Astrophysics Flashcards
What is the definition of a ray?
A representation of a light path
What is the definition of the normal line on a ray diagram?
The line perpendicular to the surface
What is the definition of the plane on a converging and diverging ray diagram?
The line drawn that is in the centre of your lens
What is the definition of the principal axis on a converging and diverging ray diagram?
The line that passes through the centre of the lens, perpendicular to its surface
What is the definition of the focal point on a converging and diverging ray diagram?
This is the point (drawn where your principal axis and plane intersect) where incoming rays travelling parallel to the principal axis will be refracted and converge/ are directed away from one another
What is the definition of divergence?
Light rays that spread apart over a distance/ spreads out incident light
What is the definition of convergence?
Light rays that come together over a distance/ focuses incident light
What is the definition of a real image?
A projected image formed from light focussing/crossing each other after refraction
What is the definition of a virtual image?
An apparent image formed on the same side of the lens - since the light rays are not focussed/ do not cross the image cannot be projected onto a screen
Which type of image can be formed on a screen?
A real image
In which type of lens are real images usually formed?
Converging lenses
In which type of lens are virtual images usually formed?
Diverging lenses - however, can be produced by converging lenses
When will a converging lens never form an image?
This is when your object is placed at your focal length - your refracted rays will be parallel to on another
When will a converging lens form a virtual image?
This is when your object is placed at a distance that is less than your focal length - your refracted rays will diverge hence you draw a dashed line backwards from them to find where your virtual image would form
What is the definition of focal length?
The distance between the centre of your lens and the principal focus
What are the 3 rays that you have to draw a converging and diverging ray diagram?
- Centre ray
- Parallel ray
- Focus ray
What is the definition of your centre ray on a converging and diverging ray diagram?
The ray that runs through the centre of your lens/ mirror curvature and does not deviate/ get refracted
What is the definition of your parallel ray on a converging and diverging ray diagram?
The ray that is drawn parallel to your principal axis that always goes through your principal focus, f:
- If your rays converge then your parallel ray will go through the principal focus on the opposite side of your object
- If the rays diverge then your parallel ray will go through the focus on the same side as your object (this focus is also focal length distance away from the centre of the lens)
What is the definition of your focus ray on a converging lens ray diagram?
The ray that runs through your focal point on the same side as your object and is then refracted parallel at the plane of your lens
What does a converging lens look like?
Has 2 convex lines facing outwards
How do you show where an image will be formed on a converging and diverging ray diagram?
Draw a line down to the principal axis from where the lines converge (real or virtual)
Where are your foci on a ray diagram?
Your foci are drawn on either side of your lens and are equidistant from one another and from the centre of the lens
What is the definition of your focus ray on diverging lens ray diagram?
The ray that runs through the principal focus virtually as it gets refracted parallel at the plane of the lens before it reaches this focus
What does a diverging lens look like?
Has 2 concave lines facing outwards
What does convex lenses normally do to rays?
Converge them - sometimes diverge when the object is placed at a shorter distance than the focal length from the lens
What do concave lenses do to rays?
Diverge them
What is the lens equation?
1/d(o) + 1/d(i) = 1/f
Where:
d(o) - is the distance from the centre of your lens to the object
d(i) - is the distance from the centre of your lens to the image you have formed
f - is the focal length of the lens (distance from the centre of your lens to the principal focus)
What is the definition of the focal plane on a converging and diverging ray diagram?
The line drawn straight down from your principal focus
What is an example of an infinitely far out object that we use lenses to view?
Stars
What are the sort of rays do we receive from infinitely far out objects?
parallel rays
Where is your f1 when you draw parallel rays coming from an infinitely far out object on a ray diagram?
Your f1 is where the first parallel ray passes the principal axis (this ray acts as your focal ray)
Where is your imaged formed when you have parallel light from an infinitely far away source coming into your converging lens?
At the principal focus
What do we use to view objects that are infinitely far away?
A telescope
What are the two types of telescopes?
- Refracting telescope
- Reflecting telescope
What are the two type of lenses used in a refracting telescope?
- Objective lens - the first lens
- Eyepiece lens - the second lens
What is the image formed from light passing through the objective lens of your refracting telescope?
- real
- inverted
- image formed at focal length (principal focus)
What is the image formed from light passing through the eyepiece lens of your refracting telescope?
