Lecture 18 - Milky Way Flashcards
what is the general structure of the milky way
disk galaxy with a spiral arm structure
describe how the milky way is a self-contained system
matter cycles itself from stars –> interstellar gas –> stars like a galactic ecosystem
what is the galactic plane?
perpendicular to the galactic plane point to where?
GALACTIC PLANE –> plane on which the disk lies
directions perpendicular to the galactic plane point to the galactic poles
what is the galactic centre?
what is in the centre?
rotational centre of the galaxy, contains a supermassive blackhole
how many galaxies are in the observable universe?
how many stars are in the milky way?
100 billion galaxies
100 billion stars in Milky Way
3 MAIN REGIONS:
1. disk
2. bulge
3. halo
explain
viewing from the side:
DISK –> spiral arms make up flat disk of galaxy’s brightest stars
BULGE –> disk bulges in the middle
HALO –> dim round halo surrounding the disk, contains groups of stars
where are the most prominent stars found in milky way? explain
in ~200 globular clusters
GLOBULAR CLUSTER = spheroidal conglomeration of stars bound together by gravity w high concentration of stars at their centres (10,000 - 1 million)
what is the main place of milky way where stars exist? are these early or late forming stars? what does this mean?
DISK
stars are from later generation so they have lots of metals from stardust of previous supernovae
describe the rotation of the milky way
which region has the most rotation?
gas interacts to collapse in a disk, then spins and collapses further
BULGE has most rotation
why can we not observe the structure of milky way?
bc we are inside it
what is the milky way part of?
what is that group part of?
what is that group part of?
the LOCAL GROUP –> 10 million light years size
local group in LOCAL SUPERCLUSTER –> 100 million light years size
local supercluster in UNIVERSE –> 100 billion light years
distances:
SUN
MOON
STAR TO NEXT STAR
ANDROMEDA
Sun –> light min
Moon –> light sec
Star to next star –> light years
Andromeda (next galaxy) –> millions of lyr
describe our galaxy
- when was it made
- its composition
- formed EARLY
- made of dust and gas, not gravitationally bound –> ratio of non-stars to stars = 10:1
POPULATION 1 STARS
- age + location
- colour
- metallicity?
- discovery
- what did they form from?
- what do they become?
POPULATION 1 STARS
- young stars in disk and arms
- bluer = hotter
- HIGH METALLICITY –> heavy elements
- population are first stars to be discovered
- formed from stardust of population 2
- can become WHITE DWARF
POPULATION 2 STARS
- age + location
- colour
- metallicity?
- size
- lifetime, when do we see them?
- what did they form from?
POPULATION 2 STARS
- older stars in halo
- redder
- less metallic
- bigger so burn quickly
- shorter lifetime, we see them at the end of their lifespan
- form from stardust of population 3
POPULATION 3 STARS
- age
- detection
- size
- metallicity?
- rate of H fusion
- FIRST stars
- extinct today + short-lived so we havent seen them
- massive
- no heavy elements
- very fast H fusion
how often does the sun orbit?
230 million years
describe the causes of the orbit of a disk star around the galaxy (orbit + bobbing)
orbital velocities
orbit occurs due its gravitational attraction toward galactic center and bobbing arises from localized pull of gravity within the disk
orbital velocities at the edge and near the center are THE SAME so stars near the center complete their orbit in less time
where do halo stars travel?
far above and below the disk in elliptical orbit with random oritentations
describe the movement of stars in the bulge
various orbital properties, some are like disk stars and some are like halo stars
give the bulge a cigar-like shape
where are later-forming stars found in the galaxy?
in bulge and halo
how long does an orbit around the galaxy take?
1/4 billion years
the ability to measure structure of galaxy via rotation of stars depends on what?
depends on whether you can measure the motion of a planet around a star AWAY from you/ TOWARDS you
how do you determine whether star is moving away/towards you?
describe the shift when light is moving:
1. AWAY from you
2. TOWARDS you
use emission lines
when moving away –> RED SHIFT
when moving towards –> BLUE SHIFT
2 ways we measure orbitals
- tangential –> observe changes in location on the sky
- radial with doppler shifts
considering the large distance btwn stars compared to their size, is it likely that 2 stars will collide?
