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Stars and Galaxies > Galaxies > Flashcards

Galaxies Flashcards

1
Q

What is a galaxy?

A

A gravitationally bound group of stars

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2
Q

What are the main features of a spiral galaxy?

A

Rotating disc, dominated by spiral arms, which contains population l stars, gas, dust and form stars.
Elliptical concentration of stars at the centre, called the bulge, which is red population ll and old population l.
Galactic halo.
Some have a bar.

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3
Q

What are the main features of an elliptical galaxy?

A 
Population ll and old population l stars
Smooth variation in intensity
Very little gas and dust
No organised rotation
Come in dwarf and giant forms
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4
Q

What are the main features of an irregular galaxy?

A

No regular structure, plenty of gas and dust and blue stars, mix of population ll and l, relatively small

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5
Q

What are Sa galaxy classified by?

A

Tight arms, large bulge, small disc, little gas

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6
Q

What are Sc galaxy classified by?

A

Loose arms, small bulge, large disc, lots of gas and dust

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7
Q

What are S0 galaxies?

A

Mix between E7 and Sa/SBa. They have both elliptical and spiral components. They have a disc, bulge as well as elliptical component. It has a little gas but no spiral arms

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8
Q

How can we view the plane of our galaxy?

A

Optical light from the shows the plane, although dust lanes obscure much of the structure. Infrared is needed to show true structure.

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9
Q

Where is our position in the Milky Way?

A

8kpc from the centre, with a disc of radius 20kpc

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10
Q

How large is the bulge of the Milky Way?

A

1kpc

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11
Q

How large is the bar of the Milky Way?

A

4kpc

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12
Q

What is the spiral arm structure?

A

It is determined using tracers of massive star formation with known distances, as well as HII and CO clouds being used.
We have 2 stellar arms, and 4 star forming gaseous arms

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13
Q

What is the velocity of stars in the disc?

A

220km/s

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14
Q

What is the wind up problem?

A

Stars travel at the same speed so there is differential rotation. If the spiral arms were a fixed pattern, the arms would wind up in a few revolutions

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15
Q

What is the solution to the wind up problem?

A

The spiral arms are modeled as a density wave pattern that rotates at a slower speed, so stars and gas pass in and out of the arm. In the arm the gas gets compressed and molecular clouds form with subsequent star formation.

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16
Q

How can you tell where the spiral arms are?

A

They are dust lanes, then molecular clouds, then blue stars

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17
Q

What is the galactic halo?

A

A spherical halo around the plane of the galaxy, consisting of globular clusters and halo stars. The radius is 100kpc.

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18
Q

In what order did the Milky Way form?

A

The galactic halo formed out of metal poor material, where there was an initial burst of star formation and none since. The disc formed later, where continuous star formation lead to a metal rich population.
Bulge also has some metal rich stars as a result of mergers.
The globular clusters follow different paths, not the same rotation as the disc.

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19
Q

What happened when two large galaxies collide?

A

The galaxies get completely disrupted. Large tidal tails can form as the galaxies orbit each other in close proximity.
If both galaxies contain gas, then this gets shocked and compressed which results in a burst of star formation.
The tidal tails can just contain Hl regions not stars so need 21cm.

20
Q

What are superwinds?

A

In starburst galaxies, there can be galactic superwinds, when large numbers of massive stars undergo supernovae explosions which blow out all the gas in the galaxy. This can remove all gas from a galaxy, stopping future star formation, leading to the formation of an elliptical galaxy.

21
Q

What are the features of an active galaxy?

A

They have a luminous point like nucleus that looks like a star. Quasars completely outshine the rest of the galaxy. It has a non-thermal continuum spectrum, and emission lines, compared to a blackbody spectrum and absorption lines of a star. Some emission lines are very broad due to very fast Doppler motion. It is strong in the radiowaves.
Most active galaxies show interaction with other galaxies.

22
Q

How can the size of the region of an active galaxy be limited?

A

The active galaxies vary in luminosity. The variability timescale multiplied by the speed of light, the fastest communication, gives the smallest size.

23
Q

How is the high luminosity of a quasar explained?

A

The release of gravitational potential energy of material falling onto a supermassive black hole

24
Q

What is the total energy released from a supermassive blackhole?

A

E=GMm/R
R is found by equating gravitational potential energy to kinetic energy.
L=0.5emc^2

25
Q

How are supermassive black holes revealed in nearby galaxies?

A

The velocities of the stars in the galaxy is taken. The fast motion near the centre reveals the Black holes mass

26
Q

What is a good standard candle?

A

Bright, easily recognisable, and well calibrated

27
Q

How are distances determined from cepheid variable stars?

A

In some stars evolution, their luminosity varies periodically. There is a good correlation between period and luminosity, so the absolute magnitude can be determined. This relationship changes with population l or ll. Intrinsic colours are also well known, so extinction can be corrected for.

28
Q

How are distances determined from Type 1a supernovae?

A

This supernovae is a controlled explosion, as well being extremely bright so can be seen to much larger distances. The brightness at maximum is shown to be well understood. Many galaxies need to be monitored nightly, as the build to maximum takes only a few days.

29
Q

What is redshift?

A

z=delta wavelength/ rest wavelength

30
Q

What is radial velocity?

A

redshift x speed of light

31
Q

What is Hubbles law?

A

The further away the galaxy, the greater radial velocity

32
Q

How can Hubbles constant be determined?

A

The velocity is revealed through redshift. The distances must be determined through standard candles.

33
Q

What is the explanation for Hubbles law?

A

Space is expanding uniformly

34
Q

How can the age of the universe be determined?

A

t=1/H

35
Q

What is the big bang?

A

The age of the universe is the time since all matter was at one point, in a black hole

36
Q

What is lookback time?

A

When we look at distant galaxies, we see them as the light was emitted

37
Q

What is the local group?

A

The milky way, andromeda and other dwarf galaxies make up the local group

38
Q

What galaxy groups?

A

a few tens of members, with a mixture of all galaxy types, with no regular structure

39
Q

What is a galaxy cluster?

A

thousands of members, containing mainly ellipticals and S0 types. It is symmetrical due to gravity and centrally concentrated with a giant elliptical at the centre.

40
Q

What causes the giant elliptical at the centre of a galaxy cluster?

A

Galaxies are relatively close together, so mergers are common.

41
Q

What are superclusters?

A

They are clusters of galaxy clusters, containing 10s of clusters

42
Q

What are voids?

A

As well as concentrations of galaxies there are large regions of empty space, as the gravity caused by the galaxies pulls in more galaxies.

43
Q

What are peculiar motions?

A

Galaxies rotate around their mutual gravitational attraction. This causes the redshift to be different than that predicted by Hubble expansion. These velocities can be found by equating the kinetic energy to the gravitational potential energy of the clusters mass.

44
Q

What is the evidence for dark matter in galaxies?

A

The stars do not rotate under Keplerian motion, where the velocity would decrease as the radius. However the velocity of stars remains constant, and the velocity is the same in the HI regions further past the stars. Therefore more mass is required for the stars to rotate at the same speed.

45
Q

What is the evidence for dark matter in galaxies clusters?

A

The peculiar velocity of the galaxy can be used to find the mass of the cluster. This is greater than the visible mass.

46
Q

What is gravitational lensing?

A

Light from a distant galaxy is bent around a galaxy or galaxy cluster due to its mass. This produces multiple images of the object.

47
Q

How is the mass of the lensing object given when an Einstein ring occurs?

A

theta = (4GM/dc^2)^0.5

Stars and Galaxies (5 decks)

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