Download Lecture 4. Big bang, nucleosynthesis, the lives and deaths

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 thick
Central 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) stars
gas (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 astronomical
observations - 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 young
star 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 and
fuse (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
neutrons
electrons
} nucleus
100 different kinds of atoms = elements (H, C, Fe, Au)
Atomic number = # of protons in the nucleus
Atomic mass = # of protons + # of neutrons
isotope:
12C
13C
14C
# protons
6
6
6
# neutrons
6
7
8
most abundant
stable isotope
least abundant, is radioactive/unstable
Chemical properties of isotopes identical, although 12C reacts faster.
Molecule = two or more atoms combined
Thermonuclear Reactions Revisited
Our Sun now:
4 1H
----->
1 proton
1 4He
+ energy (heat and light)
2 protons +
2 neutrons
small amount of
mass lost E=mc2
Our Sun 4 Ga from now:
all the H will be used up, last hundred million years will fuse
3 4 He -----> 1 12C + energy
2 protons +
2 neutrons
6 protons +
6 neutrons
Stars more massive that the Sun:
12C
-----> 16O
4He
----->
4He
20Ne
----->
24Mg
4He
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.
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.
Heavy Elements
Evidence heavy elements manufactured in stars:
1. increasing abundance of heavy elements with time
2. theoretical calculations suggesting universe began only with H and He
3. theoretical and observation that stars make heavy elements
via nuclear fusion.
Abundances:
3 of the 4 most abundant elements: C, N, O
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