MULT10011 - Astrology

describe the dawn of scientific astronomy

plato and aristole made simple models that were tested against observations

predicted planet/ star motions by placing them on rotating spheres around the earth

8 spheres with outside one for the stars

ptolemy added epicycles as motions didn’t always look like circles

heliocentric model

first mathematical version

everything orbited the sun

same epicycles as Ptolemy

heliocentric mode - arguments against

if earth is orbiting the sun, why don’t the position of stars shift during the year → parallax

delayed support for the heliocentric model

why wasn’t parallax observed

distance the Earth moves is very different to that of the stars

the eye’s resolution is too low to see

the distances are too large to see parallax → changes are miniscule

main achievement of telescopes

not everything orbits the earth → disproves geocentric model

Galileo discovered moons around Jupiter, phases of Venus and apparent rotations of the Sun

telescopes and parallax

velocuty of the Earth related to parallax → angle of stars related to motion of the earth

size of the universe

unknown → can only see 10²26m

can only say for sure how big the bit we can see is → light hasn’t reached us yet

why was it thought that the Earth was at the centre of the galaxy

light on either side of the milky way looked even

einstein and a static universe

if universe was static as previously believed, theory of general relativity would mean that the Universe would collapse to a singularity

added a repelling force (cosmological constant) to balance forces on large scales

cosmological constant was unnecessary as universe later discovered to be expanding → naturally repelling

disproving the Earth being int he centre of the Milky Way

found stars much much further away than expected with out Sun not at centre

brightness of stars used to estimate distance away

the great debate and nebulae

1. nebulae = clouds of dust in the Milky way → milky way = entire universe

2. nebulae = like the Milky Way → other galaxies that are just too far away to resolve individual stars

settled when Hubble identified stars in andromeda galaxy and measured them

• distance outside of galaxy

spectrographs function

instrument to measure intensity as a function of wavelength

different elements have diff spectra → stars = balls of burning hydrogen and not just really hot/big Earths

Hubble’s data for an expanding universe

1. everything is moving away from us

2. the further object are away → the faster they move

cosmic address

Earth → solar system → milky way galaxy → local groups of galaxies → local supercluster including Virgo → cosmic web

key features of big bang model

1. homogenous

2. expanding

3. very hot and dense at early times → cools off as it expands

meaning of a homogenous universe

if we teleport anywhere in the universe, it will look the same statistically on average

universe is homogenous in space by not time as it evolves and changes with time

implications of an expanding universe

1. there was a beginning

2. either we’re at the centre of the universe or everything is moving apart

Hubble’s law (how fast the universe is expanding and the age of the universe)

• V = H*d

  • H = Hubble's constant

  • t = time

• V = d/t

  • t = 1/H -> age of the universe

cosmological redshift meaning

as the universe expands, distances between objects becomes larger and light become redder as wavelengths become longer

if universe was contracting → wavelengths would become shorter → bluer

theory of beginning of the universe

going back in time, universe gets denser, smaller and hotter until it reaches an impossibly small size → beginning of the universe

data support a hot and dense early Universe

cosmic microwave background radiation = remnant of radiation from primordial fireball

at start → early universe = hot plasma with protons, photons and electrons tightly coupled → when matter becomes very hot → electrons and protons separate => as universe expands it cools until protons and electrons combine into hydrogen and photons escape (3000K)

could we have seen the primordial fireball a billion years ago or in the future

yes, the fireball would show us at bluer or redder wavelengths

components of the universe (3)

1. dark energy - 73%

2. dark matter - 22%

3. baryonic matter - 4.4%

dark energy spectroscopic instrument

one of many experiments seeking clues into dark energy by using galaxies to trace particles to study how the universe is expanding

what gravitational lensing has shkwn when 2 stars clash

mass from stars collide but dark matter would continue through the mass

how is the weight of astronomical objects measured

by the rate and distance of its orbit around an object

is the universe perfectly homogenous

No.

it started as having a very smooth distribution of dark matter and baryonic matter but it wasn’t uniform → small over-densities that collapsed under gravity

gravitational cascade

small tiny gravitational pull towards slightly denser regions → increased gravity as more mass falls into dense area and gravitational attraction increases

how are stars formed

supernova triggers clouds of gas (mainly helium and hydrogen) to collapse under gravity, increasing its spin and forming a dense object at its core

if density and temp becomes high enough, thermonuclear burning woll start and the star will shine

what does the mass of a star determine (3)

how hot it burns

how long its lasts

how it dies

blackbody radiation

type of electromagnetic radiation emitted by an object that absorbs all radiation falling on it → re-emits energy at a characteristic spectrum that depends only on its temperature.

