Astronomy

what is the universe made of?

• stars

• planets

• moons (satellites)

• asteroids

• comets

• solar (star) systems

• galaxies

• universe

what are stars

• large, glowing balls of gas that generate heat and light through nuclear fusion

• stars are fully plasma

• hot enough to fuse, nuclear fusion is how they generate their energy

• all stars are mostly hydrogen and helium producing their energy by fusing hydrogen into helium

• sun is 99% of the solar system by mass

what are planets

• moderately large objects that orbit a star; they don’t glow they shine by reflected light

• if there is no star to illuminate them it is hard to see them

• planets may be rocky, icy, or gaseous in composition

what are moons (satellites)

• an object that orbits a planet

• Ganymede is the moon of Jupiter which is the biggest moon in our solar system

• moons are also called satellites because satellites are anything that orbits

• not all planets have moons

• it is a natural satellite not artificial

what are asteroids?

• come in a variety of sizes

• relatively small and rocky objects that orbit a star

• they don’t have a lot of gravity so they don’t really look like a sphere

what are comets?

• relatively small and icy objects that orbit a star

what are solar (star) systems

• another name for the sun is sole

• a star and all the material that orbits it, including its planets, moons, asteroids, and comets

what are nebulae

• can get very dense; basically gas clouds but they are very very huge

• interstellar clouds of gas and/or dust where stars are born

what are galaxies

• great islands of stars in space, all held together by gravity and orbiting a common center

what is the universe

• the sum total of all matter and energy

• that is, everything within and between all galaxies

Organization of the Universe

1. universe →

2. supercluster of galaxies (thousands of galaxies held together by gravity; super big) →

3. groups of galaxies (smaller amount of galaxies held together by gravity) →

4. galaxies (groups of stars, around 100 billion stars in each) →

5. solar system (planets, asteroids, comets orbiting around one star) →

6. planets and moons (all very diverse)

foundations of astronomy

• astronomy is the study of the universe: the totality of all space, time matter and energy

• it is a subject like no other, we need to change our pespective of size, scales and time unfamiliar to us

powers of 10: large numbers

a positive exponent on the number 10 tells us how many zeros there are

power of 10: small numbers

negative exponent tells us how many decimal places are in the number

scientific notation

• a way of expressing large or small numbers

• 2,230,000 = 2.23 × 10^6

• 0.0095 = 9.5 × 10^-3

basic unit of length in astronomy

meters (m)

astronomical unit (AU)

• is the average earth-sun distance: 1 AU = 149.6 million km

→ example is that Jupiter is 5 AU away from the sun which means it is 5 times further from the sun than earth is, Mars is 1.5 AU, Neptune is 30 AU

→ 1 AU = 1.5 × 10^8km

light year (ly)

• is the distance light travels in a year 1 ly = 9.461 trillion km = 63, 000 AU

→ it would take light 100,000 years to cross the milky way

→ where we are is 6,000 light years

→ to get to andromeda it is 2 million light years away

→ virgo cluster is 60 million light years away

parsec (pc)

an astronomical unit that is about 3.26 ly

units in space

• on the earth we use km

• in the solar system we use astronomical units

• once we leave the solar system its best we start talking about light years

• the nearest star to us is 4 light years away

• our galaxy is 100,000 light years across

• the local group is million of light years

• and the universe is billions of light years

speed of light

• light has a finite speed so it takes time for light to travel great distances

• the speed of light is 300,000km per second

• when something sends out light it takes time for it to arrive

• for example, usually you see lightning before you hear thunder because light travels much faster than sounds

how can we know what the universe was like in the past?

• we see objects as they were in the past

• the farther away we look the farther back we see in time

• we see objects in the sky as they were some time agi

• we see the moon as it was 1 second ago

• we see the sun as it was 8 minutes ago

• we see proxima centauri as it was four years ago

• many distant galaxies are billions of light-years away or more, so we can see what the universe was like billions of years ago when the universe was newly born

light travel time to moon

1 second

light travel time to sun

8 minutes

light travel time to sirius (the dog star, brightest star in our sky)

8 years

light travel time to andromeda galaxy

2.5 million years

why do we need to understand light?

