Download Final review - Physics and Astronomy

Astronomy’s Next 10 Years
Kepler
SOFIA mid-infrared image of part of
the Orion star-forming region.
GMT -- a 25m telescope
TMT -- 30m effective aperture diameter
JWST
WFIRST – Understanding Dark Energy
(formerly the “Joint Dark Energy Mission” [JDEM])
International X-ray Observatory
New Astronomies – Gravitational Waves
LISA
“Astroparticle” Physics
REVIEW
How Far Away are the Stars?
