Ever stare off into space looking at the stars and wonder "how do they know that...?" We will answer some of those questions!
D.1 – Stellar quantities (CORE)
Essential idea: One of the most difficult problems in astronomy is coming to terms with the vast distances in-between stars and galaxies and devising accurate methods for measuring them.
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
Guidance:
• For this course, objects in the universe include planets, comets, stars (single and binary), planetary systems, constellations, stellar clusters (open and globular), nebulae, galaxies, clusters of galaxies and super clusters of galaxies
• Students are expected to have an awareness of the vast changes in distance scale from planetary systems through to super clusters of galaxies and the universe as a whole
Data Booklet reference:
• d (parsec) = 1 / p (arc-second)
• L = sAT4
• B = L / ( 4pd2 )
Theory of knowledge:
• The vast distances between stars and galaxies are difficult to comprehend or imagine. Are other ways of knowing more useful than imagination for gaining knowledge in astronomy?
Utilization:
• Similar parallax techniques can be used to accurately measure distances here on Earth
Aims:
• Aim 1: creativity is required to analyse objects that are such vast distances from us
• Aim 6: local amateur or professional astronomical organizations can be useful for arranging viewing evenings
• Aim 9: as we are able to observe further into the universe, we reach the limits of our current technology to make accurate measurements
Essential idea: A simple diagram that plots the luminosity versus the surface temperature of stars reveals unusually detailed patterns that help understand the inner workings of stars. Stars follow well-defined patterns from the moment they are created out of collapsing interstellar gas, to their lives on the main sequence and to their eventual death.
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
Guidance:
• Regions of the HR diagram are restricted to the main sequence, white dwarfs, red giants, super giants and the instability strip (variable stars), as well as lines of constant radius
• HR diagrams will be labelled with luminosity on the vertical axis and temperature on the horizontal axis
• Only one specific exponent (3.5) will be used in the mass–luminosity relation
• References to electron and neutron degeneracy pressures need to be made
Data Booklet reference:
• lmaxT = 2.9´10-3 m K
• L µ M 3.5
Theory of knowledge:
• The information revealed through spectra needs a trained mind to be interpreted. What is the role of interpretation in gaining knowledge in the natural sciences? How does this differ from the role of interpretation in other areas of knowledge?
Utilization:
• An understanding of how similar stars to our Sun have aged and evolved assists in our predictions of our fate on Earth
Aims:
• Aim 4: analysis of star spectra provides many opportunities for evaluation and synthesis
• Aim 6: software-based analysis is available for students to participate in astrophysics research
Essential idea: The Hot Big Bang model is a theory that describes the origin and expansion of the universe and is supported by extensive experimental evidence.
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
Guidance:
• CMB radiation will be considered to be isotropic with
• For CMB radiation a simple explanation in terms of the universe cooling down or distances (and hence wavelengths) being stretched out is all that is required
• A qualitative description of the role of type Ia supernovae as providing evidence for an accelerating universe is required
Data Booklet reference:
• z = Dl / l0 » v / c
• z = R / R0 – 1
• v = H0d
• T » 1 / H0
International-mindedness:
• Contributions from scientists from many nations made the analysis of the cosmic microwave background radiation possible
Utilization:
• Doppler effect (see Physics sub-topic 9.5)
Aims:
• Aim 1: scientific explanation of black holes require a heightened level of creativity
• Aim 9: our limit of understanding is guided by our ability to observe within our universe