Topic 7 Components Of The Universe Flashcards
What is an astronomical unit?
This is a distance of approximately 150 million km. Roughly the distance earth is from the sun.
What is a light year?
The distance that light can travel through a vacuum in one year.
What is angular resolution?
This is the smallest angular size that can be distinguished with an instrument.
Angular size equation
Angular size (in radians) = actual size of object / distance between observer and object
The actual size and distance can be any units as long as they have the same units.
Why put telescopes in space?
- Atmosphere absorbs certain wavelengths ( UV and xray)
- no light pollution
- no atmospheric distortion
- no clouds
Keplers first law
This states that each planets rotation around the seen is an elliptical orbit. The Sun is always at one of the two foci in the ellipse.
The degree of elongation of an objects orbit can be described as its eccentricity, e.
The longest distance from the centre of the ellipse to the orbit is called the semimajor axis, a.
Keplers second law
This states that objects in a circular orbit maintain constant speed around the central body but objects in elliptical orbits move faster when they are closer to the central body and more slowly when they are further away.
Keplers third law
This relates the length of the semi major axis, a, and orbital period, P, of a planet through the equation:
P^2 = ka^3 / (M +m)
Where M is the mass of the central body (star), m is the mass of the orbiting body and k is the constant 5.91518 XI0^11 kg s^2 m^-3.
Keplers third law equations
When the central mass is far larger than the orbiting body use the formula
P^2 = ka^3 / M
where P is orbital period, a is the semi major axis, M is the mass of the central body and K is the value 5.91518×10^11
For plants in the solar system the formula can be used with units of years and AU.
P^2 = [1 y^2 AU^-3] a^3
What is an exoplanet?
This refers to any planet orbiting a star that is not our sun.
What is transit technique?
This is a method used to detect exoplanets by measuring the periodic decrease in brightness of a star as a planet passes directly in front of it along our line of sight.
A Jupiter sized exoplanet will block 1% of the light from a sun sized star.
What is radial velocity technique?
This method of detecting exoplanets measures the Doppler shift in the spectrum of a star as it is tugged back-and-forth by the gravitational pull of the planets that orbit around it.
Transit depth equation
Transit depth = R^2 planet / R^2 star
Where R is the respective radius
Why are the masses of exoplanets determined by the radio velocity technique mostly only lower limits of their true masses?
If the planetary orbit is inclined to an unknown angle to our line of sight we are only measuring the component of the stars velocity along our line of sight. This will be smaller than the actual velocity of the star so it will imply a smaller than actual mass for the planet.
Radial velocity amplitude formula
Radial velocity amplitude = M planet sin(i) / M star^(1/2) a planet^(1/2)
Where M is mass, a is distance from centre of mass and i is angle of inclination.
When would a planet be easiest to detect using the radial velocity method?
The radial velocity amplitude will be largest for massive planets orbiting low mass stars at very short distances seen edge on from our line of sight.
When would a planet be easiest to detect using the transit technique?
The transit will be deepest for large planets orbiting small stars when the orbit is seen edge on from our line of sight.
Planets with starter orbital periods will be easier to detect as their transits will repeat more regularly.
What is a spiral galaxy?
A type of flattened, dise- like galaxy exhibiting arms that form a spiral pattern emanating from its centre.
What are elliptical galaxies?
The stars in elliptical galaxies are not confined to a disk but have an ellipsoidal distribution. The stars tend to be older than in spiral discs (much like the bulges of a spiral galaxies) and they tend to have little ongoing star formation.
What are irregular galaxies?
These have no clear common identifying features. They are irregular in shape.
What is the Hubble tuning fork?
This is a method of sorting galaxies into three categories:
Elliptical galaxies, E1-E7 (where one is circular and seven is elliptical)
Lenticular galaxies (intermediates between elliptical and spiral galaxies), S0 and SB0
Spiral galaxies, Sa, Sb, Sc
Barred spiral galaxies, SBa, SBb, SBc
and irregular galaxies, Ir
What is dark matter?
Matter that cannot be seen, but which makes its presence felt by the gravitational influence it has on other objects in the universe.
Why do stars at the edge of galaxies orbit faster than predicted by Kepler‘s third law?
It is believed that galaxies are inside dark matter halos. These halos would distribute the mass evenly throughout the galaxy and therefore the period at any radius would be the same. this means stars at the edge of the galaxy would appear to move faster than expected.
What is dark energy?
A mysterious form of ‘negative gravity’ which constitutes about 69% of the critical density of the universe and is responsible for the acceleration of the expansion of The universe.
