AST2034 - Stars and the Nuclear Arms Race - Prof. Rana Ezzeddine - University of Florida

How can we tell what elements are present in astronomical objects?

The atomic hypothesis:

All things are made of atoms, little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

Chemical elements:

- Atoms make up all the matter around us, there are only 118 distinct types of atoms which do not further decompose on their own. These are called elements found in the periodic table of elements.

- The elements combine in an infinite numbers of different ways in order to yield huge variety of substances/ecosystems/life.

- 115 different elements have been confirmed to exist, and researchers claim to have discovered 3 different elements. Of the 115 confirmed elements, 90 exist in nature and 15 are man-made.

Sub-atomic Particles:

Proton : positive charge

Electron : negative change

neutron : neutral charge

Mass proton = Mass neutron

>>> Mass electron

Number of Protons: Z — "Atomic number"

Number of Neutrons: N

Number of nucleons: A = N + Z — "Mass number"

Nuclide

Each type of atom that contains a unique combination of protons and neutrons.

Uranium-235

U-235 = ^235^U = 235/92 U

ground level

n=1

excited level

n>1

Light is emitted as an electron jumps from a higher-energy state, or "orbit," to a

lower-energy state.

Light is absorbed as an electron jumps from a lower-energy

state to a higher energy state in an atom.

Bohr model of the hydrogen atom

A simplified model that is successful at predicting the energy (ΔE) of light that is emitted or absorbed in an atom.

ΔE = En(initial) − En(final) = h . ν

h is called Planck's constant

v is the frequency of light (speed of light/wavelength of light)

The dark patches on the Sun's spectrum (rainbow)

are due to absorption of electrons in the atoms corresponding to the specific elements, which can be seen at specific positions (wavelengths).

Spectroscopy:

quantitatively measuring the amount of light we receive at different frequencies (or wavelength or energies).

spectrograph (or spectrometer)

produces a spectrum (plural, spectra).

Splits light into its component frequencies/wavelengths.

Diffraction Grating

a plate of glass or metal ruled with very close parallel lines, producing a spectrum by diffraction and interference of light.

Photon detectors

converts photons into electrical signals that a computer interprets to measure the strength (brightness/intensity) of the different frequencies/wavelengths.

Apparent Brightness/Magnitude

the brightness of a star as seen from Earth

Absolute Brightness/Magnitude (Luminosity)

the actual brightness of a star

Formation of stars

Stars are formed from gas clouds in galaxies. The gas clouds gradually

collapse due to gravity. They form stars, of different sizes and masses.

Stars' mass and composition

A star is a massive ball of gas, made mainly of hydrogen (75%) and helium (23%). Bigger stars have more mass, greater gravity and are hotter than smaller stars.

Main sequence

a diagonal area on an H-R diagram that includes more than 90 percent of all stars

The main sequence is a mass sequence.

All the properties of a star are dictated by its mass. High mass stars are bigger in size, bluer and hotter than low mass stars which are smaller, cooler and redder

How do stars create Energy?

The enormous pressure and heat in a star's core converts

matter into energy.

What is the source of heat in stars?

The energy of stars is fueled by nuclear reactions, a process known as Nuclear Fusion

Nuclear Fusion

reactions in stars releases a huge amount of energy.

Nuclear fusion in the Sun releases energy equivalent to 100 billion

nuclear bomb explosions every second!!

In nuclear fusion, hydrogen atoms fuse to form helium

atoms. The total mass of the H atoms is larger than the He atoms.

During each step of the process, mass is lost and energy is released.

Why do stars evolve?

Stars do not remain stable for their whole lives, because they eventually consume all their hydrogen fuel.

At this stage stars start changing with time (size, temperature, luminosity): they evolve.

Life cycle of a low mass star

Nebula -> protostar (baby star) -> low mass star -> red giant -> white dwarf

Life cycle of a high mass star

Nebula -> protostar (baby star) -> high mass star -> red supergiant -> supernova explosion

Basic Facts about the Sun

The nearest star - 8 light minutes away.

