Topic 4 Birth And Life

Question 1

What is the intercloud medium?

Answer 1

This is a component of the interstellar medium that is characterised by its very low density. Other denser components are typically embedded within this.

Question 2

What does embedded mean in the content of star formation?

Answer 2

This is a region lying within a larger body, for example a forming star being obscured by a dust cloud that surrounds it.

Question 3

What are molecular clouds?

Answer 3

Dense parts of the interstellar medium comprised of molecular gas and dust particles from which stars begin to form.

These are generally a low temperature and opaque to optical and UV radiation I so will appear dark in images at those wavelengths.

Question 4

What is the Jean’s criterion?

Answer 4

A region of gas will contract if the force of gravity causing the matter to be pulled inwards (towards the centre of gravity) and become denser is stronger than the pressure of the gas acting outwards to resist the inward motion.

Question 5

What is Jeans mass? (equation)

Answer 5

This is the critical mass where a gas cloud will collapse to form a star.

Mj = 9/4 x (1/(2πn))^(1/2) x 1/(m^2) x ((kT)/G)^(3/2)

Where m is the mass of an individual gas particle, T is the gas temperature and n is the number density.

Question 6

What assumptions are made for the jeans criterion?

Answer 6

The jeans criterion assumes that the gas cloud is spherical with a uniform temperature and density throughout.

Question 7

What is fragmentation?

Answer 7

The process by which smaller parts of a giant molecular cloud (GMC) become dense enough to exceed the jeans mass and so undergo collapse independently.

Question 8

What are dense cores?

Answer 8

These are the densest regions, which collapse from the inside outwards in a process known as infall. This process leads to the accretion (build up) of gas which will eventually become a star.

Question 9

How can the Eg equation be used for star formation?

Answer 9

Eg = -((GMm) / R)

Where M is the total mass of the fragment, m is the mass of the gas parcel, G is the gravitational constant, R is a given radius and the negative sign shows its potential energy.

Question 10

The kinetic and thermal energy of a gas parcel (equation).

Answer 10

Ek = Eth = (3/2)NkT

Where N is the number if particles in the gas cloud, T is the temperature and k is the Boltzmann constant.

Question 11

What is the virial theorem? (and equation)

Answer 11

This states that for a gravitationally bound system, the total kinetic energy of the constituent particles must be half the magnitude of the gravitational potential energy.

Ek = -(1/2)Eg

Question 12

What are the difficulties with observing protostars of mass greater than 10 solar masses?

Answer 12

We can expect that the largest protostars form from the largest and densest giant molecular clouds (GMC). These are extremely opaque to optical and infrared radiation so are very difficult to observe. ( there clouds are known as infrared dark clouds).

The nearest known examples are also much further away making them harder to observe.

Question 13

What are the two potential star forming processes for massive stars (> 10 solar masses)?

Answer 13

1. Scaled up gravitational collapse with accretion
2. Stellar mergers (the combination of two protostars)

Question 14

What are hierarchical star systems?

Answer 14

A situation where three or more stars orbit eachother. Gravity causes stars to be grouped hierarchically, e.g. As an inner binary and a third orbiting star. An example is the binary α centauri A and B, and their companion proxima Centauri.

Question 15

What is limb darkening?

Answer 15

This is the decrease in brightness towards the edge of a star when observed compared to its centre. This is due to our line of sight through the photosphere, as we see hotter material at greater depths near the centre of the solar disc.

Question 16

What are solar granules?

Answer 16

Mottled features of the surface of the sun caused by convective cells that rise to its surface. In the case of another star they are called stellar granules.

Question 17

What is hydrostatic equilibrium?

Answer 17

This is the condition where the outward force on the layer perfectly balances the inward force on a layer of a star due to gravity.

Question 18

Relationship between Temperature, Mass and radius of a star (equation)

Answer 18

T ~ (GMm) / kR

Where T is the stars average internal temperature, G is the universal gravitational constant, M is the mass of the star, m is the average mass of a gas particle and k is the boltzmann constant.

