D. Astrophysics

D. Astrophysics

Core

D.1 – Stellar quantities

Nature of science:

  • Reality: The systematic measurement of distance and brightness of stars and galaxies has led to an understanding of the universe on a scale that is difficult to imagine and comprehend.

Understandings:

  • Objects in the universe

  • The nature of stars

  • Astronomical distances

  • Stellar parallax and its limitations

  • Luminosity and apparent brightness

Applications and skills:

  • Identifying objects in the universe

  • Qualitatively describing the equilibrium between pressure and gravitation in stars

  • Using the astronomical unit (AU), light year (ly) and parsec (pc)

  • Describing the method to determine distance to stars through stellar parallax

  • Solving problems involving luminosity, apparent brightness and distance

D.2 – Stellar characteristics and stellar evolution

Nature of science:

  • Evidence: The simple light spectra of a gas on Earth can be compared to the light spectra of distant stars. This has allowed us to determine the velocity, composition and structure of stars and confirmed hypotheses about the expansion of the universe.

Understandings:

  • Stellar spectra

  • Hertzsprung–Russell (HR) diagram

  • Mass–luminosity relation for main sequence stars

  • Cepheid variables

  • Stellar evolution on HR diagrams

  • Red giants, white dwarfs, neutron stars and black holes

  • Chandrasekhar and Oppenheimer–Volkoff limits

Applications and skills:

  • Explaining how surface temperature may be obtained from a star’s spectrum

  • Explaining how the chemical composition of a star may be determined from the star’s spectrum

  • Sketching and interpreting HR diagrams

  • Identifying the main regions of the HR diagram and describing the main properties of stars in these regions

  • Applying the mass–luminosity relation

  • Describing the reason for the variation of Cepheid variables

  • Determining distance using data on Cepheid variables

  • Sketching and interpreting evolutionary paths of stars on an HR diagram

  • Describing the evolution of stars off the main sequence

  • Describing the role of mass in stellar evolution

D.3 – Cosmology

Nature of science:

  • Occam’s Razor: The Big Bang model was purely speculative until it was confirmed by the discovery of the cosmic microwave background radiation. The model, while correctly describing many aspects of the universe as we observe it today, still cannot explain what happened at time zero.

Understandings:

  • The Big Bang model

  • Cosmic microwave background (CMB) radiation

  • Hubble’s law

  • The accelerating universe and redshift (z)

  • The cosmic scale factor (R)

Applications and skills:

  • Describing both space and time as originating with the Big Bang

  • Describing the characteristics of the CMB radiation

  • Explaining how the CMB radiation is evidence for a Hot Big Bang

  • Solving problems involving z, R and Hubble’s law

  • Estimating the age of the universe by assuming a constant expansion rate

Additional higher level

D.4 – Stellar processes

Nature of science:

  • Observation and deduction: Observations of stellar spectra showed the existence of different elements in stars. Deductions from nuclear fusion theory were able to explain this.

Understandings:

  • The Jeans criterion

  • Nuclear fusion

  • Nucleosynthesis off the main sequence

  • Type Ia and II supernovae

Applications and skills:

  • Applying the Jeans criterion to star formation

  • Describing the different types of nuclear fusion reactions taking place off the main sequence

  • Applying the mass–luminosity relation to compare lifetimes on the main sequence relative to that of our Sun

  • Describing the formation of elements in stars that are heavier than iron including the required increases in temperature

  • Qualitatively describe the s and r processes for neutron capture

  • Distinguishing between type Ia and II supernovae

D.5 – Further cosmology

Nature of science:

  • Cognitive bias: According to everybody’s expectations the rate of expansion of the universe should be slowing down because of gravity. The detailed results from the 1998 (and subsequent) observations on distant supernovae showed that the opposite was in fact true. The accelerated expansion of the universe, whereas experimentally verified, is still an unexplained phenomenon.

Understandings:

  • The cosmological principle

  • Rotation curves and the mass of galaxies

  • Dark matter

  • Fluctuations in the CMB

  • The cosmological origin of redshift

  • Critical density

  • Dark energy

Applications and skills:

  • Describing the cosmological principle and its role in models of the universe

  • Describing rotation curves as evidence for dark matter

  • Deriving rotational velocity from Newtonian gravitation

  • Describing and interpreting the observed anisotropies in the CMB

  • Deriving critical density from Newtonian gravitation

  • Sketching and interpreting graphs showing the variation of the cosmic scale factor with time

  • Describing qualitatively the cosmic scale factor in models with and without dark energy

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