Space Flashcards
(33 cards)
What is a black body radiator?
A theoretical that is a perfect absorber and emitter of radiation. All radiation emitted is due to the body’s temperature. We assume stars behave as black bodies
What is the Stefan-Boltzmann law?
The output power of a black body is proportional to the surface area and T^4
L = σAT^4 (luminosity = constant x area x temperature)
What is Wien’s law?
For a black body, the wavelength at which the radiation intensity curve peaks (λmax) is inversely proportional to the Kelvin temperature of the surface
Tλmax = 2.898 x10^-3
How can we accurately identify brightness?
Use the star’s output power, known as luminosity (L)
What affects the brightness of a star we see?
The size of the star, the type of star, the distance from Earth
What are the key points of the intensity against wavelength graph?
- As temperature increases, peak wavelength decreases (and frequency increases)
- The area under the curve represents total luminosity
- Visible radiation is the only part of the spectrum emitted
- Low temperature stars are red
- High temperature stars are blue
What are the initial steps to form any main sequence star?
A large cloud of hydrogen gas and dust known as a nebula collapses over time (due to gravity). The molecules collide, kinetic energy increases so the temperature also increases, leading to the formation of a protostar.
Eventually, the temperature gets so high that nuclear fusion takes place and a star is born. Inside the star hydrogen nuclei fuse together to make helium, a process by which mass is turned into energy which fuels the star
What happens to an average star once the supply of hydrogen begins to run out?
The star becomes unstable and begins to collapse, increasing the temperature further so that helium starts to fuse to make heavier elements. The star then swells up to make a red giant.
When the helium runs out, it collapses into a white dwarf which is very hot. Eventually, this cools down to become a dark, cold star known as a black dwarf
What happens to a very large star once the supply of hydrogen begins to run out?
They can grow into a red super giant once helium begins to fuse to make heavier elements, and the temperature in these becomes so hot that heavier elements can be made in the fusion process.
When the fuel runs out, the star collapses then explodes as a supernova, which then either collapses into a neutron star which is only a few km across, or it may collapse to a microscopic point and form a black hole (which is so dense that nothing can escape from it)
What are the axis of the Hertzsprung-Russel diagram?
The y axis has luminosity in watts (to powers of 10 - logarithmic, which is 1,000,000 at its highest and 0.000001 at its lowest) and magnitude, the x axis has temperature in K (highest at the left of the axis with 20000 just offset from the left and 2500 at the bottom)
What are the spectral classes and how are they grouped?
There are 8 spectral classes which are grouped according to temperature, O, B, A, F, G, K, M
What are the 3 main branches of the Hertzsprung-Russel diagram?
Main sequence stars (the centre), giant and supergiant stars (upper right), white dwarves (bottom left)
What are the key characteristics of main sequence stars?
They are all generating energy by fusing hydrogen to helium. They have the greatest range of brightness, with the brightest stars having a surface temperature of 50,000K and the dullest 2500K, and can be a million times brighter than the sun or 10,000 times duller
What are the key characteristics of giant and super giant stars?
These stars can be relatively cool , with temperatures of 3000K, but they are very bright because they are very large. In these stars, energy is generated by fusion of helium and larger nuclei
What are the key characteristics of white dwarf stars?
Very hot surfaces but they are relatively dull because they are so small. These stars are at the end of their lives; nuclear fusion has stopped, the stars have collapsed and are now cooling down
What is the astronomical unit (AU) and what is its value in metres?
The mean distance from the centre of the Earth to the centre of the sun.
1.5 x10^11 m
What is the parsec and what is its value in metres?
One parsec is the distance from a baseline of length 1 AU when the angle is one second of an arc (1/3600 of a degree, 4.848 x10^-6 radians). 1 parsec is an abbreviation of 1 parallax second
3.09 x10^6 m
What is a light year and what is its value in metres?
The distance light will travel through a vacuum in one year
9.46 x10^15 metres
What is the condition to use the parallax method to work out distance to a star?
Has to be a nearby star so a background of distant stars can be used and so the baseline of 1 AU is meaningful
What is a standard candle and how can it be used?
It is a star with known luminosity. Its intensity on Earth can be measured and therefore distance from Earth can be calculated using the inverse square law as can the distance to other stars or galaxies
What is the Doppler effect?
The apparent change in frequency and wavelength due to the motion of the object emitting waves. The amount by which the wavelength changes is related to the speed of the source
What is meant by redshifts and what is the significance?
- A moving wave source will appear to have a change in wavelength due to the Doppler effect
- The amount by which the wavelength changes is related to the speed of the source
- The hydrogen spectrum taken from a star in a distant galaxy is the same as hydrogen on Earth but shifted towards the red end of the spectrum
- This means that all stars are moving away and the universe is expanding
What is Hubble’s Law?
The speed of recession of a galaxy is directly proportional to its distance from Earth
What is the Hubble’s Law equation and what is the approximate value of the Hubble constant?
speed of recession / distance from Earth = H0 (H nought)
Approximately 70 kms^-1 Mpc^-1
