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astrophysics > Astrophysics MIDTERM > Flashcards

Astrophysics MIDTERM Flashcards

1
Q

An excited electron will drop to a lower energy state by…

A

Emitting a specific photon.

We can observe this on emission spectra!

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2
Q

Ground State & Absorption/Excitation

A

Ground State: Lowest allowed energy (level) of that atom.

Absorption: Atoms absorb a photon with the right energy- its electron jumps to a higher energy level/orbital.

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3
Q

If atoms only emit fixed wavelengths of light, how do hot solids give blackbodies?

A

In hot solids, atoms are packed close together and MOVING. Electrons don’t like being in the same state as their neighbours.

The HIGH PRESSURE causes random shifts in energy levels (and photon energies).

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4
Q

Why are some emission lines thicker than others?

A

HIGHER PRESSURE gives a “thicker” emission line.

That’s why solids, liquids and dense gases show a very continues emission spectra of ALL RAINBOW (little black/energies non-emitted).

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4
Q

Blackbody Diagram

A

A graph showing how much of each wavelength of light an object produces/emits.

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5
Q

Energy states in atoms are…

A

QUANTIZED- restricted to certain values! Can’t be in-between these levels!

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6
Q

What happens when a light source you see moves?

A

The frequency of the light we see is altered- Doppler Shift.

Moving away = Stretched out wavelengths/lower frequency = Appears red/redshifted

Moving towards = Squished wavelengths/higher frequency = Appears blue/blueshifted

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7
Q

What do we use Doppler Shift for?

A
  1. Finding exoplanets
  2. Monitoring galaxy rotation
  3. Monitoring gas motions/flows in clouds
  4. Finding dark matter!
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8
Q

What’s the problem with Doppler Shift? What else do we need to account for?

A

The Doppler Shift only tells us how things are moving along OUR line of sight.

For proper motion (motion of cloud in the SKY’S plane), we need to account for parallax.

Doppler Shift + proper motion = TRUE “SPACE MOTION”

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9
Q

Old vs New Telescopes

A

Old: REFRACTED light/telescopes

New: REFLECTED light/telescopes

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10
Q

Index of refraction varies with ____________.

A

WAVELENGTH
(gives a different “bending angle”)

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11
Q

Celestial objects are so far away their light beams can be viewed as…

A

PARALLEL LINES / PLANE WAVES

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12
Q

Focal Length

A

The distance between the lens (or mirror) axis and focus point (where light rays meet at a single point).

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13
Q

Convex/Converging vs Concave/Diverging Lenses

A

Convex/Converging Lens: Light converges to a point behind the lens (+ focal length).

Concave/Diverging Lens: Light diverges from a point in front of the lens (- focal length).

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14
Q

Converging vs Diverging Mirros

A

Converging Mirror: Light converges to a point in front of the mirror (+ focal length).

Diverging Mirror: Light converges to a point behind the mirror (- focal length).

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15
Q

What is the main component of a refracting telescope vs reflecting telescope?

A

Refracting Telescope = Convex lens

Reflecting Telescope = Concave mirror

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16
Q

2 Main Criteria for a Telescope

A
  1. Light Gathering Power: Want to collect as much light as possible (because celestial objects are so faint as is).
  2. Resolving Power: Want to detect to closely spaced objects very well (distinguish between one big clump an two little objects).
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17
Q

Why are telescopes like giant eyeballs?

A

Because the bigger the eye, the more light you collect.

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18
Q

What are two things you can do to increase telescope sensitivity/light gathering power?

A
  1. Make a bigger telescope
  2. Longer exposure -> more time to collect (more) light
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19
Q

Resolution Limit

A

The smallest angular separation that can be measured by the telescope.

(Smaller angle is better/a sharper image)

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20
Q

Diffraction

A

The interference of advancing wavefronts (waves meeting at an angle).

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21
Q

Resolved vs Unresolved Resolution Limit

A

Resolution Limit ANGLE…

Objects separated by angle > limit ANGLE = RESOLVED (two distinct sources)

Objects separated by angle < limit ANGLE = UNRESOLVED (one clump of light)

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22
Q

What is telescope aperture and what does increasing it do?

A

Aperture: Diameter of disk used.

Larger telescope give us:
1. More light gathering power
2. Better resolution

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23
Q

Why do we frown at Earth’s atmosphere?

A

It blurs light to a few arcsec for ground-based telescopes–> increasing telescope size does not fix resolution on ground.

We have to put our telescopes in space or current the blurring with adaptive optics.

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24
Q

What is chromatic aberration and what can fix it?

A

Common optic phenomenon when a lens cannot bring all wavelengths of light to a single point, so we see rings of colours (red focus point different than blue focus point).

It can be fixed by increasing the focal length– achromatic lenses & reflector telescopes!.

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25
Q

Problems with Refracting Telescopes

A
  • Infared and UV light can be absorbed by the refracting glass
  • Some light is reflected (low efficiency)
  • Large lenses cannot be supported
  • Chromatic aberration
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26
Q

What is the Event Horizon Telescope?

