Stellar Evolution

Star birth in the Eagle Nebula Courtesy of the Space Telescope Science Institute

Introduction• Human lifetimes vs. ages of stars• How do we know . . .?

– Humans via pictures• In one day, take pictures of people, then

piece together human behavior & history; similar to finding the life history of stars

– High-energy physics• Interactions of matter/energy at

extremely high temperatures

• Theories tested, modified, some completely rewritten

• Many questions remain unanswered

The Birth of a Star

Nebulae = more than one nebula

• Vast clouds of gas in space

• Mainly hydrogen• Disturbance

– Colliding with other clouds

– Blast from nearby supernova explosion

The Birth of a Star

GRAVITY RULES!!

The Birth of a StarRotating cloud collapses in

on itselfAs center of the cloud

becomes more dense, collapse accelerates due to increased gravitational attraction between gas particles

Collapsing clouds mark the formation of a protostar(not yet a true star; no nuclear

reactions occurring yet)

Particles far apart don’t exert much gravity on each other

The same particles, now closer together, exert more gravitational force on each other

The Birth of a Star• As the clouds continue to collapse it begins to warm up

• When the gas particle collides with the center of the cloud, – it loses kinetic energy

because it slows down

– It loses potential energy because it isn’t so far away from the middle of the cloud.

• This energy turns into HEAT

Out here, the gas particle has both kinetic & potential

energy

Center of gas cloud

The Birth of a Star• Warming occurs slowly

at first• Center begins to glow,

dim to bright• When central

temperature is high enough (~15 000 K, ~15 273 C) nuclear reactions can begin

• Protostar has now become a true star

As the temperature increases, these hydrogen particles move faster. Eventually, they move so fast that when they collide they’ll stick together.*

A helium nucleus has been formed!

When this “sticking” (fusion) occurs, a bit of mass is converted to energy as in

E = mc2

The fightFusion

pressure pushes

OUTWARD from core

Gravity pulls

INWARD toward

core

The Birth of a Star• Stars can form from

extremely large interstellar clouds that have fragmented into smaller clouds.

• These clusters of stars are called . . . Star clusters (!)

• Ex: The Pleiades (Seven Sisters)

                            

  

The Birth of a Star• The haze (“nebulosity”) is

part of the original gas cloud that’s left over.

• How long does formation take?– Small low mass stars can

take billions of years to form

– More massive stars can completely form in a few hundred thousand years

Main Sequence• Star has settled into the

most stable part of its life• Converts hydrogen to

helium (H => He)• Next step depends on the

mass of the star• Three different examples of

stars:1. Stars similar to our Sun2. Stars several times more

massive than the Sun3. HUGE HUMONGOUS stars,

VERY massive

The Life of a Sun-like Star• Will remain on the main sequence (H to He) for

about 10 billion years• As more He is produced, temperature increases and

core contracts– We see this as an increase in brightness– Temperature not high enough to sustain He to C fusion

• Central core then expands as more He is produced

• Star expands, becoming a

Red GiantRed Giant• Our sun, as a red giant, will be as large as Earth’s present orbit

The Life of a Sun-like Star• Over thousands of years, the

star’s central region shrinks & heats up.

• Outer regions are pushed away

• We see:– a small, dense central star– surrounded by expanding shell

of gas• The star is now a planetary

nebula

The Life of a Sun-like Star• The object seen at the center of the gas cloud

is the core of the original star• Still very hot (~100 000 C)• Gradually cools & contracts to become a white

dwarf• Cools even more to become a black dwarf; not

much bigger than Earth, but much more dense

The Life of a Sun-like Star

The Life of a Star Several Times More Massive Than the Sun

• Enters main sequence (H to He process) at a higher temperature than smaller stars

• Core is hotter than smaller stars, causing faster “aging”

• After all H is converted to He, He is fused into carbon (requires 100 million degrees)

The Life of a Star Several Times More Massive Than the Sun

• After all the He is used, C fuses into neon (requires 500 million degrees)

• As each element is used up, star becomes a red giant. • . . . And so forth, as long as temperatures are high

enough to fuse that particular element• As particles that are colliding get larger, much more

heat (energy) is needed to get them to stick together

The Life of a Star Several Times More Massive Than the Sun

• When an iron core is formed:– Reactions STOP– Iron fusion requires HUGE

amounts of energy– Eventually, cools to white

dwarf, then black dwarf stage

– Different than smaller star’s fate because different elements will compose the core

The Life of HUGE Stars• As with all other stars, follows main

sequence• If the star is still large (>1.4 Suns) when

the core becomes iron, a supernova results

The Life of HUGE Stars•Within seconds of running out of nuclear fuel, the HUGE gravitational force (remember, large mass = large gravity) attracts all of the atmosphere into the core.

http://ircamera.as.arizona.edu/NatSci102/movies/corcoll3.gif

The Life of HUGE Stars• As particles fall to the core they lose kinetic & potential

energy and more HEAT results

• This heat triggers nuclear fusion in the outer layers, and the resulting explosion is the supernova.

• The energy released can fuse iron and other heavier elements, up to uranium.

The Life of HUGE Stars

“This next image is one of the most spectacular views of 1987A yet

acquired by the HST. The single large bright light is a star beyond the

supernova environs. Around the central supernova is a single ring but

associated with the expansion of expelled gases are also a pair of rings

further away that stand out when imaged at a wavelength that screens

out much of this bright light.”Courtesy http://rst.gsfc.nasa.gov/Sect20/A6.html

•

The Life of HUGE Stars

• The death of the largest stars results in a core more dense than anything we know on earth

• This core has such a large gravitational force that light cannot escape it.

• . . . Hence the name, black hole• Picture here

Caption: In this image, X-ray contours are overlaid on an optical image. The X-ray contours and the colors in the optical image represent brightness levels of the X-ray and optical emission, respectively. When viewed with an optical telescope this galaxy, located 2.5 billion light years from Earth, appears normal. But the Chandra observation discovered an unusually strong source of X rays concentrated in the central regions of the galaxy. The X-ray source could be another example of a veiled black hole associated with a Type 2 Quasar. This discovery adds to a growing body of evidence that our census of energetic black hole sources in galaxies is far from complete.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

CXO 0312 Fiore P3 (CXOUJ031238.9-765134): A possible Type 2 quasar veiled black hole.(Credit: X-ray: NASA/CXC/SAO; Optical: ESO/La Silla)

From http://chandra.harvard.edu/photo/2000/0312/0312_hand.html

Some artists’ conceptions of a black hole

•

The Life of HUGE Stars• How do we know a

black hole exists?• Evidence

– Strong x-ray emissions from charged particles accelerating REALLY fast

– Gravitational lensing• Light from stars is bent

when a black hole is between us & the stars

• Usually form in binary star systems

We are all made of stars . . .