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Astrophysics > Telescopes And Instruments > Flashcards

Telescopes And Instruments Flashcards

(107 cards)

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1
Q

Key considerations for telescopes

A

-wavelength coverage
-sensitivity
-spectral resolution
-spatial resolution
-field of view
-photometric stability

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2
Q

Effective throughput is a combination of

A

-Atmospheric opacity
-optics throughput
-quantum efficiency

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3
Q

Shot noise

A

The random emission of photons from astrophysical sources

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4
Q

Signal of astrophysical source

A

Ie the number of photons collected within a given exposure time

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5
Q

Spectral resolution

A

Minimal spectral width that can be measured

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6
Q

Spatial resolution

A

The smallest size of a source or feature that can be measured at some given wavelength

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7
Q

Relevant length scales for spatial resolution(replace D in the equations with whats in brackets for each one respectively)

A

-diffraction limited imaging (mirror diameter
-seeing limited imaging (fried’s coherence length)
-interferometry (longest separation between antennae

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8
Q

Black body

A

An object that absorbs all light energy incident upon it and reradiates this energy with a characteristic spectrum. It reflects no light.

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9
Q

Synchrotron radiation

A

Relativistic charged particles (electrons) accelerated in a spiral path around a magnetic field.

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10
Q

Bremsstrahlung (braking or free-free) radiation

A

-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

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11
Q

Spectral lines (bound-bound radiation)

A

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.

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12
Q

Emission line spectra

A

Optically thin volume of gas with no background light

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13
Q

Absorption line spectra

A

Cold gas lies in front of a source of radiation at a higher temperature

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14
Q

Spectral lines have finite width given by:

A

-natural line width
-collision broadening
-doppler broadening

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15
Q

Circular velocity

A

The velocity of an object that is undergoing uniform circular motion

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16
Q

Escape velocity

A

This is the minimum speed needed for an object to escape from the gravitational influence of another body.

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17
Q

Comets

A

-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

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18
Q

Asteroids

A

-minor planets with large velocity
-often locked in resonance orbits, or avoiding resonances
-mostly located in asteroid belt between mars and jupiter

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19
Q

Kepler’s 1st law

A

Each planet moves in an ellipse with the sun at one focus

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20
Q

Kepler’s second law

A

The line connecting a planet and the sun sweeps out equal areas in equal times

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21
Q

Keplers third law

A

For all planets, the orbital period P squared divided by the semi-major axis a cubed is constant

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22
Q

Protoplanetary disks

A

-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

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23
Q

Formation of planetary systems size order

A

Dust (microns) - Pebbles/rocks (cm-m) - planetesimals (km) - protoplanets- Planets (10^3km)

