astronomy chapter 13

brown dwarf

"failed star" radiate primarily in the infrared and look deep red or magenta -they do not sustain steady fusion in their cores so they cool with time and radiate away their internal thermal energy

close binary system

*angular momentum is part of the reason so many stars belong to binary systems. -as a molecular cloud contracts, breaks into fragments and forms protostars, some of the protostars end up close to one another which is what we call close binary systems.

how do stars form?

*stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction -stars are born when gravity causes a cloud of interstellar gas to contract to the point at which the central object becomes hot enough to sustain nuclear fusion in its core

fusion of heavier nuclei

-helium capture reactions-reactions in which a helium nucleus fuses into some other nucleus.

how massive are newborn stars?

-masses depend on processes that govern the clumping and fragmentation -stars with low masses greatly outnumber stars with high masses MAX MASS= 150Msun MIN MASS= 0.08 Msun **stars more massive than 100Msun blow off their outer layer, while protostars smaller than 0.08 Msun become brown dwarfs that never get hot enough for efficient hydrogen fusion.

protostar to main sequence

-protostar becomes a true star when its core temp reaches 10 million K; hot enough for hydrogen fusion -the protostar then becomes a main sequence star when it achieves energy balance between hydrogen fusion in its core and radiation from the surface. (along with gravitational equilibrium)

pressure in brown dwarfs

-the source of pressure that stops gravity from squeezing a brown dwarfs core is degeneracy pressure

planetary nebula

-when the star dies, it will eject its outer layers into space as a planetary nebula, leaving its exposed core behind as a white dwarf

fate of earth

-when the sun begins to get more luminous it will cause a runaway greenhouse gas effect on earth somewhere between 1 to 4 billion years from now, earths ocean will then boil away.

2 things that help gravity win out over pressure and star the collapse of a cloud of gas:

1) higher density; because packing the gas particles closer together makes the gravitational forces between them stronger 2) lower temperature; because lowering a clouds temp reduces gas pressure

life stages of low mass stars

1. main sequence stage: spend 90% of their lives shining steadily as a main sequence star because it is able to fuse hydrogen into helium by nuclear fusion and the self regulating processes including gravitational equilibrium and energy balance work together to keep the fusion rate steady 2. red giant stage: enters this stage when the core hydrogen becomes depleted and nuclear fusion ceases. It becomes a red giant. Will become larger in radius and more luminous. *it will be powered by hydrogen fusion in a shell surround the core. *the stars core will continue to shrink and hydrogen shell fusion will continue to intensify as the sun grows into a red giant. 3. helium white dwarf: when the temp in the helium core reaches 100 million K the nuclei will fuse together and it will enter this stage. The core collapse will be halted by degeneracy pressure and the stars corpse will become a white dwarf.

life of high mass stars

1. protostar: a star system forms when cloud of interstellar gas collapses under gravity 2. blue main sequence: in the core a high mass star; four hydrogen nuclei fuse into a single helium nucleus by the series of reactions known as CNO cycle. 3. red supergiant: after core hydrogen is exhausted, the core shrinks and heats. Hydrogen fusion begins around the exert helium core, causing the star to expand into a red supergiant 4. helium core fusion: helium fusion begins when the core temp becomes hot enough to use helium into carbon. The core then expands, slowing the rate of hydrogen fusion and allowing the stars outer layers to shrink. 5. multiple shell fusion supergiants: after the core runs out of helium, it shrinks and heats until fusion of heavier elements begins. Late in life, the star fuses many different elements in a series of shells while iron collects in the core 6. supernova: iron cannot provide fusion energy, so it accumulates in the core until degeneracy pressure can no longer support it. Then the core collapses, leading to the catastrophic explosion of the star. 7. neutron star or black hole: the core collapse forms a ball of neutrons, which may remain as a neutron star or collapse further to make a black hole.

life of low mass star

1. protostar: a start system forms when a cloud of interstellar gas collapses under gravity. 2. yellow main sequence stars: in the core of a low mass star, four hydrogen nuclei fuse into a single helium nucleus by the series of reactions know as the proton-proton chain 3. red giant star: after core hydrogen is exhausted, the core shrinks and heats. hydrogen fusion begins around the inert helium core, causing the star to expand into a red giant. 4. helium core fusion star: helium fusion begins when the core becomes hot enough to fuse helium into carbon. The core then expands, showing the rate of hydrogen fusion and allowing the stars outer layers to shrink. 5. double shell fusion red giant: helium fusion begins around the inert carbon core after the core helium is exhausted. The star then enters its second red giant phase, with fusion in both a hydrogen shell and a helium shell 6. planetary nebula: the dying star expels its outer layers in a planetary nebula, leaving behind the exposed inert core. 7. white dwarf: the remaining white dwarf is made primarily of carbon and oxygen because the core of the low mass star never grows hot enough to produce heavier elements.

