sci238.txt

# L3 - Astronomy as a science

* What is the astronomical unit?

The average distance of the Sun from the Earth.

* What are a Parsec?

The arcseconds in which a star shifts in the course of a year.

* How do we measure the distances of very far stars?

By comparing the apparently brightness of the stars. Since we can assume that
similar stars have the same intrinsic brightness.

# Earth

* How is the Earth heated?

- Leftover heat from formation
- Gravitational compression
- Radioactive decay
- Heavy elements sinking creates friction

* How are Earth's magnetic fields generated?

From the convection of metals in the Earth's core.

* What is the Earth's magnetic field do?

- Protects the Earth from Sun's solar winds (protects atmosphere).

* What is the line between Earth's atmosphere and space?

100 km.

* Where is the Earth's atmosphere warmest?

The surface.

* What is Ozone used for?

Protects against UV radiation.

* How are the northern lights formed?

Solar wind particles interacting with the atmosphere.

* What are the two types of Earth's crust?

Continental and oceanic.

* What does CO2 do in Earth's atmosphere?

Traps Infrared radiation, heating up the Earth.

# Moon

* How many moons are in the solar system?

Around 160.

* What is the rank of the moon's size?

5th.

* What is the internal core of the Moon?

Molten, hot core, much like the Earth.

* What are the highlands, and the marias?

Highlands: Cratered. Older rocks.
Maria: Smooth. No craters, younger rocks. Formed from lava overflow.

* What is the surface like on the far side of the Earth?

There are no maria. Thicker crust.
This is caused because during the formation of the Moon, the Earth was hot,
which caused the maria.

* What is the Giant Impact Hypothesis?

During the formation of the Moon, there were lots of large orbiting objects.
There was a big grazing collision from another "planet" and Earth.
The material ejected from this collision formed the Moon (from both Earth and
the other planet).

Evidence: Composition of some rocks on the Moon seem to be from the other
planet.

* What is the Late Heavy Bombardment?

A bombardment of comets from the outer solar system.
Caused the highland craters.

* What is Tycho?

A big, very visible crater on the moon.

* What are skylights?

Holes leading to the underground lava tunnels of the Moon.

* Where does water exist on the moon?

Near the poles, where in craters where there is no sunlight. Exists as ice.

# L7 - Terrestrial Planets

## Mercury

* What are some quirks about Mercury's orbit?

- Ellipetical
- Tidally locked at 2 years/3 days.

* What are the features of Mercury's surface?

- No atmosphere
- Lots of craters

# Lecture 9 - Small solar system bodies

## Asteroids

* What is the section that exists between Mars and Jupiter?

The asteroid belt. ("The Main Belt")

* What is the makeup of asteroids?

Usually carbon or silicon.

* What are Kirkwood Gaps?

Ring gaps in the asteroid belt due to Jupiter's orbital influence.

* What is the size of all the asteroids combined?

Smaller than the moon.

* What are some names of asteroids?

Ida, Ceres, Vesta.

* What is Ceres? What is its makeup?

- One of the larger asteroids in the asteroid belt.
- Has water in its core
- Has craters in the surface.

* How did the main asteroid belt form?

During the formation of the solar system, Jupiter ate a lot of the mass, but
there were still some left over. This leftover mass formed into smaller spheres
that were then influenced by Jupiter's mass.

## Comets

* What are comets usually made of?

Ice and dust.

* What are the two parts of comets?

The coma (the tails) and the nucleus (the core).

* What are the two tails of comets?

The dust tail, which glows white.
The ion tail, which glows in a color related to the Gas.

* How are comets classified?

By their orbits:
- Short-period comets (<200 years). Usually near the ecliptic
- Long-term comets (>200 years). Can appear anywhere

* What is outgassing?

The release of gas in comets due to the Sun heating the comet up. The gases
usually come from discreet places in the comet. There are "gas vents".

* Where do comets come from?

The Oort cloud.

* What did we find from comet samples?

Carbon molecules, including amino acids.

## Oort Cloud

* What is the Kuiper Belt?

A region just outside of Neptune where things there have stable orbits.
Pluto is part of the Kuiper Belt.
100000 objects, >100 km wide.
Source of short-period comets.

* What is the Oort Cloud?

The origin of long-period comets. A sphere of material around the solar system,
that extends very far away.

* What are some Kuiper Belt objects?

