Telescopes And Instruments Flashcards
Key considerations for telescopes
-wavelength coverage
-sensitivity
-spectral resolution
-spatial resolution
-field of view
-photometric stability
Effective throughput is a combination of
-Atmospheric opacity
-optics throughput
-quantum efficiency
Shot noise
The random emission of photons from astrophysical sources
Signal of astrophysical source
Ie the number of photons collected within a given exposure time
Spectral resolution
Minimal spectral width that can be measured
Spatial resolution
The smallest size of a source or feature that can be measured at some given wavelength
Relevant length scales for spatial resolution(replace D in the equations with whats in brackets for each one respectively)
-diffraction limited imaging (mirror diameter
-seeing limited imaging (fried’s coherence length)
-interferometry (longest separation between antennae
Black body
An object that absorbs all light energy incident upon it and reradiates this energy with a characteristic spectrum. It reflects no light.
Synchrotron radiation
Relativistic charged particles (electrons) accelerated in a spiral path around a magnetic field.
Bremsstrahlung (braking or free-free) radiation
-electrons in a plasma are accelerated when feel the Coulomb field of an ion
-at these temperatures, atomic processes become a less important coolant, and spectrum is a continuum
Spectral lines (bound-bound radiation)
Radiation can be emitted or absorbed when electrons make transitions between different states. Electrons can be either excited or relaxed, causing them to move between two bound states in an atom or ion. A photon is then emitted or absorbed at a discrete energy.
Emission line spectra
Optically thin volume of gas with no background light
Absorption line spectra
Cold gas lies in front of a source of radiation at a higher temperature
Spectral lines have finite width given by:
-natural line width
-collision broadening
-doppler broadening
Circular velocity
The velocity of an object that is undergoing uniform circular motion
Escape velocity
This is the minimum speed needed for an object to escape from the gravitational influence of another body.
Comets
-primordial remnants from the early solar system
-dirty snowball (ice and dust)
-volatiles vaporise and carry dust
-gas more affected by solar wind than dust
-very eccentric orbits
Asteroids
-minor planets with large velocity
-often locked in resonance orbits, or avoiding resonances
-mostly located in asteroid belt between mars and jupiter
Kepler’s 1st law
Each planet moves in an ellipse with the sun at one focus
Kepler’s second law
The line connecting a planet and the sun sweeps out equal areas in equal times
Keplers third law
For all planets, the orbital period P squared divided by the semi-major axis a cubed is constant
Protoplanetary disks
-Made of gas and dust.
-particles initially collide and stick together through electrostatic forces-dissipate energy of relative velocity on impact
-they later become large enough that their own gravity attracts other bodies
Formation of planetary systems
Dust (microns) - Pebbles/rocks (cm-m) - planetesimals (km) - Planets (10^3km)
Rocky planets/outer gas divide
Our solar system is made up of inner rocky planets, but gas giants further out, understood to be a result of a temperature gradient in the protoplanetary disk
The snow line
This is defined to be the distance from the sun where the protoplanetary disk has temperature T=273K, beyond which ice can form
Formation of rock and gas giant planets
-surface density of planetesimals was larger beyond snow line allowing for more rapid formation of planets, leading to outer planets to catch dust as well as gas
-As sun heated up and radiation field increased, gas protoplanetary disk blew out the gas
-in inner solar system process of planet formation was too slow for planets to capture gas prior to it being evaporated by the Sun
-All orbits near circular since they formed a protoplanetary disk
Albedo A
Fraction of incident sunlight reflected (1-A is absorbed)
Subsolar temperature
Appropriate for very slowly rotating planets and assumes that the absorbing area equals the emitting area (Tss)
Equilibrium temperature
Appropriate for planets with atmospheres or in rapid rotation.
Planet will lose atmosphere if
V_esc < 10 x V_rsm
Detection methods of exoplanets
-Radial velocity
-Astrometric wobble
-Transit
-Direct imaging
Hot jupiters
Jupiter-mass exoplanets that are are at very small orbital disrances from their host stars
Migration scenario
A model in which giant planets form at large radii, loose energy and angular momentum through interaction with disk, and migrate to orbits closer to the star
Star is defined by
-bound by self gravity
-radiates energy that is primarily released by nuclear fusion reactions in the stellar inferior
Stellar birth
Before the interior is hot enough for significant fusion, gravitational potential energy is radiated as the radius of the protostar contracts.
Stellar death
Remnants of stars radiate stored thermal energy and slowly cool down
Star XYZ composition
X-hydrogen fraction
Y-helium
Z-metals
Parallax
The apparent stellar motion due to Earth’s orbit around the sun.
Bolometric
Integrated over all wavelengths
Absolute magnitude
Apparent magnitude a source would have if it were at a distance of 10pc. It is an intrinsic property of the source
Distance modulus
The difference between the apparent magnitude m and the absolute magnitude M
Constant in apparent magnitude equation
The ‘zero-point’, magnitude of a star that has a flux of 1ct/s
Two main ways to measure stellar mass of a star
-Stellar spectrum
-Binary stars
Using Stellar spectrum to measure stellar mass
Certain details in the absorption spectrum of stars depend on surface density
Using binary stars to measure stellar mass
Use the motion of the stars to calculate their masses
Visual binary
We can resolve each of the stars in the binary individually
Eclipsing binary
The line of sight to the observer lies in the orbital plane such that the forground star blocks out light from the background star as they orbit each other
Spectroscopic binary
This is where we see periodic doppler shifts in the positions of spectral lines from both stars in the binary
Roche limit
The distance at which a satellite of density p_m held together by self gravitation is torn apart by tidal forces from the primary with size R_M and density P_M
