Download Foundation 1 - Discovering Astronomy

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 starforming
regions
around
Orion
Stars form out of enormous
volumes of dust and gas
• Interstellar medium
– H2 (mostly), CO,
H2O, NH3, H2CO
– Most is concentrated
in giant molecular
clouds
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 premain-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-Check
1: 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.