IB HL Physics - ASTROPHYSICS, draft
Terms in this set (47)
the average distance between the Sun and the Earth
a celestial body in orbit around the Sun that dominates its neighbourhood and is spherical under gravitation
A lump of rock and ice with an elongated elliptical orbit. Becomes visible when close to the Sun as the solar wind blows ice off in the form of a tail.
two stars that orbit each other.
a group of stars that are near to each other.
group of up to a few thousand stars loosely held together by gravity
A tight group of stars that looks like a ball and contains up to 1 million stars
A large cloud of dust and gas in space
A huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity
A measure of stellar distance.
Stellar parallax equation
d = 1/p(arcsec)
1 pc =
limitations of stellar parallax
there is a smallest angle that can be measured (0.01 arcsec for terrestrial telescope & 0.001 for space telescope like Hubble space shuttle)
also only used up to 100pc
Apparent brightness (b)
The power received per unit area on Earth by a star.
apparent brightness equation
the amount of power radiated through a given area
Equation linking luminosity and radiation flux
L = F4πd²
Wien's displacement law
λ(max) = 2.9E-3/T
Visible lines on an observed color spectrum that reveal the chemical composition of the light source
A hotter star will...
have a bluer colour
composition of most stars
72% hydrogen, 25% helium, 3% other
Hertzsprung-Russell (HR) diagram
temperature vs luminosity
L ∝ M⁷⁽²
How can you use an HR diagram to determine characteristics of a star?
use L = σAT⁴
Life cycle of a star
nebula, protostar, main sequence star, red (super) giant,
Giant Molecular Cloud - a swirling cloud of gas and dust
Main sequence star
star that uses hydrogen or helium as fuel
class of astrophysical objects which have known luminosity due to some characteristic quality possessed by the entire class of objects
Properties of cepheid variables
- brightness (radiative flux) varies over a time period. Luminosity doesn't change
- By knowing the time period, we can use a graph to find its luminosity
- This allows us to use √(L/F4π) = d to calculate the distance
an unstable star that undergoes periodic expansions and contractions, leading to a periodic change in the apparent brightness of the star
How can you use cepheid variables to work out the distance to a star?
- find star's period
- use the direct relationship to find its luminosity.
- use luminosity in the apparent brightness equation to find distance
How does a star not collapse?
There is an outward pressure from the heat produced by fusion in the star (pV = nRT) which balances out the gravitational force
Why may a GMC not collapse or scatter?
They're held together by gravity but are kept from collapsing by the pressure of the molecules moving about in random motion
the minimum mass of gas needed to collapse into a star
Jean's Criterion equation
Mj = 3kTR/2Gm
(k = Boltzmann)
What often triggers the formation of a star
What happens when a star is a protostar?
Gravity keeps on collapsing the star, until it becomes dense and hot enough for hydrogen to fuse and form helium, making the star a main sequence star
What is the chemical reaction in the proton-proton cycle?
4(1,1)H → (4,2)He + 2e⁺ + 2vₑ + 2 γ
Why does fusion release energy?
the mass of the helium atom is less than the original hydrogen atoms, so some mass is converted to energy
Does a larger or smaller star have a longer main sequence state?
smaller star, as a bigger star will have a higher rate of fusion (think L∝ M⁷⁽²) and will therefore run out of fuel faster.
What happens as a star leaves its main sequence state?
hydrogen fusion continues until hydrogen used up → fusion slows → pressure less → core collapses → temp. increases → outer layers expand
What role does mass play in the life cycle of a star?
it determines whether the star follows the red giant or red supergiant cycle
When does a star become a red supergiant after its main sequence?
if mass > 106*(mass of sun)
Pauli exclusion principle says two electrons cannot occupy the same quantum mechanical state
What does electron degeneracy describe?
the maximum density of the core
has a changing luminosity, so its position on the HR diagram is not constant
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