Krus, Charon, Pluto.

## Meteors

* What makes meteors hot?

The compression of the material in the meteor as it falls.

* Where do meteor showers come from?

Comets create a cloud of debris as they travel around the Sun. When the Earth
runs into this cloud, we get meteor showers.

* How do we date meterorites?

Carbon dating, or radioactive decay.

# Lecture 10 - Extrasolar planets.

* How are exoplanets first detected?

The star's orbit is also shifted by the planet. We can detect that shift as a
Doppler shift in the star's light.

* What are hot Jupiters?

Short-period, big planets. These were the first exoplanets that were discovered.

* What is a transit?

The exoplanet traveling in front of the star from the perspective of the Earth.
We can measure the drop in brightness caused by transits, and the amount the
sun dims by also tells us how large the planet is.

# Lecture 12 - Stars

* How are the fundamental distances to stars determined?

From stellar parallax, leveraging the orbit of the Earth throughout the year.

* What is the parsec?

The distance from which one would have a parallax of 1 arcsecond.

* What are the first stars measured using parallax?

61 Cyngni, by Dessel and Vega, by Struve.

* What is a star's brightness proportional to?

1 / distance^2

* What is the difference between brightness and luminosity?

Brightness is the total light energy collected from an object, while luminosity
is the total light energy emitted.

* What is the magnitude system for measuring brightness?

Invented by Hipparcos and refined by Ptolemy. A log scale for brightness.
First magnitude stars are 100 brighter than 6th magnitude stars.

* What is the difference between apparent and absolute magnitude?

Apparent: Brightness measured or observed.
Absolute: Brightness a star would have a distance of 10 parsecs.

* What determines the colour of a star?

Mostly temperature. Red (1000K) is cooler than blue (10000K).
Dust can also redden starlight.
The ratio of the brightnesses in two wavelengths.

* What is the standard classification of stars?

The OBAFGKM classification, which was later added to include subtypes (#s) and
widths of spectral lines (roman #s).

* What are brown dwarfs?

Objects that formed like stars, but didn't have the mass necessary for hydrogen
fusion. However, they can fuse Lithium or deuterium.

* What were brown dwarf's impact on the spectral classification system?

They didn't fit the classification system, and instead were found to be two
distinct types of stars.

L dwarfs: cooler than M9 stars, strong lines of metal hydrides, water absorption
T dwarfs: even cooler, dominated by bands of water and methane.

* What is the differences between stellar spectra and photometry?

Spectra requires bigger telescopes, but gives fine details.
Photometry gives good estimates, but is easier to do.

* What is the HR diagram?

Named after the people who invented it, it is a mass/spectra plot of stars.

* What is the main sequence w.r.t stars?

The line where most stars lie around in the mass/spectra plot.
The place where stars spend most of their lives, they do not move in the main
sequence.

* What are binary stars?

Two or more stars in orbit around a common centre of mass.

* What are the 4 kinds of binary stars?

Visual: Can see both stars
Astrometric: Only one star is visible, which moves around in a loop/wave.
Spectroscopic: Only one star is visible, shows no visible motion, but we can
detect with doppler shifts.
Eclipsing: One star passes in front of the other.

* What determines the lifetime of stars?

1. The amount of fuel (mass); and
2. The rate at which the star uses up the fuel.

Most massive stars have the shortest lives.

* What is the assumption we make about star clusters?

All stars in a cluster are roughly the same age, because they are form at the
same time in the same gas cloud.

* What are the 3 types of star clusters?

1. Associations: 100 stars
2. Open: 1000s stars
3. Globular: 100000 - 1000000 stars

* What does stellar motion tell us about stars?

Motion tells us about the gravity acting on the stars. More motion means
stronger gravity.

# Lecture 13 - Star clusters

* What are open clusters?

Loosely bound clusters of stars. Just formed by mutual gravity.
All stars orbit their collective center of mass, but might not be all in the
same plane.

* What are globular clusters?

Clusters of 100000s of stars in a spherical shape. These clusters are usually
very old. We assume they were the first stars to form.

* What is an assumption with star clusters?

All stars in the cluster are approximately the same age. This means that you can
determine the age of the cluster by measuring which stars are still on the
main sequence.

* What is an major assumption in stellar astrophysics?

If two stars have identical spectra and are on the Main Sequence, then they
are identical stars.

* What are pulsations in variations stars?