Mass = 2 x 10^33 g

Radius = 6 x 10^10 cm = 370, 000 Miles

Energy: converting 600 million tons of hydrogen to

Helium per second via Nuclear Fusion —> Main

Sequence Star

Age: 4.6 billion yrs

Sun rotates on average once every 27 days

Surface Temperature ~ 5500K

Central Temperature ~ 15 million K

Sunspots are slightly cooler so they appear darker

The surface is highly turbulent and gas flows at high speeds creating cooler granules

Why doesn't the Sun collapse or Expand?

The Nuclear Fusion energy exerts an outward force and pressure, while the Gravity exerts an equal and inwards pressure. This keeps the Sun and most stars stable stars in Hydrostatic Balance.

Nebula

A large cloud of dust and gas in space

Star Formation

When stellar nurseries or nebulas, collapse to form stars.

Protostar

A contracting cloud of gas and dust; the earliest stage of a star's life

proto-planetary disk

a rotating circumstellar disk of dense gas surrounding a young newly formed star

Main Sequence

a diagonal area on an H-R diagram that includes more than 90 percent of all stars

Evolving stars

Low mass stars will turn into a Red Giant, whereas more massive stars will turn into a larger Red Supergiant.

During this evolved stage, stars start fusing helium into heavier elements, including carbon, oxygen, calcium... all the way to iron!

Planetary Nebula

A huge cloud of gas that is created when the outer layers of a red giant star drift out into space

White Dwarf

Red giants transform into white dwarfs by ejecting its outer layer of gas and dust into a planetary nebula.

Supernova Explosion

Supergiant stars end their lives as they explode into supernova explosions, throwing away all their outer layers.

Supernova Remnant

An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.

black hole

An object in space whose gravity is so strong not even light can escape.

Neutron Star

the small, dense remains of a high-mass star after a supernova

The enormous density of a neutron star means a teaspoon of neutron star material would weigh 10 million tons.

At only about 12 miles in diameter, a neutron star would fit inside the boundaries of Chicago.

Neutron stars have exceptionally strong magnetic fields around them

Stella Spectra

reveal that stars are different, and can

be classified according to their temperatures: O, B, A, F,

G, K, M

Pulsars

A rapidly spinning neutron star that produces radio waves

Binary Stars

a system of two stars in which one star revolves around the other or both revolve around a common center. (50% of stars are born binary)

brown dwarfs (failed stars)

have mass less than 8% of the Sun and never heat enough to have nuclear reactions in their core

How do elements heavier than iron form in the Universe?

The formation of elements beyond iron in the periodic Table requires more extreme environments, usually triggered by the end of a star's life, and often includes two objects interacting.

rapid neutron-capture process (r-process)

neutrons are added to iron in fast reaction creating heavier elements until bismuth

beta decay

After neutrons enter the nuclei, the atom will become unstable and radioactive

radioactive decay of an atomic nucleus that is accompanied by the emission of a beta particle

This will transform the atom into a different element with larger Z.

r-process

1) Neutron gets captured by atom

2) Atom becomes unstable

3) Neutron becomes a proton by decaying and releasing a beta-particle (electron)

4) Atomic number becomes bigger by 1 (Z Z+1)

beta-decay steps

1) Mass Number (A = Z + N) remains the same

2) Atomic number (Z) increases by 1

Kilonova

a powerful, transient astronomical explosion occurring when two neutron stars or a neutron star and a black hole collide.

It releases immense energy and eject heavy, radioactive debris.

How do elements heavier than iron form in the Universe?

Elements heavier than iron are formed during more energetic processes via the rapid neutron-capture process (r-process).

This triggers the beta-decay where neutrons are converted

to protons leading to more stable, but heavier (higher Z) atoms to form.

r-process

1) Neutron gets captured by atom

2) Atom becomes unstable

3) Neutron becomes a proton by decaying and

releasing a beta-particle (electron)

4) Atomic number becomes bigger by 1 (Z -> Z+1)

This process repeats itself until all the heavy elements are formed.

beta-decay

1) Mass Number (A = Z + N) remains the same

2) Atomic number (Z) increases by 1

kilonova

transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other. it is powered by radioactive debris

radioactive

A material containing unstable nuclei is considered

Radioactivity

is the act of a nucleus emitting radiation spontaneously, to try to become more stable

beta-decay

too many neutrons in a nucleus lead it to emit a beta particle (electron), which changes one of the neutrons into a proton

alpha-decay

Too much mass leads a nucleus to emit an alpha (Helium atom) particle, discarding four heavy particles (two protons and two neutrons).

half-life (t1/2)

is the amount of time required for one-half of a sample of a particular radioactive isotope to decay into some other isotope or state.

