Astrobiology Block II

What are the different regions of the EM spectrum, from longest to shortest wavelength?

Radio, Microwave, Infrared, Visible, Ultraviolet, X-Rays, Gamma Rays

What is the relationship between frequency and wavelength?

Wavespeed = Frequency x Wavelength

What is the Doppler effect?

change in wavelength / original wavelength = relative velocity / speed of light

What is the quantum nature of light?

Light travels in discrete packets of energy (quanta) called photons, E = hf

How do you calculate the energy required to move an orbital electron in a hydrogen atom from level m to level n?

E(n) - E(m) = R(1/n2 - 1/m2)

What is the difference between an absorption line and an emission line?

Absorption line = light emitted by a white light source after passing through a gas. Emission line = light emitted by a glowing gas

What is a blackbody?

An object which perfectly absorbs all EM radiation

What are the characteristics of the black body spectrum?

Emits energy at all wavelengths, hotter blackbodies emit more energy per unit area per unit time, hotter objects emit a greater proportion of light at shorter wavelengths

What is the Stefan-Boltzmann Law?

F = 𝜎 T4, hotter blackbodies emit more energy per unit area per unit time

What is Wien’s displacement Law?

𝜆max = R/T, hotter objects emit a greater proportion of light at shorter wavelengths

What are Kirchhoff’s 3 Laws?

A hot, dense object emits a continuous spectrum; a hot low-density gas produces bright emission lines at characteristic wavelengths; cool gas in front of a continuous light source produces dark absorption lines

What does it mean for there to be a balance between pressure and gravity?

In massive objects, Fgravity = Fpressure so that the object does not collapse in on itself or dissipate into Space

What does E = mc2 mean?

Energy and mass are interchangeable

What are Newton’s 3 Laws?

An object in motion stays in motion, F=ma, every force has an equal and opposite reaction force

What is Newton’s Law of Universal Gravitation?

F = GMm/r2

What is centripetal force?

The force acting perpendicular to an object’s motion and towards the centre of the circle, when that object is in a circular orbit. FCP = mv2/r

What are the 3 cosmic velocities?

1st: minimum horizontal speed for stable circular orbit, 2nd: minimum speed to completely break free from Earth’s gravity, 3rd: minimum speed to break free from Earth’s and the Sun’s gravity

What are Kepler’s 3 Laws?

1st: the orbit of each planet is an ellipse with the Sun at 1 focus, 2nd: radius vector to a planet sweeps out equal areas in equal time, 3rd: T2 = kr3 / P2 = ka3

How do we know the universe is expanding?

Objects far away from us are red-shifted, so they are moving away from us.

What does the expansion of the universe tell us about the origins of the universe?

Universe is expanding -> everything used to be much closer together -> the universe used to be much hotter -> there was a hot Big Bang

How do we know dark matter exists?

Galaxies don’t rotate faster at the centre, suggesting there is mass we cannot measure on the outside of galaxies which causes them to rotate at a consistent speed throughout.

How can we see the past?

Light has a finite speed

Why is there a finite limit to the observable universe?

The universe is 13.8 billion years old, so light from further than 13.8 billion lightyears away has not had time to reach us yet

In the cosmic calendar (where 13.8 billion years = 1 year), when did humans first evolve?

8pm on Jan 31st, 1 human life is 0.23 seconds

What are the proportions of dark vs ordinary energy and matter in the universe?

70% dark energy, 25% dark matter, 5% normal matter

How many stars are there in the Milky Way?

1011

What defines a high-mass star?

Has a mass greater than 8 Solar Masses

Under what circumstances did the first stars form?

The universe was 100 million years old, they were made entirely of H and He, they formed the first heavy elements

How does a star form?

The force of gravity overcomes thermal pressure, forming a compressed cloud of rising temperature, resulting in nuclear fusion

What is radiative diffusion?

Inside a star, photons are continually absorbed and re-emitted, gradually diffusing towards the outside (cooler region) of the star, so that it emits light

Why is nuclear fusion a source of energy?

The mass per particle is smaller after the reaction than before the reaction, so energy is released

Why do high-mass stars have short lifetimes?

The huge mass causes gravity to be so strong that nuclear fusion happens faster and uses up the material in the star’s core

What is the life cycle of a low-mass star?

Hydrogen core fuses into helium core via proton-proton chain. Star in equilibrium in main sequence for a long time until hydrogen is exhausted. When inert helium core remains, hydrogen shell burning begins, core heats up, star expands into red giant. Helium fusion beings in core, size & luminosity of star begin to decrease. When helium is exhausted, inert carbon core left, star expands, outer layers of star are shed (planetary nebula), leaves behind a white dwarf.

What is the life cycle of a high-mass star?

Hydrogen core fuses into helium core via CNO cycle very quickly due to high gravity. Star in equilibrium in main sequence for a short time until hydrogen is exhausted. Advanced nuclear burning forms heavier elements. Hydrogen shell burning, then outer layers expand to form a red supergiant. Successive core burning (He → C → Ne → O → Si ) until iron-rich core remains. Core collapses under gravity, gravity pushes electrons into protons to form neutrons, core collapses into a ball of neutrons, releases enormous amount of energy as a supernova. If collapse halted by neutron degeneracy pressure, leaves a neutron star. Otherwise leaves a black hole.

Why does core burning in a high-mass star stop when only iron is left?

Iron has the lowest mass per nucleon of any element, so fusion of iron requires pressure

How is a protoplanetary disc formed?

A molecular cloud of hydrogen and helium begins to collapse due to gravity, spinning faster (conservation of angular momentum, mvr) as it collapses. A star is born as the cloud of dust collapses, and the remaining cloud settles into a thin proto-stellar disc around the star.

How are planets formed in the protoplanetary disc?

Dust grains condense in the cool disc, then collide to form aggregates, then grow to pebbles, then planetesimals. Planetesimals collide to form planetary embryos. Large embryos sweep up small embryos.

What is the snow line?

The distance from the sun at which temperature is such that heavy elements can condense to form small grains (rock and ice)

What are the types of planets in the solar system?

4 rocky planets (inside snow line), 2 gas giants & 2 ice giants (outside snow line)

What is the nature of the orbits of the planets in the solar system?

Planets all orbit in the same direction, orbits are near circular, on the same plane, mostly rotate in the same direction as orbits (except Venus & Uranus)

What & where is the asteroid belt?

The asteroid belt is between Mars and Jupiter, and is made up of the remnants of planet building. Asteroids are rocky and metal rich of varying sizes (

What are comets?

Comets are balls of dust and ice orbiting the sun with highly elliptical orbits

How does astrometry work?

Astrometry is the tracking of celestial objects’ positions over time. When a planet orbits a star, the star also wobbles due to that planet’s gravity. The movement of the star can be used to detect the existence of an exoplanet. Astrometry can be used to detect that movement.

What are the limits of astrometry?

Astrometry is only useful for detecting exoplanets which are very near to the star

What is the Doppler technique for detecting exoplanets?

When a planet orbits a star, the star also wobbles due to that planet’s gravity. The movement of the star can be used to detect the existence of an exoplanet. In the Doppler technique, the redshift/blueshift of the star can be detected, and from this we can calculate its velocity and detect the existence of an exoplanet.

What information can you get about a star using the Doppler technique?

The period of its orbit

What formula combines the semimajor axis, period and mass?

a3 = (GMstarP2planet)/(4𝜋2) where a = semimajor axis, P = period, M = mass

What is the Doppler shift formula?

Δ𝜆 / 𝜆 = vr / c

What formula combines radial velocity, mass and planet’s period?

Mplanet = Mstarvstar / vplanet

What is the transit technique?

When an exoplanet moves between Earth and its host star, some of the host star’s starlight is blocked. The amount that is blocked tells us the radius of the planet.

How do you calculate a planet’s radius given its host star’s fractional dimming?

fraction of light blocked = (area of planet’s disc) / (area of star’s disc)

What is gravitational lensing?

Gravitational lensing is where the presence of a large mass warps the fabric of space time, causing light to follow the curve created. This causes a new image of the light source (e.g. a star to be created).

What is microlensing?

In microlensing, a foreground star can gravitationally focus light from a background star. If the foreground star has an orbiting planet this may produce an anomaly. So we can detect low mass planets.

What are the limits of direct imaging?

Only works for very dim stars and very bright planets

What is a Hot Jupiter?

A planet with a similar mass and radius to Jupiter with a period of only a few days

How do Hot Jupiters form?

A possible explanation is that they form further from their star and then migrate towards the star over time due to gravity

Why is there a large range of eccentricities in the orbits of exoplanets?

A possible explanation is that in a multi-planet system the gravities of the different planets interact, causing their orbits to become more elliptical

Are other planetary and stellar systems similar to the Solar System?

No, planetary systems are ubiquitous and surprisingly diverse

How can spectroscopy be used for exoplanets?

Spectroscopy can be used to detect the atmosphere of an exoplanet

What is transit spectroscopy?

Primary eclipse: transmission spectrum when the planet transits in front of the host star, absorption spectrum. Secondary eclipse: the difference between the combined star and planet flux, and the star only flux, emission spectrum.

What are the conditions for a planet to be habitable?

It must be in the habitable zone, it most likely must have an atmosphere, and it must have geological activities

What is the habitable zone?

The zone in which it is possible for a planet to have liquid water on its surface

How do we find the surface temperature of a planet?

Tss = (R/rp)1/2To where R is the radius of the star, rp is the distance from the star to the planet, and To is the temperature of the Sun

What is the surface temperature of a planet?

The temperature at the very top of the planet’s atmosphere

What is an atmosphere?

A layer of gas surrounding a planet, made up of different gases. Atmospheres are thing for terrestrial planets

Why are atmospheres important?

Keeps surface warm enough for liquid water, drives weather & climate, scatters light, protects from radiation, destroys small asteroids

What is the greenhouse effect?

The atmosphere traps heat, giving it a second chance to be absorbed by the planet’s surface

What are the sources of atmospheric gas?

Volcanic outgassing, evaporation & sublimation from water, micrometeorites, life

How can a planet lose its atmosphere?

Condensation of gases, chemical reactions, major impacts, solar winds, thermal escape (when gas has enough energy to escape into space)

How can a planet retain its atmosphere?

With a large mass and low temperature can retain all gases in their atmosphere

What geological activities are important for life?

Volcanic outgassing produces an atmosphere, carbon recycling stabilises the climate, a planetary magnetic field drives currents

How is internal heat acquired in a planet?

Accretion (accreting planetesimals releases GPE), differentiation (dense materials sink into planet and release GPE) and radioactivity

How do planets cool down?

Convection in the liquid mantle, conduction to the surface and radiating light into space

What property causes certain planets to cool faster than others?

Small planets cool faster