28,000 light years200 billion stars

Milky Way Galaxy

100 billion galaxies in the observable Universe

10 day exposure photo!

Over 1500 galaxies in a spot 1/30 the diameter of the Moon

Farthest and oldest objects are 12-13 billion light years away!

Proxima Centauri (Alpha Centauri C)Closest star (4.2 light-years from the Sun)

Voyager 1: 12 light-hours from the Sun (90 AU)

Launched in 1977

The most distant human-made object in space

How can we learn about the life of stars??

• Our life span is ~ 80 years

• Human civilization exists ~ 5000 years

• Our Sun exists at least 4.6 billion years!

Star Clusters – “School Classes” Star Clusters – “School Classes” for Starsfor Stars

They consist of stars of the same age !

Open clusters100’s of stars

Globular clusters100,000 of stars

Giant molecular clouds – stellar nurseries

Great Orion Nebula

The Horsehead Nebula

Star Forming Region RCW 38

Coldest spots in the universe:T ~ 10 K

Composition:• Mainly molecular hydrogen• 1% dust

Protostars: warm clumps of gas surrounded by infalling matter

Disks: planet formation?!

• The matter stops falling on the star• A star becomes hot enough to sustain the pressure of gravity

Contraction stops when the gravity is balanced by thermal pressure

Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity.

Surface temperature 6000 KTemperature at the center 14,000,000 K!

A puzzle: the Sun and other stars radiate away huge amounts of energy. They should lose all their heat in less than a million years!

There must be an internal energy source: nuclear fusion reactions

However, the Sun lives 4.6 billion years

“Planetary” model of atom

Proton mass: 1.7x10-27 kgElectron mass: 9x10-30 kg

Nuclear reactionsNuclear reactions• Fission: decay of heavy nuclei into lighter fragments

•Fusion: synthesis of light nuclei into a heavier nucleus

A star will live until all hydrogen is exhausted in its core

star mass star mass (solar masses)(solar masses)

Lifetime Lifetime (years)(years)

6060 3 million3 million

3030 11 million11 million

1010 32 million32 million

33 370 million370 million

1.51.5 3 billion3 billion

11 10 billion10 billion

0.10.1 1000's 1000's billionsbillions

Our Sun will live 5 billion years more

What happens when all hydrogen is converted into helium in the core??

Mass defines the fate of the star

Fate of the collapsed coreFate of the collapsed core

White dwarf if the remnant is below the White dwarf if the remnant is below the Chandrasekhar limit 1.4 solar massChandrasekhar limit 1.4 solar mass

Neutron star if the core mass is less than Neutron star if the core mass is less than ~ 3 solar masses~ 3 solar masses

Black hole otherwiseBlack hole otherwise

Death of StarsDeath of Stars

“All hope abandon, ye who enter here” Dante

Outer layers expand due to radiation pressure from a hot core

The star becomes a Red Giant

• Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times

In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.

p. 192

p. 193

p. 193

p. 193

p. 193

What is left??

A stellar remnant: white dwarf, composed mainly of carbon and oxygen

It is extremely dense

All atoms are smashed and the star is supported by pressure of free electrons

White dwarf

White DwarfsWhite DwarfsDegenerate stellar remnant (C,O core)

Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!

White Dwarfs:

Mass ~ Msun

Temp. ~ 25,000 K

Luminosity ~ 0.01 Lsun

Chunk of WD material the size of a beach ball would outweigh an ocean liner!

As it cools, carbon crystallizes into diamond lattice.Imagine single diamond of mass 1030 kg!Don’t rush, you would weigh 15,000 tons there!

White dwarfs in a globular cluster

(SLIDESHOW MODE ONLY)

Death of a massive star

The iron core of a giant star cannot sustain the pressure of gravity. It collapses inward in less than a second.

The shock wave blows away outer layers of a star, creating a SUPERNOVA EXPLOSION!

For several weeks the supernova outshines the whole galaxy

Eta Carinae: will explode soon

Distance 7500 ly

Supernova RemnantsSupernova Remnants

The Cygnus Loop

The Veil Nebula

The Crab Nebula:

Remnant of a supernova

observed in a.d. 1054

Cassiopeia A

Optical

X-rays

Crab nebula: the remnants of supernova 1054

Formation of Neutron StarsFormation of Neutron StarsCompact objects more massive than the

Chandrasekhar Limit (1.4 Msun) collapse further.

Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object:

p + e- n + e

Neutron Star

Properties of Neutron StarsProperties of Neutron Stars

Typical size: R ~ 10 km

Mass: M ~ 1.4 – 3 Msun

Density: ~ 1014 g/cm3

Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!

Isolated neutron stars are extremely hard to observe

Neutron stars have been theoretically predicted in 30s.Landau, Oppenheimer, Zwicky, Baade

However, there are two facts that However, there are two facts that can help:can help:

Neutron stars should rotate extremely fast Neutron stars should rotate extremely fast due to conservation of the angular due to conservation of the angular momentum in the collapsemomentum in the collapse

They should have huge magnetic field due They should have huge magnetic field due to conservation of the magnetic flux in the to conservation of the magnetic flux in the collapsecollapse

Jocelyn Bell

Discovery of pulsars: Bell and Hewish, 1967

When the core is too massive, nothing can prevent collapse into a black hole

2

2

c

GMRs

Schwarzschild radius: event horizon for a nonrotating body

To make a black hole from a body of mass M, one needs to squeeze it below its Schwarzschild’s radius

Rs

Gravitational collapse: the body squeezes below its event horizon

Black holes are NOT big cosmic bathtub drains!

Far from a black hole R >> Rs weak field

Approaching a black hole R Rs (strong field): gravity pull very strong

If our Sun collapses into a black hole, we won’t see any difference in the gravitational pull (but it will be VERY cold)

How to observe a black hole if it How to observe a black hole if it does not emit any radiation? does not emit any radiation?

Good news: most stars are in binary systemsGood news: most stars are in binary systems

The Universe in gamma-ray eyes

Gamma-ray bursts

Gamma-ray bursts

Our Earth and our bodies are made of atoms that were synthesized in previous generations of stars