Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Exploring the Cosmos 1Y > Galaxies and Large Scale Structures > Flashcards

Galaxies and Large Scale Structures Flashcards

1
Q

What is the most suitable distance scale and why?

A
  • most conveniently given scale is the speed of light

- needed because of the large size of the galaxy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How fast does light travel?

A

3 x 10(8) metres per second

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why is it light seconds / minutes / years?

A
  • because the distance is expressed by giving the time required for the light to travel that distance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the parallax method?

A
  • as the Earth orbits the Sun, the apparent position of nearby stars seen against the background of very distant stars changes
  • the angular displacement of the star being investigated against this background can be measured
  • if this angle is not too small to measure accurately, it can give the distance to the star
  • if p is half of the angle measured, the distance to the star is proportional to 1/p
  • the distance can be expressed as 1/p where p is in seconds of arc
  • the resulting unit of distance is called the parsec
  • equal to approximately 3.3 light years
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are the basic features of the Milky Way galaxy?

A
  • it is basically disc shaped with a central bulge
  • the disc is about 25 kpc to 30 kpc
  • this is where new stars are being created
  • the stars and dust in the disc are referred to as the disc component of the galaxy
  • the disc has several arms and contains open star clusters, loose grouping of stars
  • there is also a significant amount of dust and gas in the disc
  • there is a central nuclear bulge with a diameter of about 6 kpc
  • the disc and nuclear bulge are surrounded by the halo
  • this has a spherical distribution of single stars and globular star clusters - called the spherical component of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Gas and Dust in the Galaxy

A
  • there are lots of this in the galaxy
  • dark patches = gas and dust
  • because of this, much of the galaxy, particularly in the plane of its disc, is hidden from observations using normal optical telescopes and visible wavelengths of light
  • we get a very incomplete picture of our galaxy if only optical telescopes are used
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What features can be seen by optical telescopes?

A
  1. MW - the plane of the galaxy but much is hidden & the arm structure is not obvious
  2. Globular Clusters - mainly away from the plane of the galaxy
  3. Open Clusters - nearby in the plane of the galaxy in the halo
  4. Star Associations - nearby in the plane of the galaxy
  5. Nebulae - emission, reflection and dark - nearby in the plane of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are Open Clusters?

A
  • found in the disc component of the galaxy
  • contain about 10 to 10,000 stars
  • very variable in size, typically about 25pc across
  • have an open appearance since the stars are not crowded together
  • loosely gravitationally bound
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What are Star Associations?

A
  • groups of stars that have been created recently
  • all have similar ages and velocities (proper motions)
  • not gravitationally bound unlike the star clusters described about but they all move in approximately the same direction
  • associations are part of the disc component of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are Globular Clusters?

A
  • typically contain 10(5) or 10(6) stars in a region that is only 10 to 40 pc across
  • have a crowded appearance because the high density of stars within the cluster
  • because of the closeness, they are quite tightly bound by gravity
  • have populations of stars that are known to be much older than stellar groups found in the galactic disk
  • have orbits with a random orientation that can line well out of the plane of the galaxy and are part of the spherical component of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the Milky Way?

A
  • OUR GALAXY!
  • our Sun and Solar system lie within it
  • the naked eye shows it in the night sky as an irregular luminescent bad that crosses the sky
  • it is formed by thousands of millions of distant stars
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How do we see the Milky Way clearly?

A
  • choose a moonless night far away from any street or other light pollution
  • allow time for the eyes to become properly dark adapted
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How do we explore our galaxy?

A
  • if we only use visible light by using optical telescopes is very limited because of the gas and dust in the galactic
  • fortunately visible light is only a small part of the EM spectrum!
  • many other parts of the spectrum at different wavelengths, at different wavelengths are now used
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the different types of EM spectrum and their different telescopes?

A

1) Radio waves by radio telescopes, eg Jodrell Bank and Aricebo
2) Infra-red radiation - IR telescopes in space and on Earth, eg UKRIT and Spitzer
3) X-rays - x-ray telescopes in space eg CHANDRA and ROSAT
4) Gamma rays - gamma ray telescopes in space eg Compton, Swift Gamma-Ray Burst Mission

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

RADIO TELESCOPES

  • how they investigate
  • important wavelengths
A
  • investigate the radio emission from the galaxy at many wavelengths but several are particularly important

> atomic H, 21cm wavelength radio waves
molecular H which radiates at 115 GHz
molecular CO with a characteristic signal at 2.6mm wavelength

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

INFRA RED TELESCOPES

  • how they investigate
  • examples
A
  • has a longer wavelength than visible light
  • can penetrate the dust that blocks visible light so that it can be used to see into the centre of our galaxy
  • also heated dust radiates strongly at some IR wavelengths and this gives a measure of the density of dust in the plane and centre of the galaxy

Earth-based telescopes eg UKRIT

Satellites eg COBE, Spitzer

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

X-RAY TELESCOPES

  • what they are
  • examples
A
  • these are emitted by very hot gases and pass easily through the dust

Satellites eg CHANDRA, ROSAT

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

GAMMA RAY TELESCOPES

  • what they are
  • examples
A
  • are emitted by unusual high events

Satellites eg Compton

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Using Visible Light to give us information about the nature of the galaxy

A
  • because of the dust and gas in the plane of the galaxy its only possible to see a few kpx across
  • BUT the O and B class stars are bright and hot and therefore easily seen
  • give indications of spiral structures since they are only found in the spiral arms of nearby galaxies and almost certainly in ours
  • O and B stars are very short lived and do not exist long enough to escape from the spiral arms where they are formed
  • they are in or close to the regions where star formation is taking place
  • these stars are often surrounded by ionised, glowing nebulae
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Using other parts of the spectrum to give us information about the nature of the galaxy

A
  • its now possible to explore the galaxy using wavelengths away from the visible the spectrum that can pass through the dust and gas more easily
  • visual, radio and IR telescopes have given us information about the spiral arm structure
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Describe the internal motion of our galaxy

A
  • the stars in he disc are moving in almost circular orbits about the galactic centre
  • have orbits in or close to the plane of the disc
  • often their motion has a small oscillation perpendicular to the plane of the disc so that periodically, they pass through the thin layer of dust and gas that lies inside the disc, a region called the inner disc
  • the Sun moves in the same way
  • our solar system is moving at about 220 km/s round the centre of the galaxy
  • it takes about 240 million years for one rotation about the centre
  • the Solar system is 4.5 thousand million years old
  • it has been round the galaxy 15 to 20 years
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Describe the differential motion of stars

A
  • at different distances from the centre, the stars of the disc are not moving at the same angular speed
  • there is differential rotation unlike all points on a turntable, which have an identical angular speed, ie increasing orbital speed towards the outer angle
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

How is it possible to estimate the mass of our galaxy?