the hotter objects are → the more high energy photons emitted → the shorter the wavelengths of light

hertzsprung-russel diagram

plots luminosity against temp of each star

location on main sequence completely determined by star’s mass

location off main sequence shows stars at later times in their evolution

stellar evolution (5)

1. star forms as gas cloud collapses

2. star burns hydrogen in its core

3. star burns hydrogen in its outer shells

4. star throws off its outer shells

death of our sun steps

1. gradual warming and denser core as star burns hydrogen to helium

2. star becomes red giant and will encompass all of the solar system

3. star becomes white dwarf and will slowly cool as it radiates off its energy

what determines the end point of a star

its mass

low mass star death

up to 1 solar mass

red giant → hydrogen burning moves out towards other shells of star → less mass flowing in → size of star increases → core collapses into white dwarf whilst outskirts blown off as planetary nebula

red giants

big yet relatively cold

expand because fusion is happening closer to its surface as it begins to lose mass

eventually helium undergoes fusion to form carbon and star gets slightly smaller as it collapses

planetary nebula

illuminated gas clouds

white dwarf

how ember of degenerate matter which cools with time

high mass star death

>10 solar masses

after burning all the hydrogen, will burn heavier elements until iron -. final collapse and burning will be fast and furious → star forms super-heavy core while remaining as is blown off as supernova

core will either become neutron star or black hole if original star was heavier

neutron star

degenerate star made of tightly packed neutrons → usually observed as pulsars

neutron star matter = most extreme form of matter known

form and behaviour studied by looking at characteristics of pulsars

black holes

region of space of such high density that not even light can escape

can’t be seen, only the stuff being sucked in

does have a minimal orbit before being sucked in

escape velocity

the speed at which something needs to move to escape the gravitational force of a second object

V = sqrt( (2GM)/r) ) where M = mass of object, r = radius, G = gravitational constant

as r gets very small or M gets very large, v becomes very large

escape velocity from earth. = 22.2km/sec

where do new elements next gen of stars and planets come from

supernovas and nebulae

waves are…

regular oscillations

types of waves

involve moving particles in the medium:

• pressure/ density → sound

• displacement → vibrating string

• surface → some ocean waves

don’t involve particles:

• electromagnetic → light

• gravitational

electromagentic waves have ____ and ____ fields perpendicular to each other and direction of motion

electric; magnetic

if the frequency of light is ___, the wavelength is ____ and the speed stays ____

doubled; halved; constant

issues with treating light as waves - summary

1. atoms exist

2. photovoltaic effect and photosynthesis

issues with treating light as waves - atoms exist

light as a classical wave predicted that atoms could not exist → ultraviolet catastrophe

electrons would emit energy and spiral into the atomic nucleus

issues with treating light as waves - photovoltaic effect and photosynthesis

power in a wave is proportional to the square of its amplitude times the square of its frequency- → same instantaneous power can be produced at any frequency of light

solar panels and plant chloroplasts can only absorb specific frequencies of light regardless of total power

1905 Einstein proposed the photon → light is not just a wave but make of tiny particles (photons) which carry a specific energy based on its frequency

why can we only see visible light

eyes have evolved to work where the Sun is the brightest → ca only see the majority of the lgiht that hits the surface of the Earth

nuclear reactions - definition

reactions that change elements

can only take place at a very high density, pressure and temperature

nuclear reactions - conditions

very early universe, a few seconds to minutes after the big bang, was extremely hot and dense → still ongoing in stars

• photons had the energy like protons, neutrons, etc

• photons has so much energy they could create or break matter matter when they collided

as the universe expanded and cooled - photons and matter

there was a characteristic temperature of the photons and matter that corresponded to an energy for the photon → photon of sufficient energy could form a proton → all electrons, protons and neutrons formed this way

Big bang nucleosynthesis (2)