- virtual
- inverted
- image formed at infinity
What is the main purpose of the objective lens?
To collect the light
What is the main purpose of the eyepiece lens?
To magnify the image
What is the same about the objective and eyepiece lens in your refracting telescope?
- Both convex lenses
- Have the principal focus at the same point (doesn’t mean they have the same focal length)
What happens when your refracting rays from your objective lens meet your eyepiece lens?
They refract to become parallel to one another
Describe the ray diagram of light coming into a refracting telescope from infinity
- Your rays come into the lens parallel to one another, the first ray to cross your principal axis acting as your focal ray and the ray that goes through the centre of your lens acting as your centre ray
- At the plane of your lens your rays will refract: the focal ray parallel and the centre ray will not deviate therefore you can find where they cross and draw all your other refracted rays to meet at this point.
NB: this point is your principal focus, therefore it should roughly be as far away from the centre of your lens as the focus on the other side. - Continue the paths of your rays until you reach the eyepiece lens (this should not be too far away as your want to maximise your magnification) This distance that your rays have just traveled is your focal length as the principal focus of your eyepiece lens in the same as your objective lens.
- At the plane of the eyepiece lens your rays will refract, becoming parallel to one another and going towards the principal axis. The parallel line that reaches the principal axis at the focal length is your focal point.
- You draw dashed lines going back from your refracted rays as we observe a virtual image at infinity
What is the equation for your telescope length in a refracting telescope?
f(o) + f(e) = length of telescope
What are two properties that an objective lens should have?
- Long focal length
- Large - in order to have a large collecting power
What is the magnification equation?
M = Eyepiece angle/Objective angle = beta/alpha = f(o)/f(e)
What is the definition of the eyepiece angle?
The angle subtended by the height of the real image from the principal axis, h, a distance, d, away (focal length)
What is the objective angle?
The angle subtended by the height of your object, h, and distance d away
How do you derive the equation for magnification, M?
For small angles tan(theta) = theta
Objective angle:
alpha = h/f(o)
Eyepiece angle:
beta = h/f(e)
beta/alpha = h/f(e) x f(o)/h
= f(o)/f(e)
How do you increase the magnification of a refracting telescope?
Increase the objective focal length and decrease the eyepiece focal length
What is the ultimate goal of a telescope?
To collect parallel rays of light and focus them onto a singular point
What is spherical aberration (description)?
Spherical aberration is when light is passed through a spherical lens and instead of the light being refracted to meet at a certain point, the light rays cross at different points due to the radial line being the normal
How do you draw spherical aberration on a diagram?
All your rays come in parallel into your spherical lens or mirror. Then your outer rays are refracted/reflected most and so cross each other first followed by the next two rays crossing further away etc.
What is the definition of spherical aberration?
The image blurring and distortion produced from a lens due to its curvature as the rays of light at the edge are focussed in a different position to the ones in the centre
In which types of telescopes will spherical aberration occur?
Refractive and reflective
How can you reduce the impact of spherical aberration in reflecting telescopes?
By using a parabolic mirror
What is the definition of chromatic aberration?
The image produced having coloured fringing due to the different focal lengths of the colours in white light as they are refracted by different amounts
In which types of telescopes will chromatic aberration occur?
Refractive
NB: it will occur a little bit in reflective telescopes but only in your eyepiece lens
What is the equation that demonstrates the order that the colours focus in, in chromatic aberration?
n = c/v = f(lambda)c/f(lambda)v = lambdac/lambdav
Therefore, the smaller the wavelength the more it is refracted, hence blue is refracted the most, then green and red is refracted the least.
Which colour is refracted the most in spherical aberration?
Blue -> Green -> Red
What are the two types of reflecting telescopes you need to know about?
- Cassegrain Telescope
- Newtonian Telescope
What type of primary mirror do both Cassegrain and Newtonian telescopes use?
Concave, parabolic with a long focal length
Which type of telescopes have longer focal length and, therefore, a greater magnification?
Reflective telescopes
What is the magnification equation for reflecting telescopes?
M = f(o)/f(e) (same equation for both types of telescopes)
Describe and draw parallel light rays entering a Newtonian telescope
- Your light rays enter in parallel to the rectangular telescope, one on either side of the plane mirror in the centre of your telescope/rectangle
- Your parallel light rays hit the concave, parabolic mirror at the back of the telescope and are reflected towards the central plane mirror. NB: they have not crossed at this point.
