AST 201 Second test

Stellar spectral classification — what do we already know?

• Surface temp (wiens law)

• Distance (parallax)

• Luminosity (apparent brightness and distance)

• Chemical composition (spectra)

1800s Harvard college ovservatory

Hired women to process mass data

Williamina Fleming

• Analyzed stellar spectra

• Created Henry Draper Catalogue

Henrietta Swan Leavitt

• Calculating large astronomical distances

After some reordering…

Stellar Spectral types

Organized stellar spectra into alphabetical groups

• Absorption lines become stronger or fainter as move from one class to another

Henry-Draper catalogue

Revised scheme is a sequence of surface temperatures (not the temp of the whole star)

Why would differences in surface temperatures produce the observed pattern of spectral lines? (balmer series)

Hydrogen spectra lines originating and ending at n=2 level is called Balmer series

• In order for star to show balmer lines, have to be electrons in n=2 shell

• NOT ground state

• Needs HEAT for excitation

Why would differences in surface temperatures produce the observed pattern of spectral lines? — continued

Cool star

• Most electrons in ground state, cant make balmer lines

Warmer star

• More electrons into n=2, make balmer lines 

Hot stars

• A large fraction in n=2, balmer lines reach their darkest/strongest 

Very hot stars

• Most electrons above n=2, balmer lines weaken again (or completely ionized)

Each atom or molecule has a similar pattern: weak at one temp, stronger at higher temp, then weak again at very high temp

Why would differences in surface temperatures produce the observed pattern of spectral lines? — molecules

Very cool stars are cool enough for MOLECULES to be intact in atmospheres

• Makes huge number of spectral lines

• Atoms can make molecules

• Only cool stars bond remains

Building the Hertzsprung-Russell diagram (HR diagram)

Stellar luminosity vs Surface temp (colour, spectral class)

• Temp higher on the left

• Luminosities in solar luminosity (1 = luminosity of sun) (logarithmic)

• Diagonal lines of stellar radius

Building the Hertzsprung-Russell diagram (HR diagram) — GAIA spacecraft

Recorded the properties of more than billion stars

Building the Hertzsprung-Russell diagram (HR diagram) — Main sequence

Cluster in a group along a diagonal

• from red and low luminosity to blue and high luminosity

MAIN SEQUENCE

• Still fusing hydrogen in the core (alive)

• Anything below is dead

• Anything above is close to death

Luminosity and Surface temp correlation

Surface temp measures amount of light emitted per square metre

Luminosity = (surface area) X (amount of light emitted per unit area)

• Amount of light per square metre given Stefan-Boltzmann equation for blackbody radiation

Stars on main sequence

Refered to as dwarf stars — even though they can be very large

• All M stars are small and red

• All O stars are large and blue

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• Top main sequence have higher mass & shorter lifespans

  • Fuses hydrogen fast

  • Few million years

• Bottom main sequence and lower mass and longer lifespans

  • Fuses hydrogen slowly

  • Trillions of years

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Main sequence is not a time sequence 

• Stars start on the main sequence, and stay there for most of their lifespan

Interpreting HR Diagrams

The H-R diagram and the study of stellar evolution 

• As a star ages, it changes places on the HR diagram

• Stars not on main sequence have ceased to fuse hydrogen in their cores

Interpreting HR Diagrams — continued

Above main sequence

• subgiants, giants, supergiants

• are in the process of dying

Below

• Mainly white dwarfs, dead

Vast majority of stars in universe are M and K red dwarfs

But, very dim so hard to see without telescope

• Most stars near the sun are red dwarfs

• Mostly SEE giants and supergiants — dead or dying suns, very bright

Luminosity classes

Luminosity class tells you where a star is in its lifecycle, but not directly about its luminosity

Why do more massive stars have shorter lifespans?

