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 MSun (over 30 MSun very rare)

• Upper limit 150 MSun

• Lower limit 0.08 MSun

• Below the lower limit, not enough gravity (mass) to produce the temp and pressure needed to sustain hydrogen fusion.

• 0.016 MSun to 0.08 MSun 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