1. heavier elements built by combining lighter ones

2. given enough time, we’d get all the way to iron → ran out of time and stopped after making helium because universe gets too cold

how does changing the rate of the universe’s expansion affect the relative abundance of elements

changes the amount of time for BBN → slowing down rate allows more heavier elements to form

nucleosynthesis in stars

core of stars are very hot and dense → nuclei fuse to form more massive nuclei → favourable if energy is released

why is the end stage of stars short

energy difference between helium-4 and iron-56 is very small

why does helium-4 form carbon-12 instead of lithium-7

forming lithium would be energetically unfavourable as it consumes energy rather than released it

however helium-4 and carbon-12 is very slow → needs three helium-4 to fuse together

how are elements formed

hydrogen and helium - big bang

lithium, beryllium, boron - cosmic rays

C, N, O, Ne, S → small stars

top half → mostly large stars

bottom half → supernovae

cosmic rays

particles travelling through space near the speed of light → still debated how they are accelerated

how do cosmic rays produce very light and unstable elements in interstellar space

high speed cosmic rays strike an object and send spray of light nucleons → light and unstable elements

• very slow and rare

eg. beryllium, lithium, boron

how are the star’s elements spread

shedding of outer shells in red giant phase

supernovae sprinkle the galaxy with metals created inside stars → re-enrich clouds that form new stars

ending of a white dwarf

1. birth of white dwarves starts off very hot

2. hot objects radiate away heat

3. cools to absolute zero over 10s or 100s of billions of years

4. invisible black dwarf is left behind

solar nebular hypothesis

soalr system formed from the gravitational collapse of a giant molecular cloud 4.6 billion years ago

collapse of the molecular cloud may have been triggered by a shock wave from a nearby supernovae → cloud was enriched with elements that only form in supernova explosions of massive stars

observations of our solar system

all planets orbit the sun in the same direction and very nearly int he same plane

sun has 99.9% of mass and planets have 99.7% of angular momentum

most planets rotate int he same direction as they revolve around the sun

inner planets are terrestrial

outer planets are jovian

all giant planets have regular moon systems

theres left over junk → asteroids and comets

sequence of solar system’s formation - summary

1. collapse

2. contraction

3. formation of planetesimals

4. formation of planets

5. leftover junk

sequence of solar system’s formation - collapse

large, irregular, weakly rotating cloud of gas begins to collapse due to gravity

sequence of solar system’s formation - contraction

most of the mass collects at the centre → when hot enough, sun turns on

rest of cloud forms a disk → increases its spin, flattens, heats up and becomes more dense → pre-stellar disk

sequence of solar system’s formation - formation of planetesimals

once the disk reaches a stable configuration, it begins to cool → because sun is heat source, will cool from the outside in

solid grains start to stick together to form larger grains → eventual building to planetesimals

sequence of solar system’s formation - formation of planets

once planetesimal is big enough, will gravitationally attract others as well to accumulate mass to grow into planets

composition of planets is determined by distance from the sun → forming further away from the sun = cooler → hydrogen and helium doesn’t evaporate away

planet formation ended when Sun turned on and solar wind cleared out all the remaining gas and dust

sequence of solar system’s formation - leftover junk

outer solar system = comets and some larger dwarf planets

inner solar system = small rocky objects, asteroids

in outer regions past Neptune, disk wasn’t dense enough to form new planets

light sails

a way to travel to the stars using radiation pressure to reach stars

comets

small icy bodies leftover from the formation of Jovian planets

comets near jovian planets either deflected into the inner solar system to evaporate near the sun or defelcted to the edges of the solar system to the Ort cloud or Kuiper Belt where they remain in cold storage

dwarf planets - properties

big enough to be round → gravity is strong enough to pull mass into core

orbit the sun → moons orbit a planet, not the sun

too small to clear their orbital neighbourhood → ceres has not cleared its orbital neighbourhood (in asteroid belt)

formation of the moon - giant impact theory

a body, a bit bigger than Mars, hit the Earth 50 million years after its formation

loss of material from Earth was ejected to form an orbiting debris field around the Earth → ring of debris quickly condensed to from a single body

Evolution of the Earth-moon system

tidal forces occur when there is a difference in the gravitational force across a single body

side of the Earth nearest the moon feels a stronger force than the side furtherst away → oceans react to this difference to create twice daily tides

tidal locking - our moon

friction between ocean and land causes the loss of energy in the orbital system → Earth and moon are moving further apart and days are getting longer

when it formed, the moon was much closer → ocean tides were much larger and the day was shorter → over time, orbital period of the moon and day on Earth will be the same

moon is already locked

importance of the moon for evolution of life

1. moon stabilises rotation axis of the Earth and stops it from changing rapidly → stable climate

