Orbital Mechanics and Ellipses Quiz
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Questions and Answers

What parameters define the shape of closed orbits?

  • Eccentricity (e) and period (P)
  • Semi-major axis (a) and eccentricity (e) (correct)
  • Semi-major axis (a) and inclination (i)
  • Eccentricity (e) and argument of periapsis (ω)
  • If an ellipse has a semi-major axis of $a = 4$ and a semi-minor axis of $b = 3$, what is the area of the ellipse?

  • $7π$
  • $14π$
  • $12π$ (correct)
  • $24π$
  • An orbit is defined by how many orbital elements?

  • 5
  • 4
  • 6 (correct)
  • 3
  • What is the eccentricity of a circular orbit?

    <p>e = 0 (B)</p> Signup and view all the answers

    What does the variable 'e' represent when defining an orbit?

    <p>Eccentricity (D)</p> Signup and view all the answers

    Given the distance from the origin to a focus is $c = ae$, what does 'e' stand for?

    <p>The ellipse's eccentricity. (D)</p> Signup and view all the answers

    If the semi-major axis (a) of an ellipse is equal to the semi-minor axis (b), what shape does the ellipse become?

    <p>Circle (B)</p> Signup and view all the answers

    What is the range of possible values for eccentricity (e) in an elliptical orbit?

    <p>0 ≤ e &lt; 1 (A)</p> Signup and view all the answers

    Which pair of parameters uniquely defines the shape of an ellipse?

    <p>Semi-major axis ($a$) and eccentricity ($e$) (A)</p> Signup and view all the answers

    Besides the semi-major axis and eccentricity, what additional information is necessary to fully describe an orbit for observational purposes?

    <p>The orientation of the orbit and the orbital plane (B)</p> Signup and view all the answers

    What does the orbital element 'inclination' ($i$) represent?

    <p>The angle between the orbital plane and the observer's line of sight. (A)</p> Signup and view all the answers

    What does the term 'longitude of ascending node' ($Ω$) define?

    <p>The angle measured in the observer's plane, defining the orientation of the orbital plane. (B)</p> Signup and view all the answers

    Which of the following terms refers to the point in an orbit that is closest to the Earth?

    <p>Perigee (C)</p> Signup and view all the answers

    Which of the following terms is a general term used to describe the point of closest approach in an orbit around a star?

    <p>Periastron (B)</p> Signup and view all the answers

    What is the general term for the point in an orbit that is farthest from the central body?

    <p>Apapsis (B)</p> Signup and view all the answers

    What do the orbital elements omega ($ω$) and true anomaly ($ν$) define together?

    <p>The relative orientation of the orbit within its plane and the position of the body along the orbit. (B)</p> Signup and view all the answers

    According to Kepler's Third Law, how is the orbital period ($P_b$) related to the semi-major axis ($a$) and the total mass ($M$) of a binary system?

    <p>$P_b^2 = \frac{4\pi^2}{GM} a^3$ (B)</p> Signup and view all the answers

    What do 'post-Keplerian parameters' describe?

    <p>The time evolution of orbital elements in a binary system. (A)</p> Signup and view all the answers

    Why are 'post-Keplerian parameters' useful for testing theories of gravity?

    <p>They can be measured independently of any specific gravity model and then compared to theoretical predictions. (C)</p> Signup and view all the answers

    In the context of binary systems, what does the inclination angle refer to?

    <p>The angle between the orbital plane and the plane of the sky. (A)</p> Signup and view all the answers

    A binary system has a total system mass of $3 M_\odot$ and an orbital period of 5 hours. Which formula should be used to calculate the size of the orbit in metres?

    <p>$a = \left( \frac{GM P_b^2}{4\pi^2} \right)^{1/3}$ (A)</p> Signup and view all the answers

    If a binary system's orbital elements are observed to be constant over time, what can be inferred?

    <p>The system is well-described by Newtonian gravity with no perturbations. (B)</p> Signup and view all the answers

    What does $dA/dt = const$ represent in Kepler's Laws?

    <p>Conservation of angular momentum (D)</p> Signup and view all the answers

    What is a significant implication of Kepler's Second Law ($dA/dt = const$) for a binary system's orbit?

    <p>The orbital speed is fastest when the stars are closest together. (A)</p> Signup and view all the answers

    What does the mass-luminosity relation primarily allow us to infer about stars?