A
  • we use the velocity of the Sun and the radius of its orbit
  • can calculate the mass of the galaxy inside the radius of the Sun’s orbit
  • this is the mass that generates the gravitational pull that keeps the Sun in its orbit
  • result = 10(11) solar masses
  • since there is mass outside the Sun’s orbit, we must concluded that the total galactic mass is significantly greater than this
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is the rotation curve?

A
  • in the general vicinity of the Sun, stars closer to the galactic centre move faster than the Sun and those further away go more slowly
  • the way in which the speeds of the disc stars change with the distance from
  • the edge of the disc of the galaxy is effectively at about 15 kpc from the centre since there are few stars beyond this distance
  • the rotation curve is flat or even increases further out
  • this would not occur if most of the mass of the galaxy was concentrated near it centre and indicates there is unseen mass out on the edges of the galaxy
  • this is the effect of dark matter, material that is not the type of matter we see in stars and planets
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

How can you measure the surface temperature of a star?

A
  • by looking at its colour and especially at the absorption lines in its spectrum caused by the presence of different chemical elements and sometimes in molecules in its outer layer
  • star with a high temperature = white or blue
  • cool star = orange or red
  • intermediate star = yellow, like our sun
  • at different temperatures, the constituents of the atmosphere are also different and this changes the absorption lines in the spectrum
  • the presence or absence of the different absorption and emission lines is a good measure of the surface temperature of a star
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is a Hertzsprung-Russell (HR) diagram?

A
  • obtained by plotting the luminosities of a set of stars against their surface temperatures (or spectral type).
  • stars do not lie in random parts of the HR diagram but in particular regions of it and can be classified from this
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What are the several main categories in the HR diagram?

A
  • Main sequence
  • Supergiants
  • Giants
  • White dwarfs
  • Red dwarfs
  • Brown dwarfs
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Describe main sequence stars

A
  • for most of their lives, stars are on the MS
  • this lies on a narrow band in the HR diagram
  • a MS star is classified by its temperature as O, B, A, F, G, K and M
  • O is the hottest & brightest whilst M is the coolest and least bright
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

How do stars get on the main sequence?

A
  • when a star forms and contracts and it heats up
  • the nuclear reactions start in its core and it moves on to the MS
  • remains here until the elements that give the nuclear reactions become used up
  • this happens fastest for the largest, brightest, hottest, bluest stars, O stars then B stars
  • at this stage, the star moves off the MS and usually expands to become a red giant or supergiant
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Main Sequence - Important Parts

A
  • the time on the MS depends on the size & brightness of the star
  • O (blue giant) stars followed by B stars are the brightest stars and spend a very short time on the MS
  • the smaller, cooler A, F and G type stars stay on the MS for much longer times
  • the dwarf K and M type stars live so long that our galaxy is not old enough for them to leave it
  • this allows the measurement of the ages of star clusters
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Measurement of Distance - Spectroscopic Parallax

A
  • if a star is eg on the MS, its spectral class can give its absolute magnitude, intrinsic brightness
  • this can be used to give its brightness
  • since its apparent brightness falls as 1/distance(2), a comparison of its apparent magnitude, measured directly, and the absolute magnitude from its spectrum, gives it distance
  • can be used to distances at which its spectrum and therefore its class can be measured
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What is the name of the process that creates elements in nuclear fusion reactions?

A

NUCLEOSYNTHESIS

  • produced by the nuclear fusion reactions in the interior of the stars
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Describe Nucleosynthesis reactions

A
  • these reactions generate the heat and light energy output of the star
  • heavy elements, or ‘metals’, produced in this way were scattered about the galaxy by stars that reached the end of their lives with novae or supernovae explosions
  • early in the history of a galaxy, stars were metal poor as there had been few supernovae and novae
  • suceeding generations of stars containing the heavier elements became more and more ‘metal rich’
  • the amount of metals in the outer layers of a star can be measured spectroscopically and gives an indication of when the star was formed
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What is metallicity?

A
  • the amount of heavy elements in a star
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Describe the abundance of elements in the Universe

A
  • significant amounts of the elements heavier than H, deuterium, He and Li found throughout the universe were created by stars, especially large stars
  • these elements have been scattered by the supernova explosions
  • have been building up steadily in our galaxy and elsewhere
  • explosions of several supernovae caused enormous jets of material, including the heavy chemical elements produced by the explosions
  • they rose above and below the plane of the disc & then fell back into the disc
  • in this way, the ‘metals’, heavy elements were distributed about the galaxy and over time more and more accumulated
  • as a consequence, as new star creation continued, the alter stars became increasingly ‘metal rich’
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

What are ‘metal poor’ stars?

A
  • stars formed just after the formation of our galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

What are ‘metal rich’ stars?

A
  • stars formed later after the formation of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

What are the two basic classes of stars in the galaxy?

A

Population I and Population II

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

Describe Population II stars

A
  • old and ‘metal poor’
  • formed early in the history of the galaxy from pure H with an admixture of primordial He with only small amounts of metals
  • since they are ‘metal poor’, they have relatively few absorption lines
  • many of these stars have completed their lives on the MS and left it
  • the most luminous of these stars are red giants
  • white dwarfs, stars near the end of their lives, are common
  • egs Type II Cepheid and RR Lyrae stars
  • found predominantly in the nucleus and the halo of the galaxy including the globular clusters that surround the disk of the galaxy
  • these regions often have an orange colour because there are no hot, blue O class stars present
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

Describe Population I stars

A
  • young stars with ages up to a few thousand million years that have formed fairly recently in the spiral arms and later than the Population II stars
  • young stars that haven’t had time to complete their lifetime on the MS so they still lie mostly here
  • created relatively recently, they are ‘metal rich’ and they have many absorption lines in their spectra
  • effectively they are 2nd generation stars formed partly from the debris of exploded population I stars
  • located in the disc as single stars, star associations or in open clusters
  • particularly concentrated in the interstellar dust of the spiral arms where new stars are continually beign formed
  • the very brightest Population I stars are not distributed at random, but are grouped in loose associations of several hundred stars that move with the general galactic rotation and are believed to have common origin
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