- Your plane mirror is angled in a 45 degree position facing downwards. After the two rays are reflected off of this plane mirror they cross before reaching the eyepiece lens. NB: continue your light rays as dashes through your plane mirror till they cross as this is your principal focus.
- The eyepiece lens is drawn just below your plane mirror and comes out of your rectangle slightly. When the light rays go through the lens they are refracted to become parallel to one another and then go into your eye.
Describe and draw parallel light rays entering a Cassegrain telescope
- Your light rays enter in parallel to each other and the rectangular telescope, one on either side of your convex secondary mirror that is placed in the centre of your telescope/rectangle.
- Your parallel rays hit the concave parabolic mirror at the back of the telescope and are reflected towards the central convex secondary mirror. NB: The rays have not crossed at this point and remember your concave primary mirror has a gap in the middle.
- Your convex secondary mirror faces your concave primary mirror. After the two rays are reflected off of the convex secondary mirror they cross before reaching the eyepiece lens. NB: continue your light rays as dashes through your convex secondary mirror till they cross as this is your principal focus.
- The eyepiece lens is drawn just behind your concave primary mirror and comes out of the rectangle slightly. When the light rays go through the lens they are refracted to become parallel to one another and then go into your eye.
Which reflecting telescope has a greater magnification? Why?
The Cassegrain telescope has a greater magnification. This is because its effective focal length of the objective is made greater by using a convex secondary mirror.
Which reflecting telescope is easier to manoeuvre? Why?
The Cassegrain telescope is easier to manoeuvre. This is because it is shorter than a similarly powered Newtonian telescope.
State the differences, advantages/disadvantages between refracting and reflecting telescopes
REFLECTING TELESCOPES
- Can be made much larger/have wider objectives than refracting telescopes (therefore greater collecting power and so can view fainter objects) as mirrors can be supported from behind, whereas a lens can only be supported from its edges
- Suffer from much less chromatic aberration
- Suffer from spherical aberration, however this can be eliminated by using a parabolic mirror
- Large magnifying power with small diameters
REFRACTING TELESCOPES
- A large diameter lens cannot be used as it can only be supported by its edges and may break under its own weight
- Suffer from both chromatic and spherical aberration
- Large magnifications require large diameters
Which types of telescopes are preferred to use in the modern day?
Reflecting telescopes
What causes a slight reduction in the amount of light viewed from a reflective telescope?
The secondary plane/convex mirror
What are the two regions of the EM spectrum that are least effected by the earths atmosphere?
Radio waves and Visible light (although still preferred to be placed in space)
Why do most telescopes have to be placed in orbit?
- Due to light pollution and other interference at ground level
- Due to absorption of the EM waves by the atmosphere
- As the wavelength of light that they are receiving would be distorted when entering the atmosphere (many different refractive indexes so the ray gets bent many times)
What is the definition of collecting power?
A measure of the ability of a lens or mirror to collect incident EM radiation
What are the 2 advantages of larger diameter telescopes?
- Greater collecting power so images are brighter
- Greater resolving power so images are clearer
What is the definition of resolving power?
A measure of the ability of a telescope to produce separate images of close together objects
What is the relationship between collecting power and the area of the objective lens?
Collecting power is directly proportional to the area of the objective lens
Since area is given by pi(d)^2/4 it is generally said that the collecting power is directly proportional to the (diameter)^2
What is required for an image of two objects to be resolved?
It is required that the angle between the 2 straight lines from earth to each object must be at least the minimum angular resolution (theta)
What is the equation for the minimum angular resolution?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
What is the equation of Rayleighs Criterion?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
What is the equation of resolving power?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
When analysing the resolving power of different telescopes, is it better to have a smaller or larger resolving power?
Smaller - as this means the telescope is able to resolve objects that are separated by a smaller angle
What are two ways, other than increasing diameter, that you can increase the collecting power of an aperture?
- Increase exposure times
- Use very sensitive detectors e.g CCD’s
What is the Rayleigh Criterion used to determine?
The smallest angular separation between two point objects which can be resolved by a telescope
What is meant by the Rayleigh Criterion in terms of diffraction patterns?