The star increases and decreases in size, which make also cause the surface
temperature to also vary.

* What are the 3 types of variable stars?

1. Pulsating stars.
2. Eruptive variables.
3. Rotating "spotted".

* What are the 3 common types of pulsating stars?

Cepheid: Period of 3-50 days, 0.2 magnitude variation, -1.5 to -5 absolute.
RR Lyrae: Period <1day, 1 absolute magnitude.
Mira: Period 80-600 days, large range in brightness

* What is the instability strip?

The place in the HR diagram that contains pulsating stars. This is caused by the
fact that heat cannot get out of the star, so it must pulsate instead to
release the heat.

* What is the period-luminosity law of cepheids?

Knowing L, b, we can find their distance (b = L/4pid^2)

Cepheid's pulsation period is related to its luminosity (and therefore radius).

* What are novae?

Stars that, one in a while, become much brighter (10000x) than before. Some
stars will repeat this behaviour decades/centuries later.

* How common are stellar collisions?

Very rare, but they can happen in dense clusters of stars.

## Nebulae

* What is a nebula?

Clouds of dust and gas in space.

* What is the difference between emission and reflection nebulae?

Emission: Illuminated by their own light.
Reflection: Illuminated by nearby stars' light. (More dusty)

# Lecture 14 - Star Formation

* What is the Interstellar Medium (ISM)?

Gas and dust between the stars.

* What are molecular clouds?

Higher density interstellar clouds. They have enough dust to block visible light
from stars. Stars are born in molecular clouds.

* When do interstellar clouds collapse?

When gravity and pressure become unbalanced.

* What is the Jeans Mass?

The maximum mass at which an interstellar cloud is stable. (Won't collapse.)

* What are the 3 main stages of early star formation?

1. Collapse begins; Caused by a shockwave
2. Fragmentation; Many cores formed from a single core
3. Core to protostar; Gas falls to central core

* How do cloud cores collapse?

Into a disk, with a bipolar outflow along the disk rotation axis.

* What is a protostar?

A central star with an accertion disk. Luminosity of star is dominated by
accretion.

* What is a pre-main sequence star?

The early stages of a star, where the surface cools allowing for further
collapse. Luminosity is dominated by contraction of star.

* What factors can inhibit star formation?

Magnetic fields; which inhibits the cloud from contracting.
Radiation from nearby stars; which changes the flow of the cloud.

* How does a pre-main sequence star form into a main sequence star?

Thermal energy escapes from the star, causing the star to contract, causing
even higher pressure and temperature inside the star. At some point, the core
is so hot that hydrogen can fuse into helium.

* How do stars join the main sequence?

Temperature increases. They join in from the "right" of the HR diagram. The more
luminous the star the faster they join the main sequence.

* What does the Stellar Initial Mass Function tell us?

It appears that far more low mass stars are produced than higher mass stars.

* What is the CNO cycle?

In massive stars (with hot enough cores), stars can use Carbon as a catalyst
for H -> He fusion.

* How does energy get out of a star?

High mass: Convective core, radiative envelope.
Intermediate mass: Radiation, outer convection zone.
Low mass: Convective "to the core".

# Lecture 15 - Stellar Evolution

* What happens when there is not enough hydrogen in the stars' core?

Core contraction accelerates.

* What are a star's death fusion cycles?

Successive fusion cycles until core temperature/pressure cannot be reached for
the next reactions to occur. Each cycle is shorter than the last, and more
temperature sensitive.

Eventually, the envelope is shed, leaving just the core behind.

* What is a helium flash?

Because low mass (<2.5Msum) stars struggle to fusion helium, the core becomes
degenerate and sudden ignition causes the core to expand and then shrink
suddenly.

* What happens after Helium is burned?

If the star is above 8Msun, it can still burn Carbon and Oxygen, but otherwise
that's about it.

* What are red giants?

Stars near the end of its lifetime, where it as expanded and cooled down.

* What is a white dwarf?

A star near the very end of its lifetime, it has blown off all of its
surrounding layers, and a carbon-based core remains.

Properties:
- Very dense
- Lots of radiation
- Glowing gas
- More massive the star, smaller the white dwarf.
- Supported by electron degeneracy pressure.

Created when a stellar core less than 1.4Msun dies.

* What is a planetary nebula?

Glowing gas due to energy imparted by a white dwarf shedding its envelope.