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24
Q

Rocky planets/outer gas divide

A

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

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25
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
26
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
27
Albedo A
Fraction of incident sunlight reflected (1-A is absorbed)
28
Subsolar temperature
Appropriate for very slowly rotating planets and assumes that the absorbing area equals the emitting area (Tss)
29
Equilibrium temperature
Appropriate for planets with atmospheres or in rapid rotation.
30
Planet will lose atmosphere if
V_esc < 10 x V_rsm
31
Detection methods of exoplanets
-Radial velocity -Astrometric wobble -Transit -Direct imaging
32
Hot jupiters
Jupiter-mass exoplanets that are are at very small orbital disrances from their host stars
33
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
34
Star is defined by
-bound by self gravity -radiates energy that is primarily released by nuclear fusion reactions in the stellar inferior
35
Stellar birth
Before the interior is hot enough for significant fusion, gravitational potential energy is radiated as the radius of the protostar contracts.
36
Stellar death
Remnants of stars radiate stored thermal energy and slowly cool down
37
Star XYZ composition
X-hydrogen fraction Y-helium Z-metals
38
Parallax
The apparent stellar motion due to Earth’s orbit around the sun.
39
Bolometric
Integrated over all wavelengths
40
Absolute magnitude
Apparent magnitude a source would have if it were at a distance of 10pc. It is an intrinsic property of the source
41
Distance modulus
The difference between the apparent magnitude m and the absolute magnitude M
42
Constant in apparent magnitude equation
The ‘zero-point’, magnitude of a star that has a flux of 1ct/s
43
Two main ways to measure stellar mass of a star
-Stellar spectrum -Binary stars
44
Using Stellar spectrum to measure stellar mass
Certain details in the absorption spectrum of stars depend on surface density
45
Using binary stars to measure stellar mass
Use the motion of the stars to calculate their masses
46
Visual binary
We can resolve each of the stars in the binary individually
47
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
48
Spectroscopic binary
This is where we see periodic doppler shifts in the positions of spectral lines from both stars in the binary
49
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
50
Natural Broadening
Results from Heisenberg’s uncertainty principle
51
Doppler Broadening
The photon-emitting atoms have thermal motions covering a range of speeds and directions
52
Collisional/Pressure broadening
Other particles affect the photon-emitting atom-> increased uncertainty in photon energy, cause small changes in the atomic energy levels
53
The Hertzsprung Russell diagram
A plot of T against L (theorists) or colour index against absolute magnitude. (Observers
54
The main thing strip of the HR diagram represents
The main sequence
55
Origin of different lines in spectra
Which lines are present in the spectrum depends on the ionisation state if the stellar atmosphere
56
Origin of different lines strength in spectra
Determined by temperature more than composition
57
Origin of line width in spectra
Determined by density in the stellar atmosphere
58
Hipparcos HR diagram
Stars on Hr diageam with measured parallaxes (calibrated for the absolute magnitude)
59
Cycle of matter
During its life cycle a star expels gas and metals during late evolutionary stages, returning this material to the interstellar medium and providing a reservoir for future star formatiom
60
The interstellar medium consists of:
Ionised, atomic and molecular gas as well as dust
61
Interferometry
Using multiple telescopes to use the interference of waves to make very precise observations
62
Forces acting on a star forming unit
Gravity (collapses the cloud) competing against pressure, magnetic field and bulk motions (act against collapse)
63
Main sequence stars burn..
Hydrogen into helium in their cores.
64
The jeans mass
The minimum mass a cloud must have if gravity is to overwhelm pressure and initiate collapse (ignore magnetic fields and bulk motions)
65
Free fall time
The time it takes for a molecular cloud to collapse
66
Shell fusion/ shell burning
Core contracts and heats up once hydrogen stops burning and helium starts burning. H burning continues in a shell around the core, leading to the expansion of the star
67
Elements heavier than _are formed in supernovae explosions
Iron
68
SubGiant branch
H burning stops in core, but continues in shell around the core. Core contracts, heating layer of burning hydrogen that heats the surrounding envelope. Causes radius of the star to increase, the luminosity to increase and the temperature to decrease. The star has not yet begun He-burning
69
Red Giant Branch and Helium Flash
As core contracts, core temperature reaches 10^8k and He-burning ignites the core. This is a runaway process, resulting in helium flash
70
Horizontal Branch
He-burning in core continues for 100 Myr , forming a carbon core. Luminosity goes down compared to peak during the helium flash phase.
71
Asymptotic Giant Branch
Core He-burning stops but continues in shell around the core. Carbon core contracts and Similar to subgiant branch, as core contracts, radius and luminosity of star increases and temperature decreases. This is the second red giant phase
72
Planetary Nebula
After a further 1Myr core fusion ends. He-burning in shell continues, causing the star to become unstable and ejects outer layers via super winds. star will then evolve towards a white dwarf as it cools, contracts and luminosity decreases
73
Planetary nebulae
Core fusion ends in AGB star, thermal pulsations eject hot, ionised outer layers of gas-, hot central star remains as white dwarf
74
Initial mass of white dwarf
<8 solar masses
75
Initial mass for neutron star or black hole
>8 solar masses
76
White dwarfs
These are remnant cores of low-mass stars, supported against gravity by electron degeneracy pressure (Pauli exclusion principle). They cool off and grow dimmer with time.
77
White dwarfs with a mass similar to the sun are about the same size as
The earth (they are very dense)
78
Higher mass white dwarfs are
Smaller
79
Neutron stars
For cores 1.4 < 3 solar masses the electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. The degeneracy pressure of neutrons supports a neutron star against gravity
80
A neutron star is about the same size as
A small city (10km)
81
Stellar-mass black hole
If the core of a star that went supernova exceeds 3 solar masses, gravity wins. The gore becomes infinitely small.
82
To quantify the size of the black hole we define the
Event horizon (schwarzchild radius) by equating the escape velocity to the speed of light
83
Pulsar
Rapidly rotating neutron star
84
Disk
A flat rotating disk consisting of stars, gas and dust, with star formation mostly concentrated in spiral arms
85
Bulge
A central stellar bulge consisting of redder (more evolved) stars in a more spherical configuration)
86
Halo
A halo of stars and dark matter (non-baryonic material) that is more spherically distributed around the disk. Within the halo there are globular clusters
87
Globular clusters
are spherical structures tightly bound by gravity, consisting of around 10^5 low metallicity old stars
88
Late-type galaxies
Blue and star forming (hottest, short-lived, massive stars still alive)
89
Early-type galaxies
Red and evolved stellar populations (hottest stars have died)
90
Galaxy dynamical support
Motion of stars and gas within a galaxy supports it against gravity. Late-type: rotationally supported Early-type: dispersion supported
91
Stellar initial mass function (IMF)
Relative number of stars of each mass formed out of a unit gas
92
Baryon dominated galaxy disk; galaxy core:
If surface density is constant, velocity varies with root(r)
93
Baryon dominated galaxy disk; galaxy outskirts:
If surface density is constant, velocity varies with root(1/r)
94
Evidence of dark matter
Observed rotation velocity remains constant far outside the visible disk. Implies an additional dark matter component that is maintaining circular velocity high out to large radiu
95
Stellar feedback
Ways in which stars influence their surrounding environment (stellar winds, supernovae, etc..)
96
Active galactic nuclei (AGN)
Supermassive black hole at centre of galaxy that ejects large amounts of gas
97
Cepheid Variables 1st rung
Cepheid Variables are stars that pulsate with a period that is directly related to their luminosity
98
Brighter CVs pulsate more
Slowly
99
Supernovae 1a 2nd rung
Supernovae type 1a are energetic explosions caused by the accretion on a white dwarf in a binary star system. Peak luminosity is directly related to the shape of the light curve.
100
0 - 0.3 Myr
Universe ionised. Photons coupled to plasma, do not travel freely
101
= 0.3 Myr
Hydrogen turns neutral (recombination). Light travels freely, referred to as the surface of last scattering, which is mow observed as the cosmic microwave background
102
0.1 - 1 Gyr
First stars ad later galaxies form, ionising bubbles around them. Once ionised bubbles overlap, reionisation has completed
103
1- 13.8 Gyr
Galaxies evolve and enrich the universe with metals, and their distribution on the sky forms so called large scale structure
104
Stellar population models
Library of stellar evolutionary tracks and stellar spectra
105
Metallicity
A second order parameter affecting stellar evolution
106
Star formation history
Description of past episodes and duration if star formation
107
Dust
Amount , distribution composition and size will affect how much ligjt is absorbed at a given wavelength

Astrophysics (3 decks)

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