ASTR 1200 Announcements Website Second problem set due next Tuesday in class. Observatory Sessions all now at 8:30pm

Solar Structure

Magnetic Fields Stronger when closer together Charged particles follow field lines Magnetic field holds energy

Solar Magnetic Fields

Sunspots Erupt in Groups

Solar Granules TGSdi4https:// TGSdi4

Solar Atmosphere Photosphere 5500K Chromosphere 10 4 K Transition Region 10 5 K Corona 2x10 6 K Solar Wind 5x10 5 K

Solar Wind Passes Earth

Coronal Mass Ejection

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

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

Summary: Sun as a Star Formed from cloud 4.6x10 9 years ago Collapsed to present size –stabilized by nuclear reactions Emits 4x10 26 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 Contains (100 billion) stars Sun is 3x10 20 m from center of MW You are here Each star orbits center Disk Stability Again

Each Star Orbits the Center

How Long does that Take? Takes about a hundred million years to circumnavigate the galaxy Orbital Period: r = 3x10 20 m, M = 2x10 42 kg, G = 6.7x m 3 kg -1 s -2

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

Distances to the Stars Closest Star, Proxima Centauri is 4x10 16 m away. (Alpha Cen ~4.3x10 16 m) Need a more convenient unit

The Light Year Speed of light is constant c = 3x10 8 m/s (300 million meters/second) Measure distance by how long it takes light to travel

The Light Year Earth circumference: 0.13 light seconds Distance to Sun: 8 light minutes Distance to Pluto: 5.5 light hours

The Light Year One year is 3.15x10 7 seconds long In one year light travels (3.15x10 7 s) x (3x10 8 m/s) = m This is the definition of a light year. Proxima Centauri is at 4ly.

Parallax How to measure the distance to stars? Look at how they appear to move relative to each other

Parallax I year cycle

The Parsec 1AU 1 parsec 1′′ 360 degrees (360°) in circle 60 arcminutes (60′) per degree 60 arcseconds (60′′) per arcminute Parsec (pc) ---- parallax second

Measure Parallax Distance in parsecs = 1/(parallax in arcseconds) If parallax is.04′′: d = 1/0.04 = 25 pc 1 pc = 3x10 16 m = 3.3 ly Measuring Parallax was first successful way to measure distances to stars after centuries of trying

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.

Gaia Parallax for a billion stars Distances out to 10 4 ly Launched Dec 2013

Proper Motion All stars move Nearby stars appear to move faster Appear to move against fixed field Can take many years Use old photographic plates

Brightness Around the sky stars vary in brightness and in color. Brightness is the result of two factors 1. Intrinsic Luminosity 2. Distance Each Sphere has area A=4πr 2 Star Emits N photons per second Brightness is r photons/m 2 /s

Question If the distance between Earth and the Sun were cut in half, how much brighter would the sun appear in our sky? a.2x brighter b.4x brighter c.8x brighter d.16x brighter

Question If the distance between Earth and the Sun were cut in half, how much brighter would the sun appear in our sky? a.2x brighter b.4x brighter c.8x brighter d.16x brighter Brightness is a function of the inverse square of distance, so if distance was cut by half it would get brighter by 4x=1/(.5) 2

Brightness Brightness e.g Watts/m 2 Simple and easy to understand If your eye is m 2, then it collects W 4 stars at W/m 2 together have 4x W/m 2 But this would be too easy for astronomers. We use a brightness system invented by Ptolemy in the 400’s

The Magnitude System Ptolemy Broke Stars into 5 magnitude groups m=1 the brightest, m=5 the faintest In 1700’s it was found this was a logarithmic scale, as that is how the naked eye responds. Also, faintest were about 100x fainter than brightest. Break the factor of 100 into 5 equal factors: Start with Vegam=1 Polaris 2.51x fainter m=2 2.5x fainter than Polaris m=3 2.5x fainter than that m=4 etc

Magnitudes (2) Every 5 magnitudes is a factor of 100 m=5 is 100 times fainter than m=0 m=10 is 100x100 =10,000 times fainter than m=0 m=15 is (100) 3 = 1million times fainter than m=0 Works only in the visible. Really inconvenient in modern astronomy because we observe across the spectrum from radio to gamma rays. Sunm=-26.5 Full Moonm=-13 Venusm=-4 Siriusm=-1.5 Vegam=1 Polarism=2 Faintest Visiblem=6 Faintest Detectedm=28

Absolute Magnitude We see a star of magnitude m=10 at 100 pc. What would be its magnitude (M) if it were at 10 pc instead of 100pc? The magnitude a star would have were it at 10pc At 10 times closer the star would be 100x brighter = 5 magnitudes M = 10-5 = 5