PHYS 2013/2913
Introduction to
Stellar Astrophysics
John O’Byrne
School of Physics Room 568
9351 - 3184
j.obyrne@physics.usyd.edu.au
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Introduction
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‘Text’ - Introduction to Stellar Astrophysics will be
closely followed.
($8.50 from the Copy Centre - or order order R556948 from
http://www.usyd.edu.au/su/ups/notes/notes_login.html)
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Various other texts are an option
WebCT used - esp. learning goals, links
One assignment in 2 parts
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Rationale
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Astrophysics has long been one of the driving forces for
the advancement of physics in general. It has been closely
linked to advances in quantum mechanics and nuclear
physics in particular.
Stellar astrophysics in particular is also firmly rooted in
classical physics - the familiar laws of mechanics,
hydrostatics and thermodynamics.
For this reason stellar astrophysics is presented here as a
synthesis of some of the physics you know and an
introduction to astrophysics in general.
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Broad outline
ISA covers the following topics:
 An overview of the universe, introducing distance scales.
 A review of the tools of astrophysics and their limitations.
 Introduction to basic astronomical concepts: magnitudes, colour
indices, distance modulus, etc.
 Basic principles of stellar spectroscopy, including line formation.
 Stellar classification and the Hertzsprung-Russell diagram.
 The physics of stars and the fundamental equations for stellar
modelling.
 Variable stars and the mechanisms giving rise to the instability
strip.
See the text Introduction for ‘Assessment and Learning Goals’
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Motivation: Explain this image
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© Sébastien Lebouc - http://www.astrosurf.com/poisaya/astro/deepsky/sebastien.htm
The scale of things
“Cosmic Voyage”
aka
“Powers of 10”
Space is big. Really big. You just won’t believe how
vastly, hugely, mind-bogglingly big it is. I mean, you
may think it’s a long way down the road to the chemist,
but that’s just peanuts to space.”
Douglas Adams, The Hitchhiker’s Guide to the Galaxy
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Planets
Historically:
 ≥103 km in regular orbit around a star
 9 in our solar system
IAU GA 2006:
Observations are changing our understanding of planetary
systems, and it is important that our nomenclature for
objects reflect our current understanding. This applies, in
particular, to the designation “planets”…Recent discoveries
lead us to create a new definition, which we can make using
currently available scientific information.
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Planet definition
1.
2.
A “planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, and
(c) has cleared the neighbourhood around its orbit.
A “dwarf planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round)
shape,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
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Planet definition - 2
All other objects except satellites orbiting the Sun shall be referred to
collectively as “Small Solar-System Bodies”.
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype
of a new category of trans-Neptunian objects - now officially called “plutoids”.
3.
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Stars
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self-luminous spheroidal mass of gravitationally bound gas.
luminosity due to nuclear reactions in the core.
occur singly or in clusters
sizes range from
 ~km (neutron “stars”)
 ~104 km (white dwarfs )
 ~106 km (sun)
 ~109 km (supergiants)
mass from
 0.08 M to ~150 M(?)
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Groups of stars
Binary/multiple
Stellar
Open
systems
associations
(galactic) clusters
~102
- 103 stars, a few pc in
size, young, associated with
spiral arms of our galaxy
Globular
clusters
~105
- 106 stars, ~30 pc in size,
old, form a halo around our
galaxy
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Gas and dust clouds
Part
of the general Interstellar Medium (ISM)
Ranging
from small globules to Giant Molecular Clouds
Masses
from 1 to 106 M, primarily H
perhaps
1% of the mass of the stars in the galaxy
Density
up to 106 cm-3
Temperatures
Dust
from ~10 K to 200 K (in the clouds)
~1% of the mass of the gas - scatters and absorbs light
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Gas and dust clouds
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The Milky Way
Our galaxy is a spiral (disk) galaxy
usually known as the Milky Way
Disk
diameter ~ 50 kpc
Halo
diameter ~100 kpc
Mass
~ 1012 M
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Galaxies
Spirals
Mass
Ellipticals
~109 to 1012 M
Luminosity
Diameter
~108 to 1010 L
~5 to 50 kpc
Mass
~105 to 1013 M
Luminosity
Diameter
~105 to 1011 L
~1 to 200 kpc
Irregulars
Mass
~108 to 1010 M
Luminosity
Diameter
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~107 to 109 L
~1 to 10 kpc
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Galaxies
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And more..
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