    <p>The approximate lifetime of stars, given their energy source. (D)</p> Signup and view all the answers

    What realization did the initial calculations of stellar ages based on gravitational contraction lead to?

    <p>That stars must be powered by a different energy source than just gravity due to geological data indicating earth's age. (C)</p> Signup and view all the answers

    In the context of stellar ages, what does the expression age ≈ E/L represent?

    <p>An approximation of a star's age based on its total energy and rate of energy loss. (C)</p> Signup and view all the answers

    What is the Kelvin-Helmholtz timescale ($t_{KH}$) primarily used to estimate?

    <p>The timescale of energy production due to gravitational contraction. (B)</p> Signup and view all the answers

    What is the significance of Eddington's 1924 plot in the context of stellar physics?

    <p>It provided observational evidence supporting the mass-luminosity relation. (B)</p> Signup and view all the answers

    What is the formula for gravitational potential energy ($U$) used to estimate the Kelvin-Helmholtz timescale?

    <p>$U = -3GM_{\star}^2 / 5R_{\star}$ (C)</p> Signup and view all the answers

    If the Kelvin-Helmholtz timescale for the Sun is approximately 20 million years, what does this suggest when compared to geological and biological data indicating a much older Earth?

    <p>The Sun must be powered by a different, more long-lasting energy source than gravitational contraction alone. (E)</p> Signup and view all the answers

    According to the mass-luminosity relation, how does luminosity (L) generally scale with mass (M)?

    <p>$L(M) ∝ M^{3.5}$ (D)</p> Signup and view all the answers

    What is the main characteristic of a visual binary system?

    <p>Both components are detectable and spatially resolved. (B)</p> Signup and view all the answers

    How do we infer the existence of a second component in an astrometric binary system?

    <p>Through the oscillation of the detectable component in space. (A)</p> Signup and view all the answers

    What effect occurs in an eclipsing binary when one star passes in front of another?

    <p>A dip is seen in the light curve over time. (B)</p> Signup and view all the answers

    In a spectroscopic binary system, how are spectral lines affected?

    <p>The lines are Doppler shifted in opposite directions due to the orbital motion. (B)</p> Signup and view all the answers

    What allows us to determine the total system mass (M) in a binary system?

    <p>Distance measurements combined with angular separations. (D)</p> Signup and view all the answers

    Which characteristic is NOT associated with an eclipsing binary system?

    <p>It allows for easy detection of both components. (D)</p> Signup and view all the answers

    What is a common misconception about double stars being classified as binary systems?

    <p>Double stars are not generally considered binary systems. (B)</p> Signup and view all the answers

    In the context of binary stars, what does 'Pb' refer to?

    <p>The period related to the motion of the stars in the system. (B)</p> Signup and view all the answers

    What effect does a circular orbit have on spectral variations over time?

    <p>Creates sinusoidal variations (C)</p> Signup and view all the answers

    If two stars in a binary system are located 1 kpc apart, what impact does their distance have on our ability to detect them?

    <p>Detection is possible over long distances in this scenario (D)</p> Signup and view all the answers

    What is the relation describing the Doppler Effect in the provided formula?

    <p>Δν/v = ν/c (A)</p> Signup and view all the answers

    What condition makes the Doppler effect potentially undetectable?

    <p>If v/c is much less than 1 (C)</p> Signup and view all the answers

    If a signal is transmitted at 123.456 MHz, what is a key factor that affects the observed Doppler shift?

    <p>The frequency of the signal (B)</p> Signup and view all the answers

    What is the required spectral resolution to notice Doppler shifts from the Mars Rovers transmitting at 8 GHz?

    <p>Resolution finer than 1 MHz (A)</p> Signup and view all the answers

    What information can be gathered from spectral lines of a star?

    <p>Temperature and composition (B)</p> Signup and view all the answers

    How can stellar masses be determined according to the methods outlined?

    <p>By observing binary interactions (A)</p> Signup and view all the answers

    Flashcards

    Binary Stars

    A star system consisting of two stars orbiting a common center of mass.

    Kepler's Laws

    Three laws that describe the motion of planets around the sun, also applicable to binary stars.

    Types of Binaries

    Different classifications of binary star systems based on visibility and interaction.

    Doppler Effect

    The change in frequency or wavelength of waves in relation to an observer moving relative to the source.

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    Orbital Elements

    Six parameters that define the size, shape, and orientation of an orbit.

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    Eccentricity

    A measure of how much an orbit deviates from a perfect circle, ranging from 0 to greater than 1.

