Stellar Evolution

Stellar Evolution

The Birthplace of StarsThe space between the stars is not completely empty. Thin clouds of hydrogen and helium, seeded with the “dust” from dying stars, form in interstellar space.

Dark Clouds gather

Molecular CloudsSometimes (especially in spiral arms), the gas is compressed enough that the dust is thick and gravity can collapse knots in these “molecular” clouds to make new stars.

Formation of stars

• First what is formed is a protostar• Protostars with mass 0.08MS do not develop the pressure and

temperature necessary to initiate nuclear fusion reaction and will contract to become a brown dwarf.

• Protostars with mass greater than 100MS develop a pressure a pressure much higher than the gravitational pressure that disrupts the evolution of the star

• When the star is formed, it joins the main sequence the star, it location depending on the mass of the star.

Evolution on the Main Sequence (I)

Zero-Age Main

Sequence (ZAMS)

Main-Sequence stars live by

fusing H to He.

Finite supply of H => finite life time.

MS evolution

Evolution on the Main Sequence (II)

Evolution off the Main Sequence: Expansion into a Red Giant

• H in the core completely converted into He – radiation reduces and core begins to collapse due to gravity resulting in an increase in temperature

• This leads to “H burning” in the outer shell around the core

• Thus the core continues shrink while the outer shell expands and cools

• This is the RED GIANT phase

Expansion onto the Giant BranchExpansion and surface cooling

during the phase of an inactive He core

and a H- burning shell

Sun will expand beyond Earth’s orbit!

Red Giant Evolution

4 H → He

He

He-core gets denser and hotter until the next stage of nuclear burning can begin in the core:

He fusion:

3 4He → 12C

“Triple-Alpha Process”

Fusion of Helium into Carbon

This is followed by

4He + 12C → 16O

Evolution after Red Giant Phase – Low mass stars

• For mass less than 4MS the star becomes unstable. The star loses the outer envelope of the star of gases exposing the inner core of oxygen and carbon Galactic nebula. Eventually this core cools to become white dwarf

• If the mass of white dwarf is less than 1.4MS , it is able to support itself due to electron degeneracy pressure and remain stable

• When the white dwarf mass exceeds this limit (Chandrasekhar’s limit), then it collapses further due to gravity to become neutron star

Evolution after Red Giant Phase – High mass stars

• For mass greater than 4MS , fusion in the core continues resulting in the formation of Ne, Si, and finally Fe.

• No more thermonuclear reaction happen, and gravity takes over collapsing the core. This collapse is an IMPLOSION termed as the type II Supernova

Evolution after Red Giant Phase – High mass stars

• During the Supernova collapse, the protons and neutrons are crushed to form neutrons. Eventually the remnant of this collapse is a neutron star

• When the mass of the neutron star exceeds 2-3 times the mass of Sun (this limit is not precisely estimated), neutron degeneracy pressure does not allow stability. The neutron star collapses further to become a Black hole. This is known as the Oppenheimer-Volkoff limit

Summary of Post-Main-Sequence Evolution of Stars

M > 8 Msun

M < 4 Msun

Evolution of 4 - 8 Msun

stars is still uncertain.Fusion

stops at formation

of C,O core.

Fusion proceeds to formation of Fe core.

Red dwarfs: He burning

never ignitesM < 0.4 Msun

High-mass stars evolve off the main sequence (to become red giants)

earlier than low-mass stars.

=> For a given age, low-mass stars are still on the MS, while high-mass

stars are already red giants!

Red shift of light from galaxies

• Due to expansion of universe, galaxies move away from each other.• This leads to red shift in the light received from these galaxies

(Doppler Effect)

Red shift formula

cv

cvcv

off

o

z

11

1

The relativistic red shift formula is(z is red shift parameter)

At low speeds

11

1

cvcv

off

o

z

Red shift formula - problem

cv

o

z

Estimate the speed of a galaxy, if the wavelength for the hydrogen line at 434nm is measured on earth as being 610nm.

181021.1434

1768103 ms

o

cv

Hubble’s law

Hdv

"The distance to objects beyond the Local Group is closely related to how fast they seem to be receding from us"

“ v ” is the recessional speed of the galaxy, “ d” is the distance of the galaxy from us and “ H” is the Hubble parameter or Hubble constant.H = 71 km/s/Mpc = 22km/s/Mly

Hubble’s law - limitations

• Can be applied to galaxies other than local cluster• Galaxies in local cluster may even show blue shift (Andromeda moves

towards Milky Way)• Distance and speed of distant galaxies cannot be accurately estimated –

this lead to uncertainties in the estimation of Hubble’s constant

Measurement of Hubble constant

• Observing Cepheid variables in different galaxies• Observing Supernova explosions

(READ FURTHER TO COMPLETE THIS TOPIC)

Age of the universe

• Reciprocal of Hubble constant gives a rough estimate of the age of the universe (Here we make a assumption that H is really a constant)

• Rough estimate of the age of universe is 10 to 20 billion years

Sample problem based on Hubble law

Solution to problem in previous slide

Smile please ….Astrophysics class is FINALLY OVER ….