Star Birth

Question 1

Describe the stellar life cycle

Answer 1

•born in clouds of interstellar gas and dust
•shine with energy produced by nuclear fusion
•die when all fuel is exhausted

Question 2

Is the space between stars a vacuum?

Answer 2

By earth standards yes but not completely empty (1 atom/cm cubed)

Question 3

How does chemical composition vary across galaxy?

Answer 3

Largely constant

Question 4

How do density and temperature vary in interstellar medium?

Answer 4

•hot and diffuse
•cold and dense

Question 5

What type of interstellar clouds do stars form in?

Answer 5

The coldest and densest

Question 6

What are the 2 major components of the ISM

Answer 6

•gas-traced by radio emission
•dust-traced by sub-mm,IR

Question 7

What do denser parts of galaxy generally have?

Answer 7

Molecular hydrogen

Question 8

What can be used to trace molecular hydrogen?

Answer 8

Carbon monoxide which is less common gas

Question 9

Describe giant molecular clouds

Answer 9

•clumpy
•organised into discrete clouds
•masses of 1000-10000000 times solar mass
•100-1000 particles/ cm cubed (less dense than a star)
•density of best vacuum chamber 100 particles/cm
•5-300 parsecs in size
•10-30K in temp

Question 10

What is the nearest site of high mass star formation?

Answer 10

The Orion molecular cloud

Question 11

Describe the Orion molecular cloud

Answer 11

•mostly h2 other molecules and dust gains
•1% of mass if a molecular cloud is in small dust grains

Question 12

What is interstellar dust made of?

Answer 12

Carbon, oxygen, silicon and iron

Question 13

What does interstellar dust influence?

Answer 13

How light travels through the ism. It scatters and absorbs almost all visible light

Question 14

Explain how interstellar dust produces interstellar reddening

Answer 14

Transmits red light more easily than blue light so stars appear redder than they actually are

Question 15

What can scattered blue light produce?

Answer 15

Blue reflection nebulae

Question 16

How do stars form?

Answer 16

Stars forms when a molecular cloud starts to collapse under the influence of gravity. Thermal pressure can resist gravity in most interstellar clouds. So need to be dense(massive) and cold(low pressure)

Question 17

What happens to gravitational potential energy as molecular cloud collapses?

Answer 17

Converted into thermal energy ( if not lost, resulting pressure will halt collapse)

Question 18

What do collision between gas molecules do?

Answer 18

Excite rotational and vibrational energy levels which is then emitted at long wavelengths and escapes cloud to keep cloud cool. Dust can also keep cloud cool by emitting in the infrared

Question 19

What can help to overcome internal pressure to allow entire cloud to collapse?

Answer 19

External effects like supernova explosions

Question 20

When is a protostar born?

Answer 20

When the central region eventually becomes dense enough to trap the radiation and temperature in core starts to increase

Question 21

How does a protostar continue to grow?

Answer 21

By accreting mass from its surrounding meaning central temperature and density continue to rise

Question 22

When does nuclear fusion start?

Answer 22

When central temperature is greater than 10 million K (main sequence star)

Question 23

What is jeans mass?

Answer 23

Mass required for collapse. Can be determined by comparing gravitational and thermal energies.

Question 24

When will a cloud be less stable against gravitational collapse?

Answer 24

When it’s has a greater mass, smaller in size, colder temperature

Question 25

When does gravitational collapse occur isothermally(constant temperature)?

Answer 25

With constant temp of 10K until number density of 6x10^10cm^-3

Question 26

How many stars form per year in our galaxy?

Answer 26

By counting number if young stars nearby deduce that 2-3 stars

Question 27

What elements in the universe have been created in stars?

Answer 27

All heavier elements ( not hydrogen and helium)

Question 28

Do H an He radiate/cool well?

Answer 28

Not unless >100K

Question 29

Describe formation of first stars?

Answer 29

Formed in extremely dense and hot molecular cloud and are much more massive than later stars

Question 30

Why happens to velocity of molecular cloud as it collapses?

Answer 30

R decreases so to maintain angular momentum velocity must increase

Question 31

How does a disk form?

Answer 31

Centrifugal force will hold collapse eventually in rotational plane. The material collapse further above and below this plane, which leads to protostellar/accretion disk forming

Question 32

What have observations of young protostars showed?

Answer 32

That they fire high speed streams of gas, or jets, into space

Question 33

What do protostellar jets align with?

Answer 33

With disks rotation axis ( magnetic fields may link angular momentum of disk to the outflowing gas in jet)

Question 34

What do the winds and jets do together?

Answer 34

Carry away angular momentum and clear the gas surrounding the protostar

Question 35

Explain basic stages of planet formation in a disk

Answer 35

•solids condense out to form grains
•grains collide and grow(planetesimals)
•largest planetesimals sweep up smaller planetesimals and continue growing

Question 36

What happens when largest bodies attract gaseous envelope?

Answer 36

Turn into gas giants. Terrestrial planets formed too mate no gas left or too small to attract gaseous envelope)

Question 37

Describe brown dwarfs

Answer 37

•less massive than stars, more massive than planets
•may shine for a hundred million years before deuterium supply burned out (possible if >13Mj)
•most have lithium in their spectrum, stars burn lithium fairly quickly

Question 38

What does the number of brown dwarfs outnumber?

Answer 38

Stars in universe