Describe the theory of Density Wave Formation

A
  • the spiral arms are seen as regions of compression that are dynamically stable
  • they are self perpetuating
  • the gas & dust orbiting in the galactic plane overtakes the slower moving arms compressed by the collision
  • this triggers the gravitational collapse of gas clouds and form this the new formation of stars
  • these include the very bright and short lived O & B stars that are the markers for the spiral arms
  • the remaining gas passes through the arms and emerges from the front with smaller, less bright stars
  • the very bright stars do not live long enough to move out of the arms
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

Describe the stages in Density Wave Formation

A
  • an orbiting gas cloud overtakes a spiral arm from behind
  • the impact of the gas cloud with the material in the arm compresses the gas cloud
  • this triggers the formation of stars
  • the massive, very bright O and B stars that are created are very luminous and light up the spiral arm
  • the bright O and B stars are short lived and fad quickly so that they do not live long enough to leave the arm
  • the low mass stars have longer lives and they pass out of the other side of the arm
  • however they are much less bright and are not so prominent
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

What are grand design galaxies?

A
  • the theory of density wave formation star creation suggests that the density wave mechanism should produce galaxies with only two, sharply defined spiral arms = grand design galaxies
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

What are self sustaining / self propagating star formations?

A
  • the creation of new stars itself causes further star formation
  • the formation of hot new stars can also trigger further new star formation due to the compression of surrounding gas and dust by the radiation pressure and flow of ions from new massive stars
  • also because the very bright and hot stars are very short lived, they explode violently as supernovae
  • this causes additional compression and further star formation before they move out of the high density parts of the cloud
  • can produce clumps of new stars with spiral shapes
  • differential rotation can drag the clump into a shape that represents a segment of a spiral arm
  • this can evolve into galaxies with branches and spurs as seen in some spiral galaxies, like ours, and in flocculent galaxies
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

NBC 3184

A
  • basically a grand design galaxy, but it has a few spurs with young clusters of hot bright stars
  • dark lanes of dust mark the inner edges of the arms
  • in this galaxy, the processes of density wave and self propagating star formation are occurring simultaneously
  • there are many other galaxies like this and there is a continuous distribution of different types from grand design to flocculant
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

How can we estimate the ages of stars?

A
  • when a star forms and contracts, all of its nuclear reactions start
  • these supply the star with its energy for heat and light
  • the star enters the MS at a position that depends on its mass and spectral class
  • stays here for most of its lifetime until the H fuel starts to run out
  • when this H does run out, the nuclear reactions change to those involving the fusion of He nuclei
  • the core is contracting, the outer parts expand and the star moves off the MS
  • becomes either a red giant or red supergiant depending on its mass
  • the time for this to happen depends on the mass of the star and therefore its spectral class
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

O and B stars and the Main Sequence

A
  • these are the brightest, hottest and most massive stars

- spend the shortest time period on the MS before they move off of it

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

Other types of stars and the Main Sequence

A
  • spend longer on the MS
  • move off of it later
  • this depends on their mass
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

What are the events that produce elements heavier than He?

A
  1. the formation of planetary nebulae
  2. Type II supernovae
  3. Type Ia supernovae
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

Planetary Nebulae - producing heavier than He elements

A
  • occur at the ends of the lives of stars with mass of 0.8 to 8/9 solar mass
  • the interiors of these stars are not sufficiently hot enough to produce much elements heavier than C and N, but they are still an important source
  • when the nuclear fuel at the core is exhausted, the core collapses to form a white dwarf and the outer layers with C & N are expelled into the galaxy and add to the heavy elements in the galaxy
  • this only happens when stars have reached the end of their energy producing life & takes about 35 to 25 million years from the time when they reach the MS
  • the biggest and brightest ones have the shortest lives
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

Type II Supernovae - producing heavier than He elements

A
  • the final stages of the lives of large, bright stars, typically O and B with masses greater than about 10 solar masses
  • the core can be hot enough to fuse elements up to Fe as part of the processes that generate their energy output
  • these elements form in layers within the star with Fe at the centre
  • at the end of the life of the star, when the heaviest element are being produced, the process is fast - the core collapses when it runs out of fuel!
  • the outer layers fall inwards and then they rebound due to the release of gravitational energy, as a type II supernovae
  • the core becomes a neutron star or a balck hole depending on its mass
  • lots of Fe trapped in the core but it is trapped and does not escape into the galaxy
  • outer layers get ejected and contain large amounts of O and Mg
  • usually about 2 to 50 million years after the formation of the star
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

Type Ia Supernovae - formation of elements heavier than He

A
  • occur when a white dwarf is part of a close binary star system and can capture additional material from its neighbour
  • if the mass of the white dwarf becomes large enough, it exceeds the maximum possible limit for its stability and collapses suddenly
  • the white dwarf still contains light elements that can act as nuclear fuel, eg C and O, if the pressure & temperature are high enough
  • these conditions occur in the collapse causing the explosive burning of C, O etc to create heavier elements such as Fe along with much energy
  • extremely violent event and the star is blown apart, scattering iron and other heavy elements into the galaxy
  • only happens after stars with initial masses of about 1 to 8 solar masses reach the white dwarf stage plus the time required to accrete the extra mass from the companion star
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

Injection of ‘metals’ into the galaxy

A
  • since the mechanisms for creating different elements and ejecting them into the galaxy are all different and take different times, a measurement can be taken of the ratios of these elements
  • a powerful method for determined when individual stars were created
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

What is the original and traditional theory for the formation and evolution of the galaxy?

A
  • started as a large cloud of gas
  • gravity pulled the gas together and turbulence within the gas cloud fragmented it into smaller clouds with random velocities
  • stars & stars clusters formed due to the gravitational contraction of these fragments - had random orbits about the centre of the cloud
  • metal poor as they were formed before significant amounts of heavy elements were created from supernovae
  • the turbulent part of its motion was damped out, leaving a more uniform rotation of the cloud
  • the gas & dust in the cloud began to collapse into a disc
  • the stars & star clusters originally formed were left behind with random orbits giving the halo and central bulge components of the galaxy
  • the contraction took several thousand million years
  • as time passed, the biggest, early stars reached the supernova stage and produced more heavy elements to be scattered in the galaxy so newly created stars were more ‘metal rich’
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

Predictions from the original and traditional theory for the formation and evolution of the Milky Way

A
  • globular clusters should have been formed over a period of about 10(9) years or slightly more from the start of the formation about 11x10(9) years ago as the gas and dust cloud was contracting
  • as a result, their stars should have similar ages and be older than the stars of the central bulge or of the disc
  • the first stars formed should be almost ‘metal free’ because there had not been any supernovae before to generate ‘metals’
  • as time passed, the constituent of newly created stars became increasingly ‘metal rich’
  • the outer globular clusters would have been formed first and be the most ‘metal poor’
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

What are the contradictions to the original, traditional theory for the evolution of the Milky Way?