Question 19

Gas pressure equation (ideal gas law)

Answer 19

P = ρkT / m

Where P is the gas pressure, k is the Boltzmann constant, T is the temperature in kelvin and m is the average mass of the gas particles.

Question 20

How do star mass and radius affect radius the internal temperature?

Answer 20

A large difference in stellar mass corresponds to only a small difference in stellar radius. This means the M/R ratio is larger in more massive stars. therefore, since

Tcore ~ M/R

The core temperature should be higher for stars of larger mass.

Question 21

What does the term random walk refer to?

Answer 21

When Photons are radiated in the core they travel up to a few centimetres and are absorbed and remitted by other particles. The photons can be remitted in any direction so their overall path is extremely random.

Question 22

What are stellar envelopes?

Answer 22

These are the outer regions of a star where nuclear fusion doesn’t occur, and energy is transferred by convection.

Question 23

How is the energy transferred from the core outwards for stars of different masses?

Answer 23

M < 0.5 solar masses: fully convective

M = solar mass: radiative inner regions and convention stellar envelopes.

M > 1.5 solar masses: inner convective regions and radiative envelopes overlay this.

Question 24

How does a star maintain hydrostatic equilibrium if its radius were to decrease?

Answer 24

If a star’s radius were to decrease, the inner layers would compress. This would increase the density and pressure of the layer. As gravitational potential energy is released as heat energy this would also increase the pressure in the layer. The outward pressure would be greater than the increase in gravitational force (due to increased mass at the centre) and the star would expand back to its original radius.

Question 25

What is the **Russel-Vogt theorem**?

Answer 25

The theorem that a star in **hydrostatic equilibrium** will have a structure uniquely determined by its mass and chemical composition.

Question 26

What is the **pp chain**?

Answer 26

The chain of nuclear reactions starting with two protons that converts hydrogen to helium in low mass stars. There are 3 main branches ppl, ppII and ppIII.

Question 27

What is the ppI chemical equation?

Answer 27

6 1/1H > 4/2He + 2 1/1H + 2e+ + 2νe + 2γ

Question 28

What happens to the positrons released in a ppI reaction?

Answer 28

The positrons **annihilate** (completely convert two particles (one of matter the other antimatter) into photons, often high energy gamma rays) with nearby electrons to release additional energy. These travel out with gamma ray photons from ppl reactions to power the stars luminosity.

Question 29

What is a **mass deficit**?

Answer 29

The difference in mass between the particles on the left and right hand sides of a nuclear fusion equation. The deficit in mass is released as energy (photons).

Question 30

Why are very high temperatures required for nuclear fusion reactions?

Answer 30

Atomis nudie are positively charged so naturally repel each other due to electrostatic force. It requires large amounts of kinetic energy to overcome this repulsion which is why high temperatures all needed.

Question 31

What is the main difference between the pp1 and the pp2 and 3 reactions?

Answer 31

The pp2 and 3 reactions form heavier elements such as beryllium, boron and tilbum lithium which decay back to He.

Question 32

Repulsive force between two nuclei equation

Answer 32

F = (Za Zb) / (4πε0r^2) Where Za and Zb are the charges of the two nuclei, r is the distance between them and ε0 is a constant known as the permittivity of free space.

Question 33

What is the **CNO cycle**?

Answer 33

The carbon - nitrogen - oxygen cycle is a chain of nuclear reactions using a cycle of carbon, nitrogen and oxygen nuclei as catalysts that convert hydrogen to helium in higher mass stars. The CNO is neither used up or created in the reaction. Net reaction equation: R 1/1H > 4/2He + 2e+ + 2νe + 3γ

Question 34

Why isn't the CNO cycle an important process in the sun?

Answer 34

As a low mass star, the suns core temperature is not high enough to overcome the stronger repulsive forces between larger nuclei. This means the CNO cycle cannot operate efficiently.

Question 35

At what mass would a stars primary nuclear pathway change from pp to CNO?

