Life and Death of Stars

Question 1

What are star clusters?

Answer 1

The groups that stars formed in.

Question 2

Why are star clusters useful to astronomers?

Answer 2

1. All the stars in a cluster lie at the same distance from Earth
2. All the stars in the cluster formed at the same time

Question 3

The features of star clusters enable us to use them as cosmic ____?

Answer 3

Clocks

Question 4

What are open clusters?

Answer 4

Clusters containing up to several thousand stars, found in the disk of the galaxy, composed of young hot stars

Question 5

What are globular clusters?

Answer 5

Contain hundreds of thousands of stars, all closely packed together. Mainly found in the halo of the galaxy

Question 6

What are associations?

Answer 6

Loose groupings of spectral type O and B, also of T Tauri stars. Not considered true clusters as they are so far apart they are not gravitationally bound.

Question 7

How do we measure the age of a star cluster?

Answer 7

We find the main sequence turn off point of its stars on a H-R diagram. The clusters age is equal to the core hydrogen-fusion lifetime of the hottest, most luminous stars that remain on the main sequence.

Question 8

Are open or globular clusters older?

Answer 8

Globular clusters, they can be as old as 13 billion years

Question 9

What is the problem in trying to plot an H-R diagram for a cluster?

Answer 9

Distance measurements are uncertain, which we need to calculate absolute luminosity.

Question 10

What do we use instead of absolute luminosity to plot an H-R diagram for a single cluster?

Answer 10

The relative brightness of the stars in that cluster.

Question 11

How do we obtain a luminosity scale for clusters

Answer 11

Superimpose the diagrams of different clusters plotting relative brightness and scale the luminosity axis so that the main sequence parts all lie on the same line.

Question 12

Now you have a luminosity scale, what next?

Answer 12

If you know the distance to any one cluster, we will know its absolute L, and then that of the other clusters. Now we can deduce distance to other clusters.

Question 13

Luminosity is approximately proportional to? What does this imply?

Answer 13

Mass cubed, massive stars live shorter lives.

Question 14

What is the theory regarding associations?

Answer 14

That they are groups of stars newly formed out of dust

Question 15

When were globular clusters theorised to have been formed?

Answer 15

When the galaxy was spherical, before it collapsed into a disc.

Question 16

What is Population II

Answer 16

Globular clusters and other halo stars (the survivors of an older generation of stars)

Question 17

Where does dust in the disc come from?

Answer 17

Supernovae among Population II stars

Question 18

What has/is the dust in the disk formed/forming?

Answer 18

Its formed the sun, and is continuously forming stars (Population I)

Question 19

What regions are the youngest clusters found in?

Answer 19

Regions of high nebulosity (clouds of dust and gas)

Question 20

Key factors of emission/ ionised nebulae

Answer 20

Mostly hydrogen, hot new stars, ionise hydrogen, pink colour due to Balmer line H-alpha

Question 21

Stages of the Birth of the Sun

Answer 21

• Protostar formed from primarily H and He
• Contraction and gravitational heating
• Emergence from cocoon at birth line on the HR diagram
• Reaches ZAMS around 50 million years later

Question 22

In stellar equilibrium of the sun, what does gas pressure do?

Answer 22

Supports weight of the layers above.

Question 23

The more massive the sun, the hotter the gas in the centre. why?

Answer 23

So it exerts more outward pressure

Question 24

Hotter gas in the centre also results in the surface being hotter. what relation does this explain?

Answer 24

The mass - luminosity relation

Question 25

What is there a stable balance between on the main sequence?

Answer 25

Gravity and gas pressure.

Question 26

As pressure increases Temperature increases. True or False? how?

Answer 26

True Increase in temp increases fusion rate, raises core pressure, causing the core to expand and cool down, and vice versa

Question 27

For the sun, when is equilibrium reached?

Answer 27

When the gas pressure supports the stars weight.

Question 28

What is Energy generated through in the sun.

Answer 28

Nuclear fusion and proton-proton chain.

Question 29

What are the products of main sequence hydrogen fusion?

Answer 29

Helium, photons and neutrinos

Question 30

How is energy transported in stars?

Answer 30

Radiation or convection, depending on the stellar mass.

Question 31

What will likely happen in a low mass star like the sun

Answer 31

Hydrogen fuel for fusion will eventually run out (in the core)

Question 32

What are the steps for a star, like the sun, becoming a red giant?

Answer 32

contraction of inert helium core, hydrogen shell fusion around it, and expansion of outer layers of gas

Question 33

Why is the solar theromostat in a red giant not working a problem?

Answer 33

The core and shell continue to shrink and heat up, causing the sun to grow larger and more luminous.

Question 34

During the red giant stage of low mass star death, where does its H-R track move?

Answer 34

Initially horizontally as it becomes a subgiant, Then will move upward as the outer layers expand (increase in luminosity)

Question 35

What happens during the Helium Flash?

Answer 35

Initiation of helium fusion

Question 36

What happens if helium fusion is initiated?

Answer 36

Star contracts and heads to the horizontal branch of the HR diagram

Question 37

What happens after further expansion and exhaustion og the helium core?

Answer 37

The star expels its outer gasses, exposing the hot core > white dwarf

Question 38

How big do stars have to be for helium fusion to occur?

Answer 38

0.4Msun

Question 39

Low mass stars use less fuel due to their lower luminosity, what does this mean for their life span?

Answer 39

Longer than the sun

Question 40

Where do low mass stars go when their H is exhausted?

Answer 40

They remain on the main sequence and cool down. (red dwarfs)