Two sources will (just) be resolved if the central maximum of the diffraction pattern of one source coincides with the first minimum of the other
What does the intensity diagram look like for a single slit diffraction pattern?
The central maximum is twice the width than any of the subsidiary maxima. The central maxima has a much greater amplitude/intensity than any subsidiary maxima
What is the equation for a single slit/diffraction grating diffraction/interference pattern?
dsin(theta) =n(lambda)
d - spacing between adjacent slits
n - order of maxima
theta - angular separation between order of maxima
NB: if you want to find the last visible order you make theta = 90
What is the definition of quantum efficiency?
The percentage of incident photons that cause an electron to be released
What is the definition of 1AU?
The average distance between the sun and the earth
What is the value of 1AU?
1.5 x 10^11m
What is an arcminute?
1/60 of a degree
What is an arcsecond
1/3600 of a degree
What is the easiest thing to see about a star?
Its brightness
What is the definition of apparent magnitude?
The brightness of a star as observed from earth
What is the definition of the luminosity of a star?
The rate of light energy released/power output of a star
What is the equation for power?
Power = energy/time
What is the definition of a light year?
The distance travelled by light in a vacuum in one year
What is the value of a light year?
9.46 x 10^15m
What is the definition of a parsec?
The distance from which 1AU subtends an angle of 1arc second (1/3600th of a degree)/ The distance at which the angle of parallax is 1 arcsecond
What is Keplers third Law?
That the square of the time period of any planet is directly proportional to the cube of the semi major axis of its orbit
T^2 proportional a^3
What is the semi major axis of an orbit?
If an orbit is elliptical, the semi major axis is the longest distance in AU between the centre of the orbit and the orbit path
What is the value of 1 parsec in light years?
3.26 ly
What is the definition of intensity?
The power received from a star (luminosity) per unit area
What is the equation for the intensity at the surface of a star?
I = P/A = P/4(pi)r^2
UNITS: Wm^-2
How does the intensity of a star change with distance?
The intensity of a star is inversely proportional to the square distance from the the star
What is the the equation that shows how the intensity of a star change with distance?
I = 1/d^2
What are the two factors that the overall brightness/apparent magnitude of a star depends on?
- Luminosity, power output
- Distance of the star from the observer
Describe the apparent brightness scale
- The scale ranges from -25 -> 25 in 5 unit increments
- Each power of five magnitude increases as you move away from 0 (1st magnitude)
- At 0 (the 1st magnitude) you have the star vega
- At -12 (roughly) you have the moon
- At -27 (roughly) you have the sun
- At 6 you have the faintest naked eye star
- At 13 (roughly) you have the brightest quasar
- At 27 (roughly) you have the faintest object as viewed by a telescope
- As your magnitude gets more positive the stars get fainter
- As your magnitude gets less positive your stars get brighter
What is the definition of the Hipparcos scale?
The scale that classifies astronomical objects by their apparent magnitudes
What type of scale does the Hipparcos scale have?
A logarithmic scale where the intensity changes with a scale of 2.51 between each magnitude
If 2 stars had a difference in magnitude of 4, what would be the intensity/brightness difference observed between the two?
2.51^4
= 39.8 = 40
Therefore, the more negative star would have a brightness 40 times that of the other star
HOWEVER, this does not mean that the closer star is brighter
What is the definition of absolute magnitude?
The apparent magnitude of a star from the standard distance, 10pc
Descriptive answer: What its apparent magnitude would be if it was placed 10 parsecs away from earth
What is the equation that shows the relationship between apparent and absolute magnitude?
m - M = 5log(d/10)
m - apparent magnitude
M - Absolute magnitude
d - distance from earth in parsecs
If m = M, how far away if the star you are looking at?
10 pc
What is a solar mass?
The mass of the sun = 1.99 x 10^30
What is the definition of parallax?
The apparent change in the position of a nearer star in comparison to distant stars as a result of the orbit of the earth around the sun
Generally, how do you determine the parallax angle for nearby stars?
- You take the earth 6 months apart (in December and June) where the distance between earth and the sun in 1AU
- You could either be given the distance from the sun -> star or earth- -> star but just ensure that this distance is in parsecs
- Use trigonometry to find the angle bisected between the sun and the earth from the star. NB: this will be in arc seconds
- This value is your parallax angle
What is the definition of a Black Body radiator?
