ASTR 1020 First Homework Due Today Second Homework due Feb 9 Next Observatory Session Tonight at 7pm Website http://casa.colorado.edu/~wcash/APS1020/APS1020.html

Escape Velocity Fall from Large Height Same Energy Needed to Reverse and Fly Away Escape velocity is the speed at which object must be thrown upward to escape and never come down. G=6.7x10-11 in mks units

The Sun

The Sun Falls into “Disk Stability” 99.9% Ended in Sun (0.1% in Jupiter) Probably the Same Around All Stars -- Planets are Common Shoots Planet-Size Bullets into Space

Most of Mass Forms Ball in Center A Star Is a Hot Ball of Hydrogen (plus 11% Helium) One Million Miles

What Stops the Fall? Gravity Gets Stronger As Material Gets More Dense R smaller implies F greater The smaller it gets, the faster it falls in! Why doesn’t it just become a black hole? Or worse yet, a point-like singularity of mass?

Scaling Scientists often do “scaling” do avoid all those large numbers. For example, the Sun is ABOUT a million times the mass of the Earth and a hundred times the size.

Question What is the surface gravity of the Moon in gees? R=.01Re, M=.25Me? a).04 b)4 c)16 d).16

Answer d

Temperature Temperature is a Measure of the Random Kinetic Energy per Particle The faster the atoms move, the higher the temperature. But we’re talking about random motion. If they all move together, then the object moves.

Thermal Pressure Thermal Gas Pressure Balances Gravitational Pressure Balloon Every Time An Bounces Off Edge of Balloon It Keeps It From Collapsing That’s Pressure Pressure is Force per Unit Area

Pressure is Proportional to Temperature Low Temperature Atoms Move Slowly High Temperature Atoms Move Fast

A Star Is Held Up By Thermal Pressure From Below Gravity Outer Mass Individual Atoms Don’t Orbit Entire Inside of Star Like This They Jostle Each Other But Effect Is The Same

Temperature Scales Fahrenheit – Celsius Kelvin 0=salt water freeze 100=human body Celsius 0=pure water freeze 100=water boil (sea level) C=(F-32)x5/9 Kelvin 0=absolute zero 100 degrees between freeze and boil K=C+273 -273C = 0K = Absolute Zero At Absolute Zero Atoms Stop Moving

Thermal Pressure Pressure Is Proportional to Temperature x Density P Pressure V Volume n # moles R Constant T Temperature (K) Ideal Gas Law Chemistry Style P Pressure V Volume N # atoms k Constant T Temperature (K) Ideal Gas Law Physics Style Pressure Is Proportional to Temperature x Density

A Star Always Balances Gravitational Pressure with Thermal Pressure Pressure Balance A Star Always Balances Gravitational Pressure with Thermal Pressure At Each Point Inside Thermal Pressure (Jostling) Gravity

But We Have a Problem The Sun is Luminous Radiates Energy Into Space Luminosity is Power Radiated -- ergs/second The Energy Comes From Motion of the Atoms Temperature Drops What Happens When T Drops?

Luminosity Effect When T Drops Thermal Pressure Can’t Hold Off Gravity The Sun Shrinks -- Radius Drops Energy is Released as Gas Falls Deeper Into Gravity Field Temperature Rises Note – Loss of Energy Results in a) Temperature Rise b) Radius Decrease

But Wait A Minute… Isn’t the Sun Stable? The Sun has been remarkably stable for 4 billion years as evidenced by geological records. This collapse is the process by which the Sun coalesced. But then it stopped. Why? The Sun collapsed until a new source of energy offset the losses to radiation. NUCLEAR FUSION --- IT’S BURNING HYDROGEN As long as it burns H at this rate, it will be stable.

Fusion Increases with T As T in core of Sun increases so does energy production Sun shrank steadily, with T rising until, about 10 million years after it started to form, it reached its current size There is a VERY fast increase in nuclear energy production above 1,000,000K. At 15,000,000K in the core nuclear power generated finally balanced the luminosity from the surface. That’s the equilibrium we are still in.

The Nuclear Core Envelope 1 Million K core 15x106K Photosphere 5000K At Surface

Cosmic Composition H hydrogen 89% by number He helium 11% O oxygen 0.1% C carbon 0.06% N nitrogen 0.015% Pretty much the composition of the entire universe. Sun and Jupiter have this composition Earth does not.

