The tracks start once the star has evolved to the main sequence and stop when fusion stops (for massive stars) and at the end of the red-giant branch (for stars 1 M ☉ and less). The evolutionary tracks of stars with different initial masses on the Hertzsprung–Russell diagram. Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous galactic protostars and their parent star clusters. Protostars are encompassed in dust, and are thus more readily visible at infrared wavelengths. Mass is typically compared to the mass of the Sun: 1.0 M ☉ (2.0 ×10 30 kg) means 1 solar mass. Further development is determined by its mass. Ī protostar continues to grow by accretion of gas and dust from the molecular cloud, becoming a pre-main-sequence star as it reaches its final mass. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded two protostars with gas outflows. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. Dense molecular filaments will fragment into gravitationally bound cores, which are the precursors of stars. Filamentary structures are truly ubiquitous in the molecular cloud. As its temperature and pressure increase, a fragment condenses into a rotating ball of superhot gas known as a protostar.
In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As it collapses, a giant molecular cloud breaks into smaller and smaller pieces. Typical giant molecular clouds are roughly 100 light-years (9.5 ×10 14 km) across and contain up to 6,000,000 solar masses (1.2 ×10 37 kg). Stellar evolution starts with the gravitational collapse of a giant molecular cloud. Main article: Protostar Schematic of stellar evolution Star formation Simplistic representation of the stages of stellar evolution Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models. Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their existence, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red-giant phase. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star.
Nuclear fusion powers a star for most of its existence. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. The table shows the lifetimes of stars as a function of their masses. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. Stellar evolution is the process by which a star changes over the course of time. Changes to stars over their lifespans Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
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