Astrophysics Flashcards
What is the doppler effect?
The apparent change in wavelength of a wave as the source moves relative to the observer. Source moving away(longer wavelength) = Red shift, Source moving towards(shorter wavelength) = Blue shift
What are the 2 methods to detect exoplanets?
Doppler Shift/Radial velocity method and The transit method
What is the scale of the Hipparcos Scale?
- Brightest stars have a apparent magnitude of 1
- Dimmest objects the naked eye can detect have a magnitude of 6
- Intensity of apparent magnitude 1 is 100x intensity of magnitude 6
- Decrease of 1 on scale translates to 2.51x increase in intensity
What is the inverse square law?
Intensity = Power/4πd² , d = distance from the sun in metres
Assume star is spherical and power is being emitted in all directions
What is a black body?
A perfect emitter and absorber of all possible wavelength of radiation
What are the characteristics of all the spectral classes?
O - Blue - 50,000K to 25,000K - He+ and He absorption lines
B - Blue - 25,000K to 11,000K - He and H absorption lines
A - Blue white - 11,000K to 7500K - H and some metal ion absorption lines
F - White - 7500K to 6000K - Metal ion absorption lines
G - Yellow white 6000K to 5000K - Metal ion and metal atom absorption lines
K - Orange - 5000K to 3500K - neutral metal atoms absorption lines
M - Red - <3500K - neutral atoms absorption lines and absorption lines for compounds like TiO
What is the cosmological principle?
On a large scale, the Universe is:
- Homogeneous (Every part is the same as every other part)
- Isotropic (Everything looks the same in every direction, so there is no centre)
What is dark energy?
The rate of expansion of the universe should be decreasing as all the mass in the universe is attracted together by gravity, however the rate of expansion is increasing. Dark energy is a theorised type of energy which fills the whole of space which allows for this
What are Quasars?
Active galactic nuclei - supermassive black holes surrounded by a disc of matter, which cause the quasar to emit radiation from the poles when absorbed. They display very large red shifts, meaning they are very far away and their power output is around that of an entire galaxy.
What is the Raleigh Criterion?
Two objects will not be resolved if any part of the central maximum of either part of the images falls within the first minimum diffraction ring of the other.
What is Stefan’s law?
Power output of a star is directly proportional to its surface area and absolute temperature to the 4th power
What is spherical abberation?
An effect caused by the curvature of a lens or mirror which leads to there being no focal point for the light rays and the image becomes blurry or distorted.
What is Doppler shift/ radial velocity method?
Observation of the host star’s line spectrum. Red shifted when moving away from Earth, blue shifted when moving towards Earth. Suggests star is ‘wobbling’ so there must be an exoplanet exerting a gravitational force on it.
What is the transit method?
Observation of the host star’s light intensity. If an exoplanet crosses in front of it, the intensity decreases. Info on timings and other data can be used to figure out speed and size of exoplanet.
What factor of a telescope leads to a brighter image?
Collecting power
What telescope is affected by chromatic aberration?
Refracting telescopes
What telescope is affected by spherical aberration?
Reflecting and refracting telescopes
What is parallax?
The apparent change of position of a nearer star in comparison to distant stars in the background. Due to Earth’s orbit of the sun.
Define the parsec
The distance at which the angle of parallax is 1 arcsecond(1/3600 degrees).
What is the equation to measure distances of nearby stars?
Distance to the star(pc) = Radius of the Earth’s orbit / angle of parallax(radians)
What is the equation to measure an objects’ distance from Earth?
half the angle subtended(radians) = radius of the object / distance to the object
What is the relationship between collecting power and aperture diameter?
Collecting power of a telescope is proportional to the square of its aperture diameter
What is a “real image”?
An image which can be projected onto a screen
What is a CCD?
A charge coupled device is an array of light sensitive pixels that become charged when exposed to light for a pre-set time
Describe how a CCD works
- CCD is exposed to light for a pre-set time, charging the array of light sensitive pixels
- Charge is transferred to a capacitor, where the voltage across it is read
- Each p.d value is proportional to energy of incident light which is used to create an image
Define quantum efficiency
The proportion of incident photons falling on a light detector that are detected
What is distinguished about the image when the object is further than 2f from the lens?
- Inverted image
- Real image
- Diminished image
What is distinguished about the image when the object is 2f from the lens?
- Inverted image
- Real image
- Same size image
What is distinguished about the image when the object is located between 2f and f from the lens?
- Inverted image
- Real image
- Magnified image
What is distinguished about the image when the object is f from the lens?
- Image formed is at infinity
- No image is formed
What is distinguished about the image when the object is closer than f from the lens?
- Upright image
- Virtual image
- Magnified image
What is normal adjustment?
Where the focus of the objective lens and the focus of the eyepiece lens are at exactly the same point
What is chromatic aberration?
A problem caused by lenses refracting different colours of light with different speeds, resulting in the image for each colour being in different positions
How does a refracting telescopes work?
Refracting telescopes use lenses to focus visible or near-visible starlight
What are the 3 components a refracting telescopes consist of?
- A tube with two lenses
- An objective lens
- An eyepiece lens
What is true about an objective lens in a refracting telescope?
The objective lens is always a converging lens
What is true about an eyepiece lens in a refracting telescope?
The eyepiece lens can be converging or diverging
When is an eyepiece lens converging?
For a Keplerian telescope
When is an eyepiece lens diverging?
For a Galilean telescope
How does a converging lens work?
- By making parallel rays converge to a focus or focal point
- If the parallel rays come from an angle, then the image is formed in the focal plane
What is the focal plane?
The plane perpendicular to the principal axis of the lens, on which the principal focus lies
What is the focal length?