A

A network of telescopes all over Earth; synthesizing a telescope THE SIZE OF EARTH!

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27
Q

What do adaptive optics do (in relation to the Rayleigh Limit)?

A

Adaptive optics distort the mirror shape to match the shape of the incoming wavefront to correct atmospheric blurring.

Adaptive optics get the telescope to the Rayleigh Limit- smallest possible angle between two sources/optimal resolution.

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28
Q

Why are telescopes usually built away from cities and on mountains?

A
  • No light pollution from city (darker skies)
  • Thinner atmosphere (better “seeing”)
  • Drier conditions
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29
Q

All sorts of stellar properties, ie. distance, mass, surface temp, radius, rotation, etc. can tell us…

A

How stars form, how they live, and how they end.

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30
Q

Where did the first stellar classifications (still used today) come from?

A

Women “computers” from Harvard cataloguing stars with large collections of plates and pattern identifications (real tiresome work!!)

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31
Q

Spectral Line Classifications

A

Is based of emission line strength.

OBAFGKM (LTY) +subgroups 0-9
Obey Bees and Find Good Killing Machines! (Like Trees, Yo)

LTY are brown dwarfs/failed stars.

32
Q

What do OBAFGKM actually mean?

A

Grouped by similar line strengths.
O = Ionized helium
B = Neutral helium
A = In-between~~
F = Hydrogen
G = Ionized metals
K = Neutral metals
M = Metal oxides

33
Q

The spectral classes (horrible mnemonic) are indicative of…

A

TEMPERATURE and COLOUR

34
Q

HR Diagrams

A

See exam theory doc

35
Q

Larger stars have _________________ atmospheres which leads to ___________ pressure which leads to _____________ spectral line width. And vice versa.

A

Larger stars = thinner atmospheres = lower pressure = thinner spectral lines

Smaller stars = thicker atmospheres = higher pressure = thicker spectral lines

36
Q

Yerkes Luminosity Classification

A

Spectral + Luminosity Classification = unique HR diagram location

Spectral = O blue to M red
Luminosity = VII white dwarf to Ia bright supergiant

37
Q

Week 1

A
38
Q

Right Ascension vs Declination

A

RA = Longitude = Banana = TIME

DEC = Latitude = Pancake = ANGLE

Stars have fixed positions, but coordinates rotate with respect to us along our rotation axis.

39
Q

Earth rotates once every _______ hours. It rotates ______ degrees each hour.

A

1 rotation every 24 hours

15 degrees every hour

40
Q

How long is a sidereal day?

A

A little less than 24 hours (4 minutes shorter)

41
Q

What are the units of RA? Of DEC?

A

RA = hours:minutes:seconds

DEC = degrees:arcminutes:arcseconds

42
Q

Geocentric vs Heliocentric Model

A

Geocentric: Earth-centered, complex model due to retrograde motion.

Heliocetric: Sun-centered, became much simpler!
- Became accepted with the discovery of parallax!

43
Q

What is parallax?

A

The apparent shift of a nearby star’s position due to Earth’s orbit.

44
Q

If we were to establish an observatory on Mars (orbit is 50% larger than Earth), how would the parallactic angles of stars differ from Earth?

A

They would be smaller!

45
Q

Kepler’s 3 Laws

A
  1. Planets orbit the sun in an ellipse with the Sun at one of the foci.
  2. The vector of the Sun sweeps out an equal area in equal time. Planets move fastest at perihelion (closest to sun) and slowest at aphelion (farthest from sun).
  3. The square of the orbital period is proportional to the cibe of the semi-major axis.
46
Q

Why is light important to astrophysicists?

A

Objects are so far away in space we have to use light (radiation) emitted by these objects to study the universe (exceptions: meteorites, satellite missions, cosmic rays).

47
Q

How does light behave?

A

A. As a photon; a particle
B. As a propagating wave; energy

48
Q

Intrinsic vs Apparent Meaning

A

Intrinsic: The true property –> what we want

Apparent: What we observe from Earth –> what we have

49
Q

Luminosity vs Flux

A

Luminosity: The true brightness (total energy output)

Flux: The apparent brightness (energy received)

50
Q

You see two stars in the sky, but one appears to be four times brighter than the other. What can you conclude about these stars?

A

We don’t have enough info to compare the distances. (We need luminosity- TRUE brightness, not flux).

51
Q

Things that are warm emit ____________. The type of this depends on ______________. The colour depends on ____________.

A

Things that are warm emit radiation.

The type of radiation depends on temperature.

The colour depends on energy.

52
Q

An ideal object will… And what is this called?

A
  1. Abosrb all light
  2. Emit energy based on temperature

A perfect absorber of radiation is a “BLACKBODY”!

53
Q

What is blackbody radiation?

A

The light emitted by a blackbody of a certain temperature.

Stars emit light very close to a blackbody.

54
Q

What is the most perfect blackbody ever measured?

A

Cosmic microwave background radiation!

55
Q

All blackbodies with the same temperature…

A

Radiate with the same pattern.

56
Q

Wein’s vs Stefan’s Law

A

Wein’s Law: The peak of a blackbody curve shifts towards shorter wavelengths with higher temperatures.

