This set of slides This set of slides starts the topic of stellar evolution, overview, protostars, main sequence… Units covered: 59, 60, 61.

Stellar Evolution – Models and Observation Stars change very little over a human lifespan, so it is impossible to follow a single star from birth to death. We observe stars at various stages of evolution, and can piece together a description of the evolution of stars in general. Computer models provide a “fast-forward” look at the evolution of stars. Stars begin as clouds of gas and dust, which collapse to form a stellar disk. This disk eventually becomes a star. The star eventually runs out of nuclear fuel and dies. The manner of its death depends on its mass.

Evolution of low-mass stars

Evolution of high-mass stars

Tracking changes with the HR Diagram As a star evolves, its temperature and luminosity change. We can follow a star’s evolution on the HR diagram. Lower mass stars move on to the main sequence, stay for a while, and eventually move through giant stages before becoming white dwarfs. Higher mass stars move rapidly off the main sequence and into the giant stages, eventually exploding in a supernova.

Stars begin as a cloud of cold gas and interstellar dust, a molecular cloud. The cloud begins to collapse in on itself. –Collapse is triggered by a variety of phenomena. –Stellar winds, explosions, etc. –Collapse heats the center of the cloud – gravitational energy is being converted to heat. Rotation of the cloud forces it into a disk-shape. After a million years or so, the center of the disk develops a hot, dense core called a protostar. Interstellar Gas Clouds

Protostars Once a dense core forms in the disk, the system has entered the protostar stage. Protostars are difficult to find – they are shrouded by gas and dust. Infrared telescopes can detect them. –Sees through the dust. –Sees the radiation of the “cooler” object. The Eagle Nebula

Bipolar Flows Once the protostar heats to around 1 million K, some nuclear fusion begins. Narrow jets of gas can form, flinging stellar material more than a light-year away. These jets can heat other clouds of gas and dust.

The birth tracks of low- and high-mass stars

High versus Low Mass Low mass stars are stars like our Sun. Low mass stars are stars with mass < 8 times the mass of our Sun. High mass stars are stars with mass > 8 times the mass of our Sun. Most stars are 0.2 to 20 times M Sun (over 30 M Sun very rare) Upper limit 150 M Sun Lower limit 0.08 M Sun Below the lower limit, not enough gravity (mass) to produce the temp and pressure needed to sustain hydrogen fusion M Sun to 0.08 M Sun are brown dwarfs. Jupiter is about 75 times too small to have become a star. (17 times smaller than the smallest brown dwarf.)

Low-mass protostars become stars very slowly. –Weaker gravity causes them to contract slowly, so they heat up slowly. –Weaker gravity requires low-mass stars to compress their cores more to get hot enough for fusion to begin. –Low-mass stars have higher density. High-mass protostars become stars relatively quickly. –They contract quickly due to stronger gravity. –Core becomes hot enough for fusion at a lower density. –High-mass stars are less dense. From Protostar to Star

Flowchart of Stellar Structure

Low-mass stars rely on the proton- proton cycle for their internal energy. Higher mass stars have much higher internal temperatures (20 million K!), so another fusion process dominates. –An interaction involving Carbon, Nitrogen and Oxygen absorbs protons and releases helium nuclei. –Roughly the same energy released per interaction as in the proton-proton cycle. The C-N-O cycle

The length of time a star spends fusing hydrogen into helium is called its main sequence lifetime. –Stars spend most of their lives on the main sequence. –Lifetime depends on the star’s mass and luminosity. More luminous stars burn their energy more rapidly than less luminous stars.. High-mass stars are more luminous than low-mass stars. High mass stars are therefore shorter-lived. Cooler, smaller red stars have been around for a long time Hot, blue stars are relatively young. The Main-Sequence Lifetime of a Star