Chapter 11 The Lives of Stars

What do you think?

• Where do stars come from?

• Do stars with greater or lesser mass last longer?

Let’s consider the star-forming regions around Orion

• Interstellar medium– H2 (mostly), CO,

H2O, NH3, H2CO

– Most is concentrated in giant molecular clouds

Stars form out of enormous volumes of dust and gas

Supernova explosions in cold,

dark nebulae trigger the birth of stars.

Stars form in large groups called “open clusters” or “galactic clusters”

When a protostar ceases to accumulate mass, it, becomes a pre-main-sequence star.

It’s life path is forever determined by its initial mass

H II regions harbor young star clusters

An OB association is where O and B class stars are producing ionizing radiation which makes an HII nebula glow.

Star formation and glowing HII

regions in the Great Orion

Nebula

Plotting all the stars from a cluster on an H-R diagram reveals its age

Plotting all the stars from a cluster on an H-R diagram reveals its age

Stars spend most of their life cycle on the main sequence

• Main sequence stars are in hydrostatic equilibrium– outward thermal pressure is exactly balanced by the

inward force of gravity

• Main sequence stars are those stars fusing hydrogen into helium in their cores

• Zero-age main sequence (ZAMS) is the location where a pre-main-sequence star fusing hydrogen in its core first becomes a stable object

The more massive a star, the faster it goes through its main

sequence phase

When core hydrogen fusion ceases, a main-sequence star becomes a giant

• When hydrogen in the core is no longer fusing into helium, the star can no longer support its weight

• The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion.

• This extra internal heat causes the outer layers to expand into a giant star.

Helium fusion begins at the center of a giant

• While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million kelvins) for helium to begin to fuse into carbon and oxygen– core helium fusion– 3 He C + energy and C + He O + energy – occurs rapidly - called the Helium Flash

Some Laws of Physics are important here

• Pauli exclusion principal– two identical particles cannot exist in the same

place at the same time– this effect in stars is called electron degeneracy

pressure and is not dependent on temperature– the star is supported by the fact that the

electrons cannot get any closer together

As stars evolve, they move on the

H-R diagram - their exact track depends on their

initial mass

Globular clusters are bound groups of hundreds of thousands of old stars at the edge of the galaxy

A composite HR Diagram showing

various star clusters

Variable Stars

• Change brightness because their diameter is fluctuating – (big/bright to small/dim and back again)

• RR Lyrae variables (periods less than 24 hours)

• Cepheid variables (periods between 1 & 100 days)

• Mira variables (periods greater than 100 days)

Cepheids enable astronomers to estimate vast distances

• This period-luminosity relationship is important because if an astronomer can find a Cepheid and measure its period, she can determine its luminosity and absolute magnitude.

• Comparing the absolute and apparent magnitudes allows for the distance to be calculated.

What did you think?

• Where do stars come from?Stars form from gas and dust inside giant molecular

clouds

• Do stars with greater or lesser mass last longer?Lower-mass stars last longer because the lower

gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars.

Self-Check1: Describe the physical properties and visual appearances of objects associated with pre-main-

sequence stellar evolution.

2: Identify the defining characteristic of main-sequence stars and compare the relative lifetimes on the main sequence for stars of different mass.

3: List the names of nuclear fusion reactions and indicate the classes of stars in which each reaction is thought to be active.

4: Identify the physical property normally thought to control the life cycles of stars and planets.

5: Explain how observations of open and globular star clusters contribute to the testing and extension of current theoretical models for stellar evolution.

6: Identify the stages of stellar evolution in which mass loss is significant.

8: Compare and contrast RR Lyrae and Cepheid variable stars in terms of period, population membership, luminosity, and evolutionary status.

9: Describe how the identification of Cepheid variables can be used to determine the distance to a star cluster.