3: STARS IN GALAXIES

Question 1

What is the order of MS stars and how are they ordered?

Answer 1

Ordered by decreasing temperature.
O, B, A, F, G, K, M

Question 2

How do the top end of the MS stars (O, B) differ from the lower end (M) stars?

Answer 2

O and B are massive, hot, blue, high luminosity stars and M stars are cool, red, low luminosity stars.

Question 3

What shape does the spectra roughly match?

Answer 3

A black body

Question 4

What is the average temperature for O, G, and M stars?

Answer 4

> 30, 000 K
6000 K
< or = to 3000 K

Question 5

What does B-V equal for the bluest stars compared to the reddest?

Answer 5

B-V = -0.3
B-V = -1.5

Question 6

How are colours expressed?

Answer 6

Difference in their magnitudes in wavelength bands

Question 7

In which star are the Balmer lines weakest/strongest and why?

Answer 7

Balmer lines aren’t that strong in O stars as the hydrogen is almost all ionised. Balmer lines max in A stars.

Question 8

What causes Balmer lines?

Answer 8

The transitions between the first exited state and higher energy states of the electron

Question 9

What causes weakness in Balmer lines?

Answer 9

Reducing the temperature.

Question 10

What are the most important lines in G stars? (4)

Answer 10

The sodium D lines, the magnesium b feature, the G band of the CH radical, and the H and K lines of singly ionised calcium, CaII.

Question 11

Define the 4000 A break.

Answer 11

The combined effect of the Balmer limit and the CaII and K lines sharply cutting off the spectrum

Question 12

What is the 4000 A break a key feature of?

Answer 12

Intermediate temperature stars

Question 13

How are luminosity and mass related?

Answer 13

L proportional to M to the power of alpha

Question 14

What values do alpha take for M < 0.5 solar masses, M > 0.5 solar masses, and M > or = to 10 solar masses?

Answer 14

Alpha = 3
Alpha = 4
Alpha = 2

Question 15

Define relationship for main sequence lifetime.

Answer 15

τms ∝ M/L ∝ M^(1-α)

Question 16

Do high mass stars have a long lifetime?

Answer 16

No

Question 17

Are main sequence stars the most important in generating light?

Answer 17

No, red giants are most important in some instances (e.g. in ellipticals)

Question 18

Why are red giants luminous despite their low temperature?

Answer 18

Because they are big bois.

Question 19

How is stellar population determined?

Answer 19

SFH and IMF

Question 20

What is the Salpeter mass function relationship?

Answer 20

dN/dM ∝ M^(-2.35)

Question 21

Why are older stellar populations redder?

Answer 21

Colour of overall stellar population becomes redder as we replace blue MS stars by red giants

Question 22

What are Balmer lines also indicative of?

Answer 22

Age

Question 23

In stellar populations with no recent SF, what dominates total light?

Answer 23

The red giants

Question 24

How do old stars shed their outer layers and what does this affect?

Answer 24

Via stellar winds or the production of planetary nebulae or spectacularly in Type I or Type II supernovae. The processed material is returned to the ISM to be incorporated into the next generation of SF, producing stars of higher Z.

Question 25

What do heavier elements effect?

Answer 25

Atoms/ions of metals absorb photons, so heavier elements absorb more light.

Question 26

Where on the spectrum does absorption mainly occur? How does this affect appearance/metallicity?

Answer 26

The blue end of the spectrum. Stars with higher Z look redder than those of low Z

Question 27

Lower mass galaxies have a lower escape velocity, so it’s easier for heavier elements to be lost via galactic winds. What does this mean for ellipticals?

Answer 27

There’s a correlation between Z and M for E galaxies

Question 28

What colour is the disk/arms of a spiral and the colour of a bulge?

Answer 28

Disk/arms are blue
Bulge is yellow

Question 29

What kind of stars occur in the disk/arms and what occurs in the bulge?

Answer 29

Population I stars in the disc/arms
Population II stars are in the bulge

Question 30

Is Population II older than Population II?

Answer 30

Yes

Question 31

Where are the older stars in the Galaxy?

Answer 31

Outer parts of the Galaxy

Question 32

How are population II stars in the inner parts dispersed on Gyr timescales?

Answer 32

Gravitational interactions

Question 33

How does the LF differ from IMF?

Answer 33

Translation between MS luminosity and stellar mass (via models of stellar structure)

Question 34

What do clusters have an advantage over?

Answer 34

They have better sampling of very faint objects.