Option E - Astrophysics Flashcards
What is the general arrangement of the solar system?
The planets orbit the Sun in ellipses and moons orbit planets
What are the relative sizes of the planets?
Mercury < Mars < Venus < Earth < Neptune < Uranus < Saturn < Jupiter
What are the relative distances (positions) of the planets from the Sun?
Mercury < Venus < Earth < Mars < Jupiter < Saturn < Uranus < Neptune
What is an asteroid?
A small rocky body that drifts around the solar system
Where is the asteroid belt located?
Between Mars and Jupiter
What is a meteoroid?
An asteroid that is on a collision course with another plante
What are meteorites?
Meteors that survive the friction of the atmosphere and reach the ground
What is a comet?
A mixture of rock and ice in a very elliptical orbit around the Sun. Its tail always points away from the Sun
What is the difference between a stellar cluster and a constellation?
Constellations are stars that seem to be close to each other when looking from the Earth.
Stellar clusters are groups of stars that are physically close together
What is a light year?
The distance travelled by light in vacuum in one year
= 9.46 X 1015 m
What is a galaxy?
A large collection of stars held together by gravity
What are the relative distances between stars within a galaxy?
Approximately 1 light year
What is the relative distance between galaxies?
106 light years
What are the approximate sizes of galaxies?
103 to 105 light years
Why do stars appear to move?
Because of the Earth’s rotation
What is the main energy source of stars?
Nuclear fusion
What is the basic process of fusion?
Hydrogen is converted into helium
What is the fusion reaction that takes place inside stars?
Why do not stable stars collapse?
There is an equilibrium between radiation pressure and grativational pressure
- The continual production of energy gives the particles kinetic energy causing an outward pressure (radiation pressure) that pushes back against gravity
What is the luminosity, L, of a star?
The total amount of energy emitted (radiated) by the star per second
- Unit is watt
What does the luminosity of a star depend on?
The temperature of the star and its size
(if two different size stars have the same temperature, the bigger one will give out more energy)
What is the apparent brightness, b?
The amount of energy per second received per unit area
- Unit is Wm–2
How is the apparent brightness calculated?
d = distance to the star
What is required in order to calculate a star’s brightness?
Measuring the energy per unit second absorbed by a detector placed perpendicular to the direction of the star.
Done by using a telescope to focus an image of the star onto the CCD plate of a digital camera
Why does a hot body emit light?
Due to electron energy transitions
What is the Stefan-Boltzmann law used for?
Calculating the total intensity of light emitted from a star
Power per unit area = σT4
σ = Stefan-Boltzmann constant
How can the total power emitted be calculated with the help of the Stefan-Boltzmann law?
Total power emitted = L (luminosity)
L = σAT4
What is the Wien’s displacement law?
As the temperature incrases, the wavelength of the peak gets less:
Do stars emit continuous or discrete emission spectra?
Discrete
What does the spectrum of a star give information about?
- Its temperature
- Its chemical composition
- Its colour
How are stars classified according to their colour?
O - blue
B - blue-white
A - white
F - yellow-white
G - yellow
K - orange
M - red
(Oh Be A Fine Girl Kiss Me)
Do stars show Doppler effect in their emission spectra?
Yes
(red-shift and blue-shift)
How can the direction of a star in respect to Earth be determined from its Doppler effect?
Star moving away: longer λ - red shift
Star moving closer: shorter λ - blue shift
What are the different types of star?
- Red giants
- Red supergiants
- Cepheids
- White dwarfs
What is a red giant?
- A cool star
- Gives out a lot of energy
- Very big
What is a red supergiant?
- A cool star
- Very big, bigger than giants
- Very rare
What is a white drawf?
- A small star
- Very hot
- Hotter than the sun but only the size of the Earth
- Low luminosity
What are Cepheid variables?
- Stars with changing luminosity
- Size changes too, as it gets bigger its luminosity increases
How is an eclipsing binary star identified?
- From the analysis of the brightness of the light from the star
- Over time the brightness shows a periodic variation
- Variation is a result of its orbit, one star gets in front of the other
How is a spectroscopic binary star identified?
- From the analysis of the spectrum of light from the star
- The wavelengths show periodic shift (red and blue)
- Due to Doppler effect
- As a result of its orbit, the stars move periodically towards and away from the Earth
What is a binary star?
A star that orbits around each other (or their common centre of mass
What is a single star?
A star that is not gravitationally bound to other stars
Identify the general regions of star types on a Hertzsprung-Russel (HR) diagram. What information can be obtained from it?
- Cepheid stars are hard to position because they have variable size and luminosity
- The scale is not linear
- The mass of main sequence stars is dependent on their position on the HR diagram
What is the time period of orbit of binary stars?
d = distance between the binary stars
(M1 + M2) = total mass of the two stars
What is the parsec?
Parallel angle of one second:
What is the parallax shift?
The apparent movement of a star from Earth’s point of view due to the earth’s movement in its orbit.
How is the parallax method used to determine the distance to a star?