• in astronomy, light is the only way we get information about whats happening in the universe

• astronomers are passive observers of what is going on, light is the messenger so we need to understand it

• light is a wave

what is a wave

• if a lead is sitting on the ponds surface, it will bob up-and-down as the wave ripples, but the leaf only moves up-and-down, not sideways

• this shows that the water is not moving past, the wave is

• a wave is a pattern of motion that can carry energy without carrying matter (stuff) along with it

wave terminology

when describing waves we often speak of their:

• wavelength

• frequency

wavelength

the distance between two wave peaks

frequency

the number of times per second that a wave bops up and down

frequency and wavelength

inverse relationship:

• higher frequency → shorter wavelength

• lower frequency → longer wavelength

speed of a wave

• waves have speed and they can transmit energy

• example: every time you make a phone call, invisible radio waves (and microwaves) are depositing energy into the receivers phone

• the speed with which a wave moves depends on both its: frequency and wavelength

how to determine the speed of a wave

for any wave:

• speed of wave = (frequency) x (wavelength)

a common convention is to denote:

• frequency as f

• wavelength as λ

• speed as c

so what written using words above is seen as c=fλ

how to use this wave equation

• frequency as f, is measured in Hertz (Cycles/sec)

• wavelength as λ, is measured in Meters (m)

• speed as c, is measured in meters/sec (m/s)

what is a light (radiation)

• light is a wave

• why isnt it more obvious that light is a wave? → because the wave length of light is tiny

• depending on the vibration (frequency and wavelength), light comes in many forms

• humans can only see one form of light called the ‘visible’ form

the electromagnetic spectrum

• the term “light” or “radiation” refers to any electromagnetic (EM) wave listed

• the human eye is sensitive to a small part of the electromagnetic spectrum (visible)

• but we are familiar with ‘invisible light’: mobile phones use radio waces, microwave oven uses microwaves, TV remote uses infrared light, tanning booths use ultraviolet light, medical imaging uses x-rays, cancer radiotherapy uses gamma rays

colour and visible light

• visible light is really made up of different colour from red to violet (combine all these colour and we get white light)

• visible light = colour = wavelength

• what we see as different colours are in fact different wavelengths of light

• the range of colours the human eye is sensitive to is from violet to red, or in wavelengths, from 400nm to 700 nm

• each type of EM radiation travels at exactly the same speed (300,000 km/s)

relationship between frequency and energy

the higher the frequency of light, the more energy it has, and the more powerful the light source

light and temperature

• a piece of metal thrown into a fire will glow first red, then orange, if the fire is really hot, it may even glow yellow

• the colour of the metal object depends on the temperature

• as an object is heated, it emits light at different wavelengths (depending on the temperature)

• any object emits light (radiation)

• the type of light (radiation) depends on the temperature of the oject

• the hotter the object, the shorter the wavelength of light (radiation) emitted

do humans emit light

• yes, but not visible light → infrared light

• a special camera is needed for our eyes to detect infrared light

blackbody radiation

• all objects emit light in the form of electromagnetic radiation

• a blackbody is a theoretical or model object which absorbs all radiation falling on it, reflecting or transmitting none, it is a hypothetical object which is a ‘perfect’ absorber and a ‘perfect’ emitter of radiation over all wavelengths

• in astronomy, stars are often modelled as blackbodies, although it is not always a good approximation

spectrum

a graph of the amount of light given off at different wavelengths

blackbody spectrum

• here are three objects shown on a blackbody spectrum

• notice what happens to the peak of the graph with objects at different temperatures

blackbody curves: things to notice

• the wavelength at which a hot object (a metal rod or a star) emits the most electromagnetic radiation is inversely proportional to the temperature of the object

1. the peak wavelength is the wavelength at which most of the radiation is given off

2. the hotter the object, the peak light emitted shifts to the left: shorter wavelength

peak wavelength

• the wavelength at which the peak of the graph appears is the peak wavelength

• where this peak occurs is the most important part of this graph

• the peak wavelength can tell us how hot the object is

• hotter blackbodies give off more light than the cooler one

• the peak of the blackbody curve in a spectrum moves to shorter wavelengths for hotter objects

putting blackbodies all together

• as the object gets hotter, it changes from red to orange to yellow white and eventually to blue and beyond

• hotter objects will have shorter wavelengths

• blue is hot, red is cool

• hotter objects emit light with shorter wavelengths (towards the blue end of the visible spectrum)

• stars of different temperatures shine with different colours (blue stars are hottest red are coolest)