P7 Observing space

P7 Observing the universe

Solar and Sideral Day

A solor day is the time it takes

for the sun to appear in the

same position in the sky

It last 24 hours

A sideral day is the time for the Earyth to rotate once

It's measured against

background stars

It lasts 23 hours 56 minutes

The moon orbits the Earth every 27.3 days (sideral month)

Solar and lunar eclipse

The moon's orbit is inclined to the Earth's so eclipses are uncommon

Retrograde motion happens

with the outer planets, it happens

because both the planet and Earth are moving.

Distance (parsec) = 1/Parallax angle (arcsec)

Beyond 50 pc angle to small to be precise

1 Parsec = 3.26 lightyear

A light year is 9,460,730,472,580.8 km or

about 6 Trillion miles or the distance travelled by light in a year

Apparent Brightness (alias INTENSITY) = how much light reaches Earth

Intrinsic Brightness (alias LUMINOSITY) = total amount of light a star emits

Apparent Brightness depends upon Intrinsic Brightness & Distance

The luminosity of a star depends on its temperature and size

Cepheid Variable Star's Pulse in Brightness

Hubble

Red shift

The further away the galaxy the greater the red shift !

The furthest galaxies are receding at the greatest velocity.

refracting

telescopes

More curved lens have shorter focal length

You have 20 seconds

to complete the

last drawing

diffraction

In a telescope the aperature (the objective lens)

must be bigger than the wavelength

The apperature needs to be wide for faint sources

• objective Lens sag
• imperfections
• chromatic aberration

Reflecting telescopes

Where do you put telescopes ?

Difficult conditions

for astromoners

One solution is using remote control with computers

Advantages

• Scan large areas
• track objects
• Coordinate telescopes

all around the world

P7.4 The Sun, the stars and their surroundings

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

2. know that the removal of electrons from atoms is called ionisation and explain how electron energy levels within atoms give rise to line spectra

3. know that specific spectral lines in the spectrum of a star provide evidence of the chemical elements present in it

4. use data on the spectrum of a star, together with data on the line spectra of elements, to identify elements present in it

5. understand that the volume of a gas is inversely proportional to its pressure at a constant temperature and explain this using a molecular model

6. Be able to explain why the pressure and volume of a gas vary with temperature using a molecular model

7. understand that both the pressure and the volume of a gas are proportional to the absolute temperature

8. Be able to interpret absolute zero using a molecular model and kinetic theory

9. know that –273°C is the absolute zero of temperature, and convert temperatures in K to temperatures in °C (and vice versa)

10. Be able to use the relationships:

a) pressure × volume = constant

b) pressure / temperature = constant

c) volume / temperature = constant

11. Be able to explain the formation of a protostar in terms of the effects of gravity on a cloud of gas, which is mostly hydrogen and helium

12. understand that as the cloud of gas collapses its temperature increases, and relate this to the volume, pressure and behaviour of particles in a protostar

13. understand that nuclear processes discovered in the early 20th Century provided a possible explanation of the Sun’s energy source

14. understand that, if brought close enough together, hydrogen nuclei can fuse into helium nuclei releasing energy, and that this is called nuclear fusion

15. complete and interpret nuclear equations relating to fusion in stars to include the emission of positrons to conserve charge

16. understand that energy is liberated when light nuclei fuse to make heavier nuclei with masses up to that of the iron nucleus

17. HT only: understand that Einstein’s equation E = mc2 is used to calculate the energy released during nuclear fusion and fission (where E is the energy produced, m is the mass lost and c is the speed of light in a vacuum)

18. know that the more massive the star, the hotter its core and the heavier the nuclei it can create by fusion

19. know that the core (centre) of a star is where the temperature and density are highest and where most nuclear fusion takes place

20. understand that energy is transported from core to surface by photons of radiation and by convection

21. know that energy is radiated into space from the star’s surface (photosphere)

22. know that the Hertzsprung-Russell diagram is a plot of temperature and luminosity and identify regions on the graph where supergiants, giants, main sequence and white dwarf stars are located

23. know that in a main sequence star, hydrogen fusion to helium takes place in the core

24. know that a star leaves the main sequence when its core hydrogen runs out; it swells to become a red giant or supergiant and its photosphere cools

25. know that in a red giant or supergiant star, helium nuclei fuse to make carbon, followed by further reactions that produce heavier nuclei such as nitrogen and oxygen

26. know that a low-mass star (similar to the Sun) becomes a red giant, which lacks the mass to compress the core further at the end of helium fusion; it then shrinks to form a white dwarf

27. know that in a white dwarf star there is no nuclear fusion; the star gradually cools and fades

28. know that in a high-mass star (several times the mass of the Sun) nuclear fusion can produce heavier nuclei up to and including iron; when the core is mostly iron, it explodes as a supernova creating nuclei with masses greater than iron and leaving a dense neutron star or a black hole.

29. understand that astronomers have found convincing evidence of planets around hundreds of nearby stars

30. understand that, if even a small proportion of stars have planets, many scientists think that it is likely that life exists elsewhere in the Universe

31. know that no evidence of extraterrestrial life (at present or in the past) has so far been detected

Atoms and the Sun

• the Rutherford-Marsden-Geiger gold foil experiment
• Nuclear Fusion
• The structure of the Sun

Gold foil experiment

Summary

• days
• eclipses
• movement of planets
• Parallax
• intrinsic brightness
• cephied variables
• reflecting telescopes
• refracting telescopes
• observatories

Greatest when aperture

is same size as wavelength

Skinny Objective

Curvy eye

Draw this !

Draw This !

Skinny lens

Fo

Advantage

Really big objectives

Magnification =

Fe

Curvy lens

Measure in dioptre

Measure in metres (not cm!)

disadvantage

where do you

put the eyepiece ?

Problems with Refractors

Difficult to build

and maintain

Avoid Light Pollution

Avoid the atmosphere

Solar Eclipse

Lunar Eclipse

Sun - Moon - Earth

Sun - Earth - Moon

A Parsec is the distance of a star

when the Parallax angle

is 1 Arc Second

Motion of the stars and planets

Spectra and temperature

absolute zero

stellar evolution