Star Birth Flashcards
Describe the stellar life cycle
•born in clouds of interstellar gas and dust
•shine with energy produced by nuclear fusion
•die when all fuel is exhausted
Is the space between stars a vacuum?
By earth standards yes but not completely empty (1 atom/cm cubed)
How does chemical composition vary across galaxy?
Largely constant
How do density and temperature vary in interstellar medium?
•hot and diffuse
•cold and dense
What type of interstellar clouds do stars form in?
The coldest and densest
What are the 2 major components of the ISM
•gas-traced by radio emission
•dust-traced by sub-mm,IR
What do denser parts of galaxy generally have?
Molecular hydrogen
What can be used to trace molecular hydrogen?
Carbon monoxide which is less common gas
Describe giant molecular clouds
•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
What is the nearest site of high mass star formation?
The Orion molecular cloud
Describe the Orion molecular cloud
•mostly h2 other molecules and dust gains
•1% of mass if a molecular cloud is in small dust grains
What is interstellar dust made of?
Carbon, oxygen, silicon and iron
What does interstellar dust influence?
How light travels through the ism. It scatters and absorbs almost all visible light
Explain how interstellar dust produces interstellar reddening
Transmits red light more easily than blue light so stars appear redder than they actually are
What can scattered blue light produce?
Blue reflection nebulae
How do stars form?
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)
What happens to gravitational potential energy as molecular cloud collapses?
Converted into thermal energy ( if not lost, resulting pressure will halt collapse)
What do collision between gas molecules do?
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
What can help to overcome internal pressure to allow entire cloud to collapse?
External effects like supernova explosions
When is a protostar born?
When the central region eventually becomes dense enough to trap the radiation and temperature in core starts to increase
How does a protostar continue to grow?
By accreting mass from its surrounding meaning central temperature and density continue to rise
When does nuclear fusion start?
When central temperature is greater than 10 million K (main sequence star)
What is jeans mass?
Mass required for collapse. Can be determined by comparing gravitational and thermal energies.
When will a cloud be less stable against gravitational collapse?
When it’s has a greater mass, smaller in size, colder temperature
When does gravitational collapse occur isothermally(constant temperature)?
With constant temp of 10K until number density of 6x10^10cm^-3
How many stars form per year in our galaxy?
By counting number if young stars nearby deduce that 2-3 stars
What elements in the universe have been created in stars?
All heavier elements ( not hydrogen and helium)
Do H an He radiate/cool well?
Not unless >100K
Describe formation of first stars?
Formed in extremely dense and hot molecular cloud and are much more massive than later stars
Why happens to velocity of molecular cloud as it collapses?
R decreases so to maintain angular momentum velocity must increase
How does a disk form?
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
What have observations of young protostars showed?
That they fire high speed streams of gas, or jets, into space
What do protostellar jets align with?
With disks rotation axis ( magnetic fields may link angular momentum of disk to the outflowing gas in jet)
What do the winds and jets do together?
Carry away angular momentum and clear the gas surrounding the protostar
Explain basic stages of planet formation in a disk
•solids condense out to form grains
•grains collide and grow(planetesimals)
•largest planetesimals sweep up smaller planetesimals and continue growing
What happens when largest bodies attract gaseous envelope?
Turn into gas giants. Terrestrial planets formed too mate no gas left or too small to attract gaseous envelope)
Describe brown dwarfs
•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
What does the number of brown dwarfs outnumber?
Stars in universe