A perfect emitter and absorber of all possible wavelengths of EM radiation
What can be approximated as a black body radiator?
A star
State Stefan’s Law
The power output (luminosity) of a black body radiator is directly proportional to its surface area (A) and its absolute temperature (T)^4
What is the Stefan’s Law equation?
P = (sigma)AT^4
P - Power output (Watts,W)
(sigma) - Stefans constant (5.67x10^-8, Wm^-2K^-4)
A - Surface area (metres squared, m^2)
T - Absolute Temperature (Kelvin, K)
State Wien’s Displacement Law
The peak wavelength (lambda^max) of emitted radiation is inversely proportional to the absolute temperature of the object
When is the peak wavelength of light emitted?
At maximum intensity
What is the equation for Wien’s Displacement Law?
(lambda^max)T = 2.9 x 10^-3 (mK) = a constant
mK - metres-Kelvin
T - Absolute Temperature
What are the axis on a black body curve?
Intensity - Wm^-2 versus Lambda(wavelength) - nm
What happens as the temperature of a black body radiator decreases?
Following Wien’s law that says (lambda)T is a constant, as the temperature decreases the peak wavelength emitted by the source increases. This follows as lower temperature emitters are red in colour hence there peak wavelength is more towards the red end of the spectrum.
Describe the Black Body Curve
Y-AXIS: Intensity
X-AXIS: Lambda(wavelength)
On your graph you have multiple curves that show the peak wavelengths at different temperatures.
12000K - Highest intensity with peak wavelength emitted at blue end of visible light spectrum.
6000K (our sun) - Peak wavelength at red/orange end of the spectrum as our sun is that colour, this peak happens at roughly half of what 12000K was.
4000K - Peak wavelength at roughly 1000nm and intensity a third the height of the 6000K peak
2000K - Coolest star, has a peak wavelength at roughly 1500nm (use the law to figure out exactly) and half the height of the peak at 4000K
NB: Curves for each temperature are steep up to the peak and then decrease exponentially
Shorter wavelength = …
Increased frequency, therefore, the wave is higher in energy
Longer wavelength = …
Decreased frequency, therefore, the wave is lower in energy
Why do absorption spectra occur from stars?
This is because when light created from within a star passes through its atmosphere, absorption of different wavelengths of the light occur (due to absorption of the wavelengths by electrons in the atoms and molecules present in the atmosphere)
What happens when electrons in atoms absorb light?
Light is a form of energy and so the electrons are able to jump up to a higher energy level
What is the equation that relates the energy to the frequency of light that is incident as the atom?
E = hf
The difference between energy levels is …
discrete
How do you convert from eV to Energy?
E = QV
E = Q (of an electron) x eV
What are absorption lines dependant on?
The temperature of the star (the hotter the star the more it fuses helium rather than just hydrogen)
What are the spectral classes from hottest to coolest?
O,B,A,F,G,K,M
What are the temperature ranges (K) and colours for each of the spectral classes?
O - 25000 -> 50000 (blue)
B - 11000 -> 25000 (blue)
A - 7500 -> 11000 (blue/white)
F - 6000 -> 7500 (white)
G - 5000 -> 6000 (yellow/white)
K - 3500 -> 5000 (orange)
M - 2500 -> 3500 (red)
What are the two elements that produce absorption lines in stars?
Helium and Hydrogen
What is the range of wavelength of visible light?
400 nm -> 700 nm
What is the intensity of hydrogen balmer lines dependant on?
The temperature of the star
What are the three series in the electron transitions for Hydrogen?
Lymen series (UV light) n = 1:
- Higher energy
- Higher frequency
- Lower wavelength
Balmer series (Visible light) n=2:
- 400 -> 700nm
Paschen series (IR) n=3:
- Lower energy
- Lower frequency
- Higher wavelength
Which series in the electron transitions for Hydrogen do you need to know about?
