Lecture 8- Life Cycle of Stars PDF

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Lecture 8- Life cycle of stars
Topics covered: star birth, stages of stars, jets, degeneracy pressure, brown dwarfs, life stages of low and high mass stars, etc....

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Lecture 8- Life Cycle of stars 8.1 Star Birth Where do stars form? - Newborn stars produce white patches in the cloud where starlight illuminates surrounding gas - The cloud looks dark where dust particles block the light from more distant stars Star- Forming Clouds  Stars form in dark clouds of dusty gas in interstellar space.  The gas between the stars is called the interstellar medium ISM Molecular clouds  Most of the matter in star-forming clouds is in the form of molecules (H2, C O, etc.).  These molecular clouds have a temperature of 10–30 K and a density of about 300 molecules per cubic centimeter. Star Birth  Eagle Nebula  Hubble’s image  Young stars in open clusters  Molecular clouds –Cold and Dense form stars Observing Newborn Stars  Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed. - A visible-light image shows immense pillars of dark molecular gas - The pillars are being sculpted by ultraviolet radiation from nearby stars (not seen in this image) that heats and erodes the dark gas - Observing the infrared light from a cloud can reveal the newborn star embedded inside it. - Infrared light passes through the dark gas, so the pillars become almost transparent - Allowing us to see newborn stars that were hidden in the visible- light image Glowing Dust Grains - Dust grains that absorb visible light heat up and emit infrared light of even-longer wavelength - Regions that glow in infrared light… - Are dark in the visible-light image If gas and dust are dark, how do we know they exist in space? a. We sometimes see absorption lines from interstellar gas. b. Infrared telescopes can see cool dust. c. Radio telescopes can detect interstellar gas. d. All of the above. Stages of Star Birth

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Gravity Contracts the cloud Cloud spinning Flattening Protostar

Gravity Contraction

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Gravity is more than the pressure Gravity strengthens as the cloud shrinks

Protostar- 1st stage of star - Early stage of the star still gathering mass from its parent molecular cloud, before the start of Nucleosynthesis or Fussion - The gas becomes hottest and denser at the centre of the cloud, making a protostar in the centre and planetray disks around it. - The centre Temp and Press rises dramatically and the P pushes back against gravity, slowing the contraction marking the first stage Proto star. Formation of Jets  Rotation also causes jets of matter to shoot out along the rotation axis.  Formation process is violent it sometimes shoots jet.  Jets are observed coming from the centers of disks around protostars.  The jets ram into interstellar gas, heating it and causing it to glow. How Does Nuclear Fusion Begin in a Newborn Star? - Fusion From Protostar to main sequence  A protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion.  Contraction must continue until the core becomes hot enough for nuclear fusion.

 Contraction stops when the energy released by core fusion balances energy radiated from the surface—the star is now a main-sequence star. Protostar- true star: Core temperature exceeds 10 Million K Summary of Star Birth 1. Gravity causes gas cloud to shrink and fragment. 2. Core of shrinking cloud heats up. 3. When core gets hot enough, fusion begins and stops the shrinking. 4. New star achieves long-lasting state of balance. Birth Stages on a Life Track

 A life track illustrates a star's surface temperature and luminosity at different moments in time. Self- Sustaining Fusion  Core temperature continues to rise until star begins fusion and arrives on the main sequence. Life Tracks for Different Masses  Models show that the Sun required about 30 million years to go from protostar to main sequence.  Higher-mass stars form faster.  Lower-mass stars form more slowly.  stars spend the shortest amount of time as protostars – High mass Vs contrary The mass range of stars

What is the smallest mass a newborn star can have? - (Mass diagram) - The reason for this cut off of due a type of pressure called Degeneracy pressure, that prevents lower mass stars from contracting to the point at which fusion can bring in energy balance. Lower Limit on a Star’s Mass 0.08 Solar Mass  Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K.  Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation.  Is there another form of pressure that can stop contraction? Degeneracy Pressure Thermal Pressure:  Depends on heat content.  Is the main form of pressure in most stars.  Compression raises pressure and temp Degeneracy Pressure:  Particles can't be in same state in same place.  Doesn't depend on heat content.  Does not depend on temp  Degeneracy pressure arises because laws of quantum mechanics place limits, on how closley sub atomic particles can be packed together.  We can understand the idea with an anology :  Where students try to find seats in a classroom, if there are plenty more chairs than students, they can move freely and could be compressed into smaller spaces.  This is analogus to situation in most stars, in which degeneracy pressure is not important.  However if the no of chairs equals the no. of students, no longer move freely and cannot be squeezed into smaller space.  This represents the situation with degenercy pressure.  In which quantum mechanical limitations limit sub atomic particles from being further compressed  An object in which contraction has been stopped by degeneracy pressure can remain at the same size indefinitely, even as the star cools with time. Brown Dwarfs- “Failed Stars”  Degeneracy pressure halts the contraction of objects with...