The Milky Way and Other Galaxies

Question 1

What are the main properties of the milky way?

Answer 1

Mainly a disc with spiral arms. Also with a bulge and halo.

Question 2

About how many stars are in the milky way?

Answer 2

~3*10^11 stars

Question 3

Name some properties of the milky way.

Answer 3

• Perseus arm at far left
• Orion arm at top
• Sun near orion arm
• ~8500pc across
• ~500pc deep

Question 4

What is the distribution of globular clusters like in the milky way?

Answer 4

Roughly spherical. Not centred at Sun though.

Question 5

What radius does the sun orbit the centre of the galaxy? What velocity?

Answer 5

R0 = 8.5kpc
v0 = 220 km s^-1

Question 6

How do you work out an equation for the mass of a star?

Answer 6

Set gravitational force equal to centripetal acceleration and rearrange for mass.

Question 7

What does a galaxy rotation curve show us?

Answer 7

That stellar count rapidly drops as a function of distance to the centre.

Question 8

What is the Milky way rotation curve like and what does this suggest?

Answer 8

Is flat - suggests there is something else other than stars mass, otherwise it would decrease with radius.

Question 9

What is dark matter?

Answer 9

Unknown mass surrounding galaxies which is seen through gravity but does not appear to emit light.

Question 10

How do we know there are black holes at the centre of galaxies?

Answer 10

Because from the equation for mass, we can see that the central mass is way to large to be a cluster of stars.

Question 11

How can we estimate how far other galaxies are?

Answer 11

Need something bright which we can estimate the absolute magnitude of and then use the magnitude equation to find out distance.

Question 12

What are Cepheid variables?

Answer 12

• Cepheids undergo oscillations in their envelopes - they are in the giant star phase
• Their oscillations are visible as changes of brightness with periods of ~10 days
• Was discovered that the pulsation period is related to the luminosity of the star

Question 13

What is the equation relating luminosity and period?

Answer 13

Mv = -2.81*log P(d) - 1.43

Question 14

What are the requirements for standard candles?

Answer 14

• Bright
• Recognisable
• Can be calibrated (need a luminosity)

Question 15

What absolute magnitude do we find for Cepheids of approximate periods of 10-100days?

Answer 15

Mv ~ -2 to -7 or L = 500-50000 L(sun)

Question 16

What is the distance to the Andromeda galaxy?

Answer 16

770 pc

Question 17

What is the net velocity of the Milky way and where is it heading?

Answer 17

110 km s^-1 and is heading for the Andromeda galaxy

Question 18

How long will it take for the milky way to reach andromeda?

Answer 18

4 billion years

Question 19

What is the equation for fractional redshift?

Answer 19

z = (λ’-λ0)/λ0 = v/c

Question 20

What did Vesto Slipher find about near/far galaxies?

Answer 20

The near galaxies are getting close but the far galaxies are moving away from us and eachother

Question 21

What is Hubble’s law of universal expansion?

Answer 21

Galaxies recede from us with a velocity v that is proportional to the distance d: v = H0d, where H0 is the Hubble constant

Question 22

What is the modern value of the Hubble constant?

Answer 22

68-72 km s^-1 Mpc^-1

Question 23

What are the two main types of supernovae?

Answer 23

Type 2 - core collapse

Type 1a - white dwarfs pushed over the Chandrasekhar limit

Question 24

What does the light curve for a type 1a supernova look like?

Answer 24

Increases and then peaks and then decreases, levelling out at the end

Question 25

How can we use type 1a supernova?

Answer 25

Can use them to find distance as they are very bright and can be calibrated to give luminosity (Mv ~ -19)