The Balmer series (visible light)
Describe the prominence of Hydrogen Balmer lines in the spectral class O
The stars atmosphere is too hot, therefore, the hydrogen is likely to be ionised -> weak prominence of Hydrogen Balmer lines
Describe the prominence of Hydrogen Balmer lines in the spectral class B
The stars atmosphere is too hot, therefore, the hydrogen is likely to be ionised -> weak prominence of Hydrogen Balmer lines, however, stronger than O spectral class
Prominent absorption lines: He, H
Describe the prominence of Hydrogen Balmer lines in the spectral class A
High abundance of hydrogen in the n=2 state -> strongest prominence of Hydrogen Balmer lines
Prominent abosorption lines: H(strongest), ionized metals
Describe the prominence of Hydrogen Balmer lines in the spectral class F
The stars atmosphere is too cool, therefore, the hydrogen is unlikely to be excited -> weak prominence of Hydrogen Balmer lines
Prominent absorpotion lines: ionized metals
Describe the prominence of Hydrogen Balmer lines in the spectral class G,K and M
The stars atmosphere is way too cool for the electrons in hydrogen to be excited -> very weak/no Hydrogen Balmer lines and too little atomic hydrogen
Prominent absorption lines: neutral metals (TiO)
What type of scale does the Hertzsprung-Russell Diagram use?
Logarithmic
Describe what the Axis of an HR Diagram looks like?
Y-AXIS: LEFT: Absolute Magnitude from +15 -> -15 at the top
RIGHT: Luminosity (Sun=1) from 0.01 -> 10000 at the top
X-AXIS: Temperature and Spectral class (can be on top and bottom)
From left to right: OBAFGKM, 30000,10000,5000,3000
Describe where the stages of the life cycle of a star are on the HR diagram?
GASEOUS CLOUD: 0 magnitude and lowest temperature
PROTOSTAR: more positive magnitude but still lowest temperature
MAIN SEQUENCE: Snake through the centre with the sun at roughly +6 absolute magnitude and 1 luminosity
GIANTS: Slightly concave blob above horizontal branch at roughly 0 absolute magnitude
SUPERGIANTS: Blob above giants
WHITE DWARFS: Slanted blob from +5 -> +15 absolute magnitude and O -> A spectral class
What is the first stage in a star life cycle? Explain what happens?
NEBULA: A big cloud of hydrogen and helium gas that begins to condense under gravitational forces of attraction
What is the second stage in a star life cycle? Explain what happens?
PROTOSTAR: The nebulae have fragments of varying masses that clump together under gravity. The irregular clumps rotate and gravity/conservation of angular momentum spins them inwards to form a denser centre -> a protostar. When the protostar gets hot enough it begins to fuse elements producing a strong stellar wind that blows away the surrounding material. NB: This protostar is surrounded by a disc.
What are the two types of hydrogen -> helium fusion?
- CNO cycle
- Proton - Proton chain
In which type of main sequence stars does the CNO cycle hydrogen -> helium fusion occur?
Stars which are more massive than the sun
In which type of main sequence stars does the proton - proton chain hydrogen -> helium fusion occur?
Stars which are as massive or less massive than our sun
What is the reason why the sun’s fusion is inefficient?
The sun is not hot enough/ does not have enough energy to overcome the strong nuclear force between nucleons that repels them. When fusion does occur it is due to them nucleons quantum tunnelling. As this is a low probability event the rate of fusion in the sun is low.
How does the time a star spends in the main sequence change with its mass?
MORE MASSIVE STARS
- Greater gravity inwards
- More efficient fusion as nucleons are more likely to quantum tunnel
- Greater power output
- Lifetime in the main sequence is less as fuel source outwards runs out quicker
LESS MASSIVE STARS
- Less gravity inwards
- Less efficient fusion so nucleons are less likely to quantum tunnel
- Weaker power output
- Lifetime in the main sequence is more as fuel source outwards lasts longer
What is the average length of time that a low mass star stays in the main sequence ?
10 billion years
What is the average length of time that a high mass star stays in the main sequence?
10 million years
What is the third stage in a stars life cycle? Explain what happens?
The inward force, due to gravity, and the outward force, due to energy released from fusion, are in equilibrium with one another. A star with a mass greater than 3 solar masses will only stay in the main sequence for around 10 million years. A star with a mass less than 3 solar masses will remain in the main sequence for much longer, around 10 billion years.
Describe how a main sequence star turns into a red giant?
