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AST1002-CH. 12
Evolution of Low-Mass Stars
49
Astronomy
Undergraduate 3
11/14/2012

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Term
Low-Mass Star
Definition
A star with a main sequence mass of less then about 8 solar masses
Term
Mass-Luminosity relationship
Definition
An empirical relationship between the luminosity (L) and the Mass (M) of main-sequence stars expressed as a power law
Term
How quickly a star runs out of fuel depends on it's __ and __
Definition
mass and luminosity
Term
Evolutionary track
Definition
The path that a star follows across the H-R diagram as it evolves through its lifetime
Term
Electron-degenerate
Definition
Describing the state of material compressed to the point at which electron density reaches the limit imposed by the rules of quantum mechanics
Term
Hydrogen-Shell burning
Definition
The fusion of hydrogen in a shell surrounding a stellar core that may be either degenerate or fusing more massive elements
Term
Red Giant
Definition
A low-mass star that has evolved beyond the main sequence and is now fusing hydrogen in a shell surrounding a degenerate helium core
Term
Red Giant branch
Definition
A region on the H-R diagram defined by low-mass stars evolving from the main sequence toward the horizontal branch
Term
Triple-alpha process
Definition
The nuclear fusion reaction that combines 3 helium nuclei (alpha particles) together into a single nucleus of carbon.
Term
An evolving low-mass star moves __ and to the __ on the H-R diagram
Definition
up and to the right
Term
Helium burns via the ___
Definition
triple-alpha process
Term
Helium flash
Definition
The runaway explosive burning of helium in the degenerate helium core of a red giant star
Term
A ___ is a thermonuclear runaway--an explosion within the star
Definition
helium flash
Term
Horizontal branch
Definition
A region on the H-R diagram defined by stars burning helium to carbon in a stable core
Term
Horizontal branch stars burn __ in the core and __ in the shell
Definition
helium; hydrogen
Term
asymptotic giant branch (AGB)
Definition
The path on the H-R diagram that goes from the horizontal branch toward higher luminosities and lower temperatures, asymptotically approaching and then rising above the red giant branch
Term
The star forms a degenerate __ core as it leaves the horizontal branch
Definition
carbon
Term
Stellar mass loss
Definition
The loss of mass from the outermost parts of a star's atmosphere during the course of its evolution
Term
Planetary Nebula
Definition
The expanding shell of material ejected by a dying asymptotic giant branch star. A planetary nebula glows from fluorescence caused by intense ultraviolet light coming from the hot, stellar remnant at its center
Term
White dwarf
Definition
The stellar remnant left at the end of the evolution of a low-mass star. A typical white drawf has a mass of 0.6 solar mass and a size about equal to that of Earth; it is made of nonburning, electron-degenerate carbon
Term
Star clusters
Definition
A group of stars that all formed at the same time and in the same general location
Term
zero-age main sequence
Definition
The strip on the H-R diagram plotting where stars of all masses in a cluster begin their lives
Term
Main-sequence turnoff
Definition
The location on the H-R diagram of a single aged stellar population( such as a star cluster) where stars have just evolved off the main sequence. The position of the main sequence turnoff is determined by the age of the stellar population
Term
Roche Lobes
Definition
The hourglass or the figure eight-shaped volume of space surrounding two stars, which constrains material that is gravitationally bound by one or the other
Term
Mass transfer
Definition
the transfer of mass from one member of a binary star system to its companion. Mass transfers occur when one of the stars evolves to the point that it overfills its roche lobe, so that its outer layers are pulled toward its binary companion
Term
Nova
Definition
A stellar explosion that results from runaway nuclear fusion in a layer of material on the surface of a white dwarf in a binary system
Term
Runaway burning of ___ on a white dwarf causes a nove
Definition
hydrogen
Term
The mass of a white dwarf cannot exceed ___
Definition
1.4
Term
Chandrasekhar limit
Definition
The upper limit on the mass of an object supported by electron degeneracy pressure; approx 1.4 solar mass
Term
Type Ia supernova
Definition
A supernova explosion in which no trace of hydrogen is seen in the ejected material. Most type Ia supernovae are thought to be the result of runaway carbon burning in a white dwarf star onto which material is being deposited by a binary companion
Term
Study evolution of stars using computer models similar to:
Definition
solar model
Term
A stars ____ determines what life path (evolutionary track) it will take
Definition
initial mass
Term
stars are divided into 2 groups:
1.) low mass ____
2.) high mass____
Definition
1.) .08 solar mass < m < 8 solar mass
2.) M > 8 solar masses
Term
___ of different masses follow different paths on their way to the main sequence
Definition
prostars
Term
Isochrone
Definition
Curves of constant age for range of initial masses (not same as evolutionary tracks)
Term
___ stars convert hydrogen to helium in their cores (=hydrogen core burning)
Definition
Main sequence
Term
Nuclear reactions depend on a star's central temperature, which depends on its mass:
-less massive stars:
-more massive stars:
-in between:
Definition
proton-proton chain
CNO cycle/ Carbon cycle(later)
mix of P-P and CNO
Term
Internal structure is different for main sequence stars of different masses, partly because of ___ differences
Definition
opacity
Term
How long do these stars stay on the main sequence?
Definition
Until they burn up their core fuel (hydrogen 10%)
Term
Massive stars have more fuel, but they are also much ___, so they use it up faster
Definition
brighter
Term
Stars with higher ___ burn their fuel more quickly
Definition
masses(mass-luminosity relation)
Term
___ stars fuse hydrogen (H) to helium (He) in their cores, eventually much of the core H is converted to He.
Definition
Main sequence
Term
An expanding ___ has a contracting inert helium core surrounded by a hydrogen burning shell.
Definition
subgiant
Term
The more massive low-mass stars, including the sun, continue to expand and become more luminous as their cores keep contracting, and heating up, becoming ____
Definition
red giants
Term
EVOLUTION OF LOW MASS STARS
stages in order:
Definition
Main sequence star, Red giant branch star, Helium flash, Horizontal branch star, Asymptotic giant branch star, Planetary nebula ejection, white dwarf
Term
In the part-degenerate core of Red giants, the core temp. climbs but the core does not ___.
Energy generation is very ___-sensitive, runs away as (above black) increases
Definition
expand; temperature
Term
The prostar's energy source is ___
Definition
gravitational potential energy
Term
ELECTRON DEGENERACY PRESSURE:
packing more electrons into a given volume means putting them in higher and higher momentum levels.
electrons resist being put in higher levels-->pressure.
Pressure depends only on ___, not on temperature
Definition
density
Term
Core of red giant will stabilize, and as it does the outer layer shrinks, and the star relocates itself on the H-R diagram as a ___ star
Definition
horizontal branch star
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