The perpendicular distance between the lens axis and its focal plane
What is the principal axis?
An axis of a lens that passes through its centre, perpendicular to its surface on both sides
What is the principal focus?
The point where incident rays parallel to the principal axis of the lens/mirror converge
What is the quantum efficiency of the human eye?
1%
What is the quantum efficiency of a CCD?
80%
What is a reflecting telescope?
A telescope which uses mirrors to reflect and focus light
How does a reflecting telescope work?
A parabolic concave primary mirror converges axial rays from an object at its principle focus, forming a real image
How can an image be magnified?
By passing the image through an eyepiece lens
What light can a CCD detect?
- Visible light
- UV light
- Infrared
What is the equation for quantum efficiency?
Quantum efficiency = conducting electrons released per second/ incident photons per second
Define apparent magnitude(m)
A measure of brightness of an object as observed from Earth
Define absolute magnitude(M)
What an object’s apparent magnitude would be if it were 10 parsecs away from Earth
What is a lightyear?
The distance light travels through space in one year
What is an astronomical unit(AU)?
1AU is the average distance between the Earth and the Sun
Star X has apparent magnitude 0.12, Star Y has apparent magnitude 0.42. Which star is brighter in the sky?
Star X as the smaller the apparent magnitude, the brighter the star
What is another way to describe absolute magnitude?
The intrinsic brightness of a star
State Wien’s Law
The peak wavelength of emitted radiation is inversely proportional to the absolute temperature of the object
What is the peak wavelength for black body radiation?
The wavelength of light released at a maximum intensity
What are standard candles?
Objects with a known peak luminosity, i.e Type 1a supernovae
What is the Balmer series?
The absorption lines in an absorption line spectrum caused by the electrons in atomic hydrogen moving between first excitation level (n=2) and higher levels
Define solar mass
The mass of an object relative to the mass of the sun
What is the event horizon?
The boundary of the region around an object in which its escape velocity is greater than the speed of light
Define escape velocity
The minimum speed required for an unpowered object to leave the gravitational field of another object
What is a supermassive black hole?
A black hole 10⁶ more massive than the size of the Sun
Define Schwarzschild radius(Rs)
The radius of the event horizon of a black hole
Derive the Schwarzschild radius equation
- V = -GM/r and ΔW = mΔV
- GPE = -GMm/r
- KE = 1/2mv² and KE needed to escape = GPE needed to be gained
- 1/2mv² = -GMm/r
- r = 2GM/v² , Schwarzschild radius is distance escape velocity = speed of light
- Rs = 2GM/c²
State the life cycle of a star less than 3 solar masses
- Nebula
- Protostar
- Main sequence star
- Red giant
- White dwarf
State the life cycle of a main sequence star greater than 3 solar masses, with a core between 1.4 and 3 solar masses
- Nebula
- Protostar
- Main sequence star
- Red supergiant
- Supernova
- Neutron star
State the life cycle of a star greater than 3 solar masses, with a core greater than than 3 solar masses
- Nebula
- Protostar
- Main sequence star
- Red supergiant
- Supernova
- Black hole
Describe what happens during the protostar stage of a star’s evolution
- Cloud of dust and gas(nebula) clump together mass under gravity
- The irregular clumps rotate and gravity spins them inward to form a dense centre, a protostar
Describe what happens during the main sequence star stage of a star’s evolution
- The attractive force of gravity and repulsive force due to fusion are in equilibrium, the star is stable
- Hydrogen nuclei are fused into helium
- The greater the mass of the star, the shorter the main sequence stage as uses it fuel faster
Describe what happens during the red giant stage of a star’s evolution
- When the star is less than 3 solar masses, it evolves into a red giant
- When hydrogen runs out, the core’s temperature increases and begins fusing helium nuclei into heavier elements (carbon, oxygen)
- The outer layers of the star expand and cool
- Similar to process of red supergiant but on a smaller scale
Describe what happens during the red supergiant stage of a star’s evolution
- When the star is greater than 3 solar masses, it evolves into a red supergiant
- When hydrogen runs out, the core’s temperature increases and begins fusing helium nuclei into heavier element (carbon, oxygen)
- The outer layers of the star expand and cool
- Similar to process of red giant but on a larger scale
Describe what happens during the white dwarf stage of a star’s evolution
- When a red giant has used all its fuel, fusion stops and the core contracts as gravity > attractive force of fusion
- The star pulsates and ejects its outer layers into space as planetary nebula
- Core is now hot and very dense
- The white dwarf will eventually cool to a black dwarf
Describe what happens during the supernova stage of a star’s evolution
- When a red supergiant has used all its fuel, fusion stops and the core rapidly collapses inwards and becomes rigid
- Outer layers than fall inward and rebound off the core in a shockwave, leading to heavy metal elements being fused and sent into space.
- Remaining core depends on the mass of the star
Describe what happens during the neutron star stage of a star’s evolution
- Core of massive star contracts and the electrons in the core material get squashed onto the atomic nuclei, combining with protons to form neutrons and neutrinos
- For a star core between 1.4 and 3 solar masses, this is the limit of contraction and the core collapses to become a neutron star, mostly made of neutrons
Describe what happens during the black hole stage of a star’s evolution
- For a star core greater than 3 solar masses, the electrons in the core material contract onto the atomic nuclei, combining with protons until neutrons and neutrinos are formed
- But neutrons can’t withstand the greater gravitational force, making the core collapse into an infinitely dense point
What are pulsars?
An object which emits radio waves from their poles, i.e quasars and some neutron stars
What is a black hole?
An object whose escape velocity is greater than the speed of light
Which spectral class has the most prominent Hydrogen Balmer absorption lines?
Spectral Class A
