Lecture 14 - Star Birth Flashcards
what are the 4 phases of stellar life?
- formation and pre-main sequence
- main sequence
- post main sequence
- death
where do stars form?
in DARK, COLD, DENSE clouds of dusty gas
what is the gas btwn stars called?
what is its composition?
gas btwn stars = INTERSTELLAR MEDIUM
made of H and He
what were early stars made of, why?
early stars only made of H and He because they were the only elements made from the Big Bang
how can we determine the composition of interstellar gas? explain
we can determine the composition of interstellar gas from its absorption lines by looking at a spectrum of a star whose light has passed through an intervening cloud of interstellar gas
the cloud absorbs some of the stars light, leaving absorption lines in the star’s spectrum –> indicating which elements are in the cloud
what is the composition of our region of the milky way?
70% H, 28% He, 2% heavier elements
is all gas btwn stars in the milky way the same?
YES but may appear different due to temp and density
describe the density of a cloud if it is HOT vs COOL
hot = low density
cool = high density
do we see newborn stars?
no, we only see the starlight that illuminates the surrounding gas
where do stars form? why?
in molecular clouds
they are cold and dense enough to allow atoms to combine into molecules
what is most of molecular clouds made of? what is the density?
MOLECULES
density = 300 molecules/cm^3 (avg)
what is the temp of molecular clouds?
10-30K
explain how we visualize molecular clouds
normally, we can just see the illuminated dust aroudn the cloud
but if we visualize the clouds at longer wavelengths, we see the gas itself and can see the molecules going thru transition
do we use H2 to understand molecular clouds? why?
NO, it is the most abundant molecule but the temp is too cold for H2 to produce emission lines in spectra so we cannot detect it
what do we use to understand molecular clouds? why?
CO
it only makes up a small amount of the cloud’s mass but it makes radio emission lines that we can detect
what is interstellar dust?
solid grains of C, O, Si, Fe that are <1um in the MOLECULAR CLOUD
what does interstellar dust hide? why?
blocks our view of stars on the other side of the cloud
it easily absorbs and blocks visible light, heating up to keep heat inside the interstellar medium
how do we view stars within the gas cloud?
using infrared light
why do stars near the edges of a molecular cloud appear red?
dust grains block shorter-wavelength (blue) photons of visible light more easily than longer-wavelength (red) photons
so at the edges where stars are only partially obstructed, the blue light is blocked so they appear redder
if stars appear redder at the edge of a molecular cloud, is this doppler effect?
NO –> the wavelengths are not changing
what does the amount of reddening of a star in a molecule cloud indicate?
indicates how much dust lies btwn earth and star
what happens when dust grains absorb visible light?
dust grains that absorb visible light will heat up and emit infrared light (brightest in star-forming regions)
describe how we study star formation
we cannot follow a real gas cloud (takes millions of years) so we use a simulation and verify with clouds we currently see
describe the general 3 steps of star formation
- large ball of gas
- random thermal motions allow it to be lumpy with various densities (if gravity overcomes the thermal pressure, the dense regions collapse and get denser)
- large cloud fragments into smaller clumps which each form new stars
what causes stars to form?
why are molecular gas clouds different from normal interstellar dust clouds?
gravity overcomes interior thermal pressure, allowing a molecular cloud to contract and get hot enough to sustain nuclear fusion
normal interstellar dust clouds have thermal pressure > gravity because they are low density
BUT molecular clouds are very dense so gravity is stronger and must have very high thermal pressure to resist gravity
what can initiate star formation?
a collision btwn 2 molecular clouds causing compression and increasing gas density
describe fragmentation of a cloud
why is a star lumpy
- gravity in a molecular cloud (with large enough mass) is strong enough to cause the cloud to collapse and increase its density
- cloud is cooler so gravity can overcome thermal pressure in small fragments that break off to become a star
star is lumpy because of random motions in diff sections of the cloud
describe the formation of 1 single star
it is possible that a molecular cloud is small but it can only produce a star if it is DENSE and COLD, allowing gravity to overcome the pressure
what would happen to a contracting cloud fragment if it could not radiate away its thermal energy
its thermal pressure would increase and gravity would not overcome the pressure –> no star
describe the amount of heavy elements in the first stars?
