Exam 2 review - Astronomy 122
Astronomy 122
Mid-term Exam 2 Review
posted May 1, 2005, jeb
PRELIMINARY
Exam will cover
Main sequence star
150,000,000 kilometers from Earth
5 billion (5 x 109) years old
696,000 kilometer radius
Spectral class G2V
Solar constant: 1400 Watts/m2
Knowing solar constant (Brightness) and the distance to the Sun
. . . allow us to determine: Absolute magnitude: +5
Central temperature 15,000,000 K
Surface temperature 6,000 K
Estimated eventual lifetime : 10 x 109 years ( 5 billion more)
Most abundant elements : hydrogen and helium
- Thermonuclear Fusion
- Provides energy
- hydrogen "burns" -> helium
-
proton-proton chain
- 4(1H) -> 4He + energy + 2 neutrinos
- Note: initial energy of Sun came from gravity as the matter collapsed and heated
until fusion "turned on"
- Hydrostatic equilibrium
- balance of force of gravity and outward gas pressure
Most of the Sun's energy is generated within 1/4 of the Sun's radius
Most of the Sun's mass is contained within 60% of its visible radius
- Thermal equilibrium
- Heat produced in the core is balanced by flow from the surface
-
Heat is transferred from the core to the surface by:
- Radiation - from core to bottom of the convection zone
- Convection - from the bottom of the convection zone to the surface
- No significant heat transfer from conduction
Heat generated at the core (by thermonuclear fusion) makes its way to the surface
after tens of thousands of years
Neutrinos produced as a by-product of fusion in the core pass out in a couple of seconds
-
Sun's Atmosphere
-
Photosphere - Produces visible surface - 6000 K
-
Chromosphere - Diffuse - emission lines
- Corona - millions of km thick
. . .
1-2 million K; best seen during
solar eclipse
-
Granules - photosphere - due to convection
- Spicules - chromosphere - jets of surging gas
- Supergranules
- Coronal Hole -
seen in x-ray image of Sun
-
Sunspots - cool (4000K) dark regions on the Sun's surface
-
intense
magnetic fields
- reveal differential rotation
- 25 day period at equator
- 35 day period near the poles
- solar cycle
- Sunspot maximum every
11 years
- Magnetic polarity
reverses every 11 years -> 22 year cycle
-
magnetic dynamo model
-
Prominences -huge arching columns of cool gas above the photosphere
-
Solar Flares - violent eruptions associated with complex sunspot groups
- Solar activity such as flares and sunspot number can dramatically affect
conditions on Earth.
Parallax - method to measure the distance of closest stars
- d (parsecs) = 1/ p (in arc seconds)
- 1 parsec = 3.26 light-years
Closest other star to the Sun is about 4 light-years away
- a little more than 1 parsec
- Alpha Centauri complex
- Brightness
- measure of the amount of light reaching the Earth
- also called Apparent Magnitude
- Luminosity
- measure of the total amount of light emitted by star
- also called Absolute Magnitude
- Absolute Magnitude is what the Apparent Magnitude would be if the star were
10 parsecs away
- Brightness (or apparent magnitude) decreases as the inverse-square law
NOTE
Luminosity and Absolute Magnitude measure the same property in different units
Brightness and Apparent Magnitude measure the same property in different units
Also will see reference to Apparent Brightness, and Absolute Brightness
-
Magnitude Scale
- 5 units = 100 x brightness (or luminosity) change
- 1 unit = 2.5 x change
- Brightest star (Sirius) Magnitude -1.5
- The larger the magnitude, the fainter the star
- Magnitude 6 stars - barely visible to the naked eye
- Magnitude 30 - visibility limit of Space Telescope
- Temperature and Color
- Blackbody theory relates the
color of a star to its surface temperature
- red -> 3,000 K
- orange -> 4,000 K
- yellow -> 6,000 K
- white -> 10,000 K
- blue -> 20,000 K
-
Filters
- V (visible)
- B (blue)
- U (ultraviolet)
- Detailed Spectra
- spectral classes (spectral type)
-
O B A F G K M
- O is hottest and most massive
- M is coldest and least massive
-
Hertzsprung-Russell Diagram
- Absolute Magnitude vs. Spectral Type
- or Luminosity vs. Temperature (reverse order)
- Main Sequence
- Giants
- White dwarfs
- The cosmic distance scale can be extended with
Spectroscopic Parallax
-
Luminosity Class
- 90% of the stars are main sequence stars
- We can distiguish the non-main-sequence stars based on the width of spectral lines
- large stars are less dense in the absorbing atmosphere, and therefore have narrower lines
-
Stellar Mass
- Binary stars
- Most stars in a multiple star system (binary, triple, ...)
- Stars move in elliptic orbits about a common "center of mass"
-
Types of binaries
- Measuring orbits of binary reveals mass
-
Stellar Size
- Betelgeuse is large enough, and close enough, to have it's
size
resolved directly
- Most stars are too small, or too small, to measure the size directly
- Combining Luminosity with Temperature gives Stellar Size
- The total luminosity is the area of the star times its surface temperature to the fourth power (Blackbody theory)
- OR -> Luminosity = Area x (temp)4
- this maps out lines on the HR diagram of specific
radii of the stars
- Stars above the main sequence are giants
- The red giants are giant stars with surface temperatures making them red (about 3000 K)
- Stars below the main sequence are dwarf
- The white dwarf are very hot, small stars
- Very important conclusion from "measuring the stars"
- The evolution and properties of a main sequence star is predominantly determined by the
mass of the star
- Star clusters
-
Open (or galactic) clusters
- loosely packed, groups of younger stars
-
Globular clusters
- tightly packed, groups of the oldest stars known
- age determined from "turn-off" point in H-R diagram
99% of the mass is gas
1% of the mass is dust
Gas
- about 1 atom per cubic centimeter
- compared to Earth's atmosphere of 3 x 1019 per cubic cm
- 90% (by number) hydrogen
- 10% (by number) helium
- small amounts of other atoms (C,O,N,Ca,Na.....)
Interstellar reddening
Extinction
Polarization
HI
HII
21 cm line
. . . .
spin-flip in hydrogen
Nebulae
- Dark (or Absorption Nebulae)
- Reflection Nebulae
- Emission Nebulae
- Typically O- and B-type stars heating their surroundings
-
Orion Nebula
Molecular Clouds
- cold regions -> molecules can form as
atoms stick together
- densities are often high, as great as 106/cm3
- dominant molecule H2
- But other molecules are useful due to their distinctive lines
- like OH, H2O, NH3, H2CO, CO, HCN, etc.
- Temperature of gas determines whether it will be:
- molecular cloud (coldest): about 20 K
- HI region (warmer): about 100 K
- HII region (warmest): about 10,000 K