Fusion vs. Fission Fusion: Atoms unite and release energy (Fuse) New atom must be no heavier than iron z=26 Fission: Heavy atoms split to release energy Initial atom must be heavier than iron WWII Nukes were fission bombs made of U and Pu Sun works on FUSION of H into He

Proton-Proton Chain Bottom Line: H+H+H+H  He 1H1 +1H1  1H2 + e+ + n 1H2 +1H1  2He3 + g 2He3 +2He3  2He4 + 1H1 + 1H1 5x106 < T < 2x107K

CNO Cycle 6C12 +1H1  7N13 + g 7N13  6C13 + e+ + n 7N14 +1H1  8O15 + g 8O15  7N15 + e+ + n 2x107 < T < 108K 7N15 +1H1  6C12 + 2He4 Net: 1H1 +1H1 + 1H1 +1H1  2He4 + 2e+ + 4g +2n hydrogen -> helium + energy

Triple-a Reaction 2He4 +2He4  4Be8 + g 4Be8 +2He4  6C12 + g T < 108K 4Be8 +2He4  6C12 + g Must be very dense for this to work Be8 decays back into helium very quickly unless struck by another He4 Too low density in Big Bang Net: 2He4 +2He4 + 2He4 +2He4  6C12 + 2g helium -> carbon + energy

Solar Schematic

Seen by Ancient Persians Sunspots Seen by Ancient Persians (and me!)

Groups of Sunspots

Solar Corona Visible in Eclipse

The Sun Viewed in X-rays

X-ray Movie

X-ray Loops

Magnetic Structure

Dynamic Structure

Solar Turbulence

Differential Rotation Rotates in 25 days at Equator 28 days Mid Latitude 30 days Poles Rapidly Twists Up

Sunspots Erupt in Groups

During mid 1600’s sunspots became non-existent Sunspot Cycle During mid 1600’s sunspots became non-existent Maunder Minimum

Solar Wind 5x105K Corona 2x106K Photosphere 5500K Transition Region 105K Chromosphere 104K Photosphere 5500K

Solar Wind Passes Earth

Summary: Sun as a Star Formed from cloud 4.6x109 years ago Collapsed to present size stabilized by nuclear reactions Emits 4x1026 W Runs on proton-proton chain and CNO cycle Now 20% brighter Turbulent upper envelope Magnetic Fields from Differential Rotation Sunspots, Corona, Solar Wind Activity Cycle 11 years

STARS

Stars are grouped in Galaxies Sun and all the stars we see are part of Milky Way Galaxy We all orbit a common center Sun is 3x1020m from center of MW You are here Each star orbits center Disk Stability Again

Distances to the Stars Closest Star, Proxima Centauri is 4x1016m away. (Alpha Cen ~4.3x1016m) Need a more convenient unit

The Light Year Light Travels at 300,000km/s (186,000miles/s = 3x108m/s) That’s one foot per nanosecond One Year is 3.15x107 seconds long In one year light travels 3.15x107x3x108 = 1016m This is the definition of a light year. Prox Cen is at 4ly.

Question There’s a big black hole in the Center of the Milky Way at a distance of 3x1020m. How long does it take for its light to reach us? A) 3years B) 30 years C) 300 years D) 3000 years E) 30,000 years

Question There’s a big black hole in the Center of the Milky Way at a distance of 3x1020m. How long does it take for its light to reach us? A) 3years B) 30 years C) 300 years D) 3000 years E) 30,000 years

The Parsec Astronomers use the parsec as a measure of distance 1pc = 3ly 1pc = 3x1016m Origin of parsec comes from method of measuring distance

Each Star Orbits the Center

How Long does that Take? Takes about a hundred million years to circumnavigate the galaxy

Star Names Arabic Names Constellations Antares, Capella, Mira, etc. Constellations a Orionis, b Cygni, … then 49 Ori, 50 Ori, etc. Catalogues HD80591, SAO 733421, etc RA and Dec – just position in the sky

Proper Motion 2003 All stars move Nearby stars move faster Appear to move against fixed field Can Take Many Years Use Old Photographic Plates 1900

Parallax I year cycle

The Parsec 1 parsec 360 degrees in circle 60 arcminutes per degree 1AU 1 arcsecond 360 degrees in circle 60 arcminutes per degree 60 arcseconds per arcminute 200,000AU = 1 parsec = 3x1016m parsec ---- parallax second

Question Based on the definition of a parsec , if star A has a parallax of 0.5 arcseconds and star B has a parallax of 0.75 arcseconds which one is farther from the Earth? A. Star B is farther away because it has a higher parallax B. Star A is farther away because it has a lower parallax C. All stars are the same distance away from the Earth D. It is impossible to tell from this information.

Question Based on the definition of a parsec , if star A has a parallax of 0.5 arcseconds and star B has a parallax of 0.75 arcseconds which one is farther from the Earth? A. Star B is farther away because it has a higher parallax B. Star A is farther away because it has a lower parallax C. All stars are the same distance away from the Earth D. It is impossible to tell from this information.

Measure Parallax distance to a star in parsecs = 1/(parallax in arcseconds) e.g. measure .04” parallax, then distance is 25pc Measuring Parallax was first successful way to measure distances to stars after centuries of trying Took high speed photography in 1890’s to do it.

Question The parallax of an observed star is 0.1 arcseconds, how many lightyears is it away from Earth? a. 1 light year b. 3 light years c. 10 light years d. 30 light years e. 75 light years

Question The parallax of an observed star is 0.1 arcseconds, how many light years is it away from Earth? a. 1 light year b. 3 light years c. 10 light years d. 30 light years (10parsecs) e. 75 light years