Star Remnants, Milky Way, Cosmology

Question 1

What is a neutron star? How are they formed?

Answer 1

A neutron star is an incredibly dense sphere composed primarily of neutrons. During the core collapse of massive stars protons and electrons combine to form neutrons.

Question 2

What is the maximum mass of a neutron star?

Answer 2

2-3 M

Question 3

What are pulsars?

Answer 3

Fast spinning neutron stars.

Question 4

Give two reasons why pulsars cannot be white dwarfs.

Answer 4

White dwarfs would be torn apart by such fast rotation.
Pulsars could be found in supernova explosion where no white dwarf can remain.

Question 5

T or F. All neutron stars are pulsars.

Answer 5

False.

Question 6

What are X-ray bursters?

Answer 6

Equivalent to novae for neutron stars.

Question 7

What are black holes?

Answer 7

Black holes are the remains of explosions from very massive stars where neither light or matter can escape its gravitational force. This happens when the remaining core is too big to form a neutron star.

Question 8

T or F. A 5M black hole comes from a 5M star.

Answer 8

False. A much bigger star would be needed.

Question 9

Why does no light escape from a black hole?

Answer 9

The escape velocity exceeds the speed of light.

Question 10

What is the schwarzschild radius?

Answer 10

Is the radius of a sphere around black hole from within no light can escape.

Question 11

What is event hoizon?

Answer 11

A space-time boundary around a black hole from within which nothing can escape.

Question 12

What is singularity?

Answer 12

The centers of a black hole, where density, gravity and the curvature of space-time are infinite.

Question 13

T or F. When a star becomes a black hole, its gravitational pull becomes stronger.

Answer 13

False.

Question 14

T or F. The larger a black hole the weaker the gravity near its Schwarzschild radius.

Answer 14

True.

The singularity itself is further away the bigger the black hole.

Question 15

What is gravitational lensing?

Answer 15

Light coming from behind the black hole will curve around the black hole, distorting our view of objects behind the black hole.

Question 16

What is gravitational redshift?

Answer 16

From an observer point of view, matter falling into a black hole gets redder as it gets closer to the event horizon.

Question 17

What is gravitational time dilation?

Answer 17

From an observer point of view, as an object gets closer to the event horizon of a black hole, time seems to slow down. So much that it will take an infinite amount of time to fall inside the black hole.

Question 18

What are the two types of gamma ray burst?

Answer 18

Long duration: Collapse of very massive stars. Type 1c supernovae.

Short duration: Merging of neutron stars or black holes.

Question 19

T or F. Gamma ray bursts are the most powerful events in the universe.

Answer 19

True.

Question 20

T or F. Gamma ray bursts are the most distant sources in the Universe that we have detected.

Answer 20

True.

Question 21

T or F. All GBR have detectable optical afterglows.

Answer 21

False.

Question 22

What are gravitational waves?

Answer 22

They cause the space-time continuum to stretch and squeeze.

Question 23

How far away from the galactic center of the milky way is the sun?

Answer 23

8 kpc

Question 24

What is the diameter of the milky way?

Answer 24

50 kpc

Question 25

What is the distribution of young vs old stars in the milky way?

Answer 25

More young stars are in the spiral arms, old stars are evenly distributed.

Question 26

How can we map the arms of the milky way?

Answer 26

Using radio observations we study how stars move around the galaxy. Because they are moving they will show Doppler shifts, so we can tell whether they are moving toward or away from us.

Question 27

How many major arms does the Milky Way have?

Answer 27

4

Question 28

What is the evidence for dark matter?

Answer 28

The orbital speed of stars and gas around the galactic center is nearly uniform throughout most of our Galaxy. If the Sun and stars obeyed Kepler's third law, the orbital speed would be less for stars in larger orbits. As this does not happen there must be a large amount of mass in the galaxy that cannot be detected.

Question 29

How much of the milky way is theorized to be dark matter?

Answer 29

Almost 90% Extends 2-4 times the size of the halo.

Question 30

We expect the rotation curve of our Galaxy to decrease rapidly after the approx visible edge, what actually happens?

Answer 30

It increases.

Question 31

T or F. The black hole at the center of our galaxy is currently in a quiet state.

Answer 31

True.

Question 32

What is the name for a galaxy with an active supermassive blackhole?

Answer 32

Active Galactic Nuclei (AGN's)

Question 33

Why is it important the Solar System is a large distance from the center of the galaxy?

Answer 33

Gamma ray emissions from the supermassive black hole at the centre of the Milky Way have likely led to very high radiation levels near the centre of the galaxy.

Question 34

T or F. Spiral galaxies evolve into elliptical galaxies.

Answer 34

False.

Question 35

Define spiral galaxies.

Answer 35

Spiral arms + large central bulges High star formation rates.

Question 36

Distinguish between Sa, Sb, Sc.

Answer 36

Sa = 4% gas and dust = more tightly wrapped arms Sb = 8% Sc = 25% = less tightly wrapped arms

Question 37

Define barred spirals.

Answer 37

Spiral Arms Size of the bar is greater than the bulge. High Star formation rates.

Question 38

What type of galaxy is the milky way?

Answer 38

Barred spiral.

Question 39

Define elliptical galaxies.

Answer 39

Lack of star formation - little dust No spiral arms Majority old stars

Question 40

Distinguish between E0, E3 and E6.

