Astronomy Unit 4

Equivalence principle

Being in free fall is equivalent to being in a zero-gravity space (elevator example)

Evidence for Einstein’s theory of gravity

In Newton’s description of gravity, light shouldn’t bend due to gravity because it has no mass, but it does. Einstein’s theory says that objects with a lot of mass bend spacetime, and that’s why light bends near massive objects

Spacetime

There are three dimensions of space and one dimension of time, space and time are not separate like we think they are. Stronger gravity = slower pace of time

How Mercury supports Einstein’s theory

Mercury’s orbit is very eccentric and the orientation of the major axis changes (orbital procession). Astronomers used to think that there was another planet affecting its orbit named Vulcan, but that didn’t exist. Einstein’s theory was able to predict Mercury’s orbit using his theory of gravity

What prevents objects from collapsing

Earth - it’s hard to compress rocks

The Sun - pressure from nuclear explosions

White dwarfs - it’s hard to compress electrons together

Neutron stars - it’s hard to compress neutrons together

Black holes - no known force can fight gravity strong enough to create a black hole

Schwarzchild radius

Radius from which light cannot escape a black hole, also called an event horizon

R = 2GM/c², determines the radius of a black hole using its mass, can also be used to find the size of an object if it were a black hole

Gravitational lensing

Direction of light around a black hole changes and gets brighter, causes objects to appear brighter or in a different place

Photon sphere

Area around the black hole where light particles can remain in orbit

Spaghettification

Our best guess for what happens beyond the event horizon. Tidal forces are shallower at the surface of the black hole, so the bottom of the object is pulled in faster than the top and the object gets stretched out

Evidence for black holes

We can see weird motion of objects around them, we can see accretion disks around them, they create ripples in spacetime called gravitational waves, and also we have a picture now

Parts of the Milky Way galaxy

Halo - in the middle, contains almost entirely old, metal-poor stars from when the galaxy first formed

Disk - includes every kind of star

Spiral arms - part of the disk, formed because of differential rotation and because the arms had self-gravity

Galactic year

How long it takes the Sun to go around the galaxy - 225 million years

Weight of the galaxy

100 billion solar masses, estimate based on Kepler’s third law

Dark matter

A model that explains why everything in the galaxy is moving quickly. The stars furthest from the center should be moving slower than the stuff in the middle, but it’s not, so either our theory of gravity is wrong, or the galaxy is filled with matter we can’t detect

Sagittarius A* (Sgr A*)

The supermassive black hole at the center of the Milky Way galaxy, has a mass of 4 million Suns. One star orbits it at 124 AU

Why accretion disks around black holes are bright

The black holes pull matter in so fast that it lights up

Why accretion disks around black holes look like two circles

There’s only one accretion disk, but gravitational lensing makes it look like two

Population I stars

Bright blue stars in spiral arms of the galaxy (can also be yellow, red, and white, but blue stars are much more luminous), 0-12 billion years old, more heavy elements. Orbits are usually on the disk

Bright supergiants, main sequence stars, open clusters

Population II stars

Stars in halos, RR Lyrae stars, and globular clusters, but also found everywhere in the galaxy, 10-14 billion years old, not as many heavy elements. Orbits don’t have to be on the disk

Why young stars have more heavy elements than old stars

Old stars process the hydrogen and helium into heavier elements through fusion, and then they release it as supernovae or planetary nebulae. Young stars form from the enriched gas

Model of galaxy formation

Spherical blob of gas → starts collapsing → first stars form → forms a rotating disk → more stars form

Origin of the Big Bang Theory

Einstein’s theory of gravity predicts that the universe should either be expanding or contracting, but he didn’t believe it until Hubble discovered that it is actually expanding because he observed the redshift of the galaxies moving away from us. If we rewind expansion, everything had to be really close together at some point in the past, and that’s the Big Bang

Hubble’s constant

H = 70 km/s/Megaparsec (one million parsecs), used to estimate the age of the universe

How we know the expansion of the universe is accelerating

Astronomers used supernovae as standard candles and saw that they were accelerating away from us

Dark energy

A model that explains why the universe’s expansion is accelerating when it should be collapsing due to gravity. We have no idea where it comes from, what it is, or why it’s happening

Cosmological redshift

When wavelengths from everything get longer due to the expansion of the universe

Raisin bread model of the universe

The bread = space and the raisins = galaxies. The bread is getting bigger and the raisins are staying the same size but getting further apart

