5 stars Flashcards
nebulae
gigantic clouds of dust and gas (mainly hydrogen)
often many hundreds of times larger than our solar system
referred to as stellar nurseries, as they are the birthplace of all stars
star birth
nebulae are formed over millions of years as the tiny gravitational attraction between particles of dust and gas pulls the particle towards each other, eventually forming the vast clouds
as the dust and gas get closer tog this gravitational collapse accelerates
due to tiny variations in the nebula, denser regions begin to form. these regions pull in more dust and gas, gaining mass and getting denser, and also getting hotter as gravitational energy is eventually transferred to thermal energy
in one part of the cloud a protostar forms- this is not yet a star but a very hot, very dense sphere of dust and gas
for a protostar to become a star, nuclear fusion needs to start in its core. many protostars never reach this stage
fusion reactions produce energy in the form of KE
extremely high pressures and temps inside the core are needed to overcome the electrostatic repulsion between hydrogen nuclei in order to fuse them tog to form helium nuclei
in some cases, as more and more mass is added to the protostar, it grows so large and the core becomes so hot that the KE of the hydrogen nuclei is large enough to overcome the electrostatic repulsion
hydrogen nuclei are forced tog to make helium nuclei as nuclear fusion begins
a star is born
star life
once a star is formed, it remains in a stable equilibrium with almost a constant size
gravitational forces act to compress the star, but the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards
the force from this radiation and gas pressure balances the force from the gravitational attraction and maintains equilibrium
stars in this stable phase of their lives are described as being on their main sequence
cores of larger supergiant stars are much hotter than those of small stars, releasing more power and converting the available hydrogen into helium in a much shorter time
really massive stars are only stable for a few million years whereas smaller stars like the sun see stable for tens of billions of years
planets
an object in orbit around a star with three important characteristics:
- it has a mass large enough for its own gravity to give it a round shape
- it has no fusion reactions (unlike a star)
- it has cleared its orbit of most other objects (asteroids etc)
dwarf planets
have not cleared their orbit of other objects
asteroids
objects too small and uneven to be planets
planetary satellites
a body in orbit around a planet
this includes moons and man made satellites
comets
small irregular bodies made of ice, dust and small pieces of rock
all comets orbit the sun, many in highly eccentric elliptical orbits
as they approach the sun, some comets develop spectacular tails
galaxies
a collective of stars and interstellar dust and gas
on average a galaxy will contain 100 billion stars, a significant proportion of which have their own solar systems
red giants
stars between 0.5Mo and 10Mo will evolve into red giants
solar mass Mo is the mass of the sun, 1.99 x 1030kg
at the start of the red giant phase, the reduction in energy released by fusion in the core means that the gravitational force is now greater than the reduced force from radiation and gas pressure
the core of the star therefore begins to collapse. as the core shrinks, the pressure increases enough to start fusion in a shell around the core
red giants have inert cores. fusion no longer takes place, since very little hydrogen remains and the temp is not high enough for the helium nuclei to overcome the electrostatic repulsion between them
however, fusion of hydrogen into helium continues in the shell around the core. this causes the periphery of the star to expand as layers slowly move away from the core
as these layers expand, they cool, giving the star its characteristic red colour
white dwarfs
eventually most of the layers of the red giant around the core drift off into space as a planetary nebula, leaving behind the hot core as a white dwarf
the WD is very dense, often with a mass aroung that of our sun, but with the volume of the earth
no fusion reactions take place inside a white dwarf
it emits energy only because it leaks photons created in its earlier evolution. the surface temp of a WD can be as much as 30,000 K
electron degeneracy pressure
according to the Pauli exclusion principle, two electrons cannot exist in the same energy state
when the core of a star begins to collapse under the force of gravity, the electrons are squeezed together, and this creates a pressure that prevents the core from further gravitational collapse
this pressure created by the elecs is called the electron degeneracy pressure
but there is a limit. the EDP is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44Mo - chandrasekhar limit
chandrasekhar limit
the max mass of a stable white dwarf star
1.44Mo
stars with low mass
since the core of stars with low masses are cooler than that of more massive stars. they remain on their main sequence for much longer.
however, eventually, the begin to move off the main sequence into the the next phase of their lives
stars with large masses
stars with a mass greater than 10Mo live very different lives
since their mass is much greater, their cores are much hotter
they consume the hydrogen in their core in much less time
when the hydrogen in their core runs low, the core begins to collapse under gravitational forces
however, as the cores of these more massive stars are much hotter, the helium nuclei formed from he fusion of hydrogen nuclei are moving fast enough to overcome electrostatic repulsion, so fusion of helium nuclei into heavier atoms occur
red supergiants
these changes in the core cause the star to expand, forming a red supergiant
inside, the temp and pressures are high enough to fuse even massive nuclei together forming a series of shells inside the star
this process continues until the star develops an iron core
iron nuclei cannot fuse, because such reactions cannot produce any energy
this makes the star very unstable and leads to the death of the star in a catastrophic implosion of layers that bounce off the solid core, leading to a shockwave that ejects all the core material into space
this explosion is called a supernova
supernova
for more massive stars, at a critical point the nuclear fusion taking place in the core suddenly becomes unable to withstand the crushing gravitational forces
the star collapse in on itself, leading to a supernova
afterwards, the remnant core is compressed into either a neutron star or a black hole
supernovae are rare. they create all the heavy elements
everything above iron in the periodic table was created by a supernova
hertzsprung-russel diagram
a graph of stars in our galaxy showing the relationship between their luminosity on the y axis and their average surface temp on the x axis
the temp axis has temp increasing from right to left
when stars are plotted on the HR diagram a pattern appears
the hottest, most luminous stars are in the top left, with the coolest, least luminous stars in the bottom right. most stars on their main sequence form part of a curved line between these two points.
very hot, dim stars like white dwarfs appear along a different line in the bottom left.
red supergiants are very luminous because of their size but have a relatively low surface temp. found on a line across the top.
smaller red giants are found in a line splitting from the main sequence (mid right)
luminosity of a star
is the total radiant power output of the star
the luminosity of the star is related to its brightness- in general the greater the L the brighter the star
energy levels
when elecs are bound to their atoms in a gas they can only exist in one of a discrete set of energies
- an elec cannot have a quantity of energy between two levels
- the energy levels are negative because external energy is required to remove an elec from the atom. the neg values also indicate that the elecs are trapped within the atom or bound to the positive nuclei
- an elec with zero energy is free from the atom
- the energy level with the most negative value is known as the ground level or the ground state
energy levels of electrons in isolated gas atoms
when an elec moves from a lower to a higher energy level within an atom in a gas, the atom is said to be excited
raising an elec into higher energy levels requires external energy
each energy level has a specific negative value. an elec in the -3.0eV energy level requires at least 3.0eV to escape from the atom
when an elec moves from a higher energy level to a lower one, it loses energy (de-excitation)
energy is conserved so as the elec makes a transition between levels , a photon is emitted from the atom
in order for an elec to make a transition from -3.0eV to -6.8eV it must lose 3.8eV. it emits this in the form of a photon with a specific energy of 3.8eV. hf=3.8eV
three kinds of spectra
emission line spectra
continuous spectra
absorption line spectra