Chronometry

the science of the measurement of time, or timekeeping

Cosmo-Chronometry

The process of measuring the age of astronomical objects.

Chronometry in geology and archaeology

are based on radiometric measurements

The amount of energy released by nuclear bombs can range from

the equivalent of a ton to almost 500,000 tons (500 kilotons) of TNT (for each gram of TNT exploded, 4.184 kilojoules — or 4184 joules — of energy is released).

Mass defect

the difference between the mass of the atom and the sum of the masses of its arts. The mass of the atom is less than the mass of the sum of its parts.

This was discovered empirically, as the masses of protons, neutrons, electrons, and individual isotopes were measured to high precision.

This is a property of all atomic nuclei.

Binding energy (BE)

the amount of energy that must be supplied to a nucleus to separate it into its component parts.

Some mass is converted into energy when the nucleus forms.

- Recall Einstein's equation: E = mc^2

strong force

attractive force that acts between protons and neutrons in an atomic nucleus

137 times stronger than the electromagnetic force, s much stronger than the electromagnetic (repulsion) force by the protons.

liquid drop model of the atom

if the nucleus captures a neutron whose kinetic energy is just enough to cause the nucleus to vibrate at least "this=binding energy" much, the two sides of the nucleus will separate enough that the electromagnetic force dominates. The nucleus fissions

nuclear fission

a nucleus captures a free neutron. That neutron brings along enough kinetic energy to excite the nucleus, causing it to fission, or split.

chain reaction

occurs when one of those free neutrons is captured by another U-235 nucleus, producing U-236, and if it fissions, then then process continues over and over again

critical mass

The minimum mass needed for self- sustaining chain reaction occurs

Uranium gun ("Little Boy")

Two separate masses of U-235 (each below the critical mass) are quickly brought together, and their combined mass exceeds the critical mass. An initiator supplies a burst of neutrons to initiate the chain reaction.

This was the first bomb design used in the war. The US dropped this bomb on the city of Hiroshima, Japan (August 6, 1945).

Plutonium implosion ("Fat Man")

Conventional explosives are arranged in a sphere around a sub-critical mass of Pu-239. As they implode inward, the

Pu-239 reaches critical mass, and an initiator supplies a

burst of neutrons to initiate the chain reaction.

- This bomb designed was first tested in New Mexico (the Trinity Test; July 16, 1945) and later dropped by the US on the city of Nagasaki, Japan (August 9, 1945).

Hydrogen fusion ("thermonuclear") bomb

A primary (fission bomb, like the U-235 or Pu-239 ones) produces an initi burst of neutrons.

- In the secondary stage, these neutrons react with Li-6 to produce He-4 a H-3 ("tritium"); the H-3 reacts with H-2 ("deuterium") to release energy.

The most energetic bombs ever detonated are of this type.

How was fission discovered and who discovered it?

In 1934, Enrico Fermi bombarded uranium with neutrons, producing what he thought were the first elements heavier than uranium.

Ida Noddack, pointed out that Fermi hadn't ruled out the possibility that in his reactions, the uranium might actually have broken up into lighter elements, though she didn't propose any theoretical basis for how that

could happen.

What delayed the discovery and why was it

deemed unthinkable before by scientists?

Scientists delayed recognizing nuclear fission because they assumed neutron‑bombarded uranium would form heavier elements, making the idea of a heavy nucleus splitting into lighter ones seem impossible at the time.

Main nuclear weapons production sites during WWII:

Los Alamos, New Mexico:

design, component testing, and assembly

Clinton Engineer Works (Oak Ridge), Tennessee:

uranium enrichment (separating the U-235 from the U-238)

Hanford Engineer Works, Washington:

plutonium (Pu-239) production from uranium (U-238

Detonation of the first atomic bombs:

• Introduced the world to weapons of mass destruction.

• Created a highly competitive militaristic attitude

among world nations.

• Created worldwide anxiety regarding the threat of

nuclear annihilation.