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    Semi-major Axis

    Half of the longest diameter of an ellipse, key in defining orbits.

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    Area of an Ellipse

    Calculated using the formula πab where 'a' is semi-major and 'b' is semi-minor axis.

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    Semi-major Axis (a)

    Half of the longest diameter of an ellipse; defines size of the orbit.

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    Eccentricity (e)

    A parameter that measures the deviation of an orbit from a perfect circle.

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    Inclination (i)

    The tilt of an orbital plane regarding the reference plane, typically the equatorial plane.

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    Longitude of Ascending Node (Ω)

    The angle from a reference direction to the ascending node of the orbit.

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    Argument of Periastron (ω)

    The angle from the ascending node to the point of closest approach in an orbit.

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    True Anomaly (ν)

    The angle between the direction of periapsis and the current position of the body.

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    Periapsis Terms

    Various terms referring to the closest point in an orbit, e.g., perihelion (sun), perigee (Earth).

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    Orbital Period (Pb)

    The time it takes for a binary system to complete one full orbit around its center of mass.

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    Inclination Angle

    The angle between the orbital plane of a binary system and the plane of reference (the sky).

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    Post-Keplerian Parameters

    Parameters that modify Kepler's laws to account for additional effects in binary systems over time.

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    Newtonian Gravity

    The classical theory of gravity describing the force between two masses as proportional to their masses and inversely proportional to the square of their distance.

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    Total System Mass (M⊙)

    The combined mass of both objects in a binary system, measured in solar masses (M⊙).

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    Relativistic Orbit

    An orbit where the relative velocity of the objects involved approaches a significant fraction of the speed of light (v/c).

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    Central Potential

    A potential energy field created by a central mass around which objects orbit.

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    Relative velocity

    The velocity of one object as observed from another object.

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    Spectral resolution

    The ability to distinguish between different wavelengths in a spectrum.

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    Eccentric orbit

    An orbit with varying distance from the central body, causing more complex spectral variations.

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    H-R diagram

    A graphical representation of stars showing the relationship between luminosity and temperature.

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    Bolometric luminosity

    The total amount of energy emitted by a star across all wavelengths.

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    Stellar density

    The mass of a star divided by its volume, indicating how compact it is.

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    Stefan-Boltzmann relation

    A law describing the power radiated by a black body in terms of its temperature.

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    Visual Binary

    A binary system where both stars can be seen and resolved.

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    Astrometric Binary

    A binary system where one component is visible and the other is inferred from its gravitational effects.

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    Eclipsing Binary

    A binary system where stars pass in front of each other, causing a dip in brightness.

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    Spectroscopic Binary

    A binary system detected by observing the Doppler shifts in the spectral lines of stars.

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    Angular separation

    The angle between the two components in a binary system as seen from Earth.

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    Parallax

    The apparent shift in position of a star against the background due to Earth's movement.

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    Light Curve

    A graph showing brightness of a star system over time; used for analyzing eclipsing binaries.

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    Main Sequence Binaries

    A significant portion of main sequence stars exist in binary or multiple star systems.

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    Mass-Luminosity Relation

    The relationship stating luminosity (L) is proportional to mass (M) raised to the power of approximately 3.5.

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    Eddington's Observation

    An important 1924 plot demonstrating the mass-luminosity relation discovered by Sir Arthur Eddington.

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    Stellar Ages

    The calculation of a star's age based on its energy source, represented by age ≈ E/L.

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    Kelvin-Helmholtz Timescale

    The estimated age of a star based on gravitational potential energy and luminosity: tKH = |U|/L.

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    Gravitational Potential Energy

    The energy stored due to the gravitational attraction of a star's mass: U = -3GM²/(5R).

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    Nuclear Fusion in Stars

    The process by which stars generate energy, leading to the discovery of their long lifetimes.

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    Observational Fact

    A conclusion derived from repeated observations, such as the empirical mass-luminosity relation.

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    Study Notes

    Lecture 3: Binary Stars

    • Lecture was on binary stars, part of the "Observing the Universe" course
    • The lecturer was Prof. Evan Keane
    • The date of the lecture was Thursday 23rd January 2025 at 12:00
    • Lecture 2 covered spectral classification of stars, spectral line origins, relevant physical processes, and the Hertzsprung-Russell diagram.
    • Lecture 3 goal: Understand binarity manifestations in stars and what can be learned from quantifying binarity.
    • Lecture 3 outline: Binary parameters, Kepler's laws, types of observed binaries, Doppler effect, stellar masses.

