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
Observing the sky with the naked eye
Sidereal day is rotation to face original direction.
A solar day the rotation goes all the way back to face the Sun.
How does a telescope work?
The more powerful a convex lens, the more curved the surface.
Forming an image of an extended object
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.
Modern telescopes have very large mirrors to:
Produce a more defined/brighter/sharper image
See faint sources
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?
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.
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.
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.
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.
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
A closed Universe
An open Universe
What are stars?
All hot objects(including stars) emit a continuous range of electromagnetic radiation, whose luminosity and peak frequency increases with temperature.
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.
I’m very excited!
Taking a closer look at the first 4 energy levels..
I’m excited now!
Excitation and de-excitation
I’m even more excited!
Energy change = -0.9 - (-13.6) = 12.7 units
Energy change = -0.9 - (-1.5) = 0.6 units
Energy change = -1.5 - (-13.6) = 12.1 units
In each case the energy is emitted as photons of light.
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
Note that, for a given gas, the emission and absorption spectra are reverse versions each other.
The whole spectrum seems to be shifted towards the red end: a RED SHIFT. Why?
What is the equation relating pressure and volume?
p a 1/V
-273oC (Absolute zero)
-273oC (Absolute zero)
Note: No degree symbol
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 ..
where T is the Absolute temperature in kelvin, K.
Combining these we get:
pV a T orpV/T = constant,
This is a NEBULA, a cloud of hydrogen and dust.
Gravitational attraction pulls the hydrogen and dust together compressing it.
Temperature and pressure rise.
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.
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.
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
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
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.
The remaining core is a neutron star.
Neutron stars have a very large mass in a very small volume. They are very dense.
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.
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.
By convention, the temperature scale goes backwards.
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.
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.
Supergiants are very large and luminous, and their temperatures cover the full range from very hot to relatively cool.
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
The nucleus of a hydrogen atom can take 3 forms:
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
Which colour writing
can you read clearly first?
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