- As the hydrogen in the core runs out, no more hydrogen fusion can occur and so the star begins to contract
- As the density of the star increases due to collapse the temperature inside the core of the star increases
- Helium fusion is able to occur which halts the collapse (Helium fusion requires extremely high temperatures so will only happen in more massive stars)
- The Helium fusion only lasts for about 1 billion years as it is a less efficient fuel
- As the core is much hotter due to this helium fusion, the outer layer of the star made of hydrogen also becomes hotter.
- This means the outer layer of the star is now able to undergo hydrogen -> helium fusion
- Due to the new outer layer of hydrogen fusion there is a massive increase in the energy output of the star
- The gravitational force inwards is dramatically overpowered by the two fusion processes outwards, hence the star expands to form a Red Giant which is cooler
What is a fusion chain?
In the death/collapse of a main sequence star a new heavier element being fused repeats again and again (fusion chain), depending on how massive a star is, until there is an iron-nickel core at the centre.
What is the highest mass element that stars that have a mass less than 3 solar masses will form?
Carbon
What masses of stars (core) will form white dwarfs?
M < 1.4M(sun)
What masses of stars (core) will form neutron stars?
1.4M(sun) < M < 3M(sun)
What masses of stars (core) will form black holes ?
M > 3M(sun)
What masses of stars (core) will go from a Red Giant -> final state via a supernova
M > 1.4M(sun)
What is the limit that prevents White Dwarfs forming from cores of stars with a M > 1.4M(sun)? Why?
The Chandresekhar Limit:
This is because for star masses that are greater than 1.4M(sun) the electron degeneracy pressure outwards fails and is overcome by the gravitational forces inwards -> A neutron star is formed instead
What is the limit that prevents Neutron Stars forming form cores of stars with M> 3M(sun)? Why?
The Oppenheimer - Volkoff Limit:
This is because for star masses that are greater than 3M(sun) the neutron degeneracy pressure outwards fails and is overcome by the gravitational forces inwards -> A Black Hole is formed
What happens during the death of a star with a M < 1.4M(sun)?
After the core has fused all of its carbon, the star will begin to contract under gravity. The energy emitted from the collapse will push off the outer layers of the star to form a PLANETARY NEBULA (made of all the different elements fused in the outer parts of the star). What is left is the core of the star, a white dwarf, a hot and ultra dense ball of carbon/oxygen, which is stable due to electron degeneracy pressure pushing outwards balancing gravity forcing the core inwards.
What Happens during the death of a star with a 1.4M(sun) < M < 3M(sun)
After the core has finished fusing its heaviest element depending on how massive it is, the star will begin to contract under gravity. Due to the gravitational force inwards being so great, electron degeneracy pressure fails which means the protons and electrons in the core left behind are combined to form neutrons -> NEUTRON STAR. The outer layers of the star hit the surface of the neutron star and rebound setting up huge shockwaves -> SUPERNOVA
What Happens during the death of a star with a M > 3M(sun)?
When the most massive stars collapse, the core contracts inwards very suddenly and becomes rigid. The outer layers of the star (that were fusing elements up to iron/nickel) fall inward and rebound of the core into space like a shockwave -> A SUPERNOVA. During this supernova elements heavier than iron are fused.
As these stars are so massive neutron degeneracy pressure outwards fails and a black hole forms.
What is the definition of a pulsar?
Rotating neutron stars that emit radio waves (these radio waves can be detected as very regular radio pulses from earth)
What are the two types of detectors used in telescopes you need to know?
- Eyes
- CCD
What are the 3 comparative points you need to talk about when comparing the 2 types of detectors used in telescopes?
- Quantum efficiency
- Resolution
- Convenience of use
Compare the quantum efficiency of the two types of detectors used in telescopes
You want the highest quantum efficiency as possible in the detector of your telescope.
Eyes have a quantum efficiency of 1- 4%
CCD’s have a quantum efficiency of 70 - 90%
Compare the resolution of the two types of detectors used in telescopes
Eyes has a resolution of 100 micrometres but this value varies
CCD’s have a resolution of 10 micrometres
Compare the convenience of use of the two types of detectors used in telescopes
Eyes have no convenience of use
CCD’s:
1. Number of images captured in a time period and exposure time can be easily adjusted
2. Information stored on a CCD can be accessed remotely
3. Generated images can be stored an analysed digitally
4. Can detect a larger range of wavelengths beyond the visible spectrum
Definition of resolution in CCD’s
The total number of pixels per unit area
What is the comparative point between the two types of detectors that will make the least impact on your image produced? Why?