    L2 Recap

    • Spectral classification of stars
    • Origin of spectral lines
    • Physical processes relevant to spectral lines
    • The Hertzsprung-Russell diagram

    L3 Learning Outcomes

    • Understanding the manifestations of binarity in stars
    • Understanding what can be learned by quantifying binary systems
    • Binary parameters and Kepler's laws
    • Types of observed binaries
    • Doppler effect
    • Stellar masses

    Aside: Ellipses

    • Equation of an ellipse: x²/a² + y²/b² = 1
    • Area of an ellipse: πab
    • For a circle, a = b = r (center of circle is the focal point)
    • Distance from origin to focus: c = ae
    • For any point on ellipse: |F₁P| + |F₂P| = const = 2a

    Binary Parameters

    • An orbit is defined by 6 orbital elements
    • Closed orbits are elliptical and defined by 2 parameters:
      • a: Semi-major axis (size)
      • e: Eccentricity (0 = circle, 0<e<1 = ellipse, e=1 = parabola, e>1 = hyperbola)
      • b: Semi-minor axis (need either (a, b) or (a, e) to define ellipse shape)
    • i: Orbital plane's inclination with respect to the observer
    • Ω: Longitude of ascending node, measured in the observer's plane
    • ν: True anomaly (angle defining the epoch of the orbital body during observation, cyclic in orbital period Pb)
    • Argument of periapsis (ω) and longitude of ascending node (Ω) define the orbital plane orientation
    • Terms like periapsis, periastron, perihelion, and perigee refer to the closest point in a system's orbit.

    Jargon Buster

    • Periapsis, periastron, perihelion, and perigee are all geometrically the same
    • Terms differ based on the specific system in question (e.g., perihelion refers to the Earth's closest point to the Sun)
    • Apapsis, apastron, aphelion, and apogee refer to the furthest point in an orbit.

    Theories of Gravity

    • Orbital elements are independently measurable for any observed binary system in the sky
    • Theories of gravity define how interactions/motion happen
    • The relationships between orbital elements, physical constants (G), and charges (like masses) can be predicted by a complete theory of gravity.

    Kepler Orbits

    • Kepler's laws for orbits, based on Newtonian gravity:
      • K1: Orbits are ellipses
      • K2: dA/dt = const
      • K3: P² = 4π²a³/GM
      • (where M = m₁ + m₂)

    Theories of Gravity (cont.)

    • Orbital elements can change with time, with additional parameters such as w(t) and Pb(t).
    • These can also be used to test different gravitational theories, independently of the specific model of the system.

    Doppler Effect

    • The Doppler effect is a change in frequency of a wave (or other periodic event) for an observer moving relative to its source.
    • Δν/ν = v/c. (where Δν is the change in frequency, ν is the emitted frequency, v is the relative velocity between source and observer and c is the speed of light)

    Stellar Masses

    • Using methods described, stellar masses can be determined
    • Applicability is not limited to stars; also applies to exoplanets.

    Stellar Properties

    • Combining information from L1-L3 allows a lot to be learned about stars.
    • Measure stellar flux and distance (parallax -> bolometric luminosity)
    • Measure spectral features (spectral lines, shape) to determine stellar temperature and composition.
    • The Stefan-Boltzmann relation is used to calculate stellar radii
    • Binary systems enable mass determination

    Mass-Luminosity Relation

    • L ∝ M^3.5 (Luminosity is proportional to the mass raised to 3.5)
    • Observationally derived
    • Can be related to internal star chemistry and structure

    Stellar Ages

    • Calculating time-scales like timescale = (anything)/(d(anything)/dt)
    • Age ~ E/L (Energy divided by Luminosity) to determine how long a process can fuel a star
    • Gravitational contraction can be used to estimate the Kelvin-Helmholtz timescale (~20 million years for Sun).
    • Nuclear fusion is required for long term sustained energy in the Sun.

    Problems 3.1, 3.2, 3.3, 3.4

    • Questions regarding binary systems, including angles of inclination, size of orbits, relativistic effects, Doppler shifts, and stellar properties.

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    Lecture 3: Binary Stars PDF

    Description

    Test your understanding of the parameters and principles that define the shape of orbits, particularly ellipses. This quiz covers topics such as semi-major and semi-minor axes, eccentricity, and orbital elements crucial for defining orbits. Perfect for students studying celestial mechanics or astrophysics.

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