Life Cycle of Stars Chapter 9, Page 279

Building Blocks Matter Energy

Energy (read p279) Gravity provides the energy for the universe Newton’s Law of Universal Gravitation F G = G m 1 m 2 r 2 Gravitational Potential Energy GPE = F G d GPE = KE = ½ mv 2

Matter (read P279, p280) Matter exists between the stars Interstellar Medium 10% of observed mass Observed in the infrared or radio Isolated atoms and molecules Hydrogen – atoms and molecules Helium Carbon monoxide, carbon dioxide, water, ammonia, formaldehyde and simple sugars

Collections of Material Read P 281 Nebula (plural: nebulae) – Dense Region of Interstellar gas and dust – Embedded within much larger clouds of gas and dust Giant molecular clouds – 1,000,000 solar masses – 300 Light years across

Stellar Nurseries Reflection nebula- star light scattered (bluish) Emission nebula- glow from energy from nearby stars Dark nebula – dense enough to block light Interstellar extinction Interstellar reddening- short wavelengths (blue) scattered more by ISM

Pleiades Reflection Nebulae Read P 281 #1 Visible light

Pleiades in Infrared Light

Read P281 #2 Read P281 #3

Read Page 281 # 4

Mountains of Creation- star forming region

Matter For Stars (P302) Population III Stars – Original Stars formed first after the “Big Bang” – Formed from Hydrogen, Helium and small amounts of Lithium – Probably very massive, Burned out quickly Population II Stars- Old Stars – Metal poor (ie not much heavy elements) Population I Stars – Younger Stars – Formed from recycled star material – Contain elements heavier than lithium

Matter and Energy Come Together Giant molecular clouds – 1,000,000 solar masses – 300 Light years across Begins to collapse Process Summarized on Pages 46-47

Sources of Compression Star Starters (p283) Supernova- a star explosion (p283 #1) Supernova remnant- material left over from the explosion of a star. (Read P283 #2) – Distinctive arched appearance – Slams into a giant molecular cloud and condenses the cloud. Collision of two giant molecular clouds Radiation and material from large stars (O and B) – Solar wind (Read #3)

Supernova Remnant (p284) X-ray image visible light image

Dense Regions Bok globules- very small dark nebulae – Block light Dense cores- compact regions of gas and dust within Bok globules. Opposing forces with in a dense region – Read P283 #4 – Gravity pulling matter together – Thermal pressure pushing matter apart Must be cool : 10K or – 263 o C

Star Clusters Giant molecular clouds Hundreds or thousands dense cores Will form hundreds and thousands of stars Open cluster. Read Page 284 #1

Protostar formation (p285) A cool, dense, gaseous, dusty region thousands of times bigger than our solar system. Gravity causes the gas to collapse into the center. Accretion- the process of increasing mass in the center of a dense core due to infall of gases from outer layers. Read P285 #1 Energy released (not from thermonuclear fusion) – Compression of the gases – Energy of infalling gases

visible light imageinfrared image

Spin ? (p285 #2) If the dense core is spinning then it collapses into a disk – Star with planets – Multiple stars If dense core is not spinning then it collapses into a sphere and a single star.

Equilibrium (p286#1) The collapse of material generates energy Energy (heat) increases the pressure of the gas Pressure of the gas eventually equals the force of gravity pulling material into the center Star ceases to increase in mass Becomes a pre-main sequence star Can be seen in infrared or radio waves but not in visible light because outer layer of gas and dust blocks the light.

infrared image- open cluster

Hydrogen Fusion (286#2) A star is a star because it produces energy by thermonuclear fusion Dense core of the pre-main sequence star continues to contract due to gravity Contraction continues to build heat 10 7 K Hydrogen fusion begins Star becomes visible because TNF builds pressure which ejects the outer layer of gases.

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Stars are Not Created Equal (P286) Place “protostars” on the HR Diagram on the red side. Total Energy per second Luminosity depends on size The Luminosity/Temperature relationship path of the star depends on the mass of the star.

Pre-Main Sequence Track Stars of different masses evolve differently from protostar to pre-main sequence star to main sequence star and take different time periods. The more massive a pre-main sequence star the more rapidly it begins hydrogen fusion.