* What is the problem with fusing Iron?

Iron absorbs energy when fused, unlike the elements before it. This causes
the star to collapse.

* What is the structure of a high-mass star just before the end?

It has multiple layers, fusing different elements together.

* How does a dying high-mass star move on the HR diagram?

To the straight right, and then maybe back left a bit.

* What is a supernova?

An exploding star. It has a supernova remnant, which is the gas and material
expelled by the star.

* Why are supernovas important for our existence?

Heavier elements are created by supernova, and these get distributed throughout
the universe.

## Neutron stars

* What are neutron stars?

A star made up almost entirely out of neutrons. They are supported by
neutron degeneracy pressure. It is also spinning super-fast due to the
conservation of angular momentum.

Created when a stellar core less than 3Msun dies.

It also has a crust with just "normal" matter, that has been squeezed

* What is a pulsar?

Neutron stars that have two "polar" beams of energy. Neutron stars spin
really fast so that the pulses will wash over the Earth periodically.

* What are magnetars?

Neutron stars with unusually strong magnetic fields. They are the most
magnetic objects in the universe.

* What is a star quake?

Conflict in force between the neutron's core and crust can cause an "earthquake".
This creates a "solar flare" that is insanely powerful.

A star quake once blinded an x-ray satellite, compressed Earth's magnetic field,
ionize upper atmosphere.

## Black holes

* What are black holes?

Created when a stellar core more than 2.8Msun dies.

* What is the event horizon?

The surface of the black hole where the escape velocity is the speed of light.

# Big Bang

* What was the status of our 4 forces in the very beginning of the universe?

The 4 forces were combined into a single unified force.

* What is the timeline of the beginnings of the universe?

0.0...1s: 4 forces were split up.
3m: Subatomics could form
3-20m: Atoms could form, forming H, He, Li and deuterium
20m: Fusion stopped due to cooling.
380000yrs: Electrons could combine with nuclei.

* What is inflation?

At some point, space suddenly expanded sharply. This "smoothed" out the
cosmic microwave background as lumps had no time to form before they were
separated and flattened out.

* Why is the Era of Atoms called the cosmic dark ages?

Because hydrogen doesn't really emit protons, so little information can be
obtained.

* What is the current era of the universe?

Era of Galaxies.

* What are the problems with the theory of the Big Bang?

1. Why is there an imbalance in anti-matter?
2. Where does the structure come from?
3. Why is the large-scale universe so smooth?
4. Why is the density of matter almost exactly the critical density?

* What would have happened if the universe was not at the critical density?

If matter was 10% more dense, the universe would have collapsed by now.
If matter was 10% less dense, galaxies would not have formed.

* How old is the universe?

About 14 billion years old.

# Lecture 16 - The Milky Way

* What is the interstellar medium made of?

Hydrogen, in many forms
Some other common molecules, like water and carbon monoxide.
Cosmic rays, Magnetic fields, dust.

* How does the interstellar medium affect light?

Dust obscures visible light, and reddens visible light by scattering the
bluer photons more.

* What is the cosmic cycle of the elements?

Interstellar gas forms stars. Stars convert elements into heavier elements.
When stars die they put the enriched elements back into the cloud. Therefore
the metallicity of the universe is constantly increasing.

* What is the rotation curve of the milky way?

It is almost fast, which means that there is some other form of matter that is
causing further away objects to orbit faster.

* What is the visible mass of the milky way?

1 x 10^11 Msun

* What is the estimate mass of the milky way?

2 x 10^12 Msun, 20 times the mass of all visible matter.

* What does the Milky Way consist of?

A halo containing most of the dark matter.
A disk containing most of the visible matter
A small bulge in the center.

* What is the Milky Way?

A flat, spiral galaxy.
There are two major spiral arms, two minor arms.

* How far are from the center of the Milky Way?

24000 light years or 8kpc.

* What are the spiral arms of the Milky Way?

They are like "traffic" jams, material is piling up in the arms. The objects
in the spiral arms are transient, the arm is like a wave.

Collisions in the spiral arms also help new stars form.

* What is the bulge?

A cylindrical cluster of very old stars. There is a central bulge, where all
stars (no matter how close) orbit with the same period.

# Lecture 17 - Galaxies

* What was the first other galaxy we discovered?

Andromeda.

* What are the four types of galaxy?

