At its core, a star is formed when gravity gathers matter tightly enough to facilitate nuclear fusion in its center while also ensuring it doesn’t generate enough energy to disintegrate. The equilibrium between the gravitational forces pulling inward and the radiative forces pushing outward is referred to as: hydrostatic equilibrium. This balance constrains the size that stars can attain. This limit is known as the Eddington mass limit, which is believed to range between 150 and 300 solar masses.
When stars rotate, they have an enhanced ability to maintain their structure because a rotating body generates a force directed inward from its outer edges. This force is called centripetal force. As the star spins, it applies a centripetal force that acts alongside gravity, balancing the radiation pressure. Recently, a group of scientists investigated how the rotation of giant stars impacts their lifetimes throughout cosmic history. Massive stars contribute significantly to key cosmic phenomena, and understanding their end stages can shed light on the universe’s formation, including the creation of black holes and supernovae.
The researchers employed grid-based modeling software called the Geneva Stellar Evolution Code, also known as Genec. This tool helped simulate stellar behavior and long-term evolution based on initial characteristics. GENEC treats a star as a multi-layered system and tracks the movement of matter across these layers over time.
Two primary variables in their simulations were the star’s rotation status and its initial mass, which ranged from 9 to 500 solar masses. The researchers indicated that current science portrays very massive stars, those exceeding 100 solar masses, as inherently unstable and unpredictable. To clarify this, the team analyzed results for these colossal stars, utilizing 2 other models.
To understand how the fates of giant rotating stars have evolved, the researchers examined the ratio of stars containing elements heavier than hydrogen and helium ( metallic). They argued that since the early universe after the Big Bang had few metals, the modern universe must contain significantly more, allowing metallicity to serve as a proxy for stellar evolution. By analyzing spinning stars with low metallicity, they sought insights into the lifespan of the early universe’s rotating stars.
Following the GENEC simulations, the researchers observed distinct differences in the fates of rotating versus non-rotating stars. Spinning massive stars were more likely to collapse into black holes while being less prone to massive supernova eruptions or transitioning into dense neutron stars. The research indicated that very massive, non-rotating stars with low metallicity tend to explode as supernovae, whereas those with high metallicity collapse into black holes.
The researchers proposed that this intricate relationship arises because rotating stars tend to have more of their material mixed, increasing the fusion potential in their cores. However, this rotational dynamic can also lead to the ejection of more outer material, ultimately reducing the fusion resources available in the core.
An additional complicating factor arises from the frequent occurrence of multiple massive stars in close proximity, forming a binary system. In these scenarios, stars can exchange mass, either gaining or losing material. The researchers suggest that because massive stars in binary systems may shed mass before their lifetimes conclude, their model could underestimate the frequency of massive stars evolving into neutron stars rather than exploding or collapsing into black holes.
In summary, the team concluded that rotation intricately influences star evolution. While rotation increases the likelihood of a massive star undergoing certain outcomes, such as collapsing into a black hole, factors like composition and initial mass significantly affect its destiny. Acknowledging the multitude of variables, the researchers emphasized that the next phase in understanding massive stars’ fates should focus on identifying stars in binary systems.
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Source: sciworthy.com
