Flashcards in D.2 Stellar Characteristics And Stellar Evolution Deck (26):
What is Wien's law?
The wavelength at which the emitted energy of a black body is a maximum is related to its surface temperature.
What is a cepheid star/variable?
A star whose brightness varies with a well defined period. This period is related to the stars luminosity and so can be used to estimate the distance to the star.
What is the hertzsprung russell diagram?
A plot of the luminosity of stars versus their surface temperature.
What is the main sequence star?
A star undergoing fusion of hydrogen into helium. Such stars occupy a diagonal strip on a HR diagram from top left to bottom right.
What is a red giant?
A large, relatively cool, reddish star which occupies the upper right region of the HR diagram.
What is a red supergiant?
A very large, relatively cool, reddish star which occupies the extreme upper right region of the HR diagram.
What is a white dwarf?
A very small, hot, dense star occupying the lower left region of the HR diagram this is the end stage in the evolution of low mass stars.
What is a neutron star?
A very hot, very dense star which is the end stage in the evolution of massive stars.
What is the chandrasekhar limit?
The largest mass a white dwarf star can have it is about 1.4 solar masses.
What is the oppenheimer volkoff limit?
The largest mass a neutron star can have it is about 3 solar masses.
When stars radiate as black bodies how does their intensity vary with wavelength?
Shows a typical black body curve the peak wavelength is related to surface temperature according to Wien's law.
What happens when light has been passed trough a gas?
Dark lines appear on a background of the colours of the rainbow. The dark lines correspond to photon wavelengths that have been absorbed by the atoms of the gas. A similar thing happens when the light that is produced within a star moves outwards through the outer layer of the star. Specific elements absorb specific wavelengths and so the dark lines give us information about what elements exist in the star.
What dark lines often dominate in the star spectra?
Hydrogen and helium lines - although many other elements are present as well.
How does the hertz sprung Russell diagram plot stars?
According to their luminosity on the vertical axis and surface temperature on the horizontal axis. Temperature increases as e move to the left. The dashed lines represent stars of equal radius, so Ro is the radius of the sun.
What are the three main groups of stars on the diagram?
Main sequence stars, the broad curve running from upper left to lower right, white dwarfs at lower left and red giants and red supergiants at the upper right.
What is a cepheid star/variable?
Stars whose brightness varies periodically with time because the stars expand and contract.
What is the period of variation of a Cepheid's brightness related to?
Its average luminosity, the longer the period the higher the luminosity. So by observing a Cepheid over time we can measure its period and thus deduce its average luminosity, this makes
What makes Cepheid's standard candles?
By observing a Cepheid over time we can measure its period and thus deduce its average luminosity, this makes Cepheids standard candles since they are stars of known luminosity. By measuring its average brightness we can deduce the distance to it, distances up to 40Mpc can be measured.
How do stars stay on the main sequence for most of their life?
They fuse hydrogen into helium.
What happens when the star has used up about 12% of its hydrogen?
The star developed instabilities and will start to expand, as it expands the surface temperature of the star decrease but the increase in surface area more than compensates and so its luminosity and actually increases. The star will become a red giant if its mass is less than about 8 solar masses or a red supergiant if its mass is greater than about 8 solar masses.
what happens to the reactions in the higher mass stars?
In higher mass stars nuclear reactions continue in the core but now involve heavier and heavier elements. The more massive the star the heavier the elements produced in the nuclear reactions. The process stops at the production of iron. No heavier elements can be produced by nuclear fusion as this would require that additional energy be supplied from outside.
What happens after the star finished producing iron?
The star undergoes a catastrophic explosion prodding s plantar nebula if the mass of the star is low and a supernova if it is high.
What happens to the core of the star once most of its mass has been ejected into space via the explosions if it less than 1.4Msun?
It will become a stable white dwarf star. It does not collapse further because of electron pressure. The limiting mass of 1.4Msun is known as the Chandrasekhar limit. Even a very massive star can end up as a white dwarf if the mass of its core is less than the Chandrasekhar limit.
What happens to the core of the star once most of its mass has been ejected into space via the explosions if it is between 1.4Msun and 3Msun?
It will become a neutron star. It does not collapse further because of neutron pressure. The limiting mass of 3Msun is known as the Oppenhemier Volkoff limit.
What happens if the mass of the core is larger than the Oppenheimer Volkoff limit?
No known mechanism exists to stop its further collapse and it becomes a black hole a star from which nothing can escape.