Burns fuel more quickly

The rate of fuel use can be quantified via luminosity, the amount of energy released/sec

• Luminosity scales with mass

Most stars form in clusters

Messier 7 — open cluster

Tucanae — globular cluster, 13 bil years old

We can learn a lot about a star cluster by plotting its stars on an HR diagram

Age

• If a star cluster shows complete main sequence — young

• As cluster ages, O and B leaves main sequence, medium lifespan stars start leaving

• Even later, most of the stars would leave the main sequence

• Die in sequence from top to bottom

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At any given time, we can estimate the age looking at the lifespan of the shortest-lived star still on the main sequence

• Main-sequence turnoff point

• The age of cluster is lifespan of star that just recently finished its lifespan

Picture of galaxy — gas and dust clouds

• Pink areas are hot, ionized hydrogen gas — HII regions

• Dark areas correspond to cold dust clouds, hydrogen is molecules — emits more infrared light

Molecular clouds often have blue stars in their cores

Birthplace of stars

• Long-lived stars are far away from their origins 

• Don't see much blue stars away, associated with molecular clouds

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E.g: Rosetta nebula. Orion nebula

• Shows a cluster of blue stars forming inside the nebula

• Molecular clouds are the engine of star formation

Formation of a star from molecular clouds

Molecular cloud core can only stay stable shortly

Own gravity will destabilize and compress

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Conservation of energy —→ starts heating up

• gravitational potential energy converted to kinetic energy

Atoms fall inward and heat up —→ protostar forms at the core

• shrinks

Rotation amplifies as collapses, flatten into disk

• surrounded by protostellar disk, producing jets of materials

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Until core reaches 10mil K, no fusion due to mutual electric repulsion

Fusion stars: pre-main sequence star

• enough pressure to counteract gravity

• gravitational collapse slows and stops

Settles onto main sequence

Becomes a zero-age main sequence star (ZAMS)

Hydrostatic equilibrium of main sequence stars

The outward pressure produced by the release of energy from hydrogen fusion in the core exactly balances the inward pull of gravity

• Stars remain on main sequence for long time

• Not expanding or contracting

Gravity is always trying to win (uh oh ran out of fuel)

• When the star runs out of fuel, gravity begins to win

• The exact sequence of steps depends on the starting mass of the star

High mass vs Low mass star

8 times the mass of the star – high mass star

Less than 8 times – low mass star

Death of a low mass star — main sequence

• At zero age, it is roughly about 10% helium and 90% hydrogen in its volume

• Hydrogen fusion stars converting hydrogen to helium, percentage of helium in the core increases 

• After a long time, most of the core will be converted to helium

Death of a low mass star — Inert helium core

Isnt producing energy to sustain pressure to counteract gravity

• Core contracts until degeneracy pressure halts contraction

• Layer of hydrogen around core reaches temp and density for fusion — hydrogen shell burning

Death of a low mass star — Hydrogen shell burning

Fresh layer of fusion produces energy at a higher rate

• release of extra energy causes outer layers to expand

• outer layers cool down — becomes redder

  • subgiant stage

• Lifts off main sequence

Death of a low mass star — red giant

Cools down but increases in size

• overall luminosity goes UP

• expands in size dramatically

• moves to upper right of HR diagram

Death of a low mass star — Helium flash

H-burning adds inert helium to core

• compress and heat the core

• eventually reach enough temp for helium fusion

  • done in few minutes

• expands the core, h-burning slows down

• star shrinks and settles into new equilibrium — horizontal branch

  • powered by helium fusion and hydrogen shell burning

Death of a low mass star — asymptotic giant stage

Helium fuses into inert carbon and oxygen

• low mass star will never get hot enough to fuse carbon or oxygen

• die permanently

So why don't stars keep collapsing into a black whole?

Electrons in stars obey the Pauli exclusion principle

• Can only force them to get so close to one another before they exert a strong repulsion

• State of degeneracy = Electron degeneracy pressure

  • Electron degenerate matter is very dense (1 teaspoon is 2 SUVs)

White dwarf

The electron degenerate core is called a white dwarf, only about the size of earth

• Really hot and bright 

• But really small

Planetary nebulae

Once a helium layer around the core starts fusing, it releases energy that propels the outer layers of the star into space – leaving the white dwarfs

• will fade after 10,000 years

• white dwarf will survive until the end of the universe until it becomes a black dwarf

Tracks of stars on HR diagram

Life cycle of a high mass star — onion structure

Massive main sequence stars DO get hot and dense enough to fuse helium on main sequence

• By the time a massive star leaves the main sequence, it has fused H, He, C, O, Ne, Mg, Si

• each type of fusion begins in the core and then moves to a shell around the core — Onion structure

Why cant stars fuse past iron (Fe)?

