astro

Question 1

What is a galaxy ?

Answer 1

a huge assembly of stars, plus gas and dust, that is held together by gravity.|They are cosmic engines that turn gas–>stars and stars–>gas |all significant star formation occurs in galaxies |they can be classified by their morphology

Question 2

What are the different classifications of galaxies

Answer 2

There are ellipticals, dwarf elipticals, spiral galaxies, barred spiral galaxies and irregular galaxies

Question 3

How can we identify and further classify eliptical galaxies

Answer 3

the galaxy is pure bulge with no disk component ||we further sub-divide E0-E7 with E7 being much longer than it is wide and E0 being circular||we can define the number using the formula in the diagram

Question 4

How can we identify spiral galaxies and further classify them?

Answer 4

Spirals are classified by their relative|amount of disk and bulge components||Sa,Sb and Sc with Sa having the clearest spiral arms and a larger bulge component.|a–>b–>c decreasing disk to bulge ratio||an S0 start of disk forming

Question 5

How can we identify barred spiral galaxies and further classify them?

Answer 5

are spiral galaxies which have bars on the arms of the spirals|we can further classify to SBa, SBb, SBc|with SBa having the largest bulge and most clearly defined arms and bulge.

Question 6

How can we identify irregular galaxies

Answer 6

they dont have a set shape and are weird looking

Question 7

what do we classify stellar populations into?

Answer 7

population I and population II star

Question 8

what are the properties of population I stars and in general where do we find them?

Answer 8

these tend to be found in disk area of galaxies|-they are young and blue |-lots of metals in them|- have circular orbits

Question 9

what are the properties of population II stars and in general where do we find them?

Answer 9

tend to be found in the bulge of galaxies or halo|-they are old and red |-metal poor |-elliptical and titled orbits

Question 10

what are the properties of disks…

Answer 10

DISKS|-flattened systems that rotate |the stars and gas clouds have circular orbit, rotating about the disk axis |-star formation is ongoing; it can be fairly constant over the age of the galaxy (more disk=more star formation)|-age of the stars vary from now to age of the universe are formation on going |-10-50% dust clouds|-mainly population one stars

Question 11

what are the properties of bulges….

Answer 11

BULGES|-spherical systems with little or no rotation |-orbits of stars are randomly ordered and highly eccentric (stretched out)|-star formation largely complete- gas used up long ag|-mainly older population II star (as old as the galaxy)|- small amount of gas present|-overall structure is smooth-no clumpy areas

Question 12

What is the most abundant type of galaxy?

Answer 12

dwarf elliptical and dwarf irregular |they are not very luminous and not very massive

Question 13

describe the structure of the galaxy and define terms

Answer 13

diagram shows a spiral galaxy ||globular cluster: a spherical collection of stars that orbit a galactic core as a satellite.||Halo: nearly spherical volume of thinly scattered stars, globular clusters of stars and tenuous gas observed surrounding spiral galaxies.

Question 14

describe our galaxy

Answer 14

our galaxy is an SBc spiral galaxy |has a central bulge with population II stars and a disk with population I stars | |disk: 100,000 ly across, 2000 ly thick|bulge: 20,000 ly across contains nucleus of galaxy |Halo: 300,000 ly across contains orbiting globular clusters and dark matter|our galaxy has a supermassive black hole at its centre

Question 15

What is an active galactic nucleus, AGN?

Answer 15

most, perhaps all galaxies have a supermassive black hole at its centre.|if these are accompanied by accretion disks they reveal themselves as active galactic nuclei.||-release non thermal continuum (doesnt follow blackbody curve0|- have an emission line spectra

Question 16

What are quasars?

Answer 16

celestial object with emission spectra with a large redshift implying large distances and large intrinsic luminosity| -It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies

Question 17

describe what is meant by clusters of galaxies

Answer 17

galaxies tend to form in graviationally bound ensembles.|in these clusters, as the environment affects galaxy formation there tends to be fewer spiral galaxies than in the “field”|often have a giant elliptical galaxy at the centre

Question 18

What is dark matter?

Answer 18

matter in an unknown form, detectable by its gravitational effects on luminous matter

Question 19

What is the evidence for dark matter?

Answer 19

velocity curves from spinning galaxies |gravitational lensing |mass of the milky way

Question 20

How is velocity curves from spinning galaxies evidence for dark matter?

Answer 20

1. measure speed of stars in rotating galaxies and observe that stars in spinning galaxies were rotating at roughly the same velocity, no matter their distance to the galactic centre||2. this is a contradiction with keppler’s law which describes rotation of planets around the sun where a planet further from the sun rotates slower.|(should follow this GMm/r²=mv²/r| GM/r = v²|M is the mass that behaves as though centrally concentrated within r.)||3. stars seem as though they are not rotating around the visible centre of the galaxy but many unknown centres, all providing gravitational attraction||this could only happen if huge amounts of invisible matter exists

Question 21

How is gravitational lensing evidence for dark matter?

Answer 21

This is a method of detecting dark matter.||- Large concentrations of dark matter create gravitational fields strong enough to distort space.|- Light coming from a distant galaxy will bend when passing a massive clump of dark matter as shown |- the galaxy appears shifted, as if coming from two different places ||in 3-dimensions, all diverted light will form a ring- these techniques help to produce a map of the universe.

Question 22

How is the mass of the milky way evidence for dark matter?

