Astro 20A final

Spectral types

O B A F G K M L T Y

bluer → redder

hotter (40,000K) → cooler (>1,300K)

Luminosity classes

0 - Hypergiant

I - Supergiant

II - Bright Giant

III - Giant

IV - Subgiant

V - Dwarf (main sequence)

VI - Subdwarf

Surface gravity is measured by the width of the spectral lines, which are more defined for larger stars.

As a star evolves, its mass doesn’t change. Size is determined by radius.

Wein’s Displacement Law

wavelength of peak flux(m) = b/T

where b = Wien’s constant (3 × 10^-3)

T = temperature of blackbody

A peak in energy at a higher wavelength (bluer) suggests a hotter star.

Stellar opacity

Parts of a star where a photon cannot reach

Bound-bound transition

Atom absorbs a photon and may (or may not) emit a photon in another direction.

(source of stellar opacity)

Bound-free transition

Atom absorbs a photon and gains enough energy to emit an electron.

(source of stellar opacity)

Free-free transition

Electron absorbs a photon and gains kinetic energy.

(source of stellar opacity)

Electron (Thompson) scattering

Electron scatters photon in a different direction.

Direct imaging

Uses a coronagraph to block out starlight in order to see only the light emitted by the orbiting planets.

Bias: Large planets (~10 Jupiter masses), Bright (young and very hot) planets that give off blackbody thermal emission, not reflected light.

Allows: Method that finds the farthest planets from the star (some with ~40AU)

Astrometry

Detects a planet through its gravitational influence on its star by detecting the “wobble” of the star as it orbits with the planet around a common center of mass.

Bias: Very massive planets that are able to have such an influence (~7 Jupiter masses)

Radial velocity

Detects a planet through its gravitational influence on its star by detecting the motion of the sun using doppler shift. Red-shift if moving towards and blue-shift if moving away.

Bias: For this to work, the planetary system needs to be at a perpendicular angle to ours so that we can see the variation using Doppler. Also favors larger planets with shorter periods.

Allows: Can detect multiplanet systems. Can be used to detect black holes by observing the black hole’s large influence on the star.

Transit method

Measures the apparent luminosity of a star and detects a planet by the dip in starlight caused by the planet as it passes in front of the star.

Bias: Favors bigger planets with smaller periods that create more noticeable dips. Favors systems whose planets are inclined so the planet passes in the middle (parallel to us).

Allows: Broader range of size than other methods and can figure out the atmosphere makeup of the planet. Can observe multiple stars at once

Brown dwarf size

Planets > Brown Dwarfs > Stars

Brown dwarfs are bigger than a large gas giant but smaller than the least massive main sequence star. They form just as stars and planets do. They are self-luminous, fusing Dueterium and sometimes Lithium in their cores, but are too small to fuse Hydrogen.

Mercury

Geography: Long, linear cliffs (height 2-100km) called “scarps” and low level of cratering.

Temp: Large difference from night (100K) to day (700K)

Venus

Geography: Thick clouds of sulfuric acid that slows rotation. Lots of volcanoes, highlands, and no plate tectonics.

Temp: Hottest temperature of all the planets (740K)

Earth

Water oceans, mountains, and valleys. Low levels of cratering.

Temp: 288K

Mars

Geography: Signs of recent volcanic activity, high elevations and canyons. Frozen water as permafrost at the poles.

Temp: Coldest terrestrial temperature (213 K)

Jupiter

Geography: Rapid rotation stretches clouds to long bands. Darker bands (belts) and lighter bands (zones). Rocky core and outer ice core.

Temp: 163K

Saturn

Geography: Prominent rings, not very dense. Rocky core and outer ice core.

Temp: 133K

Uranus

Geography: Axis tilted at 98 degrees relative to the orbital plane.

Temp: Coldest planet (49K)

Neptune

Geography: Rapid rotation stretches clouds to bands. Appears to have an internal heat source.

Temp: 73K

Radiative transfer

Photon carrying energy does a “random walk” between any two points on its way out of the sun.

(Method of energy transfer from the core to the outside of the sun)

Convective transfer

Where the sun is opaque (photon can’t travel) energy is transferred through convection.

(Method of energy transfer from the core to the outside of the sun)

Photon transfer for different star masses

Very massive stars (~7 M☉): convective interiors, very large radiative zones outside interior.

Sunlike stars (~ 1 M☉): radiative interior, convective outside interior.

Very small stars (~0.1 M☉): fully convective throughout.

Why do only some solar system objects have craters?

If an object has an atmosphere, it is more protected from impacts.

Heavy volcanic activity clears craters away. If the object was still forming (with heavy volcanism) during the heavy bombardment period, the craters would also be cleared away.

Meteor classification

Meteoroid: when in space

Meteor: when in the atmosphere (usually burn up)

Meteorites: when on the ground (size of the pebble to a fist)

Layers of earth’s interior

Inner core: 1300 km thick

Outer core: 2250 km thick

Mantle: 2900 km thick

(Asthenosphere between)

Crust: 8-40km thick