Cosmic Origins 2

Star’s color

Proportional to the surface temperature

What color is the sun?

Green (appears as white)

Blackbody

Idealized body that absorbs all radiation that falls on it and perfectly emits radiation

Wein’s Law

Alpha times the peak wavelength is equal to the constant b, over the temperature

Small wavelengths

The smaller the wavelength the hotter a star

Spectral Sequence

O→M were subdivided from 0-9 (hot to cool)

What spectral class is the sun?

G2

Most common star

76% of stars are red dwarf stars (LTY on the spectrums)

Least common star

O stars are the least common because they burn through their fuel really fast (only last a few million years)

Brown Dwarfs

They can’t sustain fusion and are usually 13-18 MJ

What is lowest mass a star could be a brown dwarf

The lowest would is 80 MJ or 0.08 Ms

Stefan-Boltzmann Law

How we determine a star’s luminosity:

1.) Temperature (dominates)

2.) Radius

Hertsprung-Russle Diagram (H-R)

Main Sequence

A star that that is burning hydrogen in the core

Life of a star

Born→main sequence→death (runs out of hydrogen)

Mass

Tells you luminosity, lifespan, and temperature

Luminosity class

I.) Supergiant

II.) Bright giant

III.) Giant

IV.) Sub-giant

V.) Main sequence star (sun is a G2V)

Variable stars

Apparent magnitude of the star changes over time

Light Curve

Is a plot of the apparent magnitude over time

Extrensic Variable Stars

Exoplanet (exoplanet passes in front of the stars)

Eclipsing binary stars

Rotating variable stars (there are star spots)

Eruptive variable stars (large ejections of matter into space)

Intrensic Variable Stars

Star physically changes (pulsating variable star)

Instability Strip

Where most variable stars are

Henrietta Swan Leavitt

Investigated Variable Stars (cepheids)

Period

Time it takes a star to go from maxima to maxima (brightest back to brightest)

Period luminosity relationship

The brighter the star, the longer its period

Leavitts Law

Can use brightness of a cephid to measure the distance of stars

Limit of Leavitts Law

30million parsecs

Cepheids

Are important because they are bright and can be used to find the distance to other galaxies

Type 1 Cepheid

Are more luminous, massive, and have higher metallicity (is younger)

Type 2 Cepheid

Is the opposite of a type 1

Star’s mass at birth

Less than 2 Ms= low

2-8Ms= intermediate

8Ms+= high

Death process

1.) Inert helium core causes fusion to stop

2.) Initiates main sequence turn off (vertical)

3.) Outer layer eventually cools (moves right horizontally to the sub-giant branch)

4.) Hydrogen shell around the helim core causes burning and pushes outward (moves to red giant branch

Inert helium core

When hydrogen is exhausted

Main sequence turn off

Leaves the main sequence

Hydrogen shell

Surrounds the helium core and by proximity it ignites, causing an increase in luminosity and pushes outward to the red giant branch

What will happen to the sun when it is about to die?

It will eventually become 100x bigger and 1kxs more luminous (will only get up to luminosity class 3)

Dredge-up

During hydrogen shell burning CNO is brought to the surface and convection layer expands down to the hydrogen shell burning

Temperature to fuse helium

100 million kelvin

Tripple Alpha Process

3 Helium → 1 Carbon

Helium Flash

Runaway helium fusion event at the tip of the Red Giant Branch

Horizontal Branch

The distance it goes on this branch depends on how much mass is lost from burning (little mass lost= big dip) and has helium core burning (only one)

What happens at the end of the horizontal branch

Helium is exhausted and the core becomes carbon and oxygen (we have double shell burning with hydrogen and helium)

Asymptotic Giant Branch

Helium shell burning stops and that is when the thermal pulse start (variable star) and have dredge up too

Envelope ejection

Ends the thermal pulses and the outer layers are left (becomes planetary nebula, leaving the core behind)

White dwarfs

Are leftover core of a low to intermediate mass star (made of carbon and oxygen)

Mass classifications

Low mass star- 2 Ms or less

Intermediate mass star- 2-8 Ms

High mass star- 8+ Ms

Iron limit

Stars will never fuse iron

Iron

Z= 26 and has the highest binding energy of any element; can’t get fission or fusion

Binding energy

The minimum energy needed to disassemble a nucleus into its consituant nucleons (protons and neutrons)

High mass star’s main sequence turn off

They go to the right because mass increases but luminosity does not and all of them will produce a super giant

Iron Core

Will collapse and then rebound (will push out and explode), creating a supernova (left over gases turn into a nebula)

1572 Tycho Brahe

Discovered the nova stella (new star), which is not the same as a super nova

Nova

Is a minor detination of hydrogen fusion

Ways to measure distance

Stellar paralax (limit is 100 parsecs)

Nova (20 mega parsecs)

Leavitts’s Law (30 mega parsecs)

Chandrasethar Limit (1k mega parsecs)

Super nova

Type 1 has no hydrogen lines and type 2 has hydrogen lines

Type 1a Super nova

A progenetor that can get up to 8Ms gives rise to white dwarf (1.4 Ms)

How does a type 1a come about?