In fusion, particls are fused into nuclei of lower mass

Mass is converted as energy and release

• The mass per nuclear particle changes from atom to atom (binding energy is different for each element)

• Separating two objects requires energy, which is added on to the mass

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Only nuclear reactions that reduce the mass per particle can release energy

• fusion into lower mass particles

• fission into lower mass particles

• IRON has particles of lowest mass, neither fusion or fission will release energy

Gravity wins

After massive star has iron core

Can no longer produce energy

• Take apart iron, need energy

• Fuse iron, also need energy

• No longer produce pressure to counteract gravity

• Gravity wins

Once a massive star leaves the main sequence it becomes a supergiant

• Gravity contracts and heat up the star

• Burns heavier elements

• Star swells until IRON

Massive star collapse

All of the ways that the core of a massive star can collapse are extremely sudden and typically very violent

Neutrionization — avoiding electron degeneracy matter

In mere seconds nearly all of it is converted into neutrons

• Proton + electron → neutron + neutrino

• Dramatically implode, falling inward very fast

• Core contracts to a ball of neutrons, 20km diameter

• Neutron degeneracy pressure kicks in, stops contraction

Neutrionization — type 2 supernova

Great mass falling inward suddenly hit an immovable ball of neutrons

• All of the neutrinos come rushing out at all the same time

• Explodes in a supernova

• Type 2 supernova – core-collapse supernova

Type 2 supernova – core-collapse supernova

• Insanely bright, can briefly outshine the entire galaxy it occurs in

• A single supernova can release as much energy as the sun will in 10bil years!

• The only stars capable of doing this are rare stars

(e.g. SN1987A: closest supernova to earth, discovered by uoft student)

Type Ia supernova (not from a massive star)

Low mass stars will not die in supernova, it will become a white dwarf

Type Ia supernova — Chandrasekhar limit

White dwarfs have a maximum mass of 1.4 solar masses

• But if a white dwarf is in a binary system, it can accrete matter from the other star

• Mass can exceed Chandrasekhar limit

• Causing a Type Ia supernova

Type Ia supernova — explosion proceedings

• Immediately undergo fusion reactions, causing it to explode

• Every Type Ia supernova, it is basically the same as every other Type Ia supernova

  • Able to identify it easily (intensity, luminosity)

• CANNOT be done by itself, NEEDS companion star

We can identify a type of supernova by its spectra

Type II supernova has spectral lines of hydrogen

• Made of hydrogen

• White dwarf doesn't have much hydrogen

  • And if it is type Ia, can identify distance

Supernova remnants

Supernovae leave behind shells of expanding gas, called supernova remnants

• Expand and dissipate, rejoining gas and dust of interstellar medium

• Most elements came from supernovae

  • Enrich interstellar medium with elements heavier than hydrogen and helium

  • Gets recycled into planets and stars

  • We are made of recycled stars!

The afterlives massive stars

The final state of a dead star depends on its initial mass and the sequence of events that happened to it

• Harder to predict for massive stars — unstable, lose mass to stellar winds

• There is no one-to-tone relationship between the starting mass and the final

• Massive stars usually die where they're born

The afterlives massive stars — Options after death

• Supernova and leave no remnant

• Failed supernova and collapse into black hole (star vanish)

• Supernova and then collapse into black hole

• Supernova and leave neutron star

Neutron stars

If the mass remaining after supernova is 1.5--3 solar masses, then the remnant is a neutron star

• Ball of neutrons from neutronization

• Typical star compressed into a city sized ball about 20km

• EXTREMELY dense

• First observed by Jocelyn Bell Burnell

Understanding neutron stars — Conservation of angular momentum

If the net external force on an object is zero, angular moment must remain constant

L = mass x speed x radius

• When neutron stars form, radius decreses DRAMATICALLY

• Mass stays constant, so speed goes WAYYY up

Pulsars (radiowaves)

Pulars have strong magnetic fields

• Channel charged particles from the star into two beams

• Often misaligned with rotation axis

• If light passes us, get rapidly blinking/pulsing radiowaves

Pulsars — continued

These pulses were observed by Bell-Burnell in 1967

Whether or not you see a neutron star is a matter of perspective, if its beam sweeps past

• Very fast rotations – millisecond pulsars

• e.g. Crab nebula: Identification of neutron stars embedded in supernova remnants establishes the connections between the two