- The relative position changes during one year
- Can be found by measuring the angle between rays from the object to two fixed points
- The telescope is lined up with the star on one day and then again half a year later when the Earth is on the other side of the Sun
- The angle is calculated with the help of a distance to a very distant star (this way the distance won’t be affected by the movement of the Earth)
What is the equation for calculating the distance of a star from the Earth using distant stars as reference points?
Why is the method of stellar parallax limited to measuring stellar distances less than several hundred parsecs?
Because the star needs to be close enough to show apparent motion with respect to the Earth.
If the star does not “move” in the sky during the Earth’s orbit, the angle cannot be measured
What are arc seconds and arc minutes?
One degree can be split into 60 arc minutes (‘)
Each arc minute can be split into 60 arc seconds (‘’)
What is the apparent magnitude (m) scale?
A scale used to compare the brightnesses of stars as viewed from Earth
Describe the apparent magnitude scale
- Inverse scale
- The bigger the number the smaller the brightness
- A magnitude 1 star is 100 brighter than a 6
- Logarithmic scale
- Magnitude depends on luminosity
What is absolute magnitude (M) of a star?
The magnitude of a star viewed from a distance of 10 pc
What is the difference between apparent (m) and absolute (M) magnitudes?
The apparent magnitude of a star is a measure of its brightness as viewed from the Earth.
The absolute magnitude is a measure of luminosity
What are the formulas for the absolute magnitude?
What can be used to estimate the luminosity of a star?
Its spectrum
How can apparent brightness and luminosity be used in calculating stellar distances?
By using the inverse square law to calculate the distance from Earth
What is the limitation of spetroscopic parallax?
It is limited to measuring stellar distances less than about 10 Mpc
What is spectroscopic parallax?
A method of using the spectrum of a star to estimate is luminosity.
(also the distance after the apparent brightness is calculated)
What is a Cepheid variable?
An unstable star in which the outer layers undergo a periodic expansion and contraction, which produces a periodic variation in its luminosity
What are Cepheid variables used for?
Measuring distances to stars further than 10 Mpc with the help of stellar parallax
What is the relationship between period and absolute magnitude (luminosity) for Cepheid variables?
They are directly proportional
What is meant by using Cepheid variables as “standard candles”?
If a Cepheid variable is located in a particular galaxy, the distance to the galaxy may be determined with the help of a Cepheid variable
What is Newton’s model of the universe?
- Infinitely big, infinitely old, static, and uniform
- Gravitational force on each star was the same in each direction
Explain Olber’s paradox
- If Newton’s model of the universe were to be true, and the sky had an infinite number of stars, the sky would never be dark
- Even if some stars were blocked, the energy absorbed would be re-emitted and would eventually reach the Earth
- The most probable solution is that the universe is not infinite and there is not an infinite number of stars
- Time is not infinite, the light from some of the stars has not reached the Earth yet
What provides evidence for the fact that the universe is expanding?
The red-shift of light from distant galaxies (= the galaxies are moving away)
Is the universe expanding into a void?
No.
What is essentially meant when it is said that the universe is expanding?
The space is growing, rather than it is speading into the nothingness that surrounds it. The galaxies therefore move apart from each other because the space between them is expanding
What did the Big Bang create?
Space and time
On what fact is the Big Bang theory based on?
If the universe is currently expanding and all its particles are moving away from each other, at some point in the past all the galaxies would have been at the same point approximately 15 billion years ago.
Describe the discovery of cosmic microwave background (CMB)
- In the beginning the universe was packed tightly with photons and other fundamental particles
- The photons are still present but have a much longer wavelength (microwaves)
- They were thought to be uniform at first
- Very small variations within the microwaves were detected
- This shows that the universe was not completely uniform in the beginning, enabling galaxies to form
How is cosmic radiation in the microwave region consistent with the Big Bang model?
- The universe is cooling down
What is an open universe?
One that keeps expanding
What is a flat universe?
One of which rate of expansion tends to zero at infinite time
What is a closed universe?
One that stops expanding and starts to contract eventually
Distinguish the types of universe graphically
What is the critical density?
The density at which the closed universe becomes open.
How does the density of the universe determine its development?
- The rate of expansion is slowing down due to gravity
- Whether it is slowing down enough to stop depends on the density of it
- If the density is bigger than the critical density, it will stop expanding and start to contract, eventually ending in a bigh crunch
- If the density is lower than the critical density, it will continue to expand forever
What are the problems associated with determining the density of the universe?
- Normally the density can be measured by measuring the mass of all the stars in a given volume
- The total mass calculated is not enough to give the gravitational attraction to hold it together
- The total mass of all the stars is only about 4% of the whole mass
- The rest of this mass is dark matter
- There’s a lot of stuff we don’t know
What is dark matter?
- Any matter that does not interact with light
- May consist of neutrinos
- May consist of new particles (WIMPs or MACHOs)
What is meant by MACHOs?
