Lecture 11 - Hazards to life in the Galaxy

what do you need to understand how and where the galactic habitable zone

first understand the structure of our galaxy, the milky way

where does the light from the milky originate

from the accumulation of unresolved stars and other material when viewed in the direction of the galactic plane

how far away is the sun loacted from the galactic centre

25,000 ly away

how large is the bulge across

10,000 ly

how large is the disk in diameter

100,000 ly

what is the bulge?

primarily made of old stars, gas, and dust

what is the disk

contains mostly young stars, gas and dust, which are concentrated in spiral arms. Some old stars are also present

what is the halo

primarily contains individual old stars and clusters of old stars
("globular clusters").

how old would the milky way’s halo across

130,000 ly across

what does the milky way contain

dark matter, which is material that we cannot see but whose gravitational force can be measured

how far away is andromeda

2.5 million lightyears away

is andromeda galaxy bigger than the milky way

yes

what is in the centre of andromeda

two types of star population scould be identified: young, high velocity stars in the disk and the older, red stars in the bulge

what do spiral arms contain

contain many young, hot, luminous blue stars which makes the arms bright

why do the spiral arms contain young stars

due to the high mass density and the high rate of star formation

what is unsual about the orbit of our solar system

it is significantly less elliptical than those of other stars similar in age and type

why is the solar systems orbit important for life

it is more circularity in sol’s orbit prevents it from plunging in to the inner galaxy, which is place we want to avoid as cosmic radiation and super-novae are more common

how is the tilt of the solar system important

it has a tilt that lies about 20 ly above the plane which avoids abrupt crossings of the plane - reducing the chance of stirring up sol’s oort cloud and bombarding the earth with comets

how is co-rotation important

orbiting very close to the co-rotation radius of the galacy where the angular speed of the galacy’s spiral arms matches that of the stars within

why is does co-rotation help the earth avoid disruptions

the solar system avoids crossing the spiral arms very often which would again expose earth to more frequent supernovae and comets encounters

what would happen if metallicity in the solar system was higher

• terrestrial planets would be larged with stronger graity and richer in volatile compounds, but poor in topographical relif

  • as a result they will probably tend to be completely covered in water with extreme weather

how could high metallicity be bad for life

as mix of land and sea plays a major role in earths atmsopheric temperature control

• could also lead to the formation of a large number of gas giants, disrupting the orbital dynamics of the system and altering the habitability of terrestrial planets in the system

how does metallicity vary in the millky way

outer regions of our galaxy are unlikely to have rocky planets because of its low abundance of elements beside H and He

why is too far from the galactic centre bad

• the supply of good materials falls of with the distance

• density of stars also falls off with distance

why is too close from the galactic centre bad

• crowded have higher supernova rates and more likely that a rocky planet is exposed to its intense radiation

what is concerning about heavy-metal abundance

• probability of formation of rocky planets increases with metalicy although very low metallicity could be enough to make one or more earth like planets

how could the galactic centre be a threat to life

• the nucleus in the milky way is dormant but observations suggest that the central back hole occasionally turns on resulting in a burst of high energy radiation

why is high energy raditation a problem for life

can ionise the atmosphere and wipe out the ozone layer

how could high energy raiation be fine

the planet’s magnetic field can fend off most partucle radiation and its ozone layer can screen out danger electromagnetic radiation

why are supernovae threatining

the high concentration of older stars there

what % do supernovae peak of the suns distance from the galactic centre

60%

what is the killer radius of a supernovae

100 light years

what is the rate of occurence for supernovae

1 per 40 years in normal galacies

how many star systems are at a distace of 15kpc from us

50

how often does a supernovaw occur within 33ly of earth

240 myr

what is the killer radius of a supernovae

100 light years

what type of stars have supernovae

O and B

how far away is the closest red giant

88 ly

what are gamma ray bursts

the most powerful explosions that have been observed in distant galaxies since the big bang

why are gamma ray bursts a threat for lfe

• the radiation could quickly deplete earth’s ozone layer allowing an increase in solar UV radiation reaching the surfaceth

• The rays pass through the human body and can cause ionisation that damages tissue and DNA

when are gamma ray bursts

found at cosmological distances

• mostly occur in the early universe and hence the threat to life in our galaxy is unclear

how might gamma ray bursts have affected mars

no atmospheric shielding

what are places to avoid and why

• global clusters - low metallicity

• nebulae - newly formed giant stars with strong winds could evaporate protoplanes and ionised gas hazard affecting the atmospheres

• massive young stars - lots of UV radiation and short lives

• galactic centre - intense radiation and unstable orbits

• supernova remnants - debris from stellar explosion

why is earth a special place

• distace to sun

• atmospheric composition

• magnetic field

• plate tectonics

• role of jupiter

• role of moon

• climate and ice ages

• chemical composition of earth

• galactic location of solar system

• type of galaxy