Answer 40

Only difference is perception from Earth.

Question 41

Define lenticulars.

Answer 41

Have bulges and disks like spiral galaxies. They do not have spiral arms.

Question 42

For each type of galaxy state its primary star type in terms of population.

Answer 42

- Spirals: Spiral Arms: Population 1 Nucleus and Disk: Population 2 and Old Population 1 - Elliptical: Population 2 and Old Population 1 - Irregular: Population 1

Question 43

What is produced by gravitational lensing when a distant blue galaxy is almost entirely behind the gravitational lens?

Answer 43

An Einstein ring

Question 44

What can gravitational lensing tell us about dark matter?

Answer 44

We can measure the total mass of the overall object that has created the effect, dark matter is needed to account for this total mass.

Question 45

Give redshift formula.

Answer 45

z = (lambda observed - lambda) / lambda

Question 46

State Hubble's law.

Answer 46

The more distant a galaxy the greater its redshift and the more rapidly it is receding from us. H0 = velocity / distance Age of the universe = 1 / H0

Question 47

Give approx H0 and hence age of universe.

Answer 47

H0 = 73 km/s/Mpc 13.7 billion years old

Question 48

T or F. As the universe expands galaxies move away from each other through empty space.

Answer 48

False.

Question 49

T or F. As space expands it carries galaxies along with it.

Answer 49

True.

Question 50

T or F. Light from stars is doppler shifted by their movement, but the wavelength then remains constant as it travels to Earth.

Answer 50

False.

Question 51

T or F. As the universe is expanding, galaxies that were in a cluster are more spread out then they were billions of years ago.

Answer 51

False. Gravitational attraction binds the cluster together locally as the universe expands.

Question 52

What is the Cosmic Microwave Background Radiation?

Answer 52

Isotropic emission in all directions believed to be an echo of the Big Bang. Origin of the universe.

Question 53

To what temperature has the universe cooled.

Answer 53

2.725K

Question 54

What does a dark area/bright area mean on CMB?

Answer 54

Dark areas: cooler regions where the early Universe was more dense than average. Bright areas: warmer regions where the early Universe was less dense than average.

Question 55

What is the problem with the hubble constant?

Answer 55

The constant obtained by CMB does not match than of SN type 1a.

Question 56

What is sigma 0?

Answer 56

Is a density parameter that describes the combined average mass density of the universe, p0, in terms of a critical density, pc. sigma 0 = p0 / pc

Question 57

What happens if (a) sigma 0 (density) > 1 (critical density) (b) sigma 0 (density) = 1 (critical density) (c) sigma 0 (density) < 1 (critical density)

Answer 57

(a) spherical shape, parallel light beams converge (b) flat shape, parallel light beams remain parallel (c) hyperbolic shape, parallel light beams diverge.

Question 58

What is sigma 0 from observation?

Answer 58

= 1 Consistent with a flat universe.

Question 59

Explain why we need dark energy.

Answer 59

CMB points to sigma 0 = 1 But if matter and radiation are all there is sigma 0 = 0.24 Solution is some form of energy we cannot detect, which accounts for the missing 76% of the total energy of the universe.

Question 60

How much of the universe is dark energy in theory?

Answer 60

76%

Question 61

T or F. The rate of expansion of the universe is slowing down.

Answer 61

False. Is speeding up!

Question 62

How much of all matter is dark matter?

Answer 62

85-90%

Question 63

Explain the stages of the big bang.

Answer 63

During the inflationary epoch, the plasma of photons and charged particles expanded by an immense factor. Recombination period saw the first atoms, CMB emitted after 380,000 years. At 400 million years, first stars and later galaxies.

Question 64

Which of the following is NOT a property of a pulsar? Question 5 options: period of 1.34 seconds each pulse consisting of a 0.01 second burst of radiation over time, the period is gradually increasing time interval between pulses is very uniform emissions only in the visible part of the spectrum

Answer 64

Emissions only in the visible part of the spectrum

Question 65

In binary neutron star systems, the orbits are expected to slowly decay, eventually resulting in a merger of the neutron stars. Where does the orbital energy go? Question 13 options: It is emitted as neutrinos. It is emitted as gravitational waves. It is emitted as gamma-rays. It is converted to mass. It is lost to friction.

Answer 65

It is emitted as gravitational waves.

Question 66

T or F. Closed universe = sphere

Answer 66

True.

Question 67

Formula for radius of a black hole.

Answer 67

2GM / c^2

Question 68

What is the mass of sun? What is G?

Answer 68

2 * 10^30kg 6.67 * 10^-11

Question 69

Observed temperature fluctuations in the Cosmic Microwave Background Radiation are of what degree?

Answer 69

tens of millionths of a kelvin

Question 70

What use are 21 cm radio waves to galactic astronomers? Question 47 options: They bounce off stars like our Sun to let us precisely measure their distances. They pick up the cool, dark matter much better than can optical telescopes. They cut through the dusty cocoons to let us watch star birth. We can reflect them off the core of the Galaxy. Their Doppler shifts let us map the motions and locations of gas in the spiral arms.

Answer 70

Their Doppler shifts let us map the motions and locations of gas in the spiral arms.

Question 71

An object slightly more massive than the Sun, but roughly the size of a city, is a?

Answer 71

Neutron star

Question 72

AU to metres

Answer 72

1 AU ≈ 1.496 × 10^11 meters.