A
  • the globular clusters do not all have the same age and some of the youngest ones are in the outer halo
  • some of the stars of the central bulge are older than those of the globular clusters
  • first stars formed should be almost ‘metal free’ - they are mainly found to be ‘metal poor’ rather than ‘metal free’
  • this means that there must have been large stars that had ended their lives and detonated as supernovae prior to the formation of the oldest stars now in the halo or globular clusters
  • stars within open clusters of the disc are significantly younger than those of globular clusters
  • when stars end their lives, the majority of them turn into white dwarfs
  • there are too few in the disc for it to be as old as the rest of the galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

What are some possible modifications to the theory of the evolution of the Milky Way?

A
  • the process did not involve a single large cloud of dust and gas, rather it started with a smaller cloud that formed the stars of the central bulge
  • there was a later accumulation of gas that was ‘metal enriched’ by earlier stars that had formed the halo
  • the disc formed later when the galaxy dust and gas flattened and as even more dust and gas fell into the galaxy and settled in the disc
  • perhaps entire small galaxies were captured by the Milky Way galaxy; this could explain the different ages of the globular clusters
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

In-falling H and continued accretion into the central disc

A
  • traditional explanation is that all the material for the creation of the stars came from the original cloud
  • this is not necessarily the case!
  • possible that significant amounts of H have also fallen into our galaxy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

Describe the Halo

A
  • has been found that the stars with the least Fe, ie the oldest stars, have the most eccentric orbits and are in the halo
  • the halo stars have a homogenous chemical make-up
  • this implies that they were created at about the time and in a similar location
  • in contrast, the chemical signatures of stars in nearby dwarf galaxies are not the same
  • therefore the contribution of mergers with another galaxy to the stars of the halo appears to be small although they are likely to have contributed some anomalous stars
  • the halo appears to have been created with a primordial, ‘metal poor’ gas cloud collapsed rapidly
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

Describe the Bulge

A
  • difficult to observe but some information has been gathered using IR telescopes
  • the ‘metal’ abundance of the bulge stars ranges widely but the bulge is dominated by old stars
  • the O : Fe in these stars implies that they formed at an early age, even before the Type Ia supernovae amongst them produced much Fe
  • it has been suggested that the bulge stars were mainly formed in a fast collapse
  • there are also young stars in the bulge
  • this can be explained by the bar of the galaxy pushing more gas into the bulge causing new star formation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

What is the G dwarf problem?

A
  • there is a problem in the tradiational model due to a shortage of low mass stars in the disc stars that are old and ‘metal poor’
  • this is called the “G dwarf” problem
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

Explain the G dwarf problem

A
  • type G stars are long lived and, if no new gas has been added to the disc since it was first formed, there should be many more old, metal poor, type G stars dating from the period soon after to the formation of the disc that have been formed
  • there are too few of this type for the traditional model to be correct
  • adding more gas into the galaxy after its formation can solve this
  • allows the creation of additional, new, ‘metal rich’ stars after the new gas mixes with the ‘metal rich’ material in the disc
  • the mechanism for this is not clear but there is evidence for more gas being added to the disc over a long period of about seven thousand million years and it may still be happening
  • also the nearby Andromeda Galaxy which is similar in form to ours, seems to be acquiring new gas clouds
63
Q

What are the two components of the disc of the Milky Way galaxy?

A

The thin disc and the thick disc

64
Q

Describe the thin disc

A
  • is about 200 to 400 pc thick
  • contains dust and gas and is the main region for new star formation
  • the stars in it have an Fe : O ratio that implies they were formed later than the thick disc
  • some data suggest that the inner part of the thin disc is older than the outer parts
65
Q

Describe the thick disc

A
  • has a thickness of 1000 to 1300 pc
  • the stars have a larger oscillatory motion above and below the plane of the disc than in the thin disc
  • tend to go round the galactic centre slightly more slowly
  • their chemical composition is intermediate between that of halo stars and the stars of the thin disc
  • they are older than the thin disc stars
  • some of them are as old as the halo
  • this suggests that the two components of the disc were formed by infalling gas at two distinct times and on different time scales
  • there is evidence to support this
66
Q

What is the theory of the galaxy and its two discs?

A

Phase 1
- a cloud of gas collapsed and in the process the stars of the halo and bulge were formed

Phase 2

  • the outer disc was formed and there was the generation of stars that are younger than those of the spherical component and the bulge
  • this is very similar to the traditional theory of the formation of our galaxy but it does not end here

Phase 3

  • infalling H gas occurred after a pause in which the rate of new star formation was much lower due to a lower availability of gas
  • this new gas mixed with the ‘metals’ already in the disc, formed the inner disc and allowed the resumption of star formation in the disc resulting in a generation of newer stars in the inner disc
  • this process may still be continuing
  • the absorption of dwarf galaxies has also taken place but these have contributed only a small fraction of the stars in our galaxy
67
Q

What is Interstellar Medium (ISM)?

A
  • the regions between the stars are not empty
  • they contain gas and dust
  • this ISM is the matter from which stars are created
  • it is the material that is responsible for the nebulae that are seen
  • best known example = the Orion nebula
68
Q

Features of ISM

A
  • the amount of gas is very much greater than that of dust
  • its total mass is about 100x that of the dust
  • the gas can be atoms, ions and molecules, depending on the temperature
  • the composition of the gas is similar to that of stars, ie mainly H
  • the gas can be detected by the absorption of starlight passing through it or by its radio-emissions
69
Q

What are the four basic types of gas in ISM?