1. Spiral; like the Milky Way
2. Elliptical; Large rounded balls of stars; little gas; no disks
3. Lenticular; Disks with no spiral arms; small amounts of gas/dust
4. Barred spiral; Spiral galaxies with a bar in the middle.

* How are elliptical galaxies defined?

By their ellipticity, as a number.

* What is hubble's constant?

The relationship between galaxies velocity (away from us) and their distance.

* How are tidal tails caused?

By dynamical interaction with another galaxy?

* What is a starburst galaxy?

Star formation that is triggered by a merger between two galaxies.

* At what speed does the plasma in an energetic galactic center move at?

Nearly the speed of light.

* What are the properties of spiral galaxies?

- Grand design: Long spirals.
- Usually have a bar.

* What are the properties of elliptical galaxies?

- Have little gas.
- Probably formed from galactic collisions

* How do we think large galaxies exist?

They cannibalize smaller galaxies around them.

* How do we think ring galaxies exist?

A small galaxy collides with the direct center a big galaxy, flinging out
the stars away from the core.

* What are active galaxies?

Galaxies outputting unusually large amounts of energy. (Gamma rays.)
They are (mainly) caused by the central black holes absorbing material.

* What is a quasar?

Quasi-stellar radio source. Galaxies far away that appears like a single store.

* What is in the center of galaxies?

A huge black hole. We believe that these form at the same time that the
galaxies do.

* What is an accretion disk?

A disk outside of the black hole. Collisions and other stuff heats up the
material inside the disk, making them glow.

* What are jets?

Magnetic fields along the poles of the accretion disk, spiting out material.
We believe that active galaxies are just galaxies with the jets pointed at us.

* What is the local group?

A few dozen galaxies around the Milky Way (and Andromeda).

* What is a galaxy cluster?

10s of millions lightyears across.
Virgo Cluster is one of them.

* What are super clusters?

100s of millions lightyears across.
The milky way is part of the Virgo super-cluster.

* What is the Hubble Deep Field?

The "darkest" part of the sky, and even that has billions of stars and thousands
of galaxies.

# Gamma Ray Bursts

* What were the Vela satellites?

Satellites that scanned for gamma-rays to detect nuclear explosions.
It actually detected gamma-ray bursts from deep space.

* What is the distribution of gamma-ray bursts?

Random positions in the sky, which meant that they were not from the Milky Way
galaxy.

* What is the supernova cause of gamma-ray bursts?

A concentrated supernova burst. It comes from the jets emitted from the poles
of the accretion disks.

* What is the neutron star cause of gamma-ray bursts?

Two neutron stars collide with each other. If their combined mass is > 2.8Msun
then a black hole is formed, and the neutrons form into an accretion disk with
the same jets.

# Dark Matter

* What was Rubin's surprising discovery about gas cloud velocities?

The further away a gas cloud is, the faster they were moving. At best, the
velocities were flatten. This meant that the gravity in the galaxy was
smooth, not concentrated in the middle.

* How much dark matter is there?

6-7 times visible matter.

* What is an axion?

A particle with mass, but does not emit much light. They also don't interact
with normal matter.

* What is gravitational lensing?

Since gravity bends light, it is possible for heavy objects (i.e. black holes)
to bend light, so you can see behind the black hole.

* How did gravitational lensing let us "see" dark matter?

We were able to use gravitational lensing to see that dark matter passed through
each other when two galaxies collided.

* Why was dark matter important in history?

Without dark matter, it wouldn't be possible for large structures to form in the
early beginnings of the universe, since big structures (i.e. galaxies) could not
form with all the energy emitted by stars.

# Lecture 18 - Expanding universe

* How do we determine galaxy masses?

Rotation curve: Works well for nearby, edge-on spiral galaxies.
Velocity dispersion
Motions of galaxies around each other.

* How do we measure the mass of a galaxy cluster?

- Measure the speeds and positions of galaxies in the cluster.
- Measure the temperature and distribution of hot gas between the galaxies.
- Observe how clusters bend light as gravitational lenses.

* What is the mass/light ratio?

The ratio between a galaxy's mass and luminosity, in terms of solar masses.
The Milky Way is 6Msun/Lsun.

* What is the critical density?

The total mass density of the universe required to keep the entire universe
from expanding.

* What is dark energy?

The energy that is causing the universe to accelerate its expansion.