Stefan’s Law: The total energy emited increases with temperature.

57
Q

What are the 2 key conventions of the magnitude scale?

A
  1. Lower magnitudes = brighter
  2. Change of 5 mag = factor of 100 flux difference
58
Q

Astronomers infer the brightness of a star (or object in space) using what?

A

Colour filters! They collect light over a specific range of wavelengths- and we see how much of each “colour” (wavelength) they display.

59
Q

What do Kirchhoff’s 3 laws imply? What are they?

A

They imply the way light and matter interact.

  1. Hot dense objects = continuous spectra
  2. Cool diffuse gas in front of a hot source = absorption spectra
  3. Diffuse gas against a dark background = emission spectra
60
Q

What does a blackbody spectrum vs spectral lines give us?

A

Blackbody spectrum = temperature

Spectral lines = composition

61
Q

What did Rutherford’s experiment show?

A
  1. Atoms are mostly empty space
  2. Atoms have a central “core” that deflects positive charges
62
Q

Describe the structure of an atom & the forces that sustain them.

A

Nucleus: Protons (+) and Neutrons (0) which are most of the mass, binded by STRONG NUCLEAR FORCE (that wins over electron repulsion)

Electrons (-) surrounding nucleus in clouds of probability due to ELECTRIC FORCE (opposite charges attract –> e- and p+, like charges repel –> e- and e-, p+ and p+)

63
Q

Number of protons determine…

Number of neutrons determine…

Number of electrons determine…

A

Number of protons determine ELEMENTS

Number of neutrons determine ISOTOPES

Number of electrons determine CHEMICAL PROPERTIES

64
Q

What is a neutral atom?

A

of e- = #of p+

No net charge

65
Q

Why do atoms have specific wavelengths that they can absorb or emit?

A

Electrons (kind of) move in distinct orbitals. They move between orbitals by gaining or losing fixed amounts of energy. The spectral lines we see are electrons changing from one QUANTUM STATE 9energy level) to another.

66
Q

How do rotation and magnetic fields affect spectral lines?

A

Rotation: It broadens them through Doppler Shift. The quicker it rotates, the broader the line will be.

Magnetic Field: Can cause energy levels to split (multiple lines), and strong magnetic fields can broaden lines unless there’s high spectral resolution.

67
Q

How old is the Sun?

A

4.57 Gyr

68
Q

What keeps the Sun shining?

A

Nuclear Fusion; hydrogen fusion into helium

E=mc^2
A small change in mass –> change in energy.

69
Q

Fusion vs Fission

A

Fusion: Smaller nuclei fuse to form a larger nucleus –> exothermic/energy released.

Fission: A larger nucleus splits into smaller pieces –> endothermic/energy absorbed.

70
Q

Explain Binding Energy

A

The work required to disassemble a nucleus (what holds a nucleus together).

Nuclei have less mass than the sum of their p+/n0 masses –> the missing mass = binding energy = the energy needed to break it apart!!

70
Q

Explain the PP Chain

A

Hydrogen burning occurs in steps known as the Proton-Proton Chain.

  1. Two H(1) combine to form H(2) through the weak nuclear force
  2. H(1) + H(2) = He(3)
  3. Two He(3) combine to form He(4) and two H(1)

The first step takes the longest (because of said weak nuclear force).

Neutrinos are also created, and pass through the Sun unhindered at nearly the speed of light. Gamma rays are produced but take MUCH longer to reach the surface (Drunkard’s Walk).

PP1 always happens, then PP2 or PP3 path depends on core conditions.

71
Q

How does the FUSION energy from each of the three branches of the PP Chain compare?

How does the USABLE energy from each of the three branches of the PP Chain compare?

A

Fusion: Energy is the same

Usable: PP1 produces the most

72
Q

Explain the CNO Cycle

A

It’s another way to fuse H into He in the sun, though not as common.

C, N, and O in the Sun act as catalysts to produce He in a continuous cycle.

73
Q

Why do PP Chains dominate in the Sun?

A

It occurs at a much lower temperature –> and occurs more than CNO at the temperature of the Sun.

74
Q

Which of the following is an important source of energy generation in stars? Select
all that apply.

A) Gravitational potential energy
B) Fusion of hydrogen into helium
C) Fusion of helium into carbon
D) Fusion of iron into bigger atoms
E) None of these generate energy in stars

A

B & C

75
Q

Explain the Hydrostatic Equilibrium in stars

A

A star is a big mass of material that wants to collapse under gravity, INWARDS, and yet energy generated from fusion exerts an OUTWARD force/pressure.

This balance keeps the star from collapsing. When fusion runs out, the star DOES collapse.

76
Q

A student says that the pressure gradient (dP/dR) is constant throughout the Sun. Is this student correct?

A

NO!

77
Q

What is the best time of year to see a star with an RA of 0h?

A

September.

Stars are best seen at midnight, with the Sun at the opposite point. At the vernal equinox in March, the star is in the sky during the day. 6 months later is September.

astrophysics (2 decks)

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