A

1) Diffuse clouds
2) The intercloud medium
3) Dark giant molecular clouds
4) Very hot coronal gas

70
Q

Describe diffuse gas clouds

A
  • form about one fourth of the ISM mass
  • have a low density, 1 to 1000 atoms per cubic centimetre
  • are cool at 50 to 150K
  • contain mainly neutral, un-ionised hydrogen, called H I
  • the mass of clouds is typically one to a few solar masses
  • they can be seen as emission nebulae or reflection nebulae
71
Q

Describe the intercloud medium

A
  • forms about 50% of the ISM mass
  • it is warmer (temperatures about few 1000K to 10,000K)
  • lower density than the diffuse gas clouds
  • the density is about 1 atom per 100 cubic centimetres
  • contain mainly ionised hydrogen, called H II
  • the colder, diffuse clouds drift through this intercloud medium
72
Q

Describe giant molecular clouds

A
  • the regions of new star formation
  • form about one fourth of the ISM mass
  • are cold with a temperature of about 10K
  • atoms are bound into many different molecules
  • range from CO2, H20 to big organic molecules like benzene and ethanol
  • normally these clouds are opaque except where the light pressure & stellar wine from nearby stars make holds, eg in the Orion nebula
  • the dense clouds protect their molecules from UV light to nearby stars
  • this would otherwise destroy the molecules
  • they can be very large (15 to 60 pc across), with high densities in the range of 10(3) to 10(5) per cubic centimetre
  • mass is in the range of 100 to a million solar masses
73
Q

Describe very hot coronal gas

A
  • forms about 5% of the ISM mass
  • has a very low density with about 1 atoms per 1000 to 100,000 cubic centimetres
  • occupies a very large volume much larger than a galaxy
  • has a very high temperature, 100,000 to 1 million K
  • probably created by supernovae explosions or very hot stars
74
Q

What is the local bubble?

A
  • a region of hot coronal gas that the Solar System is inside of
  • probably the result of a nearby supernova explosion about one million years ago
75
Q

What is interstellar dust?

A
  • it is responsible for the blocking of light seen in the plane of the disc
  • there are far fewer dust particles than atoms or molecules of gas in interstellar medium
  • over short distances, the effects of the dust are negligible so that it would not be noticed but the distances inside a galaxy are so large that the dust can absorb a lot of light
  • for light coming from other parts of our galaxy, its effect can be very large so that it obscures much of the disc of our galaxy in observations using visible light
76
Q

What are nebulae?

A
  • they are regions of our galaxy where the presence of interstellar material can be seen visually, usually using telescopes although a few bright nebulae can be seen with the naked eye
77
Q

What are the three basic types of nebulae?

A
  1. Emission nebulae
  2. Reflection nebulae
  3. Dark nebulae
78
Q

Describe EMISSION nebulae

A

These glow with light emitted by excited atoms and ions

  • the excitation is usually caused by intense UV light from a nearby (or embedded) O-type star (luminous & hot)
  • the gas temperature is about 10,000K
  • most of the gas is ionised H (called H II) and the emitted light is mainly pink

Examples can be seen around the luminous type O stars in nearby galaxies & are seen around bright stars in our galaxy

Examples of these are the Trifid Nebula and the Lagoon Nebula

79
Q

Describe REFLECTION nebulae

A
  • these are due to the dust in the gas and dust clouds
  • light from nearby stars is reflected from very small particles of dust in the nebula
  • they are not due to light emitted by excited gas atoms
  • the light from nearby stars is not strong enough to do this
  • they appear as faint wispy blue regions around the stars whose light is being reflected
  • the mechanism that produces the blue colour is similar to the one that makes the sky to look blue

(blue wavelength light is more strongly scattered by small particles than longer wavelength light such as yellow and red)

80
Q

Describe DARK nebulae

A
  • these are also regions where cold interstellar material absorbs the light from stars behind them
  • these appear as dar regions in the sky, usually against the stars of the Milky Way
81
Q

Describe the nucleus of the Milky Way galaxy

A
  • cannot be examined with visible light
  • however information about it has come from radio, IR and x-ray observations
  • these are radio emissions from a source called Sagittarius A* in the galactic centre
  • extremely energetic
  • very little visible light is emitted from the centre of the galaxy reaches Earth due to the gas and dust in the disc
  • longer wavelengths of IR radiation means that it penetrates much more effectively and up to 1/10 of the IR radiation reaches us
  • modern IR telescopes can detect this radiation
  • they also show that the stars are crowded very closely together near the centre of the galaxy & they are moving very fast
  • this means that there must be a very large mass concentrated into a very small region
  • there must be a supermassive black hole in the centre with matter accreting into it from nearby stars
82
Q

Describe the x-ray images from the heart of our galaxy

A
  • CHANDRA has found an intense x-ray source that coincides with the radio position of the black-hole candidate, Sagittarius A*
  • there is also x-ray emission from an extended cloud of hot gas around the black-hole candidate
  • has been heated to a temperature of millions of degrees by supernova and perhaps by colliding winds from young massive stars
83
Q

Describe the radio telescope images of the galactic centre

A
  • reveal the centre of our Milky Way galaxy as a complex place
  • there are tortured clouds of gas energised by hot stars and round-shaped supernova remnants (SNRs)
  • some of the structures observed have still to be explained
  • this is a violent and energetic environment
84
Q

IR telescopes and the motion of stars round the galactic centre

A
  • have observed the motion of stars in the centre of our galaxy for more than 15 years
  • have seen stars moving at very high speeds very close to what can only be a black hole at the centre of our galaxy
  • the speeds of the stars can be in excess of 3 million miles per hour and the sizes of orbits can be as small as 10 light days or less
  • one may complete its orbit in about 15 years
  • if the stars are held in their orbits by gravity, the central object must be both very compact and very massive
  • its mass must be at least 2 million times that of our Sun and it must be less than a light-day across
85
Q

Other galaxies

A
  • until the early 1900’s, the MW was thought to be the whole Universe
  • around that time, observations were made of hazy, cloud-like ‘nebulae’ out in space
  • great debate was whether these objects were outside our galaxy or simply clouds within the MW
  • the techniques for measuring distances and the astronomical telescopes were not good enough at that time to resolve the matter
86
Q

Shapley vs Curtis

A
  • famous debate between the two
  • both had opposing views
  • Shapley ‘won’ the debate but Curtis was later shown to be correct
87
Q

What was Shapley’s argument?

A

the ‘local’ hypothesis

  • these objects were part of the Milky Way galaxy
88
Q

What was Curtis’ argument?