life stages as high mass star

1.hydrogen fusion in high mass stars occurs through a chain of reactions called the CNO cycle (instead of proton-proton chains found in low mass stars): which is when protons slam into carbon, nitrogen or oxygen -reaction is the same as the proton-proton chain but it rapidly fuses its core hydrogen into helium much faster so the high mass stars live a shorter life. 2.becoming a supergiant: neear end of life it becomes a supergiant as fusion proceeds furiously in its core and surround shell.

molecular clouds

The coldest and densest clouds in which stars are born -temp ranges from 10-30 K -tend to be quite large -low enough densities, but there are other regions of interstellar space that are a lot less dense.

iron: bad news for the stellar core

a high mass stars death is imminent when iron piles up in its core because fusion of iron releases no energy

protostar

a molecular cloud fragment heats up as gravity makes it contract, producing a protostar at its center. -it is the clump of gas that will become a new star *conservation of angular momentum ensures that protostars rotate rapidly and are surrounded by spinning disks of gas. -many young protostars fire high speed streams of gas called jets into interstellar space.

degeneracy pressure

a type of pressure that does not depend on temp at all. It depends on the laws of quantum mechanics that gives rise to distinct energy levels in atoms -does not weaken with decreasing pressure -this means the brown dwarfs pressure remains stable while it cools

What would you be most likely to find if you returned to the solar system in 10 billion years?

a white dwarf

neutron star

after the supernova occurs, the ball of neutrons left behind is called the neutron star

key point

all low and intermediate mass stars follow life stages similar to those of our sun, while high mass stars lie short but brilliant lives and die in supernova explosions

a brown dwarf is

an object not quite massive enough to be a star

high mass stars

are those stars born with masses greater than about 8 solar masses

stars can form most easily in clouds that are....

cold and dense

intermediate mass stars

have birth masses between about 2 and 8 solar masses

black hole

if the remaining mass is so large that gravity overcomes neutron degeneracy pressure, the core will continue to collapse until it becomes a black hole.

CNO cycle

is just another way to fuse H to Helium using carbon, nitrogen and oxygen as catalysts -used by high mass stars

what happens to the core of a high mass star after it runs out of hydrogen?

it shrinks and heats up

what happens to a low mass star after helium flash?

its luminosity goes down

how do high mass stars make elements necessary for life?

low mass stars can't make elements heavier than carbo because of degeneracy pressure but the high mass stars high temperature allows it to produce elements od which we and earth are made

evidence for origin of elements

measurement of element abundances in the cosmos confirm our models of how high mass stars produce heavy elements.

what remnant does a supernova leave?

neutron star or black hole

what would stars be like if hydrogen had the smallest mass per nuclear particle?

nuclear fusion would not occur in stars of any mass

helium fusion

occurs only when nuclei slam into one another at much high speeds than those needed for hydrogen fusion, which means that helium fusion requires much higher temperatures. *helium fusion process converts 3 helium nuclei into one carbon nucleus. *energy is then release because the carbon nucleus has a lower mass than the 3 helium nuclei and the lost mass becomes energy using keplers third law. (E=mc^2)

life track

plots on the HR diagram that shows the suns luminosity and surface temp at each point in its life.

low mass stars

stars born with less than about 2 solar masses of material

subgiants

stars that have just begun their expansion into red giants as their cores have shut down and hydrogen shell fusion has begun.

crab nebula

supernova remnant. -gases of this supernova are still extended into interstellar space

supernova

the gravitational collapse of the core releases an enormous amount of energy-more than 100 times what the sun will radiate over its entire lifetime. it drives the outer layers off into space in a titanic explosion called the supernova -supernova scatters the elements produced by the star into space and leaves behind either a neutron star or black hole

helium flash

the rising temperature causes the helium fusion rate to spike drastically -releases an enormous amount of energy into the core -the sudden onset of helium fusion in the suns core will stop core shrinkage and the sun will become smaller and less luminous than it was as a great giant.

how does a high mass star die?

when gravity overcomes degeneracy pressure in the iron core, the core collapses into a ball of neutrons and the star explodes in a supernova

How are lives of stars with close companions different?

when one star in a close binary system begins to swell in size at the ends of its main sequence stage, it can begin to transfer mass to its companion. This mass exchange can then change the remaining life histories of both stars.

how does a low mass star die?

when the helium in the core runs out and all thats left is carbon. -core shrinkage will resume after helium core fusion ends, while both helium fusing and hydrogen fusing shells make the sun bigger and more luminous than ever.