Question

Unenriched Stars. Suppose you discovered a star made purely of hydrogen and helium. How old do you think it would be? Explain.

Short answer

An unenriched star made only of hydrogen and helium would likely be very ancient, possibly from the first generation of stars.

Step-by-step solution

Understanding Stellar Composition

Stars primarily consist of hydrogen and helium, the two lightest and simplest elements in the universe. Heavier elements are usually created through nuclear fusion within stars or during supernovae from older stars.

Identify Element Enrichment Mechanism

Stars that are enriched with heavy elements are typically younger stars because the universe initially had only hydrogen and helium after the Big Bang. Other, heavier elements (‘metals’) were formed later, inside stars.

Evaluating Unenriched Star Characteristics

A star made purely of hydrogen and helium is considered to have little to no 'metallicity.' This indicates it is a very old star, as it formed before heavier elements became more common in the interstellar medium due to stellar nucleosynthesis.

Determine Origin Time Period

The first stars, known as Population III stars, were formed in the early universe from the original hydrogen and helium available after the Big Bang. Any current star with only these elements is likely to be a remnant from this earliest stellar generation.

Key concepts

Stellar Nucleosynthesis

Stellar nucleosynthesis is the process by which elements are formed within stars. Stars are like cosmic factories, where new elements are forged through nuclear fusion. During this process, lighter elements such as hydrogen and helium are fused into heavier elements like carbon, oxygen, and iron. This happens under extreme temperatures and pressures at the core of the stars.

Stars have different life stages, and the types of elements they produce can change as they burn through their fuel.

• Small stars may create elements up to carbon and oxygen.
• Larger stars, often called supergiants, can forge even heavier elements.

Eventually, when a large star ends its life in a supernova explosion, it disperses all the newly created elements into space. Hence, these elements become part of new stars, planets, and even living beings. Stellar nucleosynthesis is crucial for understanding how the universe evolved and how it became enriched with a variety of elements.

Element Enrichment

Element enrichment refers to the process by which the chemical composition of the universe changes over time. Initially, after the Big Bang, only the lightest elements existed - primarily hydrogen and helium. Through stellar nucleosynthesis, stars create heavier elements, often referred to as 'metals' by astronomers.

Every generation of stars contributes to this enrichment process:

• When stars end their life and explode as supernovae, they spread these heavier elements into the surrounding space.
• These enriched materials eventually become part of the molecular clouds from which new stars form.

With each new star formation cycle, the universe becomes richer in these heavier elements, continuing the cycle of element enrichment. This gradual increase in metallicity allows for the formation of more complex star systems, enabling planets and, eventually, life as we know it.

Big Bang Nucleosynthesis

Big Bang nucleosynthesis refers to the formation of the first elements in the universe, occurring only a few minutes after the Big Bang when temperatures and pressures were incredibly high. This primordial nucleosynthesis produced the lightest elements, mainly hydrogen, helium, and small amounts of lithium and beryllium.

• Hydrogen and helium make up almost all of the ordinary matter in the universe today, reflecting their abundance from this period.
• Due to the rapid cooling of the universe, heavier elements beyond lithium were not formed at this time.

Understanding Big Bang nucleosynthesis provides insights into the initial abundance of elements in the universe. These original elements set the stage for later formation of stars and their contribution to the creation of heavier elements through stellar nucleosynthesis.

Metallicity in Stars

Metallicity in stars is a crucial concept in astrophysics that refers to the proportion of elements heavier than hydrogen and helium within a star. Astronomers denote all elements heavier than helium as 'metals'.

• Stars with high metallicity contain a larger variety of heavy elements.
• Low metallicity stars are considered more primitive, as they formed from material that had not been significantly enriched by previous generations of stars.

The metallicity of a star reveals important information about its age and origin. Early stars, such as Population III stars, are extremely low in metallicity since they originated from the pure hydrogen and helium present right after the Big Bang.

When we find a star with low metallicity today, it may be incredibly old, possibly tracing back to the early universe when the first stars formed. This gives us a window into the past, showing us conditions in the universe shortly after the Big Bang.