Lecture 1 - The Origins of the Universe

Question 1

When did the Big Bang occur?

Answer 1

13.7 billion years ago.

Question 2

What did the Big Bang do?

Answer 2

It brought into existence all of the material in the universe which then coalesced to form the galaxies, stars and planets.

Question 3

What was the temperature of space prior to the Big Bang?

Answer 3

10³² K

Question 4

What was the density of space prior to the Big Bang?

Answer 4

10⁹⁶ g cm⁻³

Question 5

What was the temperature of the universe one second after the Big Bang?

Answer 5

10¹⁰ K

Question 6

What did the expansion of the universe cause?

Answer 6

Neutrons that were formed in the explosion to decay into protons and electrons. The half life of this reaction was 10 minutes.

Question 7

What elements were created after the Big Bang?

Answer 7

Hydrogen and helium in a ratio of 10:1 but due to Helium’s mass, it accounted for 29% of the mass of the universe.

Question 8

What percentage of the universe is composed of hydrogen and helium?

Answer 8

99%.

Question 9

Where are the other 1% of elements formed?

Answer 9

By nuclear fusion in stars.

Question 10

How is a star formed?

Answer 10

1) A ball of hydrogen and helium accumulates.

2) Gravitational attraction and pressure causes the temperature to rise.

3) Once temperatures reach 10⁷ K, the hydrogen particles are moving fast enough to fuse together.

Question 11

What is the hydrogen burning process?

Answer 11

Four hydrogen atoms combining together to form a helium atom, two positrons, two neutrinos and some energy.

Question 12

What is the equation for the hydrogen burning process?

Answer 12

4 ¹H → ⁴He + 2e⁺ + 2ν + energy

Question 13

Why is energy released during the hydrogen burning process?

Answer 13

Mass is lost which is converted into energy in the relationship:

E = mc²

Question 14

Why does the helium burning process start?

Answer 14

Once the star runs out of hydrogen to burn, the star no longer produces an outwards force and so collapses. This produces lots of heat and once this reaches 10⁸K, helium atoms start to fuse together. This releases energy which stabilises the star.

Question 15

What is the equation for helium burning?

Answer 15

3 ⁴He → ¹²C + energy

Question 16

What are the processes after the helium burning process?

Answer 16

Every second element up to iron is produced by the continued process of running out of fuel, contracting and higher temperatures allowing the fusion of heavier elements to occur.

Question 17

What is the heaviest element that can be made by nuclear fusion?

Answer 17

⁵⁶Fe.

The temperature of the star at this point is 4 x 10⁹K.

Question 18

Why does the fusion process stop at ⁵⁶Fe?

Answer 18

Iron is heaviest element that can be formed that releases energy. The formation of heavier elements requires mass to be gained which requires lots of energy.

Question 19

What are the two methods that elements are distributed around the universe?

Answer 19

Supernovae.

Mass lost from giant stars where the material from the edge is lost to space.

Question 20

What is a second generation star?

Answer 20

A star that starts its life with trace amounts of heavier elements. This allows more complex and diverse reactions to occur which produces a greater variety of elements.

Question 21

How much material has been distributed around the universe?

Answer 21

The mass of one billion suns.

Question 22

What are the equations for the Carbon Nitrogen Cycle?

Answer 22

¹²C + ¹H → ¹³N + γ Radioactive Decay - ¹³C + e⁺ + ν
¹³C + ¹H → ¹⁴N + γ
¹⁴N + ¹H → ¹⁵O + γ Radioactive Decay - ¹⁵N + e⁺ + ν
¹⁵N + ¹H → ⁴He + ¹²C

Question 23

What is the role of carbon in the carbon nitrogen cycle?

Answer 23

It acts as a catalyst for the reaction.

Question 24

What is the structure of a star?

Answer 24

A star has a layered structure. The synthesis of heavier elements occurs closer to the centre and the synthesis of lighter elements occurs near the surface.

Question 25

What is the relationship between the mass of the element being synthesised and the time the star takes to finish burning it?

Answer 25

As the mass of the element being synthesised increases, the star takes significantly less time to burn it.

Question 26

What is the name of the graph that shows the abundance of the elements in the universe?

Answer 26

The Cosmic Abundance Graph.

Question 27

What is the main factor that affects the abundance of an element?

Answer 27

How easy it is to synthesise the element.

Question 28

Which element is used as a reference point for the Cosmic Abundance Graph?

Answer 28

Silicon with an abundance of 1 x 10⁶.

Question 29

What features does the Cosmic Abundance Graph show?

Answer 29

1) Hydrogen and helium are the most common elements.
2) There are much lower than expected quantities of lithium, beryllium and boron.
3) There is a zig zag pattern.
4) There is a peak at iron.
5) There is a gap after atomic numbers 43 and 61 (Technetium and Promethium).
6) There is a gap after Bismuth.

Question 30

Why are hydrogen and helium the most common elements?

Answer 30

They were formed just after the Big Bang and require very little synthesis.

Question 31

Why are there much lower than expected quantities of lithium, beryllium and boron?

Answer 31

They are skipped in the hydrogen and helium burning process and they have been found to be unstable in the interior of stars. Instead these elements are formed by cosmic ray spallation.

Question 32

Why is there a zig zag pattern?

Answer 32

Elements with an odd atomic number are less stable than elements with an even atomic number. This is called the Oddo Harkins rule.

Question 33

Why is there a peak at iron?

Answer 33

This is the end point of synthesis in stars and so it accumulates before being distributed by a supernova.

Question 34

Why is there a gap after atomic numbers 43 and 61 (Technetium and Promethium)?

Answer 34

The elements are radioactive with very short half lives. Therefore, they have decayed away to form other elements.

Question 35

Why is there a gap after Bismuth?

Answer 35

All elements heavier than Bismuth are radioactive and have decayed away. The only exceptions are Thorium and Uranium which have very long half lives and so haven’t decayed away yet.

Question 36

What evidence is there for the Big Bang?

Answer 36

Light observed from distant stars exhibit a shift towards the red end of the spectrum which shows that they are moving away from us. This shows that the universe is expanding and so likely originated from a single point in space.

Question 37

What evidence is there for supernovae?

Answer 37

They have been observed such as in 1054 when Chinese astronomers observed a supernova that formed the Crab Nebula.

Question 38

What evidence is there for nuclear synthesis in stars?

Answer 38

Nuclear energy is the only possible energy source that is sufficient enough for stars to burn.

Elements that are absent elsewhere in the universe because they are radioactive have been observed in the material formed when stars explode.