essence, the slight presence of these heavier elements, though just about 2% of the universe, plays a fundamental role in shaping the physical and chemical properties of planets and the broader cosmic structure. This is crucial for understanding not only the formation of our solar system but also the observable universe at large.