Astronomy Chapter 9

Exoplanet definition

A planet orbiting a star other than the Sun

First exoplanets discovered

Around pulsars

Pulsar definition

Rotating neutron star emitting regular pulses

Pulsar timing method

Detects planets via variations in pulse timing

First exoplanet around Sun-like star

51 Pegasi b

51 Pegasi b type

Hot Jupiter

Hot Jupiter definition

Massive gas giant orbiting very close to its star

Exoplanet detection methods

Radial velocity, transits, direct imaging, microlensing

Radial velocity method

Detects star wobble via Doppler shifts

Doppler shift definition

Change in wavelength due to motion

Radial velocity sensitivity

Earth detection requires ~10 cm/s precision

HARPS precision

~1 m/s

ESPRESSO goal precision

~10 cm/s

Radial velocity best for

Massive, close-in planets

Radial velocity provides

Minimum mass, orbital period, eccentricity

Transit method

Detects brightness dip when planet crosses star

Transit data gives

Planet size, orbital period

Transit limitation

Requires alignment with line of sight

Direct imaging method

Takes images of planets directly

Limitation

Bright star overwhelms faint planet

Microlensing method

Uses gravitational lensing of background star

Advantage

Can detect distant planets

Mass-period diagram

Plots planet mass vs orbital period

Hot Jupiters region

High mass, short period

Warm Jupiters region

Intermediate distance

Cold Jupiters region

Long period

Super Earths / sub-Neptunes

Lower mass planets

Terrestrial planets

Low mass, rocky planets

Neptunes

Intermediate mass planets

Solar system formation age

~4.6 billion years

Evidence for age

Meteorites (~4.56 billion years), Moon rocks (>4.4 billion years)

Solar system origin

Primeval solar nebula

Solar nebula definition

Cloud of gas and dust that formed Sun and planets

Collapse of cloud

Gravity pulls material inward

Cause of collapse

Unknown trigger (possibly nearby supernova)

Conservation of angular momentum

Cloud spins faster as it collapses

Result

Flattened rotating disk forms

Protoplanetary disk definition

Disk of gas and dust around young star

Disk structure

Hot inner region, cooler outer region

Temperature gradient

Inner disk hot, outer disk cool

Condensation sequence

Only certain materials condense at certain distances

Rock formation

Occurs in inner hot region

Ice formation

Occurs in outer cooler region

Planetesimal definition

Small solid objects that collide to form planets

Accretion definition

Growth by collision and sticking

Terrestrial planets formation

From rocky material in inner disk

Gas giants formation

Form beyond frost line, accrete gas

Frost line definition

Distance where ices can form

Importance of frost line

Allows formation of massive cores

Gas giant formation

Large core attracts hydrogen/helium gas

Terrestrial planet composition

Rock and metal

Orbital properties of planets

Nearly circular, in same plane, same direction

Reason

Common origin in rotating disk

Extrasolar systems importance

Provide insight into planet formation

Goldilocks zone definition

Region where liquid water can exist

Importance

Potential for life

Brown dwarf definition

Object between star and planet

Mass range

Too small for sustained fusion

Stars definition

Luminous spheres of hot gas

Stars vs Sun

Stars are distant suns

Stellar properties determined by

Mass

Continuous spectrum

Smooth distribution of light

Produced by

Dense gas or solid

Blackbody definition

Perfect emitter dependent on temperature

Blackbody curve

Intensity vs wavelength graph

Temperature vs wavelength

Higher temp → shorter wavelength peak

Wien’s law

Relates peak wavelength to temperature

Star color vs temperature

Blue = hot, red = cool

Sun color

White

Absorption lines

Dark lines in spectra from elements

Spectral fingerprint

Each element has unique pattern

Hydrogen spectrum

Balmer series visible lines

H-alpha line

Strong red line

Spectral classification

O, B, A, F, G, K, M

Order meaning

Hot to cool

HR diagram

Luminosity vs temperature plot

Main sequence

Where stars fuse hydrogen

Main sequence lifetime

Longest phase

Higher mass stars

Shorter lives

Lower mass stars

Longer lives

Hydrostatic equilibrium

Balance of pressure and gravity

Result

Stable star

Equation of state

Relation between pressure, temperature, volume

Ideal gas law

PV = NkT

Sun composition

73% H, 25% He, 2% metals

Heat diffusion

Energy flows from hot to cool

Radiative transport

Energy via photons

Convective transport

Energy via fluid motion

Why stars shine

Hot core + energy transport outward

Fusion definition

Combining nuclei to release energy

Energy source

Mass converted to energy (E=mc²)

Hydrogen fusion

4 H → He

Proton-proton chain

Main process in Sun

Star formation

Nebula collapse → protostar → fusion

Protostar

Early stage before fusion

Main sequence start

When fusion begins

Red giant

Expanded star after hydrogen depletion

Shell burning

Fusion outside core

Helium fusion

Triple-alpha process

Helium flash

Rapid ignition in low-mass stars