A
  • some people thought the Andromeda ‘nebula’ was a planetary system, like the Solar system
  • Curtis took the view that the nebulae were outside our galaxy and were other galaxies like ours
89
Q

Resolution of the Shapley and Curtis debate

A
  • resolved by Hubble
  • used the then new 100 inch telescope at the Lick Observatory
  • able to see individual stars in M31
  • he was able to see individual Cepheid variables and using these to measure their distance and proved that M31 was well beyond the edge of the MW galaxy
  • now called the Andromeda galaxy
  • about 2.3 million light years from us
  • quite similar to our own galaxy
90
Q

Andromeda Galaxy

A
  • has been examiend extensively
  • the CHANDRA x-ray telescope has given us information about the centre of M31
  • has an x-ray source that may indicated a black hole of a million or more solar masses
91
Q

How did Hubble categorise galaxies?

A
  • categorised them according to their shape
92
Q

What are the main types of galaxies?

A
  1. Ellipticals
  2. Spirals
  3. Barred Spirals
  • there are also irregular galaxies and S0 galaxies that do not fall into any of the categories
93
Q

What are the subcategories of elliptical galaxies?

A
  • range from spherical to very elongated
  • have no apparent arm structure

E0 - spherical to
E7 - very elongated (highly elliptical)

94
Q

What are the subcategories of spiral galaxies?

A

Sa - tight bound arms and large nucleus

Sb - intermediate

Sc - open, loosely wound arms and small nucleus

95
Q

What are the subcategories of barred spirals?

A
  • these are similar to spiral galaxies but with a bar in the nucleus

SBa - tightly wound barred spiral

SBb - intermediate

SBc - loosely wound barred spiral

96
Q

What are the most common types of galaxies?

A
  • barred spirals; about 20% of the observed galaxies
97
Q

What type of galaxy is our galaxy?

A

SBb

  • although its bar is small and it was once described as type Sb
98
Q

Describe S0 galaxies

A
  • have a disc shape with a central bulge that is similar to that of spiral galaxies
  • but they have no obvious arms
  • have almost no gas and dust in the disc and therefore have virtually no new star formation
  • because there are no arms, they have no subcategories
99
Q

Describe Irregular galaxies

A
  • have no well-defined shape
  • occur in many forms
  • no subcategories
  • anything that does not fall into the other categories is one of these galaxies
100
Q

Tuning Fork Diagrams

A
  • shows all of Hubble’s classifications of different shapes of galaxies
  • DOES NOT illustrate an evolutionary process
  • galaxies do not evolve from one of these forms to another
101
Q

What are the different mechanisms that can cause the odd shapes of irregular galaxies?

A
  • can be created when galaxies collide or come close to one another so that their gravitational forces interact and distort their shape
  • may be very young galaxies that have not yet reached a symmetrical state
  • in some irregular galaxies (M82) young stars eject energetic bubbles of gas, giving the galaxy a blobby experience
  • some apparently irregular galaxies may be normal spirals that are partially obscured by dust
102
Q

Describe the abundance of the galaxies

A
  • ellipticals are the most common
  • probably account for about 60% of all galaxies
  • the most listed galaxies are spirals
  • estimated about 20% of all galaxies are spirals
  • irregulars are also faint but are probably as common as spirals, though they only account for a few percent of catalogued galaxies
103
Q

What are starburst galaxies?

A
  • seen mainly in the IR
  • look as thought they are exploding
  • actually undergoing an intense period of star formation and hence shine brightly
104
Q

Ellipticals - SIZE

A

Size range from:

  • GIANT (a few times the size of the MW) to
  • DWARF (as small as a few % of the diameter of the MW)
105
Q

Ellipticals - BRIGHTNESS

A

Varies from:

  • a few times the brightness of the MW (giants)

TO

  • a few hundreds of thousands of times dimmer than the MW (dwarf)
106
Q

Ellipticals - MASS

A

Varies from:

  • 10x the mass of the Milky Way

TO

  • a few million solar masses (dwarf)
107
Q

Ellipticals - OTHER PROPERTIES

A
  • mainly red in colour
  • very little interstellar medium (ISM)
  • the 21cm radiation from neutral H is weak
  • little active star formation
  • most of their stars are population II
  • little or no overall pattern for the stars’ orbits - just random orbits
108
Q

Spirals - SIZE

A

Ranges from:

  • LARGE (up to a few times the size of the Milky Way)
  • SMALL (a few tenths of the diameter of the Milky Way)
109
Q

Spirals - MASS

A

Varies from:

  • a few times the mass of the Milky Way

TO

  • a few thousandths of the mass of the Milky Way
110
Q

Spirals - BRIGHTNESS

A

Varies from:

  • LARGE - a few times the brightness of the Milky Way

TO

  • SMALL - a few thousandths of times dimmer than the Milky Way
111
Q

Spirals - OTHER PROPERTIES

A
  • red in the centre and blue in the disk
  • contain both population II (halo) and population I (disk) components
  • have significant amounts of gas and dust in the disc
  • relatively strong 21cm line emission from neutral H
  • much active star formation
  • the disc stars move around the centre in circular or nearly circular orbits
  • the nucleus and halo stars have more elliptical and random orbits
  • there is a distinct Doppler shift of the 21cm radiation from a particular part of the galaxy due o the H in the disc orbiting the central nucleus
112
Q

What is the inverse square law?

A
  • the apparent brightness of a star depends on the inverse of its distance
  • if the distance is doubled, its apparent brightness falls to a quarter of its previous brightness
  • if the distance is trebled the fraction is one ninth
  • if the distance is halved, it becomes four times brighter
  • for an object with known intrinsic brightness and (measured) apparent brightness, we can use the inverse square law to determine its distance
  • absorption of the light by dust etc in the interstellar & intergalactic medium can dim the light reaching Earth and this may require a correction
113
Q

What is a standard candle?

A
  • astrophysical objects (eg variable stars or supernovae) which have known luminosity due to some characteristic quality possessed by the entire class of objects
  • effectively, this is any objects whose intrinsic brightness can be measured by a method that does not depend on knowing its distance
  • can only be used if they are calibrated by finding the intrinsic brightness of one or more examples
  • this means that the distance to these examples must be measured using another, independent distance measuring technique
114
Q

What are the problems with using standard candles?

A
  • distances inside our galaxy and beyond can be measured if we find suitable standard candles
  • the main problem is the identification and calibration of standard candles that can allow the measurement of distances not just to the edge of our galaxy or even to other galaxies but right out to the edge of the observed universe
  • apart from having a known intrinsic brightness, if a standard candle is to be useful, it must be bright enough to be seen and measured across the vast distances involved
115
Q

How do we measure distances in our galaxy then?