NO stars are so small compared to the distances btwn them
how do you calculate mass within an orbit?
M = (r*v^2)/G
v = orbital speed
r = radius of object
what is galactic recycling?
makes new generations of stars while gradually changing the composition of interstellar medium
gas –> molecular clouds –> stars –> fusion –> die as gas
describe high mass stars in the milky way
in the halo, have strong stellar winds that blow bubbles of hot gas into space by supernovae or stellar winds and sweep up surrounding interstellar material
gas cools and blends with atomic hydrogen clouds
describe low mass stars in the milky way
in the disk, return gas to interstellar space thru stellar winds and planetary nebulae
results of supernovae in terms of types of light
- X-rays reveal new, heavy elements which mix into interstellar medium
- Visible light is emitted as it expands
- Radio waves are emitted from particles accelerated to near light speed
- produce cosmic rays as well
star-gas-star cycle (5)
- Multiple supernovae explosions in a young star cluster blow a superbubble in the galactic disk
- When superbubble gets thicker than the disk, hot gas blows INTO THE HALO
- Hot gas cools and rains on disk, allowing electrons to join with protons
- cool, dense molecular clouds allow atoms to combine into molecules
- gravity forms stars and completes the star-gas-star cycle
radiation from newly formed stars causes:
erosion of star-forming clouds
general summary of galactic recycling
- stars make new elements by fusion
- dying stars expel gas and new elements to make hot bubbles
- hot gas cools, allowing atomic H clouds to form
- further cooling allows molecules to form, making molecular clouds
- gravity forms new stars in molecular clouds
what will be the fate of the galaxy’s gas in 1 trillion years?
white dwarfs and low-mass stars
what blocks most of the galaxy’s visible starlight?
dusty clouds
what do radio waves in milky way show? (2)
- from H atoms –> due to warmer gas clouds and show where gas has cooled and settled into disk
- from CO –> show location of cool molecular clouds
what do long-wavelength and short-wavelength infrared waves in milky way show?
- long-wavelength –> shows where young stars are heating dust grains
- short-wavelength –> shows stars with visible light blocked by gas clouds
ultimately show the starlight! don’t you see the starlight taylor swift
what do X-rays in the milky way show?
found in hot gas above and below disk and show that hot gas fills the halo
what do gamma rays in the milky way show?
where cosmic rays from supernovae collide with atomic nuclei in gas clouds
what are ionization nebulae? where are they found? what do they signify?
star-forming regions
found near short-lived, high-mass stars
^therefore signify active star formation
how do ionization nebulae glow?
UV photons from hot stars ionize the nebula’s atoms and the atoms emit light as electrons return to lower energy levels
what do reflection nebulae do?
what colour do they have?
reflection nebulae scatter the light from stars
look bluer than stars surrounding the light due to the scattering
does star formation occur in the halo? why?
NO
no ionization nebulae or blue stars –> no star formation
does star formation occur in the disk? why?
YES
ionization nebulae and blue stars –> star formation
how does the number of collisions vary in the galaxy?
based on the pressure and temperature of gas
where does star formation occur in the galaxy?
what does squeezing in this area do?
occurs in the disk in the spiral arms
gas clouds get squeezed/compressed to allow star formation and allow young stars to flow out of spiral arms
BLUE STARS IN MILKY WAY
- young stars, formed in spiral arm
- but die before completing 1 orbit so we only see them close to the spiral arm
RED STARS IN MILKY WAY
- low-mass stars that are older and have undergone many orbits
- populate the entire disk
4 steps for galaxy formation
- cloud of intergalactic gas
- halo stars formed first bc gravity caused gas to contract
- rotates and flattens into disk
- star-gas-star cycle supports ongoing star formation in the disk
why is the central object (sagittarius A) a black hole?
anything that is very massive (4 million solar masses) in such a small space is a likely a black hole
tidal forces also tear apart chunks of matter about to fall in