Models of the universe

No expansion, decelerating expansion (U shape), constant expansion (V shape), accelerating expansion (Y shape)

Big freeze

Aka “heat death”, everything separates and no stars form

Big rip

Expansion is stronger than nuclear forces that hold nucleons and electrons together, so atoms split apart

Big crunch

Everything clumps together, space gets tighter, atoms and subatomic particles crunch together

Big bounce

Big crunch and big bang cycle repeats over and over again

Critical density

Density needed for the universe to expand forever and not contract

3H² / 8𝜋G

Matter = 30%, dark energy and dark matter = about 70% (estimate)

Models for ages of galaxies

Light from far galaxies has to travel further, so it’s redshifted more and we can use a model to estimate its age. 0 redshift = now, higher redshift = older

Primeval fusion model (or Big Bang nucleosynthesis)

No fission but a period of nuclear fusion. Started with about 75% hydrogen, 25% helium, and a little bit of lithium

Deuterium

Isotope of hydrogen with a proton and a neutron instead of just a proton, heavier than hydrogen

Timeline of the beginning of the universe

T = 0.01 seconds: soup of matter and radiation, particles collided all the time, temperature was hot enough to turn light into an electron when photons collided

T = 3 minutes: atomic nuclei formed because high energy photons stopped breaking up nuclei

T = 300,000 years: universe was no longer ionized and electrons could bind to protons, universe became transparent

T = 400 million years: first stars form

Cosmic microwave background

Microwave radiation from all directions, redshifted because it’s from the Big Bang. Insanely uniform (2.73 Kelvin, 2 mm photons in every direction and 1 part in 100,000 is different from those measurements). Good confirmation that the universe evolved from a hot initial state, matches perfectly with predictions

Fluctuations in CMB

Seeds of galaxies. If there were no fluctuations, the atoms would be evenly spaced throughout the whole universe

Standard model of cosmology

Universe age = 13.8 billion years

Hubble constant and fractions of visible matter and dark energy/matter are also included

Universe contents that we know of

Luminous matter = 1%

Hydrogen, helium, ISM = 4%

Dark matter = 27%

Dark energy = 68%

WIMPs

Weakly interacting massive particles, a model for dark matter

Cosmological inflation

Attempt at explaining why the universe is flat, homogenous, and close to the critical density, says that the universe had a phase of very rapid expansion at 10^-35 seconds

Anthropic principle

Physical laws must allow for the existence of humans because humans have to be around to document the physical laws

Copernican principle

There is nothing special about our place in the universe

Fermi’s paradox

If alien life is common, we should expect to see evidence of them

Possible explanations for Fermi’s paradox

Life is not common

Life is common, but intelligent life is rare

Intelligent life is common, but we can’t detect their signals

Intelligent life is common, but they’re leaving us alone on purpose

Maybe intelligent life always self-destructs

We have detected intelligent life, but the government is hiding it from us

Miller-Urey experiment from 1952

Produced biotic materials from abiotic materials by putting building blocks of life in sealed vials, experiment was more successful than they originally thought

Habitable zone

Area of temperature around a star where the water on the planets is neither vaporized nor frozen, varies based on type of star and greenhouse gases on planets

Extremophiles

Some microorganisms can survive in extreme temperatures (-25 degrees C to 122 degrees C). Life can survive throughout the acid-base range and through varied salt contents

Europa

One of Jupiter’s moons, has a salty ocean beneath the crust

Titan

Largest moon of Saturn, has lakes and rivers of methane so it could have a different kind of life

Enceladus

6th largest moon of Saturn, has subsurface oceans and water vapor with minerals and organic compounds come out of plumes. Further away from Earth and gravity there is 98.8% weaker than Earth’s so it’s hard to land a probe there

Biosignatures

One of the ways to detect whether conditions on a planet are good for life or not, transmission spectrum on the atmosphere to tell the composition. We can’t detect these yet but we might be able to soon

The Drake equation

N = R* x fp x ne x fl x fi x fc x L

N = number of civilizations in the Milky Way that we might be able to communicate with

R* = rate of formation of stars in the galaxy

fp = fraction of the stars with planetary systems

ne = number of planets per system with habitable environments

fl = fraction of habitable planets on which life appears

fi = fraction of habitable planets on which intelligent life appears

fc = fraction of habitable planets on which intelligent life develops interstellar communication

L = length of time the civilizations release detectable signals into space

This equation has to be true, but we don’t know these numbers, so it’s not that helpful