1.) Accretion (gets close enough to a red giant to exceed Chandrasekhar's limit)

2.) Two white dwarfs merge

Type 1b/c supernova

Massive star core collapse (blow out hydrogen envelopes before collapse)

Type 2 super nova

Massive star core collapse

Tollmin/ Openheimer limit

The lower limit of a neutron star is 1.4Ms and the upper limit is 3 Ms

Recurrent nova

When a white dwarf collect hydrogen from a red giant that results in the detonation of hydrogen fusion

What’s leftover after a supernova

Depends on the mass

Neutron star

The most dense object in the universe (beside black hole) that has beams of radiation

1967 Jocelyn Bell

Built a radio telescope and saw “little green men”; she ended up discovering pulsars, which confirms high mass stellar evolution

Pulsar

spinning neutron star where beam of radiation passes over earth

Why do pulsars spin faster

Conservation of Angular Momentum

What do you get out of stars

Low mass- Main sequence→red giant→nebula→white dwarf

Intermediate mass- Main sequence→red giant→nebula→neutron star

High mass (greater than 20 Ms)- Main sequence→red giant→nebula→black hole

Escape Velocity

The minimum speed needed to escape gravity

Katal Schwarzschild

Solved general relativity

Singularity

Point that contains the entire mass of a black hole

Escape speed for a black hole

Is the speed of light

The event horizon

The distance from the singularity that you would need to go the speed of light to escape

Who first predicted black holes

Openheimer

Accretion disk

Swirling hot plasma rotating around a black hole in a disk (the light get bent around the black hole)

Doppler Beaming

Plasma from accretion disk rotates toward you (will look brighter)

Relativistic Jets

From title event you get beams of radiation coming from the disk that are super hot/fast plasma

Types of black holes

Micro Black holes- less than 1Ms

Stellar mass black holes- 3Ms to hundreds Ms

Intermediate mass black holes- hundreds Ms to hundreds of thousands Ms

Supermassive black hole- millions to billions of Ms

Ways to detect black holes

1.) Stars orbit something that is not there (cygnus x-1 is the first confirmed black hole in 1971)

2.) Bright x-rays (our black hole is Sagitarius A*)

3.) Relativistic jets

4.) Gravitational waves (black hole mergers emit gravitational waves we can detect)

5.) We can take a picture (only since 2019)

Shadow of a black hole

Event horizon is smaller

Photons

We can see photons shoot off (they can get closer to the black hole, but it is not the accretion disk)

How do we take pictures

Use radio waves to take a picture and send those drives from sinked telescopes to be put in an algorithem that can fill the gaps

Tidal force event

The difference in strength of gravity between two points

Spagettification

The difference in gravitational force causes molecuels to be ripped appart (if the black hole is bigger than you have more of a chance to survive the encounter)

Doppler effect

Waves are shorter towards us and get longer away from us

Photon Sphere

Photons have no mass so they can get closer to black holes

General relitivity

Discovered by Einstine in 1915

Test for general relitivity

1.) Gravitational red shift

2.) Gravitational time dilation

3.) Gravitational lensing

Gravitational red shift

Light gets redshifted near massive objects (if you fell into a black hole, light would be blueshifted cause it is accelerating toward you)

Gravitational time dilation

Time runs slower near massive objects

Gravitational lensing

Light bends around massive objects (more mass=more bend)

When was general relitivity first proved

In 1919 by Arthur Eddington

Milky Way Galaxy

Spiral galaxy that is 1000 lightyears across (has bulge and disk)

Bulge vs disk

Bulge is dead (no stars are made) and the disk has active star formation

Open clusters

Found the disk (very blue and young)

Globular cluster

Larger and more dense in the stellar halo (older stars that are reder)

Fuzzy nebule

Fuzzy objects in the sky

1785

William Hershel proposed the first model of the Milky Way

The Great Debate

Debate in 1920s at the Smithsonian over how big the universe was and what are the fuzzy nebule between Heber Curtis and Harlow Shaply (no one won)

Herber Curtis

Said fuzzy nebule were “island universes” (galaxies) so the universe huge

Harlow Shapley

Said fuzzy neblue were clouds on the outer part of the Milky Way (universe is only a bit bigger than the Milky Way)