Unit 5: (Chapters 15,16,18)

Question 1

What is the size of the milky way and how many stars does it have?

Answer 1

it is about 80 000 ly in diameter and has a mass of about 400 billion solar masses. It has over 100 billion stars.

Question 2

How are Cepheid stars used to determine distance?

Answer 2

The apparent magnitude and period of Cepheid stars is used to determine their absolute magnitude which is then used to measure distance.

Question 3

Describe the following structural components of the galaxy:
- Disk component
- galaxy Halo
- Central Bulge

Answer 3

Disk Component: is the disk of the galaxy where most of the galaxy’s stars, dust, and gas is contained.
- This is the location of star formation!
- Contains mostly star associations and open clusters

Galaxy Halo: is the spherical cloud o stars and star clusters that contains almost not gas or dust.
- this area CAN’T make stars
- These stars are old cool, lower main sequence, red giants, and white dwarfs
- This area is about 10X the diameter of the disk
- Contains mostly globular clusters

Central Bulge: is a flattened cloud of billions of stars but no gas or dust. The halo and central bulge together are called the SPHERICAL COMPONENT of the galaxy.

Question 4

What is Keplerian Motion?

Answer 4

Keplerian motion is orbital motion in accord with Kepler’s laws of planetary motion. it does not describe galaxy motion.

Question 5

Describe the motion of galaxies and explain their cause.

Answer 5

The orbital velocities in the outer disk are constant or increasing at greater distances from the centre of the galaxy (this is the opposite of Keplerian motion). This type of motion is caused by the extra mass in the galactic corona (extended halo) which reaches out 10X further than the edge of the visible disk and which contains more than 1 trillion solar masses due to its dark matter.

Question 6

What is dark matter?

Answer 6

Dark matter is matter that is invisible, neither emitting or absorbing light.

Question 7

Describe the spiral arms of the galaxy.

Answer 7

The spiral arms are long spiral patterns of bright stars, star clusters, gas, and dust which extends from the centre to the edge of the disk in spiral galaxies.

It is made up of largely O and B stars, open clusters, emission nebula, and high mass variable stars.

the Spiral arms are involved in star formation.

Question 8

What is spiral density wave theory and what are the 2 main criticisms?

Answer 8

This theory suggests that spiral arms are dynamically stable regions of compressed interstellar medium that moves slowly around the galaxy. Orbiting clouds overtake the spiral arms and compress them, triggering star formation.

2 main problems:
- Dow did the enormous and complicated spiral disturbance begin?
- How do we explain variations in spiral shape?

Question 9

What is Self-sustaining star formation?

Answer 9

Self sustaining star formation suggests that differential rotation drags the inner edge of a gas cloud ahead of its outer edge, elongating the cloud. Star formation takes place in the gas cloud and produces massive stars whose high luminosity and supernova explosions can compress the surrounding gas, triggering more star formation.

Question 10

What are two types of spiral galaxies and which star formation theory works best for each?

Answer 10

Grand Design spiral galaxies have bold symmetric two armed patterns. These galaxies work best for the spiral density wave theory of star formation.

Flocculent galaxies have many short spirals. These galaxies work best for the Self-sustaining theory of star formation.

Question 11

Describe the nucleus of our galaxy.

Answer 11

Sagittarius A (Sgr. A) is close to the centre of the galaxy, it is a supermassive black how which gas flows into. It’s mass appears to be about 4 million solar masses.

The nuclues of the galaxy is about 1 AU in diameter but has a strong source of radio energy and x-ray emissions.

This area is crowded with stars that heat the dust to emit strong infrared radiation.

Question 12

What are metals according to astronomy?

Answer 12

Metals are anything heavier than helium (e.g. carbon, nitrogen, oxygen, iron, gold, platinum etc.)

Question 13

What are different types of stellar populations and where are they typically found in the galaxy.

Answer 13

population I stars: Metal rich stars (have 2-3% metals)
–> Extreme population 1 stars: only found in spiral arms
–> intermediate population I stars: found throughout the disk
population II stars: metal poor stars (have less than 0.1% metals)
–> Intermediate population II stars: found in the central bulge
–> Extreme population II stars: found in the halo

Question 14

What is the galactic fountain?

Answer 14

This is a region in the galaxies disk where gas heated by supernova explosions throws gas out of the disk where it can fall back and spread metals through the disk.

Question 15

What is the top down galaxy formation theory of galaxy formation?

Answer 15

It is an older and inaccurate theory which suggests that spherical clouds of turbulent gas give birth to the first stars and star clusters. Then the rotating cloud begins to contract towards the equatorial plan. Star clusters are left behind in the halo as the gas cloud flattens and new generations of stars will have flatter distributions.

Question 16

What is the bottom up galaxy formation theory?

Answer 16

This theory suggests that mixed clouds of normal and dark matter are drawn together. The first stars are massive and evolve fast, making metals and dust which allows a large protogalaxy to form.

More clouds of dark matter, dust and stars fall into the growing galaxy and being building the halo and disk.

Small galaxies get absorbed by the forming galaxy, adding to the halo and disk. Gas and dust settle into a thin disk where star formation continues.

Question 17

What was the Shapely-Curtis debate about?

