P7 21st Century Science OCR revision

P7.1: Naked-eye astronomy

Solar System

• Collection of planets, comets and other objects which ORBIT the sun.

• The sun is a STAR

Year

• The time it takes the Earth to complete one orbit of the sun.

• The Earth takes 365.25 days to complete one orbit

Moon

• The Moon orbits the Earth.• It takes about 28 days to complete one orbit.

• The Moon’s orbit is tilted 5 degrees from the plane of the Earth’s orbit period.

• 27 days (sidereal)• 29 days (viewed from Earth)

Axis

• The Earth rotates around an imaginary line called its axis

Movement of the sun• Sun appears to move across sky from East West• This is because Earth spins on axis

• Sun reappears in same place once every 24 hours – solar day

• Earth spins on its axis (360o) every 23h 56 minutes – sidereal day

Why is a sidereal day different to a solar day?

• During the sidereal day the Earth moves further along its orbit of the sun.

• Therefore for the same part of the Earth to face the sun again, the Earth needs to turn for a further 4 minutes.

• Solar day is 4 minutes longer than a sidereal day

Movement of the stars• Long-exposure photographs show the stars to be

moving in circles around the Pole Star.• They are not actually moving, we are simply

observing from a spinning Earth. • After 23h 56m (SIDE REAL DAY) they will appear to

be back in the same place.

Constellation• A group of stars that form a pattern.• We see different constellations in summer and

winter because we have moved around the sun.

Movement of the Moon• Moon appears to move across sky from EW• Moon takes longer to appear in same part of sky –

24h 49m• This is because:

– As well as Earth’s rotation giving different view of Moon, Moon is also orbiting the Earth

– Moon orbits from WE so during night the position of the Moon over 28 days appears to slip slowly back through the pattern of stars.

Moving planets• We can see some planets with naked eye.• They are also orbiting the sun.• This makes their positions appear to change night

by night when viewed against the background of fixed stars

Retrograde Motion• Planets usually appear to move across sky from

E W like the Sun and Moon. • Sometimes they appear to go backwards –

retrograde motion• This is because different planets have different

times to orbit the sun so the place we see a planet in the sky depends on where both the planet and the Earth are in their orbits.

Phases of the Moon• We can only see the part of the Moon that is lit up

by the Sun. • As the Moon orbits

the Earth we see different parts of the Moon lit up.

• ,What is an eclipse?

• Solar eclipse – Moon blocks the Sun’s light

• Lunar eclipse – Moon moves into the Earth’s shadow

Shadows• Moon and Earth both have shadows

• Region of total darkness – umbra• Region of partial darkness – penumbra

Why are lunar eclipses more common than solar eclipses?

• Because the Earth’s shadow is bigger than the Moon’s shadow

Solar eclipses are also rare because the Moon does not often line up exactly with the Sun because the Moon’s orbit is tilted by 5o relative to the plane of the Earth’s orbit.

Celestial Sphere

• Axis from Pole star through axis of Earth

• Celestial equator which is extension of Earth’s equator.

• Astronomers use two angles to describe the positions of astronomical objects.

• The angles are measured from a reference point in the sky.

P7.2: Telescopes and Images

Waves and refraction

• Light travels as waves• Substance that light travels through is a medium

• Speed of light depends upon medium – as medium changes speed changes

• Once a vibrating source has made a wave frequency cannot change. So if speed changes, wavelength changes.

• If a wave changes direction it is called refraction

Refraction• If a wave changes direction it is called refraction

Refraction at lenses• Refracting telescopes use convex lenses• Convex lenses are thicker in the middle than the

edges• If parallel rays enter a convex lens they come to a

point called the focus. • When rays meet at

the focus they have converged.

Images in lenses• Use these rules to draw ray diagrams

1. Use arrows to show the direction that light is travelling

Images in lenses• Use these rules to draw ray diagrams

1. Use arrows to show the direction that light is travelling2. A ray through the centre of a lens does not change

direction

Images in lenses• Use these rules to draw ray diagrams

1. Use arrows to show the direction that light is travelling2. A ray through the centre of a lens does not change direction3. A ray parallel to the principal axis passes through the focus

Images in lenses• Use these rules to draw ray diagrams

1. Use arrows to show the direction that light is travelling2. A ray through the centre of a lens does not change direction3. A ray parallel to the principal axis passes through the focus4. A ray through the focus emerges parallel to the principal

axis

principal axis

Stars and light rays• Stars – very far away• Rays reaching Earth from stars are parallel

Stars and light rays• Stars – very far away• Rays reaching Earth from stars are parallel• Convex lens refracts rays from star through a single

point• Point is the

image of the star.

