Option- Astrophysics

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Order of planets
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
1AU represents
1.5×10^11 m
AU is defined as
The distance between the earth’s and sun’s centres
Galactic Cluster
Groups of galaxies
Globular Stellar Cluster
Hundreds of thousands of stars crammed into a relatively small area, 12-13 billion years old
Open Stellar Clusters
Appear in the spiral arms of the galaxy, are between a few million and a few billion years old
How many stars are there in our galaxy?
100 billion
Stars shine because they are
Converting hydrogen gas into helium gas though nuclear fusion. The mass of products produced in the reaction is less than the mass of material used up. Therefore, energy has been released.
Birth of a star process
Hydrogen gas molecules are attracted through gravitational attraction and form a nebula. Gravitational force causes particles to accelerate inwards, converting gravitational potential energy to kinetic energy and raising the temperature (vibrations of particles). If it reaches 14×10^6 K, nuclear fusion can occur because they travel fast enough to overcome the electrostatic repulsion between them.
For a star to be stable,
Radiation pressure must equal gravitational pressure
Radiation Pressure
Pressure outwards in a star
Gravitational Pressure
Pressure inwards in a star
Power of the star
Luminosity unit
Lo means (o has dot in middle)
Luminosity of the sun
Apparent Brightness
The power per unit area of a star
Black body
A theoretical ‘perfect’ emitter that radiates all wavelengths, independent of surface
Wien’s Law (wavelength (max) x temperature= 2.9×10^-3 mK) enables one to work out
Surface temperature of a star
Area of a star
4 pie r^2
A star is thought to be (almost) a
Black body
The frequency of line emitted from stars has gaps showing
Which elements are present (being absorbed by the outer layers)
Cepheid stars are
Similar to main sequence stars but their size and luminosities vary
Binary Stars
Pair of stars orbiting each other
Visual Binary Stars
Binary stars that can be seen using a telescope
Spectroscopic Birnary Stars
Seen due to a change in the pattern of absorption lines in the spectrum of light, with lines appearing to split and recombine over a regular period
Spectroscopic Binary Stars can be seen due to the existence of
The Doppler Effect
Eclipsing Binary Stars
Identified from a periodic fall in brightness as one star eclipses the other star
Cepheid Variables occur because of
Periodic fluctuations of the radiation pressure and gravitational pressure, causing it to expand and contract
Cepheid Variables are at their brightest when they are at their
As the time period of a cepheid variable increases,
So does it’s luminosity
Life cycle of a small star
Nebula, average main sequence, red giant, planetary nebula, white dwarf, black dwarf
Life cycle of a massive star
Nebula, massive main sequence star, red supergiant, supernova, neutron star or black hole
Neutron stars are incredibly dense because
They have no electrons, so have no free space
The greater the size of a star,
The cooler it is
A main sequence star is stable because the
Radiation pressure and gravitational pressures are equal
Red giants/ Red supergiants swell because
The radiation pressure increases
A star will become a black hole when it’s mass is greater than
40 solar masses
A star will become a neutron star when it’s mass is between
12 and 40 solar masses
A star will become a black dwarf when it’s mass is less than
12 solar masses
A red giant becomes a black dwarf when
Hydrogen starts to run out, rate of fusion decreases and radiation force decrease. The star contracts, with the smaller size causing the temperature to increase. The core becomes hot enough for fusion to occur again, increasing the star to become a red giant. The increasing size causes a decrease in the strength of the gravitational force and the high temperature increases the rate of fusion until the outer layer is ejected as a planetary nebula. The white dwarf is the remaining core, where fusion does not take place, allowing it to fade to become a black dwarf.
Red supergiant to supernova process
Hydrogen runs out, rate of fusion decreases and radiation force decreases. The star contracts. Energetic photons from fusion start to rip iron nuclei apart to form protons and neutrons. Gravity is so great that contraction cannot be stopped. Electrons combine with protons to become neutrons until the core is entirely made of neutrons and is a neutron star. The continued contraction causes the star to get too dense and rebound, causing a supernova explosion
Three characteristics of the main sequence stars
Fusion is occurring, stable, lie on the line from top left to bottom right of the Hertzsprung-Russel diagram
Light year
The distance that light travels in one year (9.5×10^15m)
The distance to a star when the parallax angle is one arc second
One arc second is
1/3600 of a degree
Newton’s three universe assumptions
It is infinite, it is uniform, it is static
The cosmological principal
On a large scale, the universe has the same structure when viewed from any point
The Big Bang Theory
Galaxies are observed to be moving away from us, suggesting the universe is expanding. This means at one point they were closer together and so once were at one infinitely close and infinitely dense point.
Evidence for the Big Bang Theory
Red-shift, background microwave radiation
Red-shift occurs because
The space has increased in the time between a galaxy emitting light and the earth receiving it, suggesting the wavelength has been stretched.
The attractive nature of gravity means that
The expansion of the universe should be slowing down
Open Universe
If the density in the matter of the universe is low, gravity slows down the expansion, but not enough to stop it. It expands forever.
Closed Universe
If the density in the matter of the universe is high, gravity will stop the expansion and bring it back to a single point, possibly causing another Big Bang
Flat Universe
If the density of the universe is at the critical density, gravity will bring the expansion to a halt and after an infinite time, it would become a finite size.
Categories: Astrophysics