The milky way galaxy

• Flat disk shape

• Shows up as band on earth

• We can get the entire sky as a flat image (black spots are dust)

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• In near infrared

  • Thin disk shape is seen more easily 

• Far infrared

  • Mostly cold dust

The milky way galaxy — properties

Barred spiral — central region is like straight bar

• (bars can be strong or weak), milky way has moderate bar

• Solar system about halfway from centre, 27,000 ly from galactic centre

• Total diameter is 100,000 light years

The milky way galaxy — Three compartments

Disk

• Flat disk, spiral

Bulge

• Central part, thicker than disk

Halo

• Speckles around the galaxy

The galactic disk

Thin — 3000 light years

Contains most the dust and gas in the milky way

The galactic disk — galaxy arms (formation?)

Stars come and go from arm, but general pattern of movement, regions of enhanced density (circular orbit)

• Blue star formation

• Always associated with spiral arms 

• Clumps of blue stars with red hydrogen and black dust

Starburst galaxies (superwind)

Starburst galaxy is one that is undergoing an exceptionally rapid burst of star formation

• Formation and supernova death

• Driving superwind — blowing gas and dust, forcing hydrogen out of the galaxy

• Often triggered by gravitational interactions among galaxies (creates heat)

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e.g:

• M82

Starburst galaxies example — antennae galaxies

Interacting galaxies resultig in a starburst

• pair of spiral galaxies colliding

The galactic bulge

• Contain older, redder stars 

• Not a lot of dust and gas

• In most spiral galaxies, the bulge hosts a supermassive black hole

  • hard to see because less than 0.1% mass of galaxy

The galactic halo

Both the bulge and the halo is spherical 

• Contain old stars 

• Very few stars, cant see much colour

• Contains Globular clusters

  • most large galaxies have globular clusters around them

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eg:

• E.g. omega centauri

  • Very densely packed

  • 10 mil stars packed into a ball 150 light years across 

  • May have its own central intermediate-mass black hole

Spins of the galaxy

Spin of the milky way causes everything to be flattened into a disk 

• Stars in halo, bulge, and clusters rotate in random orientations

Where do we find the most dark matter?

The halo of the galaxy is also where we find the majority of dark matter

• The milky way is actually a glob of dark matter

Dark matter and scientific discoveries

In Newton's law of gravity, gravity falls with the inverse square of gravity 

• Prediction is in disagreement with what we observe (rubin & zwicky)

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In light of discoveries like this, either:

1. Our model of reality is wrong

2. Our observations of reality is wrong

Explaining the rotation curve

1. The observations could be wrong

2. Our model of reality is wrong

   • Understanding of gravity wrong

   • Understanding of galaxies wrong

Observations were correct for rotation curve

Always the same result when repeated

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The part of a galaxy that glows in visible light is only a small portion of what's truly there 

• Unseen parts cover much larger area 

• BUT: The flat rotation curve extends well past the visible or luminous part of each galaxy 

  • Why would the most distant objects be moving at crazy speeds?

Our theory of gravity is right

Maybe our understanding of gravity is wrong?

• Tried to explain using modified newtonian dynamics MOND 

• But they don't explain how gravity works for other observations

We are wrong about what a galaxy is

Before: assumed that the mass is concentrated in the centre

Instead of most of mass being in the centre, but actually spread out, well beyond what we can see

• The speed only has to resist the pull of gravity of everything in its orbit circles 

• Objects have to resist the gravity of more and more mass when getting further away from the centre

How much mass does the milky way have to have to reproduce this?

• Much much bigger than what we observe added up

  • “Ordinary” or “baryonic” matter, made of protons and neutrons

To get the rotation curve, the amount of total mass is ten times what we can see

• Non-baryonic matter

• DARK matter

90% of the mass of the Milky Way is dark matter

Proof for dark matter enacting gravity

Gravitational lensing is significantly more than can be explained by the luminous mass of the lensing object

We don't know what dark matter is

• Some NEW form of matter

• Like neutrinos but does not interact with light but DOES produce a force of gravity

We don't know what dark matter is — Colliding galaxy cluters: the bullet cluster

• We can use gravitational lensing to build a map of where most the mass is located 

• Dark matter passed through each other

  • doesn't interact with each other

• The hot gas around does not match the distribution of dark matter

  • does interact with each other 

• Dark matter is COLLISIONLESS