Resolution of your detector - this is because normally the resolution of a telescope is limited by the diameter of the telescope
What are the 5 points you need to talk about when comparing radio telescopes and optical telescopes?
- Structure
- Positioning
- Uses
- Collecting Power
- Resolving Power
What is the definition of an optical telescope?
One which detects wavelengths of light from the visible part of the EM spectrum
Compare the structures of optical telescopes with radio telescopes
Similarities
- Both use parabolic surfaces to reflect waves
Differences
- Radio uses a single primary mirror, optical uses two mirrors
- Radio dish does not have to be as smooth as optical mirrors
Compare the positioning of optical telescopes with radio telescopes
Similarities
- Both can be ground based as the atmosphere is transparent to most radio and optical wavelengths
Differences
- Optical must be placed up high (to avoid atmospheric distortions) and away from cities (to avoid light pollution)
- Radio must be located remotely away (from other radio sources)
Compare the uses of optical telescopes with radio telescopes
Similarities
- Both are used to detect hydrogen emission lines
Differences
- Radio waves are not absorbed by dust whereas optical waves are so radio telescopes are used to map the Milky Way
Compare the collecting collecting power of optical telescopes with radio telescopes
Radio telescopes are larger in diameter, so they have a greater collecting power compared to optical telescopes. Therefore, radio telescopes are more likely to produce brighter images although radio sources tend to be weak.
Compare the resolving power of optical telescopes with radio telescopes
Radio waves are longer than optical waves so radio telescopes have a much lower resolving power (x 10^-3 rad). Therefore, optical telescopes are more likely to produce detailed images.
What are the 2 assumptions made for the inverse square law?
The source object is:
1. perfectly spherical
2. mass density is uniform throughout the object
What are three defining properties of neutron stars?
- Incredibly dense (made up of free neutrons) and small (normally 20km across)
- Rotate very fast (up to 600 times a second)
- Some neutron stars known as pulsars emit radio waves in two beams as they rotate
Definition of the Schwarzschild radius
The distance at which the escape velocity is the speed of light
What is the boundary of the Schwarzschild radius known as?
The event horizon
When do gamma ray burst occur?
During the collapse of red supergiants into neutron stars or black holes
Definition of escape velocity (black hole)
The velocity at which a moving object has just enough kinetic energy to overcome the black hole’s gravitational field
What lies at the centre of galaxies?
Supermassive black holes
What is the equation you need to use to calculate the Schwarzschild radius?
Rs ~2GM/c^2
What is the equation you need to use to calculate the radius of the event horizon?
Rs ~2GM/c^2
What type of supernova do you need to know about?
Type 1a supernovae
What is the defining characteristic of a supernova?
A rapid and massive increase in brightness
What is a light curve?
A plot of absolute magnitude, M, against time since the supernova began
What are the two defining features of type 1 supernovae light curves?
- A sharp initial peak
- Gradually decreasing curve
Why are type 1a supernovae very important?
As they always happen in the same way with a star of the same mass
What is the same for every type 1a supernova?
Their absolute magnitude/light curve
What type of supernova can be used as a standard candle?
Type 1a supernovae
Definition of a standard candle
Objects that have a known absolute magnitude e.g if you found a type 1a supernova within a galaxy you would be able to work out how far that galaxy was away from us
How does the energy output of a supernova compare to that of the total energy output of the sun?
The energy output of a supernova is roughly 10^44J which is roughly the same as the energy output of the sun over its entire lifetime
What do some supernovae release?
Gamma ray bursts
What would happen if a supernova occurred too close to the earth?
If the high energy gamma rays (only occur through the poles so this is very unlikely to happen) were directed towards us then the ozone layer could be destroyed leading to mass extinction
Were type 1a supernovae more or less bright than expected?
less bright
Is the expansion of the universe accelerating or decelerating?
Accelerating (not what Hubble predicted)
What is the theoretical proposal behind why the universe is accelerating?
Dark energy
Why is dark energy controversial?
There is evidence for its existence but no one knows what it is or what is causing it
How is dark energy described?
As having an overall repulsive effect throughout the universe. Dark energy is constant throughout the universe so has a greater effect than gravity causing expansion to be increasing.
