Chapter 15: Stellar Evolution I
What happens 10 million years after Tau Tauri phase?
central temp. rises enough to ignite nuclear fusion rxns
When a red giant star begins to burn helium, its diameter
decreases but not to its main sequence size
A red subgiant star is one that
decreasing surface temperature and nearly constant brightness
The core of a red supergiant star stops shrinking because its
electrons touch each other
What is mass a form of?
energy
What is temperature?
energy of motion of individual atoms or molecules
What happens on HR diagram when star runs out of hydrogen at its core?
goes off main sequence into red sub-giant and red giant
What happens when central helium core continues to contract and grow hotter?
goes until reaches ignition temp, then fuses helium to make carbon
A star leaves the horizontal branch in the HR diagram when
it runs out of helium at its core.
A star leaves the main sequence when
it runs out of hydrogen at its core
When a star first moves off the main sequence,
its surface temperature drops and its brightness first stays about the same and then increases.
What happens above 50 solar masses?
light generated by the star exerts an outward force on outer layers of star that is greater than force of star's gravity; star tears itself apart
What happens in electromagnetic force?
like charges repel, opposite charges attract
What range is electromagnetic force?
long range
How much brown dwarfs exist?
lots
Energy that powers a sun comes from
nuclear fusion rxns
What happens when hydrogen fuel begins to run out at the center of a star?
only product of nuclear burning of helium is left there; helium core contracts and heats up; hydrogen burning shell burns hotter and faster and closer to surface of star; star's photosphere expands by factor of hundred, becoming cooler and much larger
What happens in weak force?
particles change identity, generating neutrinos
Nuclear Fusion:
process of building up heavier elements from lighter ones
What is in a hydrogen atom?
proton and electron
What is in a Deuteron?
proton and neutron
Which particles interact in all 3 forces?
protons and neutrons
Nuclear Fusion Ignition Temperature:
protons can get close enough to each other for the strong and weak nuclear forces to act and protons can change into other particles
What happens to protons when temp. is too low?
protons never get close to each other and nothing happens
Which red giant phase is more violent?
red supergiant phase
Our Sun is a type G2V star on the main sequence. As it ages and uses up its fuel we expect it to
remain a G2V star until it leaves the main sequence
Nuclear fusion requires high temperatures because nuclei
repel each other
Plasma:
result of atoms moving so energetically in high temperatures of a star, that their electrons cannot stay attached
When a red giant star begins to burn helium, it
returns part way to the main sequence in an HR diagram
What range is strong force?
short range
What range is weak force?
short range
The size of a typical white dwarf star is comparable to the size of
the Earth
What will happen to our sun 5 billion years from now?
will become red giant
Neutrino mass:
0 (almost)
Positron charge:
+e
Proton charge:
+e
Electron charge:
-e
What happens after helium flash?
-new equilibrium, w/ energy coming from center again, is more like normal star, so red giant contracts and surface temp. Rises -result is horizontal branch star
The mass of a carbon atom is 12.00amu while the mass of a helium-4 atom is 4.003amu. If a gamma ray photon splits a carbon atom into three Helium atoms, how much energy is converted into mass?
.009 amu
The mass of a carbon atom is 12.00amu while the mass of a deuterium atom is 2.014amu. If six deuterium atoms fuse to form a carbon atom, how much mass is converted into energy?
.084 amu
Neutrino charge:
0
Neutron charge:
0
Neutron mass:
1 amu
Proton mass:
1 amu
Electron mass:
1/1836 amu
Positron mass:
1/1836 amu
The diameter of a white dwarf star might be
10,000 miles
How heavy is Eta Carinae, and what is happening to it?
100 solar masses (binary pair), tearing itself apart
What rate do massive O stars evolve?
50 x as fast as our sun
The period of a proto-star's evolution when its size and brightness decrease just before the ignition of nuclear reactions in its core is called
Tau-Tauri Phase
Formula for converting mass to energy:
E = mc^2
Which planet are brown dwarfs similar in size to?
Jupiter
When an interstellar cloud fragment does not have enough mass to form a star powered by nuclear fusion, but gets hot enough to glow for a while, it is called
a brown dwarf
For a star to give rise to a Nova, the star must be
a member of a multiple star system
The Örst red giant stage of a one solar mass starís life usually ends with
an explosion in the helium core
The onset of Helium burning at the core of a star normally begins with
an explosion in the helium core.
Tau Tauri Phase:
as cloud collapses further, its surface area shrinks and it becomes less bright even though it is still getting hotter; is unstable and gives rise to powerful protostellar winds of ejected gas
Because of the electrical repulsion between atomic nuclei, nuclear fusion happens only
at high temps.
Helium Flash:
b/c ignition occurs in highly compressed core, there is this explosion
What happens when stars are more massive than our sun?
become much hotter and brighter and evolve through all their stages much more quickly
When nuclear fusion reactions stop in a star similar to our own Sun, the star
becomes a white dwarf
High protostellar winds of ejected gas occur
between the initial contraction and the ignition of nuclear burning.
how long do stars like our sun burn for?
billions of years
What happens after 100,000 years of collapse of a star?
growing heat and pressure at center of cloud slow collapse and surface becomes hotter and brighter
Red Supergiant:
helium-burning fire now burns outward and star swells up one more time to become this
What happens w/ hydrogen fusion in our sun?
hydrogen undergoes sequence of rxns, which results in forming a helim atom from four hydrogen atoms
Horizontal branch star:
intermediate between red giant and main sequence star
What happens in strong force?
neucleons (protons or neutrons) stick together
When a white dwarf star collects matter from a neighboring star, fusion reactions on the surface of the white dwarf cause
nova
What happens in 30 million years after Tau Tauri Phase?
star expands, then shrinks again and settles down for several billion years as main sequence star
What happens above 20 solar masses?
stars become extremely unstable
What happens about every 20 years w/ a white dwarf?
succeeds in getting enough hydrogen from its overblown companion to set off a thermonuclear reaction; creates nova
When the hydrogen fuel runs out at the center of a main sequence star, the star
swells up and becomes a red giant
The average energy of motion of an atom or molecule in a gas is called its
temperature
During the Tau Tauri phase of a protostar's evolution,
the brightness decreases
Electron degenerate matter is matter in which
the electrons are touching each other so that they cannot be compressed
The helium flash refers to
the explosive ignition of a star's helium core
Brown Dwarf:
the lowest mass that can support nuclear burning is about .08 solar masses, and below that mass, protostars simply cool and contract to become this
Core:
the region within star which the temperature is above ignition temperature for fusion
What does the size of original collapsing cloud fragment determine?
the track of the protostar and where on the main sequence it stops
What is plasma in a star's core a mixture of?
unattached protons and unattached electrons moving at high speeds
What are brown dwarfs like?
very dense, shine in infrared w/ energy from hot interiors
Which force do neutrinos interact in?
weak
What forces do electrons and positrons interact in?
weak and electromagnetic
What happens to form white dwarf?
when carbon core shrinks to point where electrons touch each other, forming electron degenerate matter; core cannot shrink anymore, and temp. never reaches ignition point for burning carbon to heavier elements; outer layers of star become unstable and blow away leaving degenerate core; hot core becomes white dwarf, and outer layers are planetary nebula
What happens to form a red supergiant star?
when helium begins to run out near center of star and carbon core begins to build
What does the planetary nebula shape depend on?
which way a star ejects its outer envelope
What happens after red supergiant phase?
white dwarf and planetary nebula
What is another name for white dwarf?
zombie star