Inverted images• Objects in the Solar system are closer than stars. • Light rays from different parts of the object arrive at

a lens at different angles • Rays from top of object go to bottom of lens and

vice versa.• This means the

image is inverted

Focal length and lens power• Focal length – distance from lens to image

Focal length and lens power• Focal length – distance from lens to image• A fat convex lens has a shorter focal length than a

thin lens – fat lens is more powerful

Measuring the power of a lens

• Units = dioptres

• Power (dioptres) = 1 / focal length (metres-1)

What’s inside a telescope?• Objective lens (long focal length = low power)

– Forms image inside telescope

• Eyepiece lens (short focal length = high power)– Magnifies image formed by objective lens

• Distance between lenses = sum of 2 focal lengths

Why do we use telescopes?• Magnification enables you to see detail you cannot

see with the naked eye.• They have a greater aperture so they collect more

light. This enables you to see dimmer stars than with the naked eye.

Magnification

• The angles between stars appear bigger with a telescope than the naked eye. This is the angular magnification of the telescope.

• Magnification = focal length of objective lens focal length of eyepiece lens

MagnificationExample problemCalculate the magnification of a telescope with an objective of focal length 1200 mm using two different eyepieces with focal lengths of: (a) 25 mm (b) 10 mm

Example problemCalculate the magnification of a telescope with an objective of focal length 1200 mm using two different eyepieces with focal lengths of: (a) 25 mm (b) 10 mm

(a) Magnification = 1200/25 = 48x(b) Magnification = 1200/10 = 120x

Reflecting telescopes• Most telescopes use a concave mirror rather than a

lens as the objective. • This brings parallel light to a focus. • An eyepiece lens then magnifies the image from the

mirror

Advantages of reflecting telescopes1. Easier to make a big mirror than a big lens2. Hard to make a glass lens with no imperfections3. Big convex lenses are fat in the middle, glass

absorbs light on the way through the lens, so faint objects look even fainter.

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Advantages of reflecting telescopes1. Easier to make a big mirror than a big lens2. Hard to make a glass lens with no imperfections3. Big convex lenses are fat in the middle, glass

absorbs light on the way through the lens, so faint objects look even fainter.

4. Mirrors reflect all colours the same, lenses refract blue light more than red distorting the image.

Dispersion• White light = mixture of colours• Violet light = higher frequency & shorter wavelength

than red light • Violet light slows down more in glass so is refracted

more• In lenses and prisms, refraction splits white light

into colours = dispersion

Dispersion at a diffraction grating• Dispersion also occurs at a diffraction grating (narrow

parallel lines on a sheet of glass). When white light shines on the grating, different colours emerge at different angles. This forms spectra.

• Astronomers view stars through spectrometers containing prisms or gratings.

• These show the frequencies of light emitted by the star.

Diffraction• Diffraction is when waves go through a gap, bend

and spread out.

• The effect is greatest when the size of the gap is similar to or smaller than the wavelength of the waves.

Diffraction and telescopes• The light-gathering area of a telescope’s objective

lens or mirror is its aperture• If diffraction occurs at the aperture, the image will

be blurred.• Optical telescopes have apertures much bigger than

the wavelength of light to reduce diffraction and form sharp images.

• Radio waves have long wavelengths. A telescope that detects radio waves from distant objects needs a very big aperture.

• These pictures show images with a 10 meter telescope and a 100 meter telescope.

P7.3: Mapping the Universe

Light year

• Distance light travels in one year

• After the sun, the nearest stars are about 4 light

years away

• We see light that left those stars 4 years ago

• Some galaxies are millions of light years away

Parallax

• As the Earth orbits the Sun, the closest starts appear to change positions relative to the very distinct ‘fixed stars’.

• This effect is called parallax• The stars have not actually moved. It

is the Earth that has moved.

Parallax angle• HALF the angle the star has apparently moved in 6

months (as we travel from one side of the Sun to the other).

• Parallax angles are tiny. They are measured in seconds of arc.

• 1 second of arc = 1/3600 of a degree• The smaller the parallax angle, the further away the star.• .