A
  • we have to start inside our galaxy & work outwards by using a succession of methods which are effect up to larger and larger distances
  • this succession of method is often referred to as the cosmic distance ladder
116
Q

What are the methods on the cosmic distance ladder?

A
  • Radar ranging
  • Parallax
  • Spectroscopic parallax (main sequence fitting)
  • Cepheid variables
  • Tully-Fisher relation
  • Type 1A supernovae
  • Hubble’s law
117
Q

Describe Cepheid Variables

A
  • stars whose brightness varies with a period that depends on their average intrinsic brightness
  • relatively young giant stars with masses between 5x and 20x times that of the Sun
  • instability causes their radii to expand & contract making their brightness fluctuate
  • fortunately, because they are large, they are bright enough to be easily seen & they provide the next rung in the distance ladder to greater distances
118
Q

Describe the Tully-Fisher relation

A
  • links the rotational speed of a galaxy with its total intrinsic brightness
  • the correlation between the two is surprisingly strong & precise
  • it allows a measurement of the intrinsic brightness of the galaxy if the rotational speed is measured
  • this can be achieved by using the Doppler effect of the motion on light or radio signals with well defined frequencies emitted by stars or gas in the galaxy as an emission line
  • the motions has the effect of broadening range of frequencies & this can be measured
  • the 21cm line emitted by neutral hydrogen is particularly
  • once the rotational speed has been measured from the broadening of the line, the T-F relations gives the total intrinsic brightness of the galaxy
  • once this is known, measurement of its apparent brightness & the inverse square law give the distance of the galaxy in the usual way
119
Q

What is the doppler effect?

A
  • the speed of approach or recession of a galaxy or any other object emitting a spectrum with lines can be measured by the amount that the lines are displaced to shorter or longer wavelengths respectively
  • these displacements are referred to as a blue or a red shift, respectively
  • the doppler effect tells the observer if the objet is approaching r receding and, from the size of the shift, how rapid it is doing this
120
Q

What are the two shifts associated with the doppler effect?

A

RED and BLUE!

121
Q

What is a blue shift?

A
  • if an object is moving towards an observer, any light, radio signal etc, from it appear to have their wavelengths shortened
  • light becomes bluer
122
Q

What is a red shift?

A
  • if an object is moving away from an observer, any light, radio signal etc, appears to have their wavelengths increased
  • light becomes redder
123
Q

Describe Type 1a Supernovae

A
  • caused by the collapse of a white dwarf in a binary star system due to accretion of mass from its companion star
  • occur quite readily
  • since the white dwarf collapsed when it reaches a certain mass, this type of supernovae all exhibit approximately the same intrinsic brightness and, as a result, they can be used a standard candle
124
Q

How do use Hubble’s law?

A
  • if the red shift of the spectrum of a distant galaxy is measured, Hubble’s law allows an estimate of its distance
  • the most distant galaxies most rapidly
  • it shows that our universe is expanding!
125
Q

What happens when galaxies and stars collide?

A
  • when two galaxies meet one another, the stars interact due to gravitation
  • this can cause enormous distortion & disruption of the galaxies
  • the dust & gas of the discs do collide
  • the resulting turbulence & instabilities can initiate intense generation of new stars
  • this is the cause of the ‘starburst galaxies’
126
Q

Describe the tidal disruption of galaxies

A
  • a galaxy is held together by the gravitational attraction of the stars and other matter within it
  • if another galaxy comes close it, the gravitational pull between the two galaxies acts on one another but they do not act evenly
  • since gravity falls off with distance, while both galaxies are pulled towards one another, the pull is not the same across across either galaxy
  • the nearest parts are pulled more than the centre and the parts furthest away are pulled least and tend to be left behind in long tails - this is called tidal disruption
127
Q

What is galactic cannibalism?

A
  • when a small galaxy merges with a larger one, it will be pulled apart slowly and its stars will spread through the larger galaxy
  • effectively, the large galaxy eats the smaller one
128
Q

How do we normally see galaxies?

A
  • in clusters, single galaxies are rare
  • most are in clusters of galaxies with numbers that range from a few to a few thousand in regions that are one to ten Mpc across
129
Q

What are the two rough categories of clusters?

A

RICH and POOR clusters

130
Q

What are rich clusters?

A
  • contain a thousand or more galaxies in a region that is roughly 3 Mc across
  • have a concentration of galaxies at their centre
  • often contain one or more giant elliptical galaxies, like M87
  • collisions are likely to occur frequently
131
Q

What are poor clusters?

A
  • contain less than a thousand and sometimes only a few galaxies
  • collisions are less common
  • these clusters contain a higher proportion of spiral galaxies
132
Q

Describe the Local Group

A
  • a poor cluster that our Milky Way galaxy is a member of
  • consists of more than 35 galaxies scattered irregularly in a region about 1 Mpc in diameter
  • the total number is not certain because of the dust in the disc of our galaxy obscures some regions
133
Q

What are super-clusters?

A
  • the clusters of galaxies are not random across the Universe and there are groupings on an even larger scale than for clusters
  • do not have a random distribution
  • linked by a set of filaments and walls of groupings of super-clusters separated by sparsely by sparsely populated voids
134
Q

Describe the origin of spiral galaxies

A
  • possibly formed in near isolation
  • likely to have avoided any serious encounter with another galaxy since their delicate structure is easily destroyed
  • loss of their gas & dust in collisions would stop the process of stellar formation
  • likely that they did not form as in the traditional theory for the formation of the Milky Way since there are some observed features that aren’t explained by this theory
135
Q

Elliptical galaxies - first theory of formation

A
  • similar to the formation of the nucleus of our galaxy
  • the rotation of the clouds from which they formed might have been less than for the spiral galaxies so that the contraction was faster
  • the stars formed more quickly leaving very little ISM so that only a minimal disk formed from the settling of the gas into a disc shape
136
Q

Elliptical galaxies - second theory of formation

A
  • formed from earlier spirals that collided
  • the rotations of the colliding spirals ‘cancelled out’ to leave a large non-rotating elliptical
  • the ISM was consumed by very active star formation as can be seen, for example in starburst galaxies
  • may also have been ejected from the galaxies by the supernovae explosions that marked he end of the short lived O & B stars that would have dominated the early stages of the galaxies
  • a few collisions & mergers would eliminate almost of the ISM so that formation of new stars died away & it was not possible to form a disc - this would explain the tenuous hot gas detected in the galactic clusters
  • seen to be more likely since galaxy mergers & collisions were and still are probably very common
137
Q

What are active galaxies?

