Physics observing the universe
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Physics Observing The Universe. revision. Observing the sky with the naked eye. Movement of celestial bodies. The sun appears to travel east-west across the sky once every 24hours. Sidereal Day.

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Physics Observing The Universe

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Physics Observing The Universe


Observing the sky with the naked eye

Movement of celestial bodies

  • The sun appears to travel east-west across the sky once every 24hours.

Sidereal Day

  • The star appear to move across the sky in a slightly shorter time(23h 56min). This is called a sidereal day.

Sidereal day is rotation to face original direction.

A solar day the rotation goes all the way back to face the Sun.

The moon

  • The moon appears to travel east to west across the sky once every 24hrs 49mins. One complete cycle takes 28days.

  • Why does the Moon take longer to cross the sky than the Sun?

  • Because it orbits the Earth in the same direction as the Earth rotates. So by the time the Earth rotates enough for a static object to have gotten all the way to the opposite horizon, the Moon hasn't quite gotten there yet because it was moving with the Earth's rotation a little.

Retrograde motion

Retrograde motion

  • At certain points in the orbits of planets, other planets (for example mars) appear to move backwards or from west to east across the sky. This is because earth is moving faster than that planet and so overtakes it.



  • Eclipses do not happen very often as the Sun and moon do not align very often.

  • The moon’s orbit is tilted relative to the plane of the Earth’s orbit.

  • Usually Earth, Sun and the Moon are not in line so no eclipse occurs.







Autumn equinox







Winter solstice





Spring equinox









Summer solstice

How does a telescope work?

Focusing parallel light

  • Stars are so far away that light arriving from them is parallel.

Power of a lens

  • Calculate the power of a lens:

  • Power (dioptre)= 1/focal length (m)

The more powerful a convex lens, the more curved the surface.

Forming an image of an extended object

Real image




Ray 1.Arrives parallel to the Principal Axis – then passes through F.

Ray 2.Passes through the optical centre – undeviated.

Ray 3.Passes through F first – then emerges parallel to the Principal Axis.

Any other rays will be refracted to pass through the same image point.

Note that the top of the image is now below the Principal Axis.


Focal length of the objective lens

Focal length of the eyepiece lens

Remember- the more powerful a lens the shorter the focal length.

Simple telescopes are made of two converging lenses of different powers. The more powerful lens acts as the eyepiece.

Concave mirrors

Modern telescopes have very large mirrors to:

Collect light/radiation

Produce a more defined/brighter/sharper image

See faint sources

Reduce diffraction

Most astronomical telescopes have concave mirrors, not convex lenses as their objectives.

What are the objects we see in the night sky and how far away the are?


  • The parallax angle is half the angle moved in a 6 month period.



  • Parallax makes some stars seem to move relative to others over the course of a year.

  • The smaller the parallax angle is the further away a star is.


  • A parsec (pc) is the distance to a star with a parallax angle of one arc second.


An hour can be broken into 60 divisions called minutes

A degree can be broken into 60 divisions called minutes. They are written as ‘ eg 20’

Each minute can be broken into divisions called seconds. They are written ‘’ eg 20’’

Each minute can be broken into 60 divisions called seconds

So as a fraction of a degree, 1 second is 1/3600 of a degree meaning there are 3600 seconds in a degree.

So as a fraction of an hour, 1 second is 1/3600 of an hour meaning there are 3600 seconds in an hour.

Star distance

  • A parsec is similar in magnitude to a light-year

  • 1 parsec = 3.1 x 1013 km

  • 1 light-year = 9.5 x 1012 km

  • Typical interstellar distances are a few parsecs.


  • Luminosity (intrinsic brightness) of a star depends on its temperature and its size.

  • Temperature: a hotter star radiates more energy every second from each square metre of its surface.

  • Size: a bigger star has more surface that radiates energy.

  • Observed Brightness- depends on the stars distance from the earth, as well as the stars luminosity. Dust or gas between Earth and the star may absorb some of its light.

Cepheid Variables

Measuring the distance to a star in a distant galaxy:

Look for a cepheid variable in the galaxy of interest.

Measure its observed brightness and its period of variation.

From the period, determine luminosity.

Knowing both the luminosity and the intensity of its light at the telescope, calculate the distance of the star.

  • These are stars that pulse in brightness. They have a period related to their brightness.

The great debate



He challenged Shapley’s claim about the universe.

Curtis was studying ‘spiral nebulae’ rather than globular clusters.

He felt that they were distant objects- galaxies on their own.

Was proved correct by Hubble’s discovery of the Andromeda galaxy.

  • Measured distance to nebulae. Observed they form a spherical cloud with a centre far from the solar system.

  • Guessed the nebula was a cluster of stars and they formed a sphere around the Milky Way galaxy. (Globular clusters)

  • He claimed milky way was the entire galaxy.


