Stars off the Main Sequence - ScienceEducationatNewPaltz

March 20, 2018 | Author: Anonymous | Category: N/A
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Stars on and off the Main Sequence Just a history of a star’s birth, life and death

Main Sequence Stars  Where are they on the H-R Diagram?  How long is their life compared to other stars?  What is their dying process?  What do they ultimately become?  Why?

A Look at the Non-Main Sequence Stars: Birth and Death of Stars        

Protostars T-Tauri stars Brown Dwarfs Red dwarfs Neutron stars White dwarfs Red Giants Super Giants

Protostars  A protostar is what you have before a star forms  If it has enough mass and begins to fuse, it becomes a T-Tauri star  If it does not have enough mass it becomes a Brown Dwarf (not red)

T-Tauri Stars  Star's formation and evolution right before it becomes a main sequence star  Occurs at the end of the protostar phase  Gravitational pressure holding the star together is the source of all its energy

T-Tauri Stars Continued  Don't have enough pressure and temperature at their cores to generate nuclear fusion  Same temperature as MS stars but brighter because they're a larger

T-Tauri Stars  Have large areas of sunspot coverage  Have intense X-ray flares  Extremely powerful stellar winds  Remain in the T Tauri stage for about 100 million years

Main Sequence Stars, Again  The majority of all stars are main sequence stars  Our nearest neighbors, Proxima Centauri, Sirius and Alpha Centauri are main sequence stars  Stars can vary in size, mass and brightness

Main Sequence Continued  All doing the same thing  Converting hydrogen into helium in their cores  Releasing a tremendous amount of energy

Main Sequence Continued  In a state of hydrostatic equilibrium  Gravity is pulling the star inward  Pressure from all the fusion reactions in the star are pushing outward

Main Sequence Continued  Lower mass limit for a main sequence star is about 0.08 times the mass of the Sun (ex: Red Dwarf)  To more than 100 times the mass of the Sun

Red Giant Star  When an average star like our Sun consumes all hydrogen in its’ core, fusion stops  No longer generates outward pressure to counteract inward pressure  Outer shell of H around core ignites, prolonging life of star

Red Giants Continued  But the shell of ignited H causes it to increase in size dramatically  Can be 100 times larger than it was in its main sequence phase

Red Giants Continued  When hydrogen fuel is used up, further shells of helium and heavier elements can be consumed in fusion reactions  Will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.

White Dwarf Stars  An average star has completely run out of hydrogen fuel in its core  It lacks the mass to force higher elements into fusion reaction  It becomes a white dwarf star

White Dwarf Stars Continued  Outward light pressure from the fusion reaction stops  Star collapses inward under its own gravity  No fusion reactions happening and cools down  Process takes hundreds of billions of years

Red Dwarf Stars  Most common kind of MS stars  Low mass  Much cooler than stars like our Sun  Able to keep the hydrogen fuel mixing into their core for a longer time

Red Dwarf Stars Continued  Can conserve their fuel for much longer than other stars  Some red dwarf stars will burn for up to 10 trillion years  The smallest red dwarfs are 0.075 times the mass of the Sun and largest up to ½ our Sun

Neutron Stars  If a stars has between 1.35 and 2.1 times the mass of the Sun the star dies in a catastrophic supernova explosion  The remaining core becomes a neutron star

Neutron Stars Continued  It is an exotic type of star that is composed entirely of neutrons  How? The intense gravity of the neutron star crushes protons and electrons together to form neutrons

Neutron Stars Continued  More massive stars do not become neutron stars  What do they become?  What do we know about these structures created by the death of super massive stars?

Supergiant Stars  The largest stars in the Universe are supergiant stars  Dozens of times the mass of the Sun  Consuming hydrogen fuel at an enormous rate

Supergiant Stars Continued  Will consume all the fuel in their cores within just a few million years  Live fast and die young  Detonating as supernovae  Disintegrating themselves in the process  Betelgeuse is a prominent example of a red supergiant star. It is located at the shoulder of Orion

Nova and Other Things to Consider  Nova means "new star"  They are actually "newly visible" stars  One model of novae suggests that they occur in binary systems

Nova Continued  One is a white dwarf  The other is on its way to becoming a red giant  The red giant can lose mass which would trigger hydrogen fusion as it falls on the white dwarf

Nova Continued  This would blow the gas off and the process could repeat itself. A notable nova example is Nova Cygni 1975.

Pulsars  Evidence: precisely repeated radio pulses  Attributed to rotating neutron stars which emit lighthouse type sweeping beams as they rotate

Pulsars Continued  Variations in the normal periodic rate are interpreted as energy loss mechanisms or, in one case, taken as evidence of planets around the pulsar

Quasars  These objects were named Quasistellar Radio Sources  Quasars are closely related to the active galaxies  The quasars have very large redshifts

Quasars Continued  Quasars are extremely luminous at all wavelengths and exhibit variability on timescales as little as hours, indicating that their enormous energy output originates in a very compact source

Black Holes  What are they?  How do they form?  What do we know about them?  What don’t we know?  How does time operate at a black hole? How does time operate inside the black hole?

Black Holes Continued  What is singularity?  What is the event horizon?  Do black holes rotate?  What are the only emissions from a black hole?

We will add more to the Hertzsprung-Russell Diagram    

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