• Distance to star = 1 / parallax angle (in seconds of arc) (parsecs)

Parsecs (pc)• A parsec (pc) is the distance to a star whose parallax

angle is 1 second of arc.• A parsec is similar in magnitude (size) to a light-year• Distances between stars within a galaxy are usually

a few parsecs• Distances between galaxies are measured in

megaparsecs (Mpc)

Distances between stars within a galaxy are usually a few parsecs

Distances between galaxies are measured in megaparsecs (Mpc)

Star luminosity• Luminosity = Amount of radiation emitted by a star

every second

• Luminosity depends on:

– size of the star– temperature of the star

• HOTTER and BIGGER = more energy radiated per second

R2Double the radius

Luminosity variables – size of star• For stars with the same surface temperature

the bigger the star the more energy it gives out. • A star with double the radius of another one will

have an area four times as great and so have a luminosity four times greater than the first star

R

Radius x2Area x4Luminosity x4

• For stars of the same size the hotter the star the more energy it gives out.

• A star with a temperature of double another one will have a luminosity sixteen times greater.

T2Double the temperatureT

Temperature x2Luminosity x16

Luminosity variables – temp of star

Observed intensity• How bright a star appears when seen

from Earth

• Brightness depends on:–luminosity of the star–distance of the star from the Earth

Brightness• For two stars of the same luminosity with one star

double the distance of the other from the Earth the closer star will look four times brighter.

Double the distance from earth

Distance x2

Luminosity x ¼

D D2

Light spreads out, so the more distant a source is the less bright it appears.

Luminosity, brightness and magnitudes

• For stars which give out the same amount of light

Brightness on Earth

as Distance from Earth

Cepheid variable stars• Star whose brightness changes with time

• Variation in brightness thought to be because the star expands and contracts in size (by 30%) causing a variation in temp and luminosity.

Calculating the distance to a Cepheid variable star• Measure the period

• Use the period to work out the luminosity

Calculating the distance to a Cepheid variable star• Measure the period – 5 days

• Use the period to work out the luminosity

• Measure the observed brightness

• Compare the observed brightness with the luminosity to work out the distance

The period–luminosity relation for Cepheid variables

Observing nebulae and galaxies• We know from telescope observations that our

galaxy (Milky Way) is made up of millions of stars.• The sun is one of the stars in our galaxy• In the 1920s astronomers were puzzled by fuzzy

patches of light seen through telescopes.• They called these fuzzy patches nebulae.• Nebulae have different shapes including spirals

Shapeley vs. Curtis

Shapeley• Thought Milky Way

was entire universe• Thought nebulae

were clouds of gas within Milky Way

Curtis• Thought spiral

nebulae were huge distant clusters of stars – other galaxies outside the Milky Way

Shapeley vs. Curtis• Neither astronomer had enough evidence to win

the argument• Later HUBBLE found a Cepheid

variable in a spiral nebula, Andromeda.• Hubble measured the distance from Earth star.• It was further than any star in the Milky Way galaxy• He concluded that the star was in a separate galaxy

• Cepheid variable stars have been used to show that most spiral nebulae are distant galaxies, of which there are billions in the Universe.

The amazing and expanding Universe!• Astronomers study absorption

spectra from distant galaxies• Compared to spectra from

nearby stars, the black absorption lines for distant galaxies are shifted towards the red end of the spectrum. This is REDSHIFT

• Redshift shows us that galaxies are moving away from us.

Speed of recession• Redshift shows us that galaxies are moving away

from us. • The speed of recession of a galaxy is the speed at

which it is moving away from us. This can be found from the redshift of the galaxy.

(you can use either set of units)

Hubble, Cepheid Variables and Recession

• Hubble measured the distance to Cepheid variable stars in several galaxies.

• He found the further away a galaxy is, the greater its speed of recession.

Hubble’s constant• Since Hubble, many astronomers have gathered

data from Cepheid variable stars in different galaxies.

• These data have given better values of the Hubble constant.

• The fact that galaxies are moving away from us suggests that the Universe began with a big bang about 14 thousand million years ago.

• Distant galaxies seem to be moving away faster than nearby galaxies, hence scientists conclude that space itself is expanding.

P7.4: The Sun, the stars and their surroundings

Star Spectra and Temperature• Hot objects (including stars) emit energy across all

wavelengths of the EM radiation spectrum• Different stars emit different amounts of radiation

and different frequencies depending on their temperature.

INCREASING TEMPERATURE

Spectrometer • Used by astronomers to:

– Measure radiation emitted at each frequency– Identify the peak frequency of a star

• The peak frequency gives an accurate value for the temperature of a star.

Peak frequency = Higher temperature

Identifying elements in stars• Astronomers use spectra from stars to identify their

elements. • The surface of the Sun emits white light. As the light

travels through the Sun’s atmosphere, atoms in this atmosphere absorb light of certain frequencies.

