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Unformatted text preview: Protostar growth
Since gas clouds are usually
slowly rotating, a protostar forms
from a rotating disk.
The protostar emerges from the
center of the disk.
Planets may well form from the
Figure Protostar Development by Mass
The initial mass of a
where on the main
sequence a star will lie.
There is a relation
between initial mass and
A one solar mass star
requires 30 million years
to reach the main
sequence for the
protostar stage. Late Stages for Protostars
It is difficult to
observe a protostar
We observe other
objects such as
HH30 is a newly
formed star at the
center of a dense
disk of dusty gas. Associations of Young Stars There are many observations such as this one showing early stage
stars which form in groups called associations .
For example T Tauri stars are in a T association.
These stars are very young.
These Star Clusters
The compression of the
initial gas cloud is probably
due to a supernova explosion.
Here we see two star
The yellow one is made up
of 50 million year old stars.
The white one is made up
of 4 million year old stars.
The cluster began forming
about 50 million years ago
due to a supernova. Structure and Evolution of Stars
• What are the basic facts we observe about
the Sun, and what does this tell us?
• Throughout our memory, our parents and
grandparents memories and all of recorded
history, the sun has remained the same.
– Radius remains the same!
– Temperature remains the same!
– Luminosity remains the same! Hydrostatic Equilibrium
•Radius constant=> A dynamically
stable star is in hydrostatic equilibrium. •The gravity of the sun is balanced by
outward gas pressure. •Each “layer” of star supports its on
weight plus those of the layer above so
pressure increases inward. •Punishment is swift when balance is Figure 11.13
Figure disturbed: it takes only one hour for
the sun adjust in pressure to restore
equilibrium. This is the timescale for
flares or prominences. Mechanical Properties of Stars
• Gravity- holds the star together from
mutual attraction of all particles
• Gas Pressure – keeps the star from
collapsing in on itself
- Boyle’s Law: P proportional to density
Blowing up a balloon
- Charles’ Law: P proportional to T
heat can, close lid => implosion
- Compressing gas causes it heat up
tube of bicycle pump warms as you pump
CO2 cartridge, spray can cool as they discharge gas Kinetic Theory of Gases
(James Clerk Maxwell)
• Kinetic theory of gases: “tiny elastic
particles (atoms) in constant motion”
• With this idea, we can understand
– Random thermal motions:
– Temperature is measure of kinetic energy
– Charles’ Law
– Boyle’s Law
– Hydrostatic Equilibrium! Thermal Equilibrium
• Heat in = heat out
– Energy flows from Hot => cold
• Temperature profile in a star can be
determined from the condition that the
temperature at the surface is constant Energy Transport Energy can be transported from the star's core in several ways.
Conduction, radiation or convection.
These three modes are the main energy transport system within a
star. Energy transport in our star … ..\animations\TheSun.swf Opacity
• Absorption of radiation by matter is called
• In interior of Sun it is electron scattering that is
important opacity: it takes radiation about 1
million years to reach the surface
• Opacity in a region of a star determines the rate
of energy flow through it, if it becomes too large
(partial ionization zone) we get convection Convection in a Star
The core radiates
its energy outward.
The upper regions
of the star form a
transported to the
Granules are the
visible evidence of
convection. Mass Luminosity Relation
(Sir Arthur Eddington)
• Now we can explain it: As Mass increases,
gravity increases, so Pressure (P) must increase
to maintain HSE; Charles’ Law then says if P
increases, Temperature (T) increases; S-B says if
T increases, Luminosity increases and we are
• Eddington worked out the details and showed
quantitatively that Luminosity should be
proportional to Mass (to the 3.5 power), in
agreement with our observational result! Why is a Star Hot?
• Independent of the nature of the energy
source, stars radiate because they are hot
• We must ask, Why are stars hot?
– Because they have an energy source… • What is the source of energy? ...
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This note was uploaded on 11/27/2011 for the course AST 301 taught by Professor Harvey during the Spring '07 term at University of Texas at Austin.
- Spring '07