A
  • a subclass of galaxies which are abnormally bright & emit tremendous amounts of energy throughout the spectrum ranging from the radio to X-ray regions
138
Q

What are the different types of active galaxies?

A
  1. Seyfert galaxies
  2. Radio galaxies
  3. Quasars
139
Q

Describe Seyfert galaxies

A
  • 1940s/1950s - some unusual spiral galaxies were detected
  • they had small but very luminous nuclei
  • spectra showed broad emission lines form highly ionised atoms
  • these emission lines form highly ionised atom imply the presence of hot, low density gas
  • broad emission lines suggest that the atoms are moving very fast so the Doppler effect is smearing out the wavelengths of the photons being emitted
  • example of this type of galaxy = NGC 1566
140
Q

What are the different types of Seyfert galaxies?

A

Type 1 and Type 2

141
Q

Describe Type 1 Seyfert galaxies

A
  • very strong emitters of X-ray and UV radiation
  • have very broad emission lines from the atoms in them
  • this means that the speeds of stars & other matter in their centre is much higher than in normal galaxies (30x?)
142
Q

Seyfert galaxies

A
  • spirals or irregular galaxies with an active galactic nucleus
  • make up about 10% of all galaxies
  • two main types: Type 1 & 2
143
Q

Describe Type 2 galaxies

A
  • particularly strong emitters of infra red radiation

- have narrower emission lines but they still are broader than those from a normal galaxy

144
Q

Describe double lobed radio sources

A
  • radio galaxies are often flanked by double lobed radio sources
  • radio lobes are objects that emit radio signals from two neighbouring regions of the sky
  • optical telescopes have found that there is a galaxy between them
  • the radio sources seem to be due to two jets being ejected in opposite directions from the galaxy
145
Q

Cygnus A

A
  • the lobes on either side are produced by jets from the galaxy between them
  • main source of the signal from the lobes seems to be synchrotron radiation
  • this is caused by high speed electrons moving in a magnetic field
  • this gives them a circular motion and because of this motion, they emit radio waves with the same frequency as their rotation
  • magnetic fields are quite weak - however the lobes are so enormous that they contain much energy
146
Q

Centaurus A, NGC 5182

A
  • another strong radio source with two lobes
  • optically, it appears to be a giant elliptical galaxy colliding with a dusty spiral galaxy
  • strong radio signals come from the two lobes which, if they could be seen, would appear to be 10x the size of the full moon
  • radio & X-ray images show a jet of high energy particles pointing into the northern lobe
  • an infra red image of the centre of the galaxy shows a small disc of bright, hot gas surrounding the nucleus
147
Q

What is the twin exhaust model?

A
  • the radio lobes appear to be inflated by jets of gas & charged particles emitted in opposite directions from the nucleus
  • this is explained by this model
  • the radio waves emitted & the presence of radio hot-spots support this model too
  • some active galaxies have twisted jets that suggest that they have been emitted from a moving and/or rotating galactic nucleus
  • double jets are found in other structures such as proto-stars or neutron stars
  • in these, there is a rotating disc of accreting matter falling into a strong gravitational field
  • all this is consistent with the belief that there is a massive black hole at the centre of these galaxies
148
Q

3C129

A
  • this galaxy appears to be moving through the intergalactic medium & precessing
  • this causes kinks in the jets
  • clearly, the inter-galactic medium interacts with and slows down the jet
149
Q

What are the implications of the twin exhaust model?

A
  • the lobes seen on opposite sides of many active galaxies seem to be caused by two jets coming from the active nucleus
  • these jets bore through the ISM out of the galaxy into the intergalactic medium and inflate hot cavities in the ISM on both sides of the galaxy
  • these cavities are seen by radio telescopes as lobes & there are hot spots where they strike the ISM
  • the effects of gravitational & magnetic fields on the jets are too weak to confine them but the intergalactic medium does eventually contain them
  • the material cools & mixes with the intergalactic medium but there is always more matter flowing out in the jets to sustain the lobes
  • the jets emit energy in radio & x-ray wavelengths and are the brightest in the direction of the jet
  • the speed of matter in the jets can reach a large fraction of the speed of light
150
Q

What is the strongest evidence for black holes?

A
  • the strongest evidence of existence of black holes at the centre of active galaxies comes from the observable properties of orbiting matter
  • theory predicts that matter is attracted gravitationally by the black hole and should form a rapidly rotating accretion disc around it
  • if there is a very large mass at the centre, perhaps several times (a thousand million) solar masses, its effect can be measured by analysing the motion of stars, gas etc round it
  • in the presence of a black hole, the speed of the orbiting matter should be very large
  • the observation of small orbits requires that the central mass must be extremely compact & concentrated in a very small region
151
Q

What are the properties of a galaxy with a massive black hole at the centre?

A

1) the high speeds & small orbits of stars and other matters require the gravitational field from a very massive & compact object - a massive black hole will give this
2) if the central object is a black hole, it must be small in order for its brightness to change on a short time scale - direct observation support this
3) it is not known how a black hole causes the jets in active galaxies but similar effects are seen elsewhere from new stars that are forming and from neutron stars

152
Q

Describe the giant elliptical galaxy, M87

A
  • has interesting properties that support the theory for galaxies with massive black holes
  • it has a small, very bright nucleus that is only about one light week in diameter
  • a jet of ionised gas is moving out from the nucleus at high speed
  • the nucleus is at the centre of a spinning disc of gas
153
Q

What are the observed properties of the centre of active galaxies?

A

1) the matter in the accretion disc is very hot near the black hole
2) theory predicts that the disc is thickened close to the black hole which may be hidden inside a deep well
3) the outer part of the accretion disc is thickened into a torus of dusty gas
4) the inner disc is the source of the jets that are often seen
5) the jet may not be exactly at right angles to the disc

154
Q

Describe the formation of jets

A
  • the mechanism that creates jets is not clearly understood
  • however it seems to involve the flow of matter into the black hole
  • as it spirals in, it speeds up into an accretion disc that is fastest nearest the centre
  • magnetic fields get trapped in it & become tightly wrapped into spirals that eject jets at high speeds
  • these speeds can be very high & can approach the speed of light
  • much of the energy is emitted along these jets & is very directional

Exploring the Cosmos 1Y (4 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions