Download Astronomy (C) - North Carolina Science Olympiad

2013 North Carolina Science Olympiad Coaches’ Institute
Astronomy (C)
Don Warren
Physics Department
NC State University
State event leader
Event Rules
Description: Students will demonstrate an understanding
of the basic concepts of mathematics and physics relating
to stellar evolution and variable stars.
Team size: Up to 2
Event parameters: Each team is allowed two laptops, two
3-ring binders (any size), or one of each. Each member may
bring a programmable calculator. No internet access!
Scoring: Each question is worth a set amount of points.
Highest score wins. Ties broken by score on selected
questions.
Check NCSO website regularly for changes/updates!
My Philosophy
The purpose of science is not acquisition of facts.
Science is about understanding
the Universe with the simplest
theories possible.
My Philosophy
My tests require more than just rote memorization!
Understand the concepts, and know how to read graphs
and charts.
Q1: What is the most common element in the Universe?
A. Helium B. Hydrogen C. Carbon D. Iron
vs.
Q2: Stars near which letter
have the smallest radius?
My Philosophy
Finally,
Can’t do astronomy without physics and mathematics,
so students should expect to have to do math at some
point!
Brief Outline
I. How stars work
II. Stellar evolution
III. Filling out the stellar zoo
IV. Astronomical concepts
V. List of objects
I. How stars work
A star (like the Sun) is a mass of incandescent gas (really, a
miasma of incandescent plasma)
They’re
HUGE
Earth
I. How stars work
Two competing forces perpetually in near perfect balance
Gravity pulls star inward
(Thermal) Pressure from
burning fuel through
nuclear fusion
II. Stellar evolution
All stars begin as cloud
of gas (nebula), which
collapses to form
a protostar
Protostars contract
under gravity until
fusion begins
Single most important
attribute for a star:
its mass at birth
II. Stellar evolution
Stellar life cycle splits into
three paths based on
mass at birth
• Low mass stars
(d 0.5 MSun)
• Medium mass stars
(0.5 MSun – 8 MSun)
• Massive stars
( t 8 MSun)
Many, many more low mass stars than high mass stars!
II. Stellar evolution
Low Mass Stars (d 0.5 MSun)
Fuse hydrogen to helium at core
(Slowly!) Convert all fuel, and
eventually dim as fuel runs out
(Not very interesting, eh?)
No dead low-mass stars exist:
Universe not old enough
II. Stellar evolution
Medium Mass Stars (0.5 MSun – 8 MSun)
Fuse hydrogen to helium at core
Convert core to helium, and
contract due to gravity
Have inert core of helium
inside shell of burning
hydrogen
II. Stellar evolution
Medium Mass Stars (0.5 MSun – 8 MSun)
Star becomes a red giant – increased energy production
pushes outer layers away from star; outer layers cool down
II. Stellar evolution
Medium Mass Stars (0.5 MSun – 8 MSun)
Core of star continues to contract, reaching helium burning
temperatures
Burning rate increases, until
pressure is blowing away
outer layers of star
(i.e. pressure wins battle
vs. gravity)
II. Stellar evolution
Medium Mass Stars (0.5 MSun – 8 MSun)
Outer layers become
planetary nebula.
What about
the core?
II. Stellar evolution
Medium Mass Stars (0.5 MSun – 8 MSun)
Carbon core (C + O, but sometimes O + Ne) becomes white
dwarf
About size of Earth,
about mass of Sun:
incredibly dense!
(Very!) Slowly radiates heat,
cooling into black dwarf
(Again, none exist yet)
II. Stellar evolution
Massive Stars (t8 MSun)
Fuse hydrogen to helium at core
Convert core to helium, and
contract due to gravity
Have inert core of helium
inside shell of burning
hydrogen
II. Stellar evolution
Massive Stars (t8 MSun)
Star becomes a red supergiant – increased energy
production pushes outer layers away from star; outer
layers cool down
II. Stellar evolution
Massive Stars (t8 MSun)
Core of star continues to contract, reaching helium burning
temperatures
Sound familiar?
These stars too massive to
blow off outer layers.
Instead…
II. Stellar evolution
Massive Stars (t8 MSun)
Core fusion continues
until iron
Can’t fuse iron and gain
energy, so no more
thermal pressure – only
gravity
Entire star collapses
under own weight
II. Stellar evolution
Massive Stars (t8 MSun)
Rebounds off of core, becomes (core-collapse) supernova
Explosion (roughly)
10,000,000,000
times brighter than
Sun – briefly outshines
entire host galaxy
II. Stellar evolution
Massive Stars (t8 MSun)
Outer layers expand as a
supernova remnant
Detectable tens,
hundreds, to thousands
of years later.
What about the core?
II. Stellar evolution
Massive Stars (t8 MSun)
Core becomes either neutron star or black hole.
Neutron star: ≈ 2 MSun in
≈ 10 km radius
Very dense, very strong
magnetic field, very fast
rotation
Supported by strong
nuclear force
II. Stellar evolution
Massive Stars (t8 MSun)
Black hole: so dense
nothing opposes
collapse
Nothing – even light –
can escape after getting
too close (“event
horizon”)
Can’t be directly observed – must be inferred from
presence of accretion disk and/or jet
II. Stellar evolution
Summary:
• Low-mass stars: slowly burn all fuel
• Medium mass stars: burn H, become red giant, separate
into planetary nebula & white dwarf
• Massive stars: burn H all the way to Fe, explode in
supernova, leave behind remnant and either neutron star
or black hole
III. Filling out the stellar zoo
T Tauri variables
Protostars surrounded by gas
cloud in which they formed
Dusty blobs may become planets
Mira variables
Red giants just beginning
to transform into
planetary nebulae
(Sun will be one in 5
billion years)
III. Filling out the stellar zoo
RR Lyrae variables
Regular (periodic) variable star
Low mass stars (≈ 0.8 MSun)
nearing end of life (like Mira vars)
Much brighter than Sun (≈ 40x)
All stars in this class roughly the
same brightness
Used as distance indicator
III. Filling out the stellar zoo
(Classical) Cepheid variables
Regular (periodic) variable star
Massive stars (4-20 MSun) near death
Much brighter than Sun (up to
1,000,000 times brighter)
Strong relation between period of
pulsation and peak brightness
M V = −2.43log10 P − 1.62
Used as distance indicator
III. Filling out the stellar zoo
S Doradus variables
Irregular variables
Also called Luminous Blue Variables
Massive stars (up to 150 MSun), with
brightness > 1 million times the Sun
So massive it’s permanently
unstable
LBV
Very short lifetimes, very
violent ends
Sun
III. Filling out the stellar zoo
Magnetars
Neutron stars with very intense
magnetic fields (up to 1015 times
stronger than Earth’s)
Pulsars
Rapidly spinning neutron stars
Emit light from magnetic pole
As beam sweeps across Earth,
we see regular pulses of light
III. Filling out the stellar zoo
(Classical) Novae
White dwarf with companion star
Companion donates matter, which
forms layer on surface of WD
Eventually, layer dense enough to
start nuclear burning, briefly shining like Sun does
Burning finishes, star dims, but is slightly heavier
Cycle repeats until…
III. Filling out the stellar zoo
Type Ia Supernova
White dwarf becomes
too heavy to support
its own weight
Explodes in runaway
thermonuclear event
Briefly outshines entire
host galaxy (i.e. 1010
times brighter than Sun)
Uniform peak magnitude
Used as distance indicator
III. Filling out the stellar zoo
Type II Supernova
Explosion after massive star
burns all the way to iron in core
Leaves behind remnant, and
neutron star or black hole
X-ray binary system
Compact object (NS or BH) with
a companion, visible in X-rays
Could be two NSs, or NS+BH
IV. Astronomical concepts
Stellar temperature
At birth, heavy stars hotter
Surface temps from 3000 K
to over 50,000 K
Easy to accurately measure
because light curve only
depends on temperature
Temperature determines
color of star
IV. Astronomical concepts
Spectral class
Temp also determines
spectral class of star
Easy way to group
similar stars
IV. Astronomical concepts
Luminosity
Stars release a lot of energy
Solar luminosity: 3.9 x 1026 Joules/sec (93,000,000,000
Megatons of TNT each second)
Other stars measured in units of LSun, not Joules/sec
IMPORTANT:
Luminosity related to temperature
and radius of star
2
L1  R1   T1 
=   
L2  R2   T2 
Cooler star can be brighter if radius large enough!
4
IV. Astronomical concepts
Hertzsprung-Russell diagrams
Way to show information
about groups of stars
Each star plotted by
temp and luminosity
Many, many possible
questions using HR
diagrams. KNOW HOW
TO READ & USE THEM
IV. Astronomical concepts
The distance problem
Everything in astronomy depends on distance from Earth:
• Size & age of Universe
• History & fate of Universe, Sun, stars, Milky Way, etc.
• Size of Milky Way galaxy
• Nearby cosmic neighborhood
• Etc., etc., etc.
Problem: Nearest stars are light-years away. Nobody
makes rulers long enough
How to determine distance, then?