<0.1% heavier elements
why do we know that elements in the galaxy have not changed much in the past 5 billion years?
the nearby stars have similar composition to the elements we find in interstellar clouds today (70% H, 28% He, 2% heavier elements)
how could the first generation of stars form with only H and He? did they have short or long lifetime?
they formed from WARMER clouds (no CO to radiate away thermal energy and cool them down)
therefore, the clouds were more massive to overcome thermal pressure and collapse the cloud
shorter lifetime
describe the 4 steps once a cloud starts contracting
- cloud fragment continues to collapse on itself as long as it is cold
- as it gets more dense, it is harder for infrared/radio photons to escape as heat
- thermal energy builds up and increases thermal pressure
- contraction slows down and the center of the cloud fragment becomes a protostar
what allows a cloud to stay cold while it is collapsing?
photons can carry away energy, allowing gravitational contraction
why is it harder for photons to escape as heat when the cloud contracts and gets more dense?
higher density = higher chance photon is absorbed instead of released
what is a protostar? what is its source of thermal energy?
aka PRE MAIN SEQUENCE STAR
looks like normal star with similar temp and luminosity but core is not hot enough for nuclear fusion
source of thermal energy = gravitational contraction (NOT fusion)
describe the rotation of a cloud as it collapses
rotation speed increases as the cloud contracts
causes collisions btwn particles in the cloud:
- cloud flattens into a disk
- gas particles reduce their random and up/down motions
- eventually flattens as it shrinks
what does the rotation of a collapsing cloud produce?
where are these located?
how are they aligned?
why can we see them?
produces jets of matter that shoot out
- jets detected coming from the centers of disks around protostars
- jets are aligned with the disk’s rotation axis –> angular momentum is involved
- jets ram into interstellar gas, heating it and causing it to glow
what would happen to a protostar that formed without any rotation at all?
- no jets
- would not form planet
- would not be bright in infrared light
- would be round, not flattened disk
describe how a star goes from a protostar to a main sequence star
- gravitational contraction continues until core is hot enough for nuclear fusion
- then contraction stops when energy released by core fusion balances the amount of energy radiated from the surface
what allows temp to rise during gravitational contraction?
why is this important?
the main factor allowing temp to rise is radiation of energy into space
if it did not lose thermal energy:
1. gravity would not overcome the pressure
2. protostar would stop contracting
3. central temp would be fixed
what is the main form of energy transport during gravitational contraction?
CONVECTION –> gas rises until photons escape
what happens to the star when the contraction stops?
energy released by core fusion balances the energy radiated from the surface –> now it is MAIN SEQUENCE!
with very low surface temp and luminosities, where are protostars on HR?
bottom right
describe the 4 birth stages on a life track of a star (i.e. how its properties change)
- luminosity and temp increase as matter collects in a protostar
- surface temp remains near 3000K and convection is main transport mechanism –> luminosity decreases, temp is constant
- luminosity remains constant during late stages of contraction
- core temp rises until star begins fusion to balance the rate of radiative energy escaping and arrives on main sequence
describe the speed at which high vs low mass stars form, why?
higher mass form faster –> has more gas
lower mass form slower –> has less gas
can we see protostars? why?
more difficult to see because they haven’t started burning yet so they have no light
what would cause fusion to not begin in a contracting cloud?
fusion will not begin in a contracting cloud if a force STOPS CONTRACTION BEFORE CORE TEMP RISES ABOVE 10^7 K
can thermal pressure stop contraction?
no because it is constantly losing thermal energy thru radiation
what type of pressure can stop contraction? when does this occur? how?
degeneracy pressure can stop contraction if masses are too low and don’t reach 10^7 K
degeneracy pressure pushes outward against gravity
what is degeneracy pressure? use the chair analogy
temp loses its relation to pressure and volume, closely related to density
in thermal pressure: there are more available quantum states (chairs) than electrons (people) so an electron is unlikely to enter the same state as another electron –> the only pressure is due to the thermal motion of the electrons
in degeneracy pressure: there is a similar number of quantum states (chairs) as electrons (people) so an electron is likely to move faster to find an available state –> creates higher density
when does degeneracy pressure stop the contraction of an object?
why can degeneracy pressure stop contraction?
degeneracy stops the contraction of an object if its mass is <0.08 M_sun before its core temp is hot enough for fusion
it is unaffected by the gradual cooling of the core, so degeneracy pressure stays strong and WINS against gravity
what type of objects are produced when degeneracy pressure stops contraction?
how do we detect them?
what happens to luminosity?
BROWN DWARFS are produced
they emit infrared light due to heat left over from contraction –> we can use infrared telescopes to see them
luminosity gradually declines as it loses its thermal energy
what determines the maximum mass of a newborn star as it contracts?
radiation pressure
what is radiation pressure? how does it stop contraction?
photos of light exert a slight amount of pressure when they strike matter
MASSIVE stars are so luminous that the overall pressure of photons is greater than thermal pressure so gravity cannot resist the force
essentially, what does radiation pressure limit? what is the value of this limit?
limits how massive a star can be without blowing itself apart
max mass = 150 M_sun
are low-mass stars more or less common than high mass stars?
LOW-MASS –> most less massive than sun