Hubble used the data from cepheids to determine the distances to galaxies.

He discovered that all galaxies appeared to be moving away from us.

There spectrum has been redshifted.

The more distant the galaxy the faster the rate of recession.

Hubble’s Constant

Speed of recession = Hubble Constant X distance

The first time Hubble estimated the constant he found it to be 500km/s. With more reliable data from the HST the current excepted value is 72 ± 8 km/s-1Mpc-1

Moving Galaxies

  • The fact that galaxies are moving lead to two important ideas:

  • The universe itself may be expanding, and may have been much smaller in the past.

  • The universe may have started by exploding outwards from a single point- the big bang.

A closed Universe

An open Universe

flat Universe

closed Universe

open Universe

What are stars?

Alpha scattering-gold foil experiment

  • Start with a metal foil. Use gold, because it can be rolled out very thin- thickness of a few atoms.

  • Direct the source of alpha radiation at the gold foil. Do this in a vacuum as alpha is easily absorbed.

  • Watch for flashes of light as alpha particles strike the detecting material around the outside of the chamber.

  • Count the flashes at different angles, to see how much the alpha radiation is deflected.

Interpretation of results

  • Most alpha particles passed straight through the gold foil, deflected by no more than a few degrees.

  • A small fraction of the alpha particles were actually reflected back towards the direction from which they had come.

  • Must be something positive repelling the alpha particles.

What are stars?

All hot objects(including stars) emit a continuous range of electromagnetic radiation, whose luminosity and peak frequency increases with temperature.


Energy levels

A hydrogen atom has:

1 proton in the nucleus

1 electron (in the first shell)

The atom does have other shells too …

but they are all empty …

most of the time.

Why does the electron normally occupy the innermost shell?

The innermost shell has the lowest energy. The electron drops down through the shells, losing energy as it goes, until it has the lowest possible energy.

Each shell represents a specific level of electron energy.

In Physics we refer to the shells as ENERGY LEVELS.






Ground state

I’m very excited!

Taking a closer look at the first 4 energy levels..

  • An electron can absorb energy and jump to a higher energy level. This is called EXCITATION. The electron is excited!

  • But then it will fall back down to a lower energy level, giving out energy as it does so.

  • The electron can return to the Ground state in a number of ways. How many?

I’m excited now!

Excitation and de-excitation

I’m even more excited!





Ground state



  • Each jump between energy levels produces an amount of energy determined by the difference in energies between the 2 levels.

  • Level 4 to 1

    Energy change = -0.9 - (-13.6) = 12.7 units

  • Level 4 to 3

    Energy change = -0.9 - (-1.5) = 0.6 units

  • Level 3 to 1

    Energy change = -1.5 - (-13.6) = 12.1 units

    In each case the energy is emitted as photons of light.



Types of spectrum

Most very hot objects will emit a continuous spectrum.

Hot gases emit only those colours which correspond to the energy released by de-excitation. A line EMISSION spectrum

But a cold gas would absorb exactly the same colours because they have just the right energy to jump up to higher energy levels (excitation).

A line ABSORPTION spectrum

Comparing the 3 types of spectrum

Comparing the 3 types of spectrum

Note that, for a given gas, the emission and absorption spectra are reverse versions each other.

But something odd was noticed in the spectra of distant stars

The whole spectrum seems to be shifted towards the red end: a RED SHIFT. Why?

Boyles Law

What is the equation relating pressure and volume?

Boyles Law

Boyles Law graph

p a 1/V


Charles Law

Charles Law results

-273oC (Absolute zero)

Extrapolating to find temp at which volume is zero

Pressure Law

Pressure Law results

-273oC (Absolute zero)

Extrapolating to find temp at which pressure is zero

Now logically,

  • Volume of a gas cannot be less than 0.

  • Pressure of a gas cannot be less than 0.

  • Therefore both Charles Law and the Pressure Law predict a minimum temperature of -273oC.

  • Kelvin: If -273oC is the lowest possible temperature then it should be the zero of an Absolute Temperature scale.

Absolute Temperature Scale

Note: No degree symbol

  • The unit is the kelvin, K.

  • Absolute zero is assigned 0 K.

  • Same size increment as oC, so

    0 K = -273 oC

    50 K = -223 oC (-273+50)

    273 K = 0 oC

    373 K = 100 oC

    Conversion is easy.

    Absolute temperatures are represented by ‘T’.

So if we now plot the Charles Law and Pressure Law results using Absolute Temperatures instead of oC ..

The Ideal Gas Equation (equation of state)

  • Boyles Lawp a 1/V (at constant temperature)

  • Charles LawV a T (at constant pressure)

  • Pressure Lawp a T (at constant volume)

    where T is the Absolute temperature in kelvin, K.

