Astrophysics Flashcards
Age of universe
1/Ho
What is meant by the Hubble constant
Gives the ratio of the (recessional) velocity (of galaxies) to distance from earth.
Equation for quantum efficiency
(Photons detected by sensor/photons entering sensor) x 100
Spatial resolution
Min distance distinguishable on the sensor eg size of the each light sensing unit
What are CCDs
- increases amount of useful energy received by source
- have a linear response so very faint parts of a relatively bright image
CCD vs Eye
•QE- 1% at low light levels vs 80%
at low levels aperture size can increase (larger pupils) but true light adaption≈30mins. Loss of colour vision at low light levels as light detection taken over by rods not colour sensitive cones
•Spatial resolution- 100microm vs 10microm
• Resolution- CCD resolution related to size of each pixel- can be as small as 4x4 vs ≈2 for eye. Suggests similar resolution- but only simple analysis
•eyes require no extra equipment. CCDs produce digital images- easy to store. Allow for remote viewing, direct computer analysis, long exposure times
• eye- visible. CCD- IR, visible, UV
Refracting telescope- objective lens
Collects light from distant object and brings it to a focus in its focal plane.
Refracting telescope- eyepiece lens
Uses light to form a magnified image at infinity- achieved by placing lens so that it’s focal plane coincides with the focal plane of the objective.
Normal adjustment (refracting telescope)
Image is formed at infinite meaning the light rays leave the telescope parallel. This means the focal points of the 2 lenses coincide.
Angular magnification, M
=fo/fe
Ratio of the angle subtended by image at eye to the angle subtended by object at unaided eye
Angle subtended by an object
=h/d
Spherical aberration
- rays further from principal axis have focal point closer to mirror
- overcome by using parabolic mirror and mirror being very precise and smooth
Reflecting telescope- Cassegrain arrangement
- primary reflecting mirror- parabolic- collect light and brings it to a focus
- secondary convex reflector reflects light out through hole in primary parabolic mirror- light enters eyepiece
Reflectors vs refractors
- best- reflectors- larger. Large lens likely to break due to own weight- larger diameter means….
- mirror can be supported from behind vs lens supported at edge
- lenses suffer from chromatic aberration, b,r. Images have multi-coloured blurred edges
- reflectors- both secondary mirror and framework holding it in place diffract light as it passes= poorer quality image. And some refraction and chromatic aberration in eyepiece used to view final image
Other telescopes
- parabolic wire mesh dish due to the length of radio waves (light). Ground
- IR- parabolic mirror. High altitude/space
- optical- parabolic mirror. Space
- X-ray- grazing mirror. Space
Advantage of larger diameter telescope
- better collecting power- brighter images
* better resolving power- clearer images
Supernova
A star with a rapid increase in absolute magnitude
Low mass star (eg sun)
Becomes a red giant then the core contracts to become a white dwarf
High mass star
Core has a large mass so continues to contract past white dwarf density
Neutron star
- collapsed core of a high mass star
* so dense that gravitation causes electrons & protons to combine to form neutrons
Pulsar
•if oriented correctly neutron stars rotate rapidly. Emit 2 beams of radio waves. If these sweep past earth we detect them as pulses
Black holes
A region of space with a g-field so strong that no matter or radiation can escape.
Event horizon
Boundary of region at which the escape velocity = c
What does λmax represent in Wien’s law
The wavelength at which max intensity occurs
Doppler effect
Occurs due to apparent change in wavelength/frequency between a source and observer due to their relative motion (blue and red shift)
Properties of a quasar (give rise to controversies)
- very large power output
- very high luminosity (much brighter than a star)—> indicates by the fact it has stronger than expected radio emissions + a large red shift
- very large red shift (very far away)
- star like, small- very compact
What is a quasar
Very luminous objects at immense distances. They appear to light telescopes as stars but aren’t typical:
- extremely bright
- spectra show lines that correspond to no known elements however the lines were considerably red shifted
- some are intense radio sources
- small (smaller than a galaxy)
What is meant by red shift
Increase in wavelength of em radiation due to relative recessive velocity between observer and source
Why can type 1a supernovae be used as standard candles to determine distances
All type 1a supernovae have the same peak absolute magnitude
Apparent magnitude can be measured
Use of m-M = 5log(d/10) or inverse square law?
What is an event horizon of a black hole
Boundary where escape velocity = c
What’s the defining property of a black hole
Object whose escape velocity is greater than the speed of light
Doppler effect
Occurs due to the apparent change in wavelength between a source and an observer due to their relative motion
(Objects moving towards have + speed, away have - speed)
What’s the principal advantage of reflecting telescopes
- mirrors do not suffer from chromatic aberration
- mirrors can be larger—> greater collecting powers
- mirrors can be larger—> smaller minimum angular resolution
- parabolic mirrors with axial rays do not suffer from spherical aberration
Defining characteristics of neutron star
- extremely dense
- made up of neutrons
Parsec definition
1 parsec is defined as the distance to a star which subtends an angle of 1 arc second to the line from the centre of the earth to the centre of the sub
What is 1 arc second equal to
1degree/3600
Principle focus
Converging lens- point where incident beams passing parallel to principal axis converge
Diverging lens- the point from which the light rays appear to come from. (Same distance either side of lens)
Focal length
Distance between centre of lens and the principle focus
Shorter focal length = stronger lens
Real image
Formed when light rays cross after refraction
Virtual image
Formed in the same side of the lens. The light rays don’t cross so a virtual image can’t be formed on a screen
Power of a lens
A measure of how closely a lens can focus a beam that is parallel to the principal axis (ie how short the focal length is)
+ve for converging and -ve for diverging
P= 1/f
Refracting telescope- objective lens
Collects light and creates a real image of very distant object
- long focal length
- large- college as much light as possible
Refracting telescope- eyepiece lens
Magnifies the image produced by objective lens so observer can see. Produces virtual image at infinity as light rays parallel. (Reduces eye strain as don’t have to refocus when looking between telescope and sky)
Normal adjustment for refracting telescope
When distance between Obj and eye lens is sum of focal lengths (principal focus for the lens in same place)
Which spectral class has stars that have the most prominent Balmer lines
A
What’s the assumption when using Hubble’s law to estimate the age of the universe
That the universe is expanding at a constant rate
Intrinsic colour for spectral classes
O- blue B- blue A- blue-white F- white G- yellow-white K- orange M- red
Why is resolving power of a single dish radio telescope much less than that of a normal optical telescope
Wavelength of radio waves»_space; wavelength of visible light. Minimum angle of resolution proportional to wavelength