# Lecture 19 - The Big Bang

* What are the two predictions the big bang model makes?

The existence of the Cosmic Microwave Background.
The expected Helium abundance in the universe.

* What was Slipher's observation about the universe?

Slipher determined that the galaxies were all moving away from each other (
because they were all red-shifted).

* What is the velocity of galaxies vs their distance?

The further a galaxy is, the faster they are moving away from us.

* What is the lookback time?

Since light has a finite velocity, the further away a light source is, the
more "in the past" the object is.

* What is the microwave background radiation?

The redshifted light from the early stages of the universe. It was caused by the
emission of photons from the recombination of electrons binding with the
nucleus in the early stages of the universe.

* Why is the Cosmic Microwave Background not from the beginning of the universe?

Because the universe was opaque, since it was filled with particles that could
absorb photons.

* What was the purpose of the Planck Satellite?

To get a higher resolution picture of the Cosmic Microwave Background.

* What is the measured age of the universe?

13.8 billion years.

* What is the rate of the rate of expansion in the universe?

Accelerating.

* What is the observable universe?

The distance at which if light is emitted, it will never reach us. This distance
is "decreasing" everyday as the universe is expanding.

* What is the Planck Era?

The "first instant" of the universe. Our current models are unable to describe
what happened in this era.

* What is the GUT Era?

10^-42 to 10^-38 seconds.
The Universe contained two natural forces: Gravity and The Grand Unified Theory.
Electromagnetic + strong + weak were just all one force.

* What happened when the universe cooled out of the GUT Era?

When the strong force "froze out" of the GUT force, the energy released caused
a sudden and dramatic inflation of the universe.

* What was the Electroweak Era?

10^-38 to 10^-10 seconds.
3 Forces: Gravity, Strong, Electroweak.

* What is the Particle Era?

10^-10 to 10^-3 seconds.
All 4 forces were distinct.
Quarks combined to form protons, neutrons, and their anti-particles.

* What is the Era of Nucleosynthesis?

10^-3 seconds to 3 minutes.
Protons and neutrons fused into hydrogen and helium, among other atoms.

* What is the Era of Nuclei?

3 minutes to 3.8 x 10^5 years
Universe was a plasma of nuclei and electrons.

* What is the Era of Atoms?

3.8 x 10^5 to 10^9 years.
Universe was filled with atomic gas, as atoms formed.

* What is the Era of Galaxies?

10^9 years to now.
First galaxies came into existence.

* Why is the large-scale Universe so smooth?

Because when the strong force froze out of the GUT force, the release of
energy caused sudden inflation.

* Why is the density of matter almost critical?

Because if it wasn't we would not have been able to live in this universe.

# Lecture 20 - Structure formation

* How are galaxies distributed in the sky?

They seem to be clumpy. Long strings to galaxies and galaxy clusters surrounding
"voids". This implies that there is some large scale structure to the universe,
and that the Universe is "foamy".

* How does a galaxy's birth determine its type?

- Initial angular momentum determines how fast the cloud will form a disk.
- Initial density determines how fast the cloud can form stars before it
collapses into a disk.

* How do galaxy interactions determine their type?

When galaxies collide, tidal forces randomize the orbits of stars, allowing
elliptical galaxies to form.

* How is black hole mass correlated with galaxy mass?

The large the central black hole, the more massive the galaxy.

* How do very big galaxies form?

By merging and absorbing smaller galaxies.

# Lecture 21 - Extraterrestrial Life

* Why is there renewed interest in searching for extraterrestrial life?

- We discovered extrasolar planets.
- We found that liquid water can exist on other worlds.
- We found organic molecules throughout the solar system and galaxy.
- We discovered that life existed on Earth as soon as it was possible.
- Organisms can survived in extreme conditions.

* What is astrobiology?

The study of life in the universe.

* Why are we searching for oxygen in atmospheres?

It is hard to replenish oxygen, and the best mechanism we know of is biology.

* Why do we think getting life to form is easy?

Because the time life existed on Earth was close the earliest moment when
liquid water existed on Earth, which kind of implied that life could exist
in very harsh conditions.

* What is SETI?

"If aliens are out there, we can communicate with them via radio."

* What are some solutions to the Fermi Paradox?

- We are alone, the first life to attain self-awareness.
- No one has colonize the galaxy.
- There is a Galactic civilization that has concealed itself from us.