Time to Main Sequence (p286) 5 solar mass PMS start fusing, 1 million years 1 solar mass PMS take a few 10’s of millions of years. > 7 solar masses start fusing as protostars never becoming PMS. 2 to 7 solar masses become hotter with out much change in luminosity because they contract- “move” horizontally on H-R

Brown Dwarfs “Failed Stars” Fail because they don’t study. < 0.08 solar masses do not have enough gravitational force compressing and heating their cores to ever get as hot as 10 million K. Never begin TNF Result they contract to become planetlike orbs of hydrogen and helium Small, cool, hard to detect

Upper Limit on Mass ?? Greater the mass the more rapid the TNF Greater than 120 solar masses such rapid TNF that surface temperature so extreme that outer layers are expelled thus decreasing their mass. Theory- No stars greater than 120 solar masses 2004 observed star between 130 and 150 solar masses. Back to the drawing board solar masses

Main Sequence Star (P291) Main-sequence stars are those stars in hydrostatic equilibrium, in which nuclear reactions fuse hydrogen into helium in their cores at nearly constant rates. Hydrostatic equilibrium – thermal pressure balances the gravitational attraction so the star neither collapses or expands. Thermonuclear Fusion of Hydrogen to Helium The life-time of the star is determined by the star mass.

Table 9-2 Page 293

Chapter 9 Schedule Monday 3/11 Small and Mid Sized Stars Tuesday 3/12 Large Stars and Variable Stars Wednesday 3/13 Complete Study Guide Thursday 3/14 Video Friday 3/15 Review Study Guide Monday 3/16 Test-Chapter 9 Tuesday 3/17 Death of Stars

Overview <0.08 solar masses – Brown Dwarf – no TNF Small - < solar masses Mid sized (our sun) 0.4 to 4 solar masses Large ->4x solar mass- Chapter 10 – Live Fast – Love Hard – Die Young – Leave a beautiful memory

Low Mass Stars Solar Mass Red Dwarfs Page 293 Low temperature and low fusion rates in the core, dimmest of MSS Helium produced in the core rises to the outer layers while hydrogen in the outer layers falls into the core by convection currents.

Red Dwarfs Star eventually converts all hydrogen to helium. Do not have enough mass to produce enough heat to fuse helium Fuses hydrogen for hundreds of billions of years. Lowest mass, Dimmest stars Most common stars Oldest stars- all red dwarfs ever formed still exist Single stars with planetary systems.

Mid sized (P294) >.4 – 4 solar masses Do not transport helium out of the core by convection. Fusion of hydrogen in the core slows down. Pressure holding up the outer layers decreases Outer layers collapse inward compressing the hydrogen and increasing temperature.

Hydrogen Shell Fusion Hydrogen begins fusing in a shell around the helium core. Fusion in the shell generates more heat than core fusion did Outer layer of gas expands due to the extra heat Becomes a Red Giant

hydrogen Collapse He Helium builds up in core and fusion slows, core cools, less pressure, star collapses

hydrogen TNF of H He Collapse generates heat which causes TNF of Hydrogen in a shell around the core.

Heated by pressure and TNF in the shell, the core gets hotter than ever and TNF of Helium begins in the core hydrogen TNF of H TNF of He

Added heat with no added mass causes the star to swell up and outside cools down. hydrogen H TNF He TNF

Red Giant Fate of Our Sun In about 5 billion years Our sun will swell to a red giant vaporizing Mercury, absorbing Venus and incinerating Earth. Shine 2000 times more brightly than today

Helium Core Fusion (P296) Shell fusion generates enough energy to cause fusion of helium in the core Triple Alpha Process He 4 + He 4 + He 4  C 12 + gamma energy C 12 + He 4  O 16 + gamma energy Produce carbon and oxygen

Large > 4 solar masses Contracts and heats rapidly Heat/ luminosity increase but shrinking reduces luminosity causing luminosity to remain constant while temperature increases. TNF begins -main sequence

Very Large > 200x solar mass Very luminous, very hot High pressure causes expulsion of gases Reduced size then proceeds to main sequence Follow a sequence similar to our sun Because of the extra mass, there are extra chapters to the story.

Variable Stars Constant struggle between force of gravity trying to collapse a star and thermal pressure trying to expand the star.

Variable Stars After Helium Fusion begins Helium fusion in the core Hydrogen fusion in a shell around the core Fusion generates heat causing the star to expand. Expanding cools the star and reduces pressure. No longer in hydrostatic equilibrium.

Types of Variables RR Lyrae variables – low mass stars – Period < 1 day Cepheids – Higher mass stars Light output increases rapidly then drops off gradually Class I Cepheids – metal rich Class II Cepheids – metal-poor

Measuring Distance with Cepheids Direct relationship between period of pulsation of Cepheids and their luminosity Must know distance in order to measure luminosity Use nearby stars to determine the relationship between period and luminosity. Relationship depends on type of Cepheid

Measuring Distance with Cepheids Direct relationship between period of pulsation and luminosity. 1.Observe the period 2.Determine luminosity from relationship and hence, absolute magnitude 3.Observe apparent magnitude 4.Calculate distance from these two observations. Cepheids are very bright so can see far away