AST201 week 4

gravitational contraction

the sun generates energy by slowly contracting in size, a gradually shrinking sun would always have some gas moving inward converting gravitation potential energy into thermal energy

sun has 2 kinds of balance that keep its size and energy output stable

gravitational equilibrium and energy balance

gravitational equilibrium

between outward push of internal gas pressure and the inward pull of gravity

energy balance

between the rate of which fusion releases energy in the sun’s core and the rate at which the sun’s surface radiates this energy into space

in about 5 billion years…

the sun will finally exhaust its nuclear fuel and gravitational contraction will begin once again

sunspots

visible splotches that appear darker than the surrounding surface (larger in life than earth)

power

the rate at which energy is used or released

luminosity

a stars total power output

solar wind

stream of charged particles continually blown outward in all directions from the sun

corona

outermost layer of the atmosphere, temp is astonishingly high, density is very low

cromosphere

middle layer of the solar atmosphere and the region that radiates most of the sun’s UV light

photosphere

lowest layer of the atmosphere, visible surface of the earth

convection zone

where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection

radiation zone

where energy moves outward primarily in the form of photons of light, turbulence of convection zone gives way to the calmer plasma

core

source of suns energy, density is more than 100 times that of water, extreme pressure

radiative energy

the energy that light carries

joules

the unit we measure energy in

power

the rate of energy flow, measured in units called watts

spectrum

a prism split light into the rainbows of light

white light

a mix of all these colours in roughly equal proportions

black light

when we perceive no light and hence no colour

primary colours of vision

red, green, blue (RGB)

how do light and matter interact

emission, absorption, reflection

transparent

materials which transmit light

opaque

materials that absorb light

particle

can sit still or move from one place to another

waves

consist of peaks

wavelength

distance from one peak to the next

frequency

number of peaks passing by any point each second

hertz

another name for cycles per second

field

describe the strength of force that a particle would experience at any point in space

electromagnetic waves

light waves are traveling vibrations of both electric and magnetic fields

photons

light comes in individual “pieces” that have properties of both particles and waves

electromagnetic spectrum

light that we can see

electromagnetic radiation

light itself

visible light

light that we can see with the naked eye

infared

light with wavelengths somewhat longer than those of red light

Ultraviolet

lies between blue and end of the rainbow in wavelengths

atoms

all ordinary matter is composed of atoms

element

atoms come in different types and each type corresponds to a different chemical element

atoms are made up off:

• protons
• neutrons
• electrons
• nucleus

attraction and repulsion

oppositely charged particles attract and similarly charged particles repel

molecules

number of different material substances is far greater than the number of chemical elements because atoms can combine to form molecules

chemical bond

interactions between electrons that hold the atoms in a molecule together

molecular dissociation

high enough temp, the collisions become so violent they break chemical bonds

pressure

force per unit area pushing on an objects surface

energy levels

the possible energy of an atom

energy level transitions

an electron can rise from a low energy level to a higher one, or fall from a high level to a lower level

spectroscopy

the process of obtaining a spectrum and reading the info it contains

three basic types of spectra

1. continuous spectrum
2. emission line spectrum
3. absorption line spectrum

continuous spectrum

the spectrum of a traditional, or incandescent, light bulb is a rainbow of colour, because the rainbow spans a brand range of wavelengths without interruption

emission line spectrum

a thin or low density cloud of gas emits light only at a specific wavelengths that depend on its composition and temp, the spectrum consists of bright emission lines against a black background

absorption line spectrum

cloud of gas lies between us and a lightbulb, still see most of the continuous spectrum of the light bulb, but the cloud absorbs light of specific wavelengths, so the spectrum shows dark absorption lines over the background rainbow

two laws of thermal radiation

1. each square metre of a hotters object surface emits more light at all wavelengths
2. hotter objects emit photons with a higher average energy

doppler effect

if an object is moving toward us the light waves bunch up and its entire spectrum is shifted shorter wavelengths

sun’s size

diameter is 100 times Earth’s diameter

sun’s distance

8 light minutes from Earth

Cecilia Payne-Gaposchkin

showed that the sun was made mostly of hydrogen, a little helium, and tiny amounts of other elements

colour and wavelength

the wavelength of light determines its colour and energy

short wavelengths

bluer (and more energetic)

red light

low energy (700 nm)

blue light

high energy (400 nm)

the sun appears to have

a continuous spectrum: it has some of every colour of light

opaque objects

emit a special kind of continuous spectrum

blackbody spectrum

the amount of light given off by a blackbody and the wavelength where it emits the most light set by the temp of the blackbody

as you heat an opaque object

it emits more light

as you heat a blackbody

the wavelength at which it emits the most light shifts to shorter wavelengths

spectral lines

tell us what the sun and other stars are made of

electrons can be forced to jump to a higher-energy orbital by

absorbing light

different chemical elements have

different sets of orbitals

electrons can spontaneously drop to a lower-energy orbital by

emitting light

orbitals do not have colours but

transitions between orbitals do

stars are

blackbodies or “thermal emitters”

sun is not

on fire, it is mainly made of hydrogen

plasma

a gas that is so hot, the electrons break free from the atoms

hydrostatic equilibrium

the sun is in this, the pressure pushing outwards and the gravity pulling inwards is balanced

sun’s surface temperature

5800 K

sun's core temperature

15 million K

E = mc^2

the reaction between energy and mass

nuclear reactions can

convert matter to energy (or the reverse)

in the centre of the sun,

hydrogen is being converted into helium via nuclear fusion

nuclear fusion

providing enough outward pressure necessary to support stars against collapse

neutrinos

only interact with other matter via the weak nuclear force and gravity, can pass through objects that are very large and dense

neutrinos dont

respond to the electromagnetic force, so they dont interact with most matter