    Combining these we get:

    pV a T orpV/T = constant,

Life of a star – Stage 1: NEBULA

This is a NEBULA, a cloud of hydrogen and dust.

Gravitational attraction pulls the hydrogen and dust together compressing it.

Temperature and pressure rise.

Life of a star – stage 2: PROTOSTAR

As the pressure and temperature increases a ball of hydrogen forms, so hot that is glows.

This is a PROTOSTAR.

Nuclear fusion has not really started to happen yet.

Life of a star. Stage 3: Main Sequence

If there is enough mass, gravity continues to compress the hydrogen until the temperature reaches about 10 000 000 K.

Hydrogen nuclei now collide at speeds where nuclear fusion begins.

Life of a star - Stage 4: Red Giant

When the hydrogen starts to run out the star fuses helium and larger nuclei in the core.

This generates less heat than fusion of hydrogen.

The star cools down and swells becoming a RED GIANT

Life of a small/medium star – Stage 5: White Dwarf

Eventually the cool outer layers drift off into space forming a PLANETARY NEBULA.

The remaining, collapsed inner core is a WHITE DWARF.

It continues fusing larger nuclei until it runs out of fuel.

As fusion stops it cools down to become a BLACK DWARF

OR: Life of a large star – Stage 5: Supernova

The star continues to collapse, fusing increasingly larger nuclei.

Once fusion ceases the star ‘explodes’ ejecting the outer layers.

This is a SUPERNOVA.

Supernovae are very bright.

Life of a large star – Stage 6: Neutron star

The remaining core is a neutron star.

Life of a large star – Stage 6: Neutron star

Neutron stars have a very large mass in a very small volume. They are very dense.

Life of a large star – Stage 6: Pulsars

Pulsars are highly magnetized neutron stars that emit a beam of e/m radiation.

They rotate very rapidly. e,.g once every 1.4 milliseconds to 8.5 seconds.

The radiation can only be observed when the beam of emission is pointing towards Earth.

This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name.

For some pulsars, the regularity of pulsation is as precise as an atomic clock.

Life of a VERY large star – Stage 7: Black hole

If there is enough mass the neutron star continues to collapse to form a BLACK HOLE.

The gravitational force is so strong not even light can escape.

Hertzsprung -Russell diagram

By convention, the temperature scale goes backwards.

Hertzsprung-Russell diagram

The majority of stars (including the Sun) are in the main sequence - a line which runs from massive, luminous, hot stars at one end to low mass, dim, cool stars at the other end.

Hertzsprung-Russell diagram

Another group of stars, the red giants, are relatively cool - but they are very luminous, because their diameters and surface areas are very large compared with main sequence stars.

Hertzsprung-Russell diagram

Supergiants are very large and luminous, and their temperatures cover the full range from very hot to relatively cool.

Hertzsprung-Russell diagram

The white dwarfsare hot but not very luminous - because their diameters are very small.


A single proton H






A proton and 1 neutron H



A proton and 2 neutrons H


Isotopes of hydrogen

The nucleus of a hydrogen atom can take 3 forms:

called deuterium.

called tritium.























The deuterium + tritium fusion reaction can be written as:

Nuclear fusion reaction equations



2 helium nuclei can then go on to fuse:
















The simplest fusion reaction is between a deuterium and a tritium nucleus.

Fusion of hydrogen nuclei

Nuclei are positive. They repel each other.

To make nuclei collide with enough force to fuse needs very high speeds only achieved at temperatures of millions of kelvin.

Points to consider

  • Where does the energy come from to make fusion happen?

  • What conditions are necessary for the process to keep itself going?


  • More effort can be put in from international collaboration than from a single experimental group.

  • It is often impossible for individuals to deal with the huge amount of data accumulated from surveys.

Why build a telescope on the top of a mountain?

  • For optical observatories a low turbulent atmosphere(very good seeing)

  • Dark skies- no light pollution

  • High number of clear night skies

  • For radio telescopes this doesn’t matter as radiowaves pass through clouds

  • Not windy due to refraction

Non astronomical reasons for choosing a site.

  • Must have reasonable logistical supply.

  • Easy to travel to

  • Accomodation, food, drink

Which colour writing

can you read clearly first?

easteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteast middlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlerightrightrightrightrightrightrightrightrightrightrightrightrightrightrightrightright






How clearly can we see? Resolution

Is it easier or more difficult?







Now try it again looking at the screen through a small hole.

In general we can resolve blue better than green or red.

This is because blue light has a shorter wavelength than green or red light.

The shorter the wavelength the better the resolution.

Which waves in the e/m spectrum would give us the best resolution?

Which of these two telescopes would give the best resolution?

Looking at something through a small aperture (hole) makes the resolution WORSE.

Therefore the bigger the aperture the better the resolution


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