• The light that travels on has these frequencies missing. • When the light is spread into a spectrum, there are

dark lines across it. • This is the absorption spectrum of the sun

Identifying elements in stars• Each element produces a unique pattern of lines in

its absorption spectrum• Astronomers identify the elements in stars by

comparing star absorption spectra to those of elements in the lab

How do gases behave?• Stars are balls of hot gases• To understand stars, you need to understand gases• The particles of a gas move very quickly in random

directions• When they hit the sides of a container they exert a

force as they change direction this causes gas pressure

Pressure and volume• If you decrease the volume of the container,

the particles hit the sides more often and the pressure increases

Pressure and volume• Volume and pressure are inversely

proportional• For a fixed mass of gas at constant temp as

volume decreases pressure increasespressure x volume = constant

Pressure and temperature• The hotter the gas, the more energy the particles

have and the faster they move

• The faster the particles move, the harder and more often they hit the sides of the container.

Pressure and temperature• Temperature and pressure are directly

proportional• For a fixed mass of gas at constant volume as

temperature (K) increases pressure increasespressure / temperature = constant

Cooling a gas• As a gas cools, the particles lose energy & they

move more slowly• At the lowest temperatures particles stop moving

and therefore would never hit the sides of the container.

• Lowest theoretical temp = absolute zero (-273oC)

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Kelvin Scale• Starts at the

lowest theoretical temp = absolute zero (-273oC)

• Zero on the Kelvin scale is -273oC = absolute zero

Volume and temperature• If you decrease the temperature of a gas at constant

pressure, the volume decreases.• At absolute zero, the volume would theoretically be zero• For a fixed mass of gas at a fixed pressure:o As temp increases, vol increaseso Volume is directly

proportional to temp (K)

volume = constant temperature

Inside stars

Protostars• Gravity compresses a cloud of H and He gas• The gas particles get closer and closer• The volume of the gas cloud decreases• As they get closer they move faster• Temperature and pressure increase

• This mass of gas is called a protostar

Protostars and Nuclear Fusion

• When H nuclei get close enough they form He nuclei – nuclear fusion

• This process releases energy.

• Protostar star when fusion begins• Nuclear fusion happens in all stars including our Sun

Protostar formation

Nuclear fusion in the Sun

01

+ e+ (positron)

Nuclear fusion in the Sun

• The product of the previous reaction may then fuse with another hydrogen nucleus to form an isotope of helium

Positrons• Like an electron, but with a positive charge• Emitted in some nuclear reactions to conserve

charge

Nuclear equations• You must balance:

– Mass (top number)– Charge (lower numbers)

• In fusion reactions the total mass of product particles is slightly less than the total mass of reactant particles.

• The mass that is lost has been released as energy

• You can use Einstein’s equation to calculate energy released in nuclear fusion / fission reactions

Energy released = mass

lost x (speed of light in a vacuum)2

Inside stars

• Stars which fuse H to form He are main-sequence stars (e.g. the sun)

Core Radiative zone

Convective zone

Photosphere ~ surface of star

Temperature and density are highest. Most nuclear fusion happens here

Energy is transported outwards from the core by radiation

Convection currents flow here, carrying heat energy to the photosphere

Energy is radiated into space from here

Inside stars

Red giants and supergiant stars1. In main sequence stars, H nuclei fuse to form He. 2. Eventually the H runs out3. The pressure decreases4. The core collapses5. Hydrogen containing outer layers of the star fall inwards6. New fusion reactions happen in the core7. These reactions make the outer layers of the star expand8. The photosphere cools and its colour changes from

yellow red9. A red giant / supergiant has formed

Red giants and supergiant stars• While the outer layers of a red giant / supergiant

expand, its core gets smaller• It becomes hot enough for He nuclei to fuse

together to form heavier nuclei

• The more massive the star, the hotter the core, the heavier the nuclei it can produce by fusion– Red giants – fusion reactions produce nuclei of

carbon, then nitrogen and oxygen– Supergiants (core pressure and temp higher) –

fusion reactions produce elements with nuclei as heavy as iron!!

Inside stars

White dwarf stars• The Sun has a relatively low mass• When it becomes a red giant it will not be

compressed further once its He has been used up• The star will shrink to become a white dwarf star• There is no fusion in a white dwarf. • It will gradually cool and fade

Supernova• When the core of a supergiant is mainly iron, it

explodes - this is a supernova

• It is so hot that fusion reactions produce atoms of elements as heavy as uranium

After a supernova explosion,

a dense core

remains.

After a supernova explosion, a dense

core remains.

