Star Chapter 19: A Traumatic Birth Star Formation Star Chapter 19: A Traumatic Birth

Standards Understand the scale and contents of the universe, including objects such as stars Understand the concept of equilibrium

Stars evolve as they consume their fuel supply

Formation of stars like the sun Gravitational competition Star formation begins when a nebula in interstellar space starts to collapse under its own weight The nebula shrinks and heats up The center becomes hot enough to start fusion Contraction stops and a star is born

Formation of stars like the sun A star is in equilibrium: gravity pulling in exactly balances fusion pressure pushing out

Stages of Star Formation Stage 1: An Interstellar Cloud Cloud of cold atomic & molecular gas Thousands of times mass of sun Starts to collapse Begins to fragment

Stages of Star Formation Stage 2: A Collapsing Cloud Fragment Sun-like star: fragment is 1 – 2 solar masses ~100 x size of solar system Temperature & pressure increase and fragmentation stops

Stages of Star Formation Stage 3: Fragmentation Ceases Several tens of thousands of years after initial contraction Size of solar system Heats up & begins to resemble a star – called a protostar – has surface

Stages of Star Formation Stage 4: Protostar Shrinks, density & temperature increase Size of Mercury’s orbit Can be plotted on H-R diagram Violent surface activity Strong winds Called T Tauri star

Stages of Star Formation Stage 5: Protostellar Evolution ~10 x size of sun Central temperature = 5,000,000 K Gas is ionized Evolution slows

Stages of Star Formation Stage 6: Newborn Star 10 million years = true star Central temperature = 10,000,000 K = fusion starts: H2  He

Stages of Star Formation Stage 7: Main Sequence at Last Endpoint of prestellar evolution, no matter what mass star has When fusion starts & star becomes stable

Stars of Other Masses The time required for an interstellar cloud to become a main sequence star depends strongly on its mass The most massive O stars reach the 10 million Kelvin needed to start fusion in a million years (1/50 time taken by sun) An M-type star less massive than our sun takes one billion years to form

Stars of Other Masses Whatever the mass, the endpoint of the prestellar evolutionary track is the main sequence A star is considered to have reached the main sequence when hydrogen burning (fusion) begins in the core and its properties settle down to stable values (i.e., it’s in equilibrium)

Stars of Other Masses Zero-age main sequence (ZAMS) – main sequence band predicted by theory. Agrees well with observed main sequence stars, which provides strong support for the modern theory of star formation and stellar structure

Stars of Other Masses The composition of a star affects its internal structure, which affects both its temperature and luminosity, and thus its place on the main sequence Stars with more heavy elements are cooler and slightly less luminous than stars of the same mass with fewer heavy elements

Stars of Other Masses The main sequence is not an evolutionary track – stars do not evolve along it It is a way station where stars stop and spend most of their lives Once on the main sequence, a star stays in the same location on the H-R diagram until it runs out of fuel and begins to die

Failed Stars Some cloud fragments are too small to become stars Jupiter is a good example There is not enough mass to start fusion. They continue to cool, eventually becoming compact, dark, cold fragments of unburned matter They are known as brown dwarfs Note: Jupiter would need to be 80 times more massive to start fusion

Failed Stars The minimum mass needed to start fusion is about 0.08 solar masses There may be vast numbers of these objects throughout the universe, but they are difficult to detect

Observations of Cloud Fragments and Protostars The age of our entire civilization is much shorter than the time needed for a cloud to contract and form a star. We can never observe individual objects proceed through a full panorama of star birth We can, however, observe many different objects at different stages of stellar evolution