Answer 22

Total mass in the disk of the Milky Way:|Approx. 200 billion|solar masses|Additional mass in an extended halo:|Total: Approx. 1000 billion solar masses|Most of the mass is not emitting any radiation:|–>Dark Matter!

Question 23

What are the potential candidates for dark matter?

Answer 23

Main candidate are WIMPs: an exotic subatomic particle- weakly interacting massive particle||other candidates are white and brown dwarfs or neutron stars and black holes but these dont account for total mass.

Question 24

How can we use cephid variables as standard candles to find large stellar distances .

Answer 24

1. We can use e.g. Hubble Space|Telescope to measure the distances to|very distant galaxies because it can|resolve individual stars. Then we can|find the Cepheid variables |2. Luminsoity = constant x Period of variability|3. Distance from luminsoity, L and Inverse square law of light

Question 25

What is hubbles law?

Answer 25

the further away a galaxy is the greater its redshift |V=H0×D|it seems to show that everything is moving away from everything on a cosmological scale (locally not true). |we should say that the space between galaxies is expanding not the galaxies themselves||hubble’s law implies the big bang as space had a begining

Question 26

What is the hot big bang theory?

Answer 26

the universe began at a finite time in the past, as an incredibly dense and hot “fireball” that has been expanding, and cooling, ever since.

Question 27

What evidence is there for the big bang theory?

Answer 27

THE EXPANSION OF THE UNIVERSE |if galaxies are moving away from each as described by V=H0×d (due to the expansion of space), there must have been a beginning, when everything concentrated to a single point- a singularity of infinite density. |-evidence from red shift of galaxies||COSMIC MICROWAVE BACKGROUND RADIATION|early universe was a very hot place and that as it expands, the gas within it cools. Thus the universe should be filled with radiation that is literally the remnant heat left over from the Big Bang, called the “cosmic microwave background||EVOLUTION OF THE UNIVERSE||ABUNDANCES OF THE LIGHTEST CHEMICAL ELEMENTS

Question 28

explain where cosmic microwave background radiation comes from?

Answer 28

1. initially the universe was fully ionised and opaque (due to electron scattering). Photons continually scattered, absorbed and re-emitted by fog of free electrons.||2. Eventually, the universe cooled sufficiently for (hydrogen) atoms to form (T = 3000K, t =100,000yr)||3. Radiation decoupled from matter, and has flowed through the universe ever since. We see this ‘last scattering surface’ redshifted, and it appears to us as a black body at T = 3K giving off microwave radiation.||4.to validate its cosmic origin, the radiation should be isotropic as if its from a hot primeval state, the istropy of the universe at that time should set the isotropy of the radiation.|-we find only small fluctuations in isotropy of the radiation so it is very isotropic.

Question 29

What assumptions do we make about the universe when|considering it on the largest of scales…|define the assumptions…..

Answer 29

Homogeneity: |on the largest scale the local universe has the same physical properties throughout the universe e.g. mass, density, expansion rate, visible vs dark matter.||isotropy:|on the largest scale, the local universe looks the same in any direction that one observes.||Universality :|The laws of physics are the same everywhere|in the universe

Question 30

What are the 3 models of the universe which define its fate….

Answer 30

closed universe|flat universe |open universe

Question 31

What is the critical density of the universe?

Answer 31

fate of the universe depends on the matter density in the universe.|there is a critical density at which is just enough to slow down the cosmic expansion to a halt at infinity.||we use a dimensionless density parameter Ω to compare actual density with with critical density

Question 32

What is the close model of the universe?

Answer 32

matter will stop universal expansion after a finite time, followed by a re-collapse and a big crunch||has a positive curvature k=+1

Question 33

what is the flat model of the universe?

Answer 33

the density of the universe is equal to the critical density so it is enough to halt the expansion at infinity.||has no curvature k=0

Question 34

what is the open model of the universe?

Answer 34

gravity will never stop the expansion ||has a negative curvature of k=-1

Question 35

how can we interpret the curvature of the universe?

Answer 35

Curvature has a simple but limited dynamical interpretation:|- IF only gravity matters; then positive geometry corresponds to a closed universe (recollapses), |- negative geometry corresponds to an open universe (expands forever)

Question 36

What did scientist use to determine about the true expansion of the universe and what was the result of this?

Answer 36

1. By observing type 1a supernovae (standard candle so can find distance) astronomers can measure the Hubble relation at large distances. ||2. we can calculate recession speeds and get an idea about the size and rate of expansion of the universe. ||3. It was expected that this would measure the deceleration of the universe but we found out the universe is accelerating!

Question 37

How do we explain the acceleration of the universe?

Answer 37

We use the cosmological constant ∧||∧ is a free parameter in Einstein’s fundamental|equation of general relativity|Energy corresponding to ∧ can account for|the missing mass/energy (E = mc²) needed to|produce a flat space-time. ||THIS IS DARK ENERGY

Question 38

How does the presence of dark energy and dark matter affect our calculations for Ω in defining the fate of the universe

Answer 38

We generalize Ω to include not just matter, but all forms of mass/energy:|Ω=p/p (critical)|Ω = ΩM + ΩΛ|where (matter) ΩM = ΩB + ΩDM|ΩΛ- is the density parameter for dark energy.|ΩB- is the density parameter for baryonic matter|||What is the geometry (i.e., what is Ωtotal?)