Birth and Death in Stars

Evidence of Star Formation• forming stars are usually embedded in clouds

Evidence of Star Formation• forming stars are usually embedded in clouds

MAIN SEQUENCE STAR

The Life of a Star

GAS CLOUD

PROTOSTAR

WHITE DWARF

PHASES:

RED GIANT

WHAT CHANGES THINGS:

• nuclear reactions begin in star’s core

• gravity pulls part of cloud together

• hydrogen fuel runs out in center of star

.

SUPERGIANT

NEUTRON STAR

BLACK HOLE

LOW MASS HIGH MASS

SUPERNOVA• other fuels run out in center of starPLANETARY

NEBULA

Temperature

Luminosity

The HR Diagram

• position in diagram tells us what stage of life a star is in

Protostar:

PROTOSTAR SHRINKS AND HEATS UP

PROTOSTAR BEING ASSEMBLED

NUCLEAR FUSION STARTS

“pre-star” without nuclear fusion

Main Sequence Stars

TemperatureLu

min

osity highest mass

lowest mass

.

SUN

SPICA:

107 yr lifetime

PROXIMA CENTAURI

lifetime greater than age of universe

1010 yr lifetime

Thought QuestionWhich of the cut-away views below best represents

what is happening in the core of a star at the very end of its main sequence phase?(“” means nuclear reactions are happening,H = hydrogen, He = helium, C = carbon)

Running Out…

• nuclear fuel runs out at the center of the star first… where reactions happen fastest (and where it is hottest)

Rules for Stars• pressure supports star and

opposes gravity

• nuclear reactions release energy that keeps gas hot and maintains pressure

GRAVITY

CROSS-SECTION

• stars use up nuclear fuel at center first (where gas is hottest)

• when fuel runs out, gravity crushes star’s core and causes a temperature rise

Becoming a Giant

H fusion in core ends and core is crushed and heated…

Fusion continues in a “shell” surrounding the He core …

Large energy release makes envelope expand

He

mostly H

CROSS-SECTION

Thought Question:As the Sun becomes a red giant, its…

A. luminosity and temperature increase

B. luminosity increases and temperature decreases

C. luminosity decreases and temperature increases

D. luminosity and temperature decrease

Life Track of a Sun-Like Star

H fusion

He fusion

Nuclear ReactionsFUSION: small nuclei combine together IF they collide

fast enough• example: hydrogen

• higher temperature is needed for new nuclear fuels because:– nuclei have larger charges– nuclei are more massive (and slower)

Thought Question

Compared to the first step in H fusion, the electrical repulsion between nuclei in the following fusion reaction is how many times larger?

=proton

=neutron

Life stages of a low-mass star like the Sun

Planetary Nebulae

• low-mass star pushes most of its gas into space, exposing hot core

nuclear reactions stop

star cools forever…

• gas is illuminated by hot white dwarf

The Ring Nebula

ABOUT 1.3 LIGHT-YEARS

(planetary nebula)

• gas moving about 30 km/s

• How old is this nebula?

White Dwarfs

Nearest example:

Sirius B

MANY others probably out there

densely packed electrons resist being crushed

nuclear reactions no longer needed to support star

SIRIUS B

“Degeneracy”Certain kinds of particles

(like electrons and neutrons) don’t like looking EXACTLY like others…

“THE EXCLUSION PRINCIPLE”

example: electrons in atomic orbitals

if forced together, they must have:

different spin orientation

ORdifferent motion

TWIN CLOSET

Low-Mass Star Corpses: White Dwarfs

• pressure comes from “degenerate” electrons• star about same size as Earth• higher mass white dwarfs are smaller

BUT… can’t be larger than 1.4 M or collapse!

.

neutron star

High-Mass Stars (M > 9 MSun)

• massive stars zig-zag in HR Diagram using new fuels:

fuel used up

core of star shrinks, heats up

new nuclear fuel

ignites

… until last fuel is used up

SUPERGIANTS

The End for a Massive Star

iron core gets too massive and collapses…

BUT heat CANNOT be released by nuclear reactions using iron…

“onion skin” structure in core:

Supergiant:

Star Death

Learn what happened by looking at:

expanding gas cloud

energy release

star corpse

ABOUT 12 LIGHT-YEARS

Crab Nebula

Thought Question:How high will the small superball bounce when I

drop the stack of 4 superballs from 1 foot above the ground?

A. It won’t bounce – it will come to a dead stop.

B. It will bounce back to its start (1 foot).

C. It will bounce twice as high (2 feet).

D. It will bounce 4 times as high (4 feet).

E. It will hit the ceiling.

“MAXIMUM SCRUNCH”

COLLAPSE EXPANSION

CUT-AWAY VIEW OF STAR CORE

Massive Star Corpses

Iron core collapses and forms tiny neutron star:

(protons, electrons crushed together to make neutrons)

none found with more than 3 Msun

collapsing gas “bounces” off to start supernova explosion

10 km

CITY SIZE!!!

The Crab Nebula pictures taken in 1973 and 2001

CORPSE

Energy Release

• energies can help reveal what happened…

how much total energy is released?

(how violent was the star’s death?)

what kinds of energy are released?

where could the energy have come from?

SUPERNOVA

Where Does Energy Come From?• Fusion? For H fusion over star’s whole life:

• Gravitational potential energy:

How Much Energy is Released?

Light: total released

Gas kinetic energy:

Neutrinos: electrons, protons combine to form neutrons and…

PLANETARY NEBULA

(low-mass star)SUPERNOVA

(high-mass star)

Thought Question:Supernova explosions spread heavy elements

like iron into space. If you find a star that has a smaller abundance of iron than the Sun, it is

A. probably younger than the Sun.

B. probably older than the Sun.

C. not possible to tell the star’s age

You are Star StuffDying stars give important chemicals back:• supernova explosions (main source of Fe)• planetary nebulas (source of C, N, O)

POLLUTION OF GAS CLOUDS

STAR FORMATION

LOW-MASS STAR DEATH

HIGH-MASS STAR DEATH

Mass

Very important in astronomy…

Mass measurement allows us to:

• distinguish between different types of planets

• predict future lives of stars

• quickly identify how much “stuff” is in something very big (like the Milky Way)

• find matter even when it is “dark”

Listen:

Two videos involving a car horn…

Listen to the PITCH…

Racecar applet

#1 #2

Sound Waves

You hear a certain pitch when a sound source and you are stationary…

…imagine sound waves spreading out like ripples in a pond

The Doppler Effect

Moving apart (“REDSHIFT”): train’s motion adds a little distance between each wave…

larger wavelength/lower frequency

Applet

Honking horn movie

Moving closer (“BLUESHIFT”): train’s motion removes a little length from each wave…

smaller wavelength/higher frequency

Applet 2

Measuring Star SpeedsRadial velocity: rate of change of distance between two objects

negative if distance is decreasing

positive if distance is increasing

distance covered during the time it takes to make one wave

Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:

vr

vt

pure “transverse velocity”

pure “radial velocity”

vt

vr

mixture

Thought Question:The laboratory spectrum below shows emission

lines from hydrogen. If the spectrums of the other objects also show hydrogen, which is moving away from us fastest?

A.

B.

C.

D.

Thought Question:A policeman’s radar gun uses the Doppler effect, but it is

not always able to measure total speed accurately. In which situation below would the cop be able to measure your speed using just the radar gun?

A. B.

C. D.