AST 101 Introduction to Astronomy:

Stars & Galaxies

Life of a Low-Mass Star REVIEW

END STATE: PLANETARY NEBULA + WHITE DWARF

Planetary Nebulae – White dwarfs WHAS IS A WHITE DWARF?

Exposed core of a low-mass star that has died

No fusion to maintain heat and pressure to balance gravity pull Electron degeneracy pressure balances inward crush of its own gravity

Mostly made of Carbon and Oxygen

Very high density and hence gravity

Maximum mass=1.4 Msun (Chandrasekar limit)

Size Of A White Dwarf Size of Earth

Funky properties of white dwarf material!

1 Kg chocolate cake!

2 Kg chocolate cake!

0.4 Msun white dwarf!0.8 Msun white dwarf!

•  Hubble Space Telescope spies 12-13 billion year old white dwarfs –  Formed less than

1 billion years after the creation of the universe

© HST and H. Richer (University of British Columbia)

Which is correct order for some stages of life in a low-mass star? A.  protostar, main-sequence star, red giant,

planetary nebula, white dwarf B.  protostar, main-sequence star, red giant,

supernova, neutron star C.  main-sequence star, white dwarf, red giant,

planetary nebula, protostar D.  protostar, main-sequence star, planetary

nebula, red giant E.  protostar, red giant, main-sequence star,

planetary nebula, white dwarf

Clicker Question

Which is correct order for some stages of life in a low-mass star? A.  protostar, main-sequence star, red giant,

planetary nebula, white dwarf B.  protostar, main-sequence star, red giant,

supernova, neutron star C.  main-sequence star, white dwarf, red giant,

planetary nebula, protostar D.  protostar, main-sequence star, planetary

nebula, red giant E.  protostar, red giant, main-sequence star,

planetary nebula, white dwarf

Clicker Question Time scales for Evolution of Sun-like Star

H core burning Main Sequence 1010 yr 10 billion years

Inactive He core, H shell burning Red Giant 108 yr 100 million years

He core burning (unstable), ” Helium Flash Hours He core burning (stable), ” Horizontal Branch 107 yr

10 million years C core, He + H shells burning Bright Red Giant 104 yr

10 thousand years Envelope ejected Planetary Nebula 105 yr

100 thousand years Cooling C/O core White Dwarf - Cold C/O core Black Dwarf ∞

Hubble image, visible light

Chandra image, X-ray light

Sirius A & B Main Sequence & White Dwarf

The Big Bang produced only hydrogen and helium. Suppose the universe contained

only low mass stars. Would elements heavier than Carbon and Oxygen exist?

A.  Yes B.  No

Clicker Question

The Big Bang produced only hydrogen and helium. Suppose the universe contained

only low mass stars. Would elements heavier than Carbon and Oxygen exist?

A.  Yes B.  No

Clicker Question Lives of Intermediate/High-Mass

Stars •  Low mass: < 2 times the Sun

•  Intermediate mass: 2-8 times the Sun

•  High mass: > 8 times the Sun

General Principles Are the Same: Battle Between Pressure and Gravity

•  Main sequence lifetimes are much shorter

•  Early stages after

main sequence –  Similar to a low mass

star, but happen much faster

•  No helium flash

Intermediate-Mass Stars

•  May burn up to carbon but do not have enough mass to get temperatures high enough to go any higher up the periodic table

•  Degeneracy pressure stops the core from collapsing and heating enough: particles are squashed together as much as possible

•  End their lives with planetary nebulae, white dwarfs, similarly to low-mass stars.

•  Sequence of expansion/contraction repeats as higher and higher elements begin to fuse

•  Each heavier element requires higher core temperatures to fuse

High-Mass Stars (M >8 MSUN)

•  Core structure keeps on building successive shell -  Like an onion •  Lighter elements on the outside, heavier ones on the inside

•  Most elements are formed via Helium Capture –  A helium (2 protons) nucleus is absorbed, energy is

released •  The elements are created going up the periodic

table in steps of 2

Other Reactions Carbon (6), Oxygen (8), Neon (10)

Magnesium (12)….

“WE ARE ALL STAR STUFF!”

- Carl Sagan “We are all star-stuff” •  All heavy elements are created and dispersed

through the galaxy by stars

•  Without high mass stars, no heavy elements

•  Our atoms were once parts of stars that died more than 4.6 billion years ago, whose remains were swept up into the solar system when the Sun formed

HIGH mass stars keep creating elements up the period table UNTIL….

IRON (Fe, 26 protons )

•  Iron does not release energy through fusion or fission –  Remember: All

energy created by the loss of mass from the fusion or

the fission (E=mc2)

There Is No Way Iron Can Produce Any Energy to Push Back Against the Crush

of Gravity in the Star’s Core

The star is DOOMED!!!

What is the heaviest element that can be created through fusion?

A.  Carbon B.  Silicon C.  Iron D.  Uranium

Clicker Question

What is the heaviest element that can be created through fusion?

A.  Carbon B.  Silicon C.  Iron D.  Uranium

Clicker Question

No significant changes in luminosity Star travels back and forth on the HR diagram

In the most massive stars, changes happen so quickly that the outer layers do not have time to respond

Outer layers subject to strong winds

Massive red giant or supergiant: Fierce hot winds and pulsed ejecta

Hubble

Wildest of all ! ETA CARINAE Supermassive star (150 MSUN ) late in life, giant outburst 160 yr ago

Violent bipolar ejecta + disk at equator

Question: why do we see the glowing gas surrounding the star to grow in time?

Note: the star emitted a pulse of radiation some time ago.

Star V838 Monocerotis HST-ACS

`Light Echo� from pulse

Red Giant with intense brightening

•  The core of a high mass star accumulates iron as the layers above it fuse

•  Without any outward pressure, the core once again starts to contract.

•  Electron degeneracy pressure supports the core for awhile until the mass of iron gets too heavy (how heavy?)

•  When mass is too large (>1.4Msun), core collapses and iron atoms get compressed into pure neutrons

•  protons + electrons ! neutrons + neutrinos –  This takes less than 0.01 seconds

•  Electron degeneracy pressure - GONE!

–  Core collapses completely

• Eventually neutron degeneracy pressure stops the collapse abruptly • Infalling atmosphere impacts on the core.

• Time for a demo…

Big and small balls Demo

•  What do you think will happen?

A.  The little ball will bounce up together with the others

B.  The little ball will bounce higher than the others, but no higher than when the little ball is dropped alone

C.  The little ball will bounce much higher than the other balls

Supernova!

•  The lightweight atmosphere impacts on the heavy core and is “bounced” off in a huge explosion

•  Plus huge energy release from neutrinos!

The star�s former surface zooms outward ��� with a velocity of 10,000 km/s!