Light• Transverse electromagnetic wave, or electromagnetic radiation

• Includes radio waves, microwaves, infra-red, visible, UV, X-rays, and gamma rays

• The type of light is determined purely by wavelength.

Hubble Space Telescope• Observes in infra-red, visible, and UV regions of spectrum

• Orbits Earth at ~540 km above the surface

Light - Measurements• Speed of light (in a vacuum) = c = 3 x 108 ms-1

• Frequency is the number of cycles a wave completes each second. Related to wavelength by c = f x λ

• Light-year is unit of distance, not time, equal to the distance light travels in a vacuum over one Earth year (365.25 days)

• 1 ly = 9.47 x 1015 m

Doppler Effect• Change in frequency of a detected wave due to relative motion

of source and observer.

• When moving towards observer, frequency is higher

• When moving away, frequency is lower

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Doppler Shift• If an object is moving away from us, the light we see is shifted

towards the red end of the spectrum. We call this redshift.

• Likewise, if the object is moving towards us, light is shifted towards the blue end, and we call this blueshift.

• The Doppler shift can tell us a great deal about a star or galaxy.

Hubble’s Law• The further a star or galaxy is from Earth, the greater the redshift

• This means that the more distant objects are moving away from Earth faster than closer objects.

• This is Hubble’s Law

Questions• 6.3, questions 1-6

Big Bang Cosmology• Because of the way other galaxies are moving away from us, we

expect that in the past the entire universe was confined to a very small space

• Roughly 14 billion years ago, this space began to expand rapidly, and this is considered the origin of the universe as we know it

• We call the model explaining the expansion of this early universe the Big Bang theory

Early Universe• The early universe was extremely hot, but as space expanded,

the universe began to cool.

• As it cooled, particles were able to form according to a very famous equation

Early Universe• t = 10-43 s: Universe is still extremely hot, about 2x1028 times

hotter than the surface of the sun. Laws of physics break down.

• t = 10-34 s: Electrons and positrons are able to form due to the universe cooling as it expands.

• t = 10-4 s: Protons and neutrons form, but are too energetic to form nuclei

• t = 5 minutes: H, He, and Li nuclei form, but cannot trap electrons.

Early Universe• t = 300,000 years: Universe has cooled to just over 3000 K, and

electrons were captured by nuclei, forming atoms.

• These atoms then formed stars, galaxies and planets.

Evidence• Red shift of galaxies and stars supports an expanding universe

with an origin described by the big bang theory

• The amount of helium in the universe cannot be explained by production in stars. However the extremely hot early universe would be able to produce enough helium to match our observations

• Cosmic microwave background.

Cosmic Microwave Background• Earliest light from the universe

• Has been redshifted into the microwave region of the EM spectrum.

• Detection of CMB by Robert Wilson and Arno Penzias won the Nobel Prize in Physics in 1978

Formation of Stars• Early atoms formed large clouds of hydrogen and helium called

nebulae.

• The mass in these clouds is not evenly distributed. The clouds are “lumpy”

• The slightly more dense regions attract more mass, and over time this builds up to form a protostar, the earliest stage of a star that is still gathering mass.

Formation of Stars• As the protostar gains mass, gravitational pressure causes atoms

to heat up.

• When a high enough temperature has been reached, hydrogen begins to fuse and form helium.

• This process is called nuclear fusion, and is how stars generate energy.

Colour and Brightness• The colour of a star depends on its temperature.

• We can classify stars based on their temperature, and how bright they are (their absolute magnitude or luminosity)

• We also give objects an apparent magnitude, which describes how bright they appear from Earth.

• Using these characteristics, we form the Hertzsprung-Russel diagram.

HR Diagram• Almost all stars are grouped in a

line called the main sequence

• This is how stars spend the majority of their lifetime.

• What happens after they leave the main sequence depends on the mass of the star.

Low Mass• As helium builds up in the core, hydrogen fusion occurs in an

expanding shell around the core, causing the star to expand. We call this a red giant.

• After hydrogen and helium have been used up, outer layers are expelled, and the carbon-oxygen core remains.

• We call the expelled gases a planetary nebula, and the remaining core a white dwarf.

High Mass• Similar to low mass stars, high mass stars become red giants or

red supergiants as the helium core builds up.

• Stars with enough mass will be able to fuse carbon and oxygen, and continue burning.

• Eventually runs out of fuel. The resulting explosion is called a supernova, and the remaining core is extremely dense, and will form a neutron star or a black hole.

Galaxies• Galaxies form by huge numbers of stars rotating around a central

mass.

• In the case of our galaxy, the Milky Way, this mass is thought to be an extremely massive black hole called Cygnus X-1.

Constellations• Patterns of stars we see in the sky

• Stars are connected by lines to trace out an image

• Most civilisations have developed their own set of constellations, with varying meaning behind them

SETI• Search for Extra-Terrestrial Intelligence

• Combined projects all around the world working to detect signs of extra-terrestrial life

• To date, the best candidate for evidence of intelligent life is the Wow! signal.

Voyager Records• The Voyager probes were launched with golden records

attached, featuring important aspects of life on Earth.

• A range of images were included of humans and other animals

• Sounds included whale and bird songs, wind, thunder, as well as some music and greetings in different languages.

Summary• Electromagnetic spectrum• Hubble telescope• Measurements of/from light• Doppler effect and red/blueshift• Hubble’s law• Big Bang theory timeline and evidence• Nuclear fusion in stars• Lifecycles of stars

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