Answer 35

The transition between low mass stars dominated by the pp chains and high mass stars dominated by the CNO cycle occurs at around 1.5 solar masses.

Question 36

The luminosity and mass proportionality of main sequence stars

Answer 36

L α M^4

Question 37

Main sequence star lifetime mass proportionality

Answer 37

t α M^-3

Question 38

What evidence do we have for nuclear fusion in stars?

Answer 38

1- we can detect neutrinos that are emitted by the sun. By measuring the quantity of nutria neutrinos emitted by the sun we can calculate the rate of nuclear fusion reactions in its core. 2- via astroseismology we can measure the modes of oscillation on the surface of the sun letting us know about the interior density and composition.

Question 39

What is a **brown dwarf**?

Answer 39

This is a star like object that has a mass smaller than 0.08 solar masses and therefore cannot sustain nudeor fusion reactions in its core. A brown dwarfs luminosity comes from the release of gravitational potential energy as the brown dwarf contracts.

Question 40

Radiation pressure equation.

Answer 40

Prad = 1/3 *a* T^4 Where *a* is the radiation constant (7.57x10^-16 N m^-2 K^-4) and T is the temperature.

Question 41

What is the **ELS model**?

Answer 41

This is a galaxy formation model that suggests that a galaxy is formed from a large collapsing gas cloud. 1. Protogalaxy ( assembly of gas and dark matte a galaxy originates from). 2. Collapse and star formation. 3. Formation of a disc. 4. Continuous contraction forms thin disc.

Question 42

What is the **Searle and Zinn model** for the formation of the milky way?

Answer 42

This model suggests that many smaller fragments of gas, some of which may have already formed stars, gradually clump together. It is suspected that the milky ways formation was a combination of both this model and the **ELS model**.

Question 43

What similarities are there between star and galaxy formation?

Answer 43

- Gravitational collapse is the driving force. - rotation leads to the formation of flattened discs. - inflowing of material. - accretion af gas.

Question 44

What is the **freefall timescale**?

Answer 44

The time it takes (in the absence of an opposing force) for a cloud of gas to collapse under its own gravity. In reality opposing pressure forces will make the process much slower.

Question 45

Free fall timescale equation.

Answer 45

t = sqrt(3π / 32Gρ) Where ρ is the initial density of the cloud.

Question 46

Total gravitational potential energy released from a forming star equation

Answer 46

E ~ (G Mtot Mgas) / R Where Mtot is the total mass of the system, Mgas is the mass of gas and R is the final radius. In the case Mtot = Mgas E ~ GM^2 / R

Question 47

How long does it take a gas cloud to lose a significant proportion of its energy equation (thermal energy equation)

Answer 47

t = Eth / L Where L is luminosity and Eth can be calculated using Eth = 3/2 N*k*T Where N is total number of particles, *k* is Boltzmann constant and T is gas temperature.

Question 48

What is a **galaxy merger**?

Answer 48

The consequence of two galaxies colliding so that they are mutually bound by their own gravity and form into a single larger galaxy.

Question 49

What is a **major merger**?

Answer 49

The collision between two galaxies of similar mass. Both galaxies are substantially altered by the process, merging to form a galaxy that may be quite different from either of the original two components. If both galaxies are gas rich then a spiral structure may reform. If both galaxies are gas poor no spiral structure con form. If on galaxy in gas rich the outcome is uncertain.

Question 50

What is a **minor merger**?

Answer 50

A collision between two galaxies of very differed masses. Typically the morphology of the larger galaxy will remain unchanged while the less massive galaxy is likely to be completely disrupted.

Question 51

What are **hydrodynamic simulations**?

Answer 51

A model that uses grid or a set of particles to represent matter as a fluid ( e.g. In a molecular cloud or a galaxy cluster). Sometimes abbreviated to hydro simulation.

Question 52

What are **semi analytic models**?

Answer 52

A simpler faster model than a hydrodynamic simulation which follows the path of galaxies as they merge in the hierarchical model. Simple or more sophisticated rules are applied to trace how galaxy properties change with time.