The Sun (continued)
Solar
Structure
Core - the hottest part of the sun and where nuclear fusion occurs
Radiation zone - hot plasma
where photons are trapped for hundreds of thousands of years Convection zone - energy is transferred to the surface through convection
Photosphere - the visible
surface of the Sun and where photons finally escape
Chromosphere - hot part of
the atmosphere where most of the UV light is radiated
Corona - extremely hot part of Sun’s atmosphere and where
Mass of 4 protons greater than 4He
‣ Difference in mass is the energy output of the proton-proton chain (E = mc2)
‣ 600 megatons/s → 596 megatons/s = 4 megatons of mass lost per second
‣ Photons created in core must escape
‣ Interior is extremely dense
Escape of Energy
‣ Photons scatter off electrons
‣ Random walk for hundreds of thousands of years
Transports energy to the photosphere
‣ Hot gas rises ‣ Cool gas falls
Each granule lasts only a few minutes
Magnetic Fields
‣ Magnetic fields trap charged particles
‣ Charged particles spiral around the magnetic field
Magnetic field suppresses convection
Hot plasma unable to rise up to photosphere
Solar prominences connect sunspots
Chromosphere and corona caught in magnetic field
Solar Flare
‣ Occurs when magnetic field rearranges ‣ Heats nearby plasma to 100 million K
‣ Releases x-rays
Magnetic Heating
‣ Most of the solar activity involves the
chromosphere and corona ‣ Convection oscillates
magnetic field lines
‣ Magnetic field heats
plasma to several 106K
‣ Coronal holes - regions of missing coronal gas
Coronal Mass Ejection
CME - an ejection of a large number of charged particles Geomagnetic storms
‣ Induce large currents on the ground
‣ Heat the upper atmosphere
Period of 11 years between solar maxima (most sunspots)
Sunspots form at high latitudes and move to lower latitudes
Magnetic field orientation flips at solar maximum ‣ Magnetic field orientation has 22 year period Flare and CME activity follows the sunspot cycle
IBEX - Solar System Tail Voyager Missions
Stars
Ch. 11•
~98% H and He•
Fusion in the core supports the outer layers•
Range of sizes and massesKey Properties
•
Apparent brightness•
Luminosity•
Temperature / color•
Mass•
Evolutionary stateApparent Brightness
•
Depends on two things•
Intrinsic brightness of star•
Distance from Earth•
•
Amount of light decreases withInverse Square Law for Light
Apparent Brightness measured in watts per
square meter
Drops off as square of distance
Parallax
Stellar Parallax
•
Caused by motion of Earth in it’s orbit•
d = 1 / pwhere p is in arcsec and
d is in parsecs
Luminosity
(Absolute Brightness)
•
Need distance to determine luminosity•
Surveys of stars seem to show that•
There is a large range in luminosities•
Low luminosity stars are the most common•
Luminosities relative to the sun•
Logarithmic•
Large values are dim objects•
Small values are bright objectsMagnitudes
•
Apparent magnitude - how bright the star appears to be on Earth•
Can only see stars with an apparent magnitude smaller than ~ 6•
Absolute magnitude - how bright the star would appear from a distance of 10parsecsWhich has the smaller
apparent magnitude?
A. 3Lsun with distance of 7ly
OR
B. 60Lsun with distance of
7000ly?
Which has the smaller
absolute magnitude?
A. 7Lsun at a distance of 3ly
OR
B. 30Lsun at a distance of
250ly?
Which has the smaller
apparent magnitude?
A. 3Lsun with distance of 7ly
OR
B. 60Lsun with distance of
7000ly?
Which has the smaller
absolute magnitude?
A. 7Lsun at a distance of 3ly
OR
B. 30Lsun at a distance of
250ly?
Which has the smaller
apparent magnitude?
A. 3Lsun with distance of 7ly
OR
B. 60Lsun with distance of
7000ly?
Which has the smaller
absolute magnitude?
A. 7Lsun at a distance of 3ly
OR
B. 30Lsun at a distance of
250ly?
Temperature / Color
!"#$%&'()*'
!max " T = 2.9 "106
(nm " K) Peak of blackbody emission
Temperature / Color
•
Color - difference in intensity between two filters•
B-V color•
Proxy for temp•
Independent of distanceSpectral Type and
Temperature
Determined from observed spectral lines
More ionized elements
means higher temperature Fewer spectral lines point
to more ionized elements
Lame mnemonic:
Spectral Type
•
Spectral types aresubdivided for intermediate temperatures
•
Subdivisions from 0-9•
Smaller numbers are hotter•
Larger numbers are cooler•
Use binary systems to estimate mass•
Eclipsing binary - when the orbital plane is along our line of sight and we see the stars eclipse each other•
Spectroscopic binary - orbitalmotion only seen in the spectrum of system
•
Seen as two sets of absorption lines shifting in location with time•
Need orbital period and separation•
Can use velocity to get separation (only certain for eclipsing binary)The HR Diagram
•
Main Sequence•
Giants•
SupergiantsThe HR Diagram
Luminosity class gives information
on the luminosity of the star as well as the size
Which is brighter?
Which has star is physically larger?
A. O2 V OR B. K7 I
Which star is more luminous?
A. G2 V OR
Which is brighter?
Which has star is physically larger?
A. O2 V OR
B.K7 I
Which star is more luminous?
A. G2 V OR
Which is brighter?
Which has star is physically larger?
A. O2 V OR
B.K7 I
Which star is more luminous?
A. G2 V OR
•
Mass is the most fundamental property of a main sequence star•
Hydrogen fusion in the core•
Stars spend ~90% of their lifetime inthe main sequence phase
•
Length of MSlifetime depends on the mass
Main Sequence Lifetime
•
Very massive stars have very short main sequence life times•
Have more fuel but burn it much more quickly•
Low mass stars have very long main sequencelifetimes
•
Have much less fuel but burn it slowly•
Stars evolve off the main-sequence when they run out of H in their core•
Supergiants and giants expand to extremely large sizes•
Temperatures very low•
Luminosities extremely high•
White dwarfs are very hot but faint•
Small and have no energy source•
•
Very useful in understanding stellar formation and evolution•
Can use them as clocks•
All stars in a given cluster located at about the same distance from Earth•
All the stars in a given cluster formed atabout the same time or have the same age
•
Most of what we know about stars comes from studying clusters of starsGlobular Clusters
•
Extremely old: ~11-14Gyrs•
Some of the oldestobjects in the Milky Way
•
Contain mostly low mass stars•
Around 105-106 starsconcentrated in a small volume of space
Open Clusters
•
Young objects: only a few Myrs old•
Contain lots of very luminous blue stars•
Contain several thousand stars•
~30ly acrossAges of Clusters
•
Main sequence turnoff the most massive stars left on the mainsequence (MSTO)
•
The location of theMSTO gives the age of a cluster
•
The lifetime of thecluster is given by the lifetime of the stars at the MSTO
Young clusters still have their massive blue stars
on the MS
Old clusters are devoid of massive blue stars
and only have lower mass red stars
