Topic 7 Cosmic Timescales Flashcards
(18 cards)
Is look back time useful for studying the evolution of stars within our galaxy?
No because the most distant objects in the galaxy are only you tend to kill apart away which means 10 to the power of five years of look back time whereas the evolution of most stars is measured in billions of years (10 to the power of six).
What process occurs in high math stars that can account for the much larger change in radius than in small mass stars?
The highest mass stars have significant stellar winds that reduce their masses and therefore alter the structure of the star.
The higher mass stars exhibits a number of rapid increases and decreases in radius not seen in the lowest mass stars. What caused these rapid changes?
Heimer stars burn successively more massive elements as they evolve. Rapid changes in structure occur as the core contracts and heats up to enable each successive energy source before hydrostatic equilibrium is regained. The lowest mass stars do not have sufficient maths to initiate core fusion beyond helium burning.
What is aberration?
A change in the apparent direction of arrival of light from a star resulting from the combination of the speed of the earth in its orbit around the sun and the finite speed of light.
What is a forbidden transition?
An energy transition that does not obey quantum mechanical selection rules and therefore has a very low probability of occurring.
What are metastable states?
Excited energy states that have a very low transition probability to lower energy states.
What is Lookback time?
It is a time interval between the emission of light from an object and its detection on earth, observations of galaxies at increasing red shift (increasing distance, or greater look back time) allow us to probe their evolution.
How to calculate the upper limit of the size of an object based on variability.
R <= c Δt
Where R is upper limit to size, c is the speed of light and Δt is the change in time.
What is the mean solar day?
The time interval between successive Meridian crossings of the mean sun (a fictitious object with uniform motion across the sky throughout the year) so that mean solar time passes at a steady rate.
What is a sidereal day?
The spin period of the Earth with respect to the stars.
What is a sidereal year?
The time taken for the Earth to complete one orbit of the Sun, relative to the stars (during which time 365.256 mean solar days and 366.256 sidereal days have lapsed).
What is a tropical year (solar year)?
The tropical year or solar year is the time interval for one seasonal cycle of the Sun. At 365.242 mean solar days, it is about 20 minutes shorter than the sidereal year because of procession.
What must be taken into account when calculating the radial velocities of objects?
The motion of the Earth in orbit around the Sun (and its daily rotation) add a component of up to approximately plus or -30 km/s to observed radial velocities, which must be corrected when determining the velocity relative to the Sun.
What do nuclear fusion rates, r, depend on?
-Number density of particles, n
-Their relative speeds (defined by gas temperatures), ν
-The effective cross-section for the interaction, σ
rab = na nb νab σab (νab)
What determines a star is dominant nuclear pathway?
The dominant pathway is largely driven by a stars core temperature. The higher core temperatures in massive stars result in higher reaction rates and hence shorter lifetime.