Smaller core – neutron star

Bigger core – black hole (so much mass concentrated

into a tiny space that even light cannot

escape from it)

Clouds of dust and gas blown outwards by a supernova may

eventually form new protostars

Hertzsprung-Russell diagram

• H-R diagram plots luminosity against temperature

• For main sequence stars there is a correlation: the hotter the star, the more radiation emitted and so the greater its luminosity

Hertzsprung-Russell diagram• H-R diagram plots luminosity against temperature

• For main sequence stars there is a correlation: the hotter the star, the more radiation emitted and so the greater its luminosity

Hertzsprung-Russell diagram

• H-R diagram plots luminosity against temperature

• For main sequence stars there is a correlation: the hotter the star, the more radiation emitted and so the greater its luminosity

Increasing temperatureHOT COOL

Incr

easi

ng lu

min

osity

Hertzsprung-Russell diagram

• H-R diagram plots luminosity against temperature

• For main sequence stars there is a correlation: the hotter the star, the more radiation emitted and so the greater its luminosity

You may be asked to identify regions of the H-R in which different

types of star are located

Exoplanets• Astronomers have found evidence of planets

orbiting nearby stars.• These are exoplanets• Some may have the right conditions for life• Because of this scientists think there may be life

elsewhere in the Universe• No evidence of ET life has yet been discovered

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P7.5: The astronomy community

Choosing your observatory site• Astronomers use huge telescopes to collect weak

radiation from faint or very distant sources. • Major optical and infrared telescopes on Earth are

in:– Chile– Hawaii– Australia– Canary Islands

Choosing your observatory site• When choosing a site, astronomers consider a

number of factors

• If you have a question in the exam about evaluating telescope sites1. Compare the advantages and disadvantages of each site2. State with reasons which site you believe to be best

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Atmosphere reflects light

This distorts images

Choose a high altitude location (e.g. a mountain) to reduce this problem

The Sphinx Observatory in the Swiss Alps

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Atmosphere reflects light

This distorts images

Choose a high altitude location (e.g. a mountain) to reduce this problem

The Sphinx Observatory in the Swiss Alps

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Light is refracted more if the air is damp or polluted

Locate your telescope in an area with dry, clean air for higher-quality images

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Light is refracted more if the air is damp or polluted

Locate your telescope in an area with dry, clean air for higher-quality images

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Astronomical observation cannot be made in cloudy conditions

Choose an area with frequent cloudless nights

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Astronomical observation cannot be made in cloudy conditions

Choose an area with frequent cloudless nights

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Cities cause light pollution Choose an area far from cities

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory siteWhen choosing a site, astronomers consider a number of

factorsFactor Solution

Cities cause light pollution Choose an area far from cities

Clear skies, dry air and low pollution make Arizona a hotspot for astronomy.

Choosing your observatory site• When choosing a site, astronomers consider a

number of factors• Other factors:

– Cost (inc. travel to and from telescope for supplies and workers)

– Environmental impact near the observatory– Impact on local people– Working conditions for employees

Computer controlled telescopes• Allows astronomers to use a telescope thousands of

miles away• Images are recorded digitally and sent electronically

to computers• Computers can then be used to analyse images and

improve their quality (e.g. adding colour)• Observations can then be shared with

other astronomers

Why put telescopes in space???• Telescopes on Earth are affected by:

– Atmosphere (which absorbs most IR, UV, X-ray and gamma radiation)

– Atmospheric refraction (distorts images and makes stars ‘twinkle’)

– Light pollution– Bad weather

Why put telescopes in space???• Telescopes on Earth are affected by:

– Atmosphere (which absorbs most IR, UV, X-ray and gamma radiation)– Atmospheric refraction (distorts images and makes stars ‘twinkle’)– Light pollution– Bad weather

• All of these problems are overcome by putting a telescope in space

• The Hubble Space Telescope has a better resolution than any telescope on Earth

Problems with telescopes in space???

• High cost of setting up• High cost of on-going maintenance and repairing a

telescope in space• Uncertainties of future funding

International collaboration

• Allows the cost of a major telescope to be shared• Allows expertise to be pooled

• Exam tip: Know two examples showing how international co-operation is essential for progress in astronomy

International collaboration – European Southern Observatory (ESO)

• Involves 14 European countries + Brazil• Consists of several telescopes in Chile• Chile provides the base and the office staff• 1000+ astronomers from all over the world use the

facility each year

International collaboration – Gran Telescopio Canarias

• In the Canary Islands• At the top of a high volcanic peak• Funded mainly by Spain, with contributions from

Mexico and the USA• Planning involved 1000+ people from 100 companies