Lecture 4. Big bang, nucleosynthesis, the lives and deaths of stars.

reading: Chapter 1

Big Bang

Universe began in a tremendous explosion 13.7 ± 0.2 billion years ago.Supported by:- theory- observation (age of the chemical elements, oldest star

clusters, oldest white dwarf stars)

1 billion years = 1 Giga annum = 1,000,000,000 years

All matter condensed into a tiny space.Extremely high density and temperature.Matter formed (mostly hydrogen H and helium He).Matter attracted matter.Stars formed in clumps of matter.Galaxies formed where clumps of stars a few billion years.

Since then everything has been expanding and cooling.

Evidence of the Big Bang

Heat generated during the Big Bang radiated into space.Cosmic background radiation, first discovered in 1965 by two young

radio astronomers, Penzias and Wilson.Heat of Big Bang 3˚K, spread throughout the universe.“…it is the afterglow of the big bang, cooled to a faint whisper in the

microwave spectrum by the expansion of the universe”.Comic background radiation is slightly clumpy.Suggests early unevenness in the universe - could have led to galaxy

formation.

Milky Way Galaxy

100,000 light years across, Disk is 1,000 light years thickCentral bulge, contains a black hole?Rotating spiral arms.Surrounded by globular clusters of stars.Sun 28,000 light years from the center.

Formed at least 9 Ga.Sun formed ~ 4.6 Ga.

Galaxies made of :(burning) starsgas (H and He) dust (solid particles of Fe, Al, Mg, Si, C, O, N, etc.)dark matter

Clusters of galaxies held together by gravity.

Galaxies in the Universe

Taken by Hubble Space Telescope Deep Field.A speck in the sky contains 1500 galaxies.

Galaxies as far as the eye can see.

Life Cycle of a Star

“Birth”H, He, and dust attract each other due to gravity.Cloud collapses, temperature (T) and pressure (P) increases.Thermonuclear reactions begin - star ignites.

“Life”Star shines/burns

- some burn hot- some burn cool

“Death”Nuclear fuel is exhausted.May start to burn an alternative nuclear fuel.Spent gas returns blown off to space.

Origin of the Heavy Elements

Early universe mostly H, He, tiny bit of Li.But interstellar dust and Earth made up of Al, Si, Fe, O, Mg.Life is made up of C, H, N, O, P, S.Any atoms “heavier” than He are called The Heavy Elements.

Where did the heavy elements come from?

We are made of “star stuff” …….. Carl Sagan

Birth of Stars

1. Begins with interstellar cloud of gas and dust:~75% H~25% He~2% heavy elements

2. Cloud collapses due to gravitational instability

3. Cloud begins to spin

4. Cloud forms a disk- cloud collapses further and spins faster (like an ice skater)- a spinning cloud naturally flattens into a disk- T rises as density and P increases- when T high enough, thermonuclear reactions begin- star ends up with same composition as the cloud- young star surrounded by a disk of gas and dust- process takes millions of years

Observations

Theory of the spinning disk supported by astronomicalobservations - young starts contain flat clouds of dust.

If central object rotates very fast, it could split into two stars -binary star system.

infrared image of a youngstar showing hot disk of dust

How do You Get the Heavy Elements?

Stars of different ages have different atomic compositions.

Very old stars:~99.9% H and He~0.1% heavy elements(composition of the early universe)

Young stars:~98% H and He~2% heavy elements

Suggests the amount of heavy elements is increasing over time.

Thermonuclear Reactions

T and P so high that atomic nuclei bump into each other andfuse (nuclear fusion) into a single larger nucleus:

Basic Properties of Atoms

Atom is 100,000 times larger than the nucleus.Nucleus contains almost all the mass of the atom.

Atoms consist of:protons neutronselectrons

100 different kinds of atoms = elements (H, C, Fe, Au)

Atomic number = # of protons in the nucleusAtomic mass = # of protons + # of neutrons

isotope: # protons # neutrons12C 6 6 most abundant13C 6 7 stable isotope14C 6 8 least abundant, is radioactive/unstable

Chemical properties of isotopes identical, although 12C reacts faster.

Molecule = two or more atoms combined

} nucleus

Thermonuclear Reactions Revisited

Massive starts will eventually explode as supernovae- scattering heavy elements into space.

Huge explosions also generate heavy elements like Fe.Clouds of H, He, and heavy atoms can collapse and spin.

Our Sun now:4 1H -----> 1 4He + energy (heat and light)

Our Sun 4 Ga from now:all the H will be used up, last hundred million years will fuse3 4 He -----> 1 12C + energy

Stars more massive that the Sun:12C -----> 16O -----> 20Ne -----> 24Mg

1 proton 2 protons + 2 neutrons

6 protons +6 neutrons

2 protons + 2 neutrons

4He 4He 4He

small amount ofmass lost E=mc2

Hot Stars, Cold Stars

Hot stars:- large, high T and P- H exhausted in few million years- fuse most of the heavy heavy elements- have short life spans (burn hot and fast)- die in supernovae

Medium stars (like the Sun):- H exhausted in ~9 billion years- generated most of the C in the universe

Cold stars:- small, low T and P- mostly generate He- have long life spans (burn cold and slow)

Amount of heavy atoms in the universe is increasing.

Evidence heavy elements manufactured in stars:1. increasing abundance of heavy elements with time2. theoretical calculations suggesting universe began only with H and He3. theoretical and observation that stars make heavy elements

via nuclear fusion.

Abundances:3 of the 4 most abundant elements: C, N, O

Heavy Elements

Galaxies

Gravity in Milky Way traps gas and dust (heavy elements).Most of the gas is in the Milky Way disk (not the halo), so most

new stars form in the disk (stars in the halo are very old).Rotation of arms of Galaxy generate gravity waves which can

perturb clouds, causing them to collapse.Get birth of new generations of stars - our own Sun is a product

of several previous generations of stars.Galaxies are like giant recycling plants.

Heavy Elements in the Galaxy

Heavy elements are not evenly distributed throughout the galaxy.Are more heavy elements toward the center of the galaxy.

Lecture 5. Origin of the Solar System, Formation of the Earth.

reading: Chapter 4