Astronomy 101 The Solar System Tuesday, Thursday Tom Burbine

Course Course Website: – Textbook: –Pathways to Astronomy (2nd Edition) by Stephen Schneider and Thomas Arny. You also will need a calculator.

There is an Astronomy Help Desk that is open Monday-Thursday evenings from 7-9 pm in Hasbrouck 205. There is an open house at the Observatory every Thursday when it’s clear. Students should check the observatory website before going since the times may change as the semester progresses and the telescope may be down for repairs at times. The website is

Exam #2 February 25 th –Covers from last exam up to Feb. 18 Formulas: T (K) = T ( o C) c = f* E = h*f KE = 1/2mv 2 E = mc 2 Power emitted per unit surface area = σT 4 λ max (nm) = (2,900,000 nm*K)/T Apparent brightness = Luminosity 4  (distance) 2

Review Session 6 pm in Hasbrouck 20 on Wednesday (Feb. 24)

HWs #6, #7, #8, and #9

Milky Way Galaxy

Interstellar Medium – matter between stars Made up of gas and dust

Life of a Star A star-forming cloud is called a molecular cloud because low temperatures allow Hydrogen to form Hydrogen molecules (H 2 ) Temperatures like K

Region is approximately 50 light years across

Condensing Molecular clouds tends to be lumpy These lumps tend to condense into stars That is why stars tend to be found in clusters

Protostar The dense cloud fragment gets hotter as it contracts The cloud becomes denser and radiation cannot escape The thermal pressure and gas temperature start to rise and rise The dense cloud fragment becomes a protostar

When does a protostar become a star When the core temperatures reaches 10 million K, hydrogen fusion can start occurring

Because nuclear reactions have not yet begun in the protostar’s core, this luminosity is due entirely to the release of gravitational energy as the protostar continues to shrink and material from the surrounding fragment

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Brown Dwarfs Failed stars Not enough mass for fusion Minimum mass of gas need for fusion is 0.08 solar masses (80 times the mass of Jupiter)

Main Sequence Is not an evolutionary track –Stars do not evolve on it Stars stop on the main sequence and spend most of their lives on it

Sun ends it time on the main sequence When the core hydrogen is depleted, nuclear fusion stops The core pressure can no longer resist the crush of gravity Core shrinks

Why does the star expand? The core is made of helium The surrounding layers are made of hydrogen

And.. Gravity shrinks the inert helium core and surrounding shell of hydrogen The shell of hydrogen becomes hot for fusion This is called hydrogen-shell burning

And … The shell becomes so hot that its fusion rate is higher than the original core This energy can not be transported fast enough to surface Thermal pressure builds up and the star expands

And.. More helium is being created Mass of core increases Increases its gravitational pull Increasing the density and pressure of this region

When When helium core reaches 100 million Kelvin, Helium can fuse into a Carbon nucleus

Helium Flash The rising temperature in the core causes the helium fusion rate to rocket upward Creates a lot of new energy

However The core expands Which pushes the hydrogen-burning shell outwards Lowering the hydrogen-burning shell’s temperature

And Less energy is produced Star starts to contract

Now In the core, Helium can fuse to become Carbon (and some Oxygen) Star contracts Helium fusion occurs in a shell surrounding the carbon core Hydrogen shell can fuse above the Helium shell Inner regions become hotter Star expands

–Triple Alpha Process – 4 He + 4 He ↔ 8 Be – 8 Be + 4 He ↔ 12 C + gamma ray MeV

Some carbon fuses with He to form Oxygen 12 C + 4 He → 16 O + gamma ray Harder to fuse Oxygen with Helium to produce Neon

Planetary Nebulae There is a carbon core and outer layers are ejected into space The core is still hot and that ionizes the expanding gas

Planetary Nebulae

White Dwarf The remaining core becomes a white dwarf White dwarfs are usually composed of carbon and oxygen (can not fuse carbon) Oxygen-neon-magnesium white dwarfs can also form (hot enough to fuse carbon but not neon) Helium white dwarfs can form

High-Mass Stars The importance of high-mass stars is that they make elements heavier than carbon You need really hot temperatures which only occur with the weight of a very high-mass star

Stages of High-Mass Star’s Life Similar to low-mass star’s Except a high-mass star can continue to fuse elements When the fusion ceases, the star becomes a supernova Supernova is a huge explosion