• Began a greater culture of awareness concerning

moral responsibilities during times of warfare.

nuclear arms race

was an arms race competition for

supremacy in nuclear warfare between the United States, the Soviet

Union, and their respective allies during the Cold War. During this

same period, in addition to the American and Soviet nuclear

stockpiles, other countries developed nuclear weapons, though

none engaged in warhead production on nearly the same scale as

the two superpowers.

United Nations Atomic Energy Commission

The General Assembly asked the Commission to "make specific

proposals":

(a) for extending between all nations the exchange of basic scientific

information for peaceful ends

(b) for control of atomic energy to the extent necessary to ensure its

use only for peaceful purposes

(c) for the elimination from national armaments of atomic weapons

and of all other major weapons adaptable to mass destruction

(d) for effective safeguards by way of inspection and other means to

protect complying States against the hazards of violations and

evasions.

Hydrogen fusion ("thermonuclear") bomb - H bomb

A primary (fission bomb, like the U-235 or Pu-239 ones) produces an initial burst of neutrons.

In the secondary stage, these neutrons react with Li-6 to produce He-4

and H-3 ("tritium"); the H-3 fuses with H-2 ("deuterium") to release

energy.

The yield can be designed to be arbitrarily high by adjusting the amount

of lithium and fissionable material.

The first hydrogen bomb test occurred in 1952.

The most energetic bombs ever detonated are of this type.

A- bomb vs. H- bomb

A thermonuclear bomb differs fundamentally from an atomic bomb in that it utilizes the energy released when two light atomic nuclei combine, or fuse, to form a heavier nucleus.

The hydrogen nuclei that combine to form heavier helium nuclei must lose a

small portion of their mass (about 0.63 percent) in order to "fit together" in a single larger atom. They lose this mass by converting it completely into energy, according to Albert Einstein's famous formula: E = mc^2

Characteristics of nuclear fusion reactions make possible the use of non-

fissile materials as the weapon's main fuel, thus allowing more efficient use of

scarce fissile material such as U-235 or Pu-239.

Hydrogen bombs would result in a yield of about ~100,000 kilotons of TNT,

as compared to the Little Boy and Fat Man which resulted in ~ 20, 000 kilotons

of TNT.

Hydrogen bombs are also harder to produce but lighter in weight, meaning they could travel farther on top of a missile, according to experts.

US Thermonuclear tests

The United States detonated the first hydrogen bomb on November 1, 1952, on Enewetak, an atoll in the Pacific Ocean. Code-named "Ivy Mike", the project was led by Edward Teller, a Hungarian-American nuclear physicist.

Strategic Bombers vs. Ballistic Missiles

Strategic bombers were the primary delivery method at the beginning of the Cold War.

Missiles had long been regarded the ideal platform for nuclear weapons, and were potentially a more effective delivery system

than bombers.

Starting in the 1950s, medium-range ballistic missiles and intermediate-range ballistic missiles ("IRBM"s) were developed for delivery of tactical nuclear

weapons, and the technology developed to the progressively longer ranges, eventually becoming intercontinental ballistic missiles (ICBMs).

Multiple independently targetable reentry

vehicle (MIRV)

an exoatmospheric ballistic missile payload containing several warheads, each capable of being aimed to hit a different target.

Mutual Assured Destruction (MAD)

A doctrine of military strategy and national security policy in which a

full-scale use of nuclear weapons by two or more opposing sides would cause the complete annihilation of both the attacker and the defender.

Deterrence

Practice on how threats or limited force by one party can convince another party to refrain from initiating some course of action.

Nash equilibrium

In game theory, it is a collection of strategies where there is no benefit for any player to switch strategies. In the Nash equilibrium, each player's strategy is optimal when considering the decisions of other players.

Limited (Partial) Test Ban Treaty (1963)

prohibited nuclear weapons tests in the atmosphere, outer space, and underwater to reduce radioactive fallout during the Cold War. It marked a major step toward arms control by forcing nuclear powers to shift testing underground and opening the door to future nonproliferation agreements.

Strategic Arms Reduction Treaty

a nuclear arms reduction treaty between the United States and Russia. It was signed on 8 April 2010 in Prague and, after ratification, entered into force on 5 February 2011. It is expected to last until 5 February 2026, having been extended in 2021.