Shorthand for massive astronomical compact halo objects
- There is some evidence that lots of matter does exist in these groupings
- Can be thought of as low-mass ‘failed’ stars or high-mass planets, or black holes
- Produce little or no light
- Possibly a part of dark matter
What is meant by WIMPs?
Shorthand for weakly interacting massive particles
- New particles that we don’t know about
- Possibly a part of dark matter
What does current scientific evidence suggest about the nature of the universe?
That it is open
How is the critical density of the universe calculated?
H0 = Hubble’s constant
Why are astrophysical projects often international?
Because they are extremely expensive and require the funding of several countries
e.g. Gamma-ray Large Area Space Telescope (GLAST), which involves the US, France, Germany, Japan, Italy, and Sweden
Is investing significant resources into researching the nature of the universe worth it?
In terms of the advancement of science, yes.
In terms of money, no one knows.
What are the conditions necessary for the birth of a star?
- Huge clouds of gas and dust need to be compressed
- Usually caused by an exploding supernova or a collision between two dust clouds
- As the gas atoms are pulled together, they gain kinetic energy and the temperature increases
- The increase in temperature causes an outward pressure that pushes against the gravitational force
- As the atoms get closer, the gravitational force increases, so the gas continues to collapse and get hot infinitely
Describe the start of fusion inside a star
- As the cloud collapses, a dense coreis formed surrounded by a cloud of gas and dust
- The centre of the core rapidly contracts, resulting in high temperature and pressure
- After a while, the radiation from the star blows away the dust cloud and the mass of the star stabilises
- The core continues to contract and heat up until the atoms are moving fast enough for fusion to take place
- Once fusion starts, the increase in temperature causes greater pressure, balancing the inward force of gravity.
- The star stops contracting and becomes a main sequence star
What happens to a star when it runs out of hydrogen?
- When all the hydrogen in the core has become helium, the core is no longer producing energy and is no longer in equilibrium
- The thermal pressure drops, causing the core to rapidly compress
- The compression results in an increased temperature, which heats the hydrogen gas outside the core, causing it to start fusing
- As this happens the outer layers expand and the star becomes a giant
- The helium in the core starts to fuse to carbon
- If the star has sufficient mass, the red giant can continue to fuse higher and higher elements and nucleosynthesis can continue (layered structure)
What is the mass-luminosity relationship?
The luminosity of massive main sequence stars is greater than stars of small mass; this enables us to know where the different stars join the main sequence line.
How is the Chandrasekhar limit used to predict the fate of stars?
Chandrasekhar limit: 1.4 MO
- The mass of a white drawf cannot exceed this
- Results from stars with a mass of max 4 MO
- If the mass of the star is greater than this, it will not become a white drawf
How is the Oppenheimer-Volkoff limit used to predict the fate of stars?
Oppenheimer-Volkoff limit: 3 MO
- The maximum mass of a stable neutron star
- Neutron stars of bigger masses become black holes
Compare the fate of a red giant and a red supergiant
- A red giant forms a planetary nebula and then becomes a white dwarf
- A white drawf is stable due to electron degeneracy pressure
- A red supergiant experiences a supernova and becomes a neutron star or collapses to a black hole
- A neutron star is stable due to neutron degeneracy pressure
Draw the evolutionary path of a low-mass star on an HR diagram
Draw the evolutionary path of a high-mass star on an HR diagram
Describe pulsars
- As a star collapses, the speed of rotation increases
- A neutron star rotates very fast
- The magnetic field increases when a star collapses into a neutron star
- The rotating magnetic field causes charged particles to accelerate, which emit EM radiation along the ling of the poles
- When the neutron star rotates, its beam of radiation sweeps around like a lighthouse in regular intervals (0.03 s to 1.3 s)
How are galaxies distributed in the universe?
- Not randomly
- Tend to be found clustered together (galactic cluster)
- Galactic clusters and also group together to form galactic superclusters
Why do distant galaxies show red-shift?
Because the universe is expanding
How can the velocity of a galaxy relative to Earth be calculated?
∆λ = change in wavelength (shift
λ = original wavelength
v = relative velocity
c = speed of light
What is Hubble’s law?
The recession of speed is directly related to the distance to the galaxy:
What is the problem with Hubble’s law?
- The law assumes that the velocity is constant
- Gravitational attraction slows the galaxies down; the recessional velocity we measure today is smaller than it was
How can the age of the universe be calculated?
How did the expansion of the universe make possible the formation of light nuclei and atoms?
- In the beginning there were no atoms because the temperature was so high that the photons had enough energy to ionise atoms
- Any electron combining with a proton to form a hydrogen atom would be knocked off
- When the universe expanded and cooled down, the photons did not have enough energy to ionise atoms
- Electrons were able to start combining with protons to form atoms
How does the formation of atoms provide evidence for the fact that the expanding was not uniform?
If the universe had spread out uniformly the graviational attraction between each particle would have been balanced and the particles would not have been brought together to form galaxies.
Thus there must have been some irregularities to provide the starting point for all the galaxies