Answer 17

This debate was whether there existed island universes (other galaxies - Curtis) or whether the milky way was the only galaxy (Shapely)

Question 18

How many galaxies are there in the universe?

Answer 18

we have found evidence of a few 100 billion galaxies

Question 19

What are the different classifications for galaxies?

Answer 19

1. Elliptical galaxies: round or elliptical, containing no visible gas or dust and no hot bright stars.
2. Spiral Galaxies: have disks and spiral arms. Contains dust, gas, and hot bright O and B stars.
3. Irregular galaxies: a mix of gas, dust, and stars with no obvious nuclear bulge or spiral arms

Subclasses based on size of nucleus
- Sa - large nucleus
- Sb - medium nucleus
- Sc - small nucleus

Subclasses based on presence of bar
- Barred: SB

Subclasses based on lenticular galaxies (obvious disk and bulge but no gas or dust or bright stars)
- Lenticular = S0

Question 20

What does the colour of a galaxy mean?

Answer 20

Galaxies with a blue tint: have lots of gas and dust because they have active star formation and hot bright stars.

Galaxies with a red tint: have less gas and dust and lack emission nebula and bright stars

Question 21

What is look back time?

Answer 21

Look back time is the idea that some galaxies are so far away that it takes many years (sometimes billions) for their light to reach us.

For example, the furthest known galaxies are 10 billion ly away which means it has taken 10 billion years for their light to reach us. So what we are actually seeing is an image of what that galaxy looked like 10 billion years ago. We would have to wait an additional 10 billion years to see what the galaxy looks like today.

Question 22

What is the relationship between black holes and galaxies?

Answer 22

most galaxies have a supermassive black hole at their centre where the size of the black hole is proportional to their central bulge

Question 23

How much dark matter is in the universe?

Answer 23

About 90% of the universe is expected to be dark matter

Question 24

What is the pattern of galaxy clustering?

Answer 24

Galaxies cluster in either rich or poor clusters

Rich clusters have 1000 or more galaxies in 3Mpc diameter.
- These galaxies are most crowded at the centre and tend to be ellipticals. They have a hot gas between galaxies

Poor clusters dent to have between a few dozen to a few hundred galaxies in a 3mpc diameter.
- These galaxies tend to have more spirals.

The milky way’s local group is a poor cluster.

Question 25

What happens when Galaxies interact?

Answer 25

Galaxies interact quite often and interactions may: - trigger spiral arm formatoin - result in a smaller galaxy being absorbed - trigger rapid star formation Large galaxies often collide with other galaxies and absorb them. This is especially true for ellipticals but less common for spirals.

Question 26

What are blue dwarf galaxies?

Answer 26

These are small irregular clouds forming massive hot stars at rapid rates. They have only been seen with substantial look-back time and may be the foundation for early galaxy formation.

Question 27

What is Cosmology?

Answer 27

The study of the universe as a whole

Question 28

Given the number of stars out there, why is it even possible for our sky to be dark at night?

Answer 28

Because the universe is not infinite in age, the look back time prevents us from seeing all the stars out there. Some stars are so many light years away that the universe has not existed long enough for their light to reach us. When we look at the night sky, we are looking at some regions that are so far away, the look back time shows us a time before the stars existed.

Question 29

What is the idea of the expanding universe?

Answer 29

This is the idea, supported by data, that space is stretching and carrying galaxies further away from each other.

Question 30

What is the basics of the big bang theory?

Answer 30

This is the idea that the universe began with a violent explosion from which the expanding universe of galaxies eventually formed. We can use the rate of expansion to estimate the age of the universe

Question 31

Explain the evolution of the universe at 1/millionth of a second, 3 minutes, 30 minutes, 50 000 years, and 400 000 years.

Answer 31

1/Millionth of a second: filled with high density photons, temp of 2 trillion K. At this energy level photons can convert to matter and anti-matter. Anti matter and matter can convert back to photons of gamma rays. basically the universe was filled with energy constantly flickering between photons and particles. As the universe expanded, it cooled and was able to begin to form deuterium (heavy hydrogen) 3 minutes: Deuterium could be converted into helium and minute amounts of lithium 30 minutes: nuclear reactions ended and 25% of the universe was helium 50 000 years: the density shifted to the point where gravity drew ordinary matter together 400 000 years: free electrons were so far apart photons could travel thousands of parasecs and not be deflected. The universe was transparent. Free electrons were captured to form neutral hydrogen. The universe entered a 400 million year "dark age" until it formed it's first star (massive luminous short lived stars that created UV light which ionized the gas).

Question 32

What is the cosmological principle and what two facts about the universe lead to this?

Answer 32

The cosmological principle is the idea that any observer in any galaxy sees the same general properties of the universe. This is based on two facts: 1. The universe is isotropic: it looks the same no matter which direction you look based on the number of galaxies and background radiation 2. The universe is homogeneous: it is the same at all locations.

Question 33

What is a cosmological red shift?

Answer 33

This is when the space between galaxies stretches. This gives the appearance that the galaxies moved but they have actually stayed in the same spot, the universe has simply stretched.

Question 34

What does current data tell us about the nature of the universe?

Answer 34

That it is flat, accelerating, and will expand forever.