Fusion in High-Mass stars Besides fusion of Hydrogen into Helium The high temperatures allow Carbon, Nitrogen, and Oxygen to be catalysts for converting Hydrogen into Helium

CNO cycle

Fusion The interior temperatures of high-mass stars in its late-stage of life can reach temperatures above 600 million Kelvin Can fuse Carbon and heavier elements Helium Capture can also occur where Helium can be fused into heavy elements

“Deaths” of Stars White Dwarfs Neutron Stars Black Holes

White Dwarfs White Dwarfs is the core left over when a star can no longer undergo fusion Most white dwarfs are composed of carbon and oxygen Very dense –Some have densities of 3 million grams per cubic centimeter –A teaspoon of a white dwarf would weigh as much as an elephant

White Dwarfs Some white dwarfs have the same mass as the Sun but slightly bigger than the Earth 200,000 times as dense as the earth

White Dwarfs Collapsing due to gravity The collapse is stopped by electron degeneracy pressure

Electron Degeneracy Pressure No two electrons can occupy the same quantum state

Electron Degeneracy Pressure As electrons are moved closer together Their momentum (velocity) increases Due to Heisenberg Uncertainty Principle

So What Does This Mean Electron Degeneracy Pressure balances the gravitational force due to gravity in white dwarfs

One Interesting Thing More massive white dwarfs are smaller

White Dwarf Limit The mass of a White Dwarf can not exceed approximately 1.4 Solar Masses Called the Chandrasekhar Limit

The Sun Will end up as a White Dwarf

Black Dwarf Black dwarf – Theoretical cooled down white dwarf Not hot enough to emit significant amounts of light Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe of 13.7 billion years, no black dwarfs are expected to exist in the universe yet

Neutron Star Neutron stars are usually 10 kilometers acroos But more massive than the Sun Made almost entirely of neutrons Electrons and protons have fused together

How do you make a neutron star? Remnant of a Supernova

Supernova A supernova is a stellar explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.

The last person to see and chronicle a supernova outburst in our galaxy was Johannes Kepler. That was in 1604 rivaled Venus in brightness.

Type Ia Supernova Type II Supernova

This stops with Iron Fusion of Iron with another element does not release energy Fission of Iron with another element does not release energy So you keep on making Iron

Initially Gravity keeps on pulling the core together The core keeps on shrinking Electron degeneracy keeps the core together for awhile

Then The iron core becomes too massive and collapses The iron core becomes neutrons when protons and electrons fuse together

Density You could take everybody on Earth and cram them into a volume the size of sugar cube

Explosion The collapse of the core releases a huge amount of energy since the rest of the star collapses and then bounces off the neutron core Joules Annual energy generation of Sun is Joules

How do we know there are neutron stars? The identification of Pulsars Pulsars give out pulses of radio waves at precise intervals

Pulsars Pulsars were found at the center of supernovae remnants

Pulsars Pulsars were interpreted as rotating neutron stars Only neutron stars could rotate that fast Strong magnetic fields can beam radiation out

Black Holes If a collapsing stellar core has a mass greater than 3 solar masses, It becomes a black hole

Black Hole After a supernova if all the outer mass of the star is not blown off The mass falls back on the neutron star The gravity causes the neutron star to keep contracting

Black Hole A black hole is a region where nothing can escape, even light.

Event Horizon Event Horizon is the boundary between the inside and outside of the Black Hole Within the Event Horizon, the escape velocity is greater than the speed of light Nothing can escape once it enters the Event Horizon

How do calculate the radius of the Event Horizon? It is called the Schwarzschild Radius Radius = 2GM/c 2 This is a variation of the escape velocity formula Escape velocity = square root (2GM planet /R planet )

Black Hole Sizes A Black Hole with the mass of the Earth would have a radius of meters A Black Hole with the mass of the Sun would have a radius of 3 kilometers

Can you see a Black Hole?

No Black Holes do not emit any light So you must see them indirectly You need to see the effects of their gravity

Evidence The white area is the core of a Galaxy Inside the core there is a brown spiral- shaped disk. It weighs a hundred thousand times as much as our Sun.

Evidence Because it is rotating we can measure its radii and speed, and hence determine its mass. This object is about as large as our solar system, but weighs 1,200,000,000 times as much as our sun. Gravity is about one million times as strong as on the sun. Almost certainly this object is a black hole.

Any Questions?