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

GravitationFG = G m1m2

r2

• Gravitational Potential Energy• GPE = FGd• GPE = KE = ½ mv2

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

PleiadesReflection NebulaeRead 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 CompressionStar 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 imagevisible 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 – 263oC

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 image infrared 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• 107K Hydrogen fusion begins• Star becomes visible because TNF builds

pressure which ejects the outer layer of gases.

Page 292

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 - 200 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 - <0.08 - .4 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 0.08-0.4 Solar MassRed 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

HeTNF

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• He4 + He4 + He4 C12 + gamma energy• C12 + He4 O16 + 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 period2. Determine luminosity from relationship and

hence, absolute magnitude3. Observe apparent magnitude4. Calculate distance from these two

observations.• Cepheids are very bright so can see far away