2. tidal forces form the moon and the sun created tidal pools → nutrient-rich and flushed twice daily → might have been cradles for the formation of the first biological cells

terrestrial planets in our solar system - key properties

composition of rock and metals

comparable size to Earth

close to the sun and therefore warm and hot

atmosphere present on Earth and Venus, traces on mars

few moons

jovian planets in our solar system - key properties

sun-like composition -. hydrogen, helium, trace amounts of other stuff

big size

far from the sun and therefore cold

gaseous atmosphere, no surface

many moons in a miniature solar system

have no promise of life, but their moons are promising

keeping an atmosphere - physical characteristics (2)

1. temperature of the gas

2. gravitational field at the surface

larger velocities allow an object to escape

keeping an atmosphere - temperature of gas

all moelcules at the same temperature have the same kinetic energy → for a fixe temperature, lighter molecules have a higher velocity than heavier molecules

kinetic energy and velocity increases with temperature

keeping an atmosphere - gravitational field at the surface

gravity is higher for more massive or dneser planets

what kind of planets can keep what type of atmosphere

small hot planets can’t keep light molecules like hydrogen but can keep heavier ones like CO2

big cool planets like Jupiter can keep everything from hydrogen onwards

why does Earth not have a hydrogen and helium atmosphere

too small to have strong enough gravity to hold onto very light gases like hydrogen and helium

too warm so any hydrogen or helium it picked up would have escaped into space

How did Earth get its atmosphere

outgassing from rocky interior → volcanoes

comet impacts

formation and escape → some gases escaped into space and other chemically absorbed by surface rocks

chemical reactions with rocks → CO2 dissolved in water and reacted with rocks, reducing its amount in the air

evidence of Earth being bombarded with space debris

comet mrkos, asteroid ida and its moon, tunguska event in siberia

brownlee particles → tiny dust grains from comets found in the upper atmosphere = continued comet dust capture

what is the argument about the moon’s formation and the Earth’s water

the impact which formed the moon would have vaporised all the water on Earth and left the surface superheated

however Zircon crystals contain oxygen isotopes suggesting they formed near liquid water → strong evidence that Earth had water shortlet after the moon formed

implications of zircon crystals

the earth must have cooled quickly and or the water was delivered again via impacts after the moon-forming event

where did Earth’s water come from

likely a combination of volcanic outgassing and impacts by icy comets and asteroids

Early ideas on canals on Mars

believed they were built by intelligent beings, novel written → war of worlds

better telescopes showed no constructed canals in 1909

requirements for life - general

heavy elements: O, C, H, N, P, S

protective shield → atmosphere or liquid

energy source → probably a star

liquid water or suitable substitute

stability

maybe liquid or solid surface, and a moon

requirements for life - out solar system

heavy elements: O, C, H, N, P, S

protective shield → atmosphere or liquid

energy source → probably a star

stability

moon

important properties of water and presence in the solar system

large temperature range for a liquid

relatively warm liquid range

solid form is less dense and floats

polar molecule

present on Euopa and Enceledus

unlikely to be present on Mercury and Venus

potentially present on the moon and Mars

how to find water on the moon

hit the moon really had with booster rocket to induce a plume → debris and sublimating H2O comes up → UV radiation ionises water → OH atmosphere with residual hot debris

water is likely widespread deep under its surface

frozen water in shadowed craters on moon’s surface and ice under gray dust

argued that water is only stable at high latitudes and varied with the lunar day towards the equator → water molecules would move around until trapped in a local cold spot

requirements for life - europa

has ice and oceans → fairly smooth surface which is young and cracked

contains heavy elements

has a protective shield

hydrogen peroxide is abundant on surface → highly oxidising and decays O2 when mixed with water → could provide chemical energy for life forms

is stable

what was detected in plumes of water vapour off Enceladus

>200amu organic molecules

water on mars - polar ice caps

has polar ice caps which grow and shrink with seasons

mars has seasons due to tilt of its axis → ice caps appear and disappear at the poles seasonally