IV. Astronomical concepts
Parallax
Use motion of object against
distant background to get
distance
New unit of measurement:
1 parsec = distance at
which parallax is
one arcsecond
(1 arcsecond: size of ping pong ball from 5 miles away)
Distance-parallax relation:
Dpc = 1
parcsec
IV. Astronomical concepts
Magnitude
Another way to measure brightness
2 kinds: apparent magnitude and absolute magnitude
Apparent magnitude: how bright star appears from Earth
(depends on distance and type of star)
Lower number means brighter star (i.e. magnitude 1
brighter than magnitude 6)
Absolute magnitude: how bright star would be at 10
parsecs from Earth
IV. Astronomical concepts
Distance modulus
Relates apparent (m) and absolute (M) magnitudes to get
distance (d) to star
IV. Astronomical concepts
Light spectrum
Know where radio, infrared (IR), visible/optical, ultraviolet
(UV), and X-ray fall on the spectrum
V. List of objects
Mira
Star at right of UV image
Namesake of Mira variable class of stars
Moving through space, trailing outer layers (so no
planetary nebula)
V. List of objects
W49B
Supernova remnant
Pictured in radio, IR & X-ray
Barrel shape suggests SN was
gamma-ray burst
≈ 1000 years old
≈ 26000 ly from Earth
V. List of objects
Tycho’s SNR
Type Ia supernova remnant
(seen here in IR and X-ray)
SN seen in 1572 (so 440 years
old), observed by Tycho Brahe
≈ 9000 ly from Earth
Expanding at t5000 km/s
V. List of objects
Vela SNR
Type II supernova remnant
(seen here in optical)
Old SNR (≈ 12,000 yrs)
Close to Earth (800 ly)
Left behind pulsar, which
was first confirmation of supernova/pulsar relationship
Huge in night sky (16 times wider than full Moon)
V. List of objects
G1.9+0.3
Type Ia supernova remnant
(shown in X-ray)
Youngest remnant in Milky Way
(just 140 years old)
25,000 ly away, toward galactic
center
V. List of objects
Eta Carinae
Luminous blue variable
(a.k.a. S Doradus variable)
Surrounded by nebula
ejected in 1841 outburst
Binary system, with primary
≈ 100 MSun and ≈ 5x106 LSun
Shown in visible light
Strong contender for next galactic supernova
V. List of objects
SS Cygni
Dwarf nova (recurring bursts
due to accretion disc, not
burning on WD)
Outbursts every 7-8 weeks
Goes from 12th to 8th mag
≈ 370 ly away
Close binary system (stars orbit every 6.5 hrs, d 100,000
miles separation)
V. List of objects
T Tauri
Not a star yet: powered by gravitational contraction, not fusion
Protostar surrounded by dusty
disk
Shown in optical
Strong wind coming off star, heavy accretion of disk matter
onto star (hasn’t reached birth mass yet)
Trinary+ system, & T Tauri may have been ejected
V. List of objects
GRS 1915+105
X-ray binary with star &
black hole
Observed in X-ray as well as
radio (pictured at right)
Famous for “faster-than-light”
expansion of jets
About 40,000 ly distant
V. List of objects
47 Tucanae
Globular cluster
Millions of stars, of
multiple populations
Many pulsars, but no
(observed) planets
≈ 17,000 ly distant
Size of full moon, but very difficult to see from northern
hemisphere – impossible from Europe
V. List of objects
The Trapezium
Open cluster in Orion nebula
(false color visible image here)
5 primary stars, all of which are
O-B stars, > 15 MSun
Only 1600 ly distant, but partially
obscured by nebula
Very compact – telescope needed to resolve individual
stars in cluster
V. List of objects
T Pyxidis
Recurrent (but irregular) classical
nova with WD & companion
WD estimated to be near max.
allowed mass, so may become
Type Ia supernova soon
Normally 15.5 mag, bursts to 7
(2500 times brighter)
V. List of objects
Abell 30
Planetary nebula (shown in
visible light)
Very rare “two-stage”
expulsion of nebula material
≈ 5500 ly distant
Spherical shell marks interaction between two stages
of nebula emission
V. List of objects
RX J0806.3+1527
WD-WD X-ray binary system
Orbital period 320 sec, with
50,000 mile separation
Radiating energy in form of
gravitational waves, moving
inwards at 60 cm/day
Only 1600 ly distant
V. List of objects
V1647 Orionis
Very new protostar of T Tauri type
Rapidly accreting matter from
surroundings, so very variable
brightness
Characterized by large outburst
in 2004 that lasted for 18 months
V. List of objects
M31 V1
Cepheid variable star in nearby
Andromeda galaxy (a.k.a. M31)
Identified by Edwin Hubble
Used to prove that Andromeda
did not lie within Milky Way, i.e.
was an entirely different galaxy
V. List of objects
NGC 1846
Globular cluster in Large
Magellanic cloud, satellite
galaxy of Milky Way
As with 47 Tucanae, more
than one population of
stars visible
Suggests that many globular
clusters have rich history of interaction with galaxies &
molecular clouds
V. List of objects
NGC 3132
Planetary nebula lit by new
white dwarf at center
Central WD over 100,000 K
but only size of Earth
About 2000 ly away
0.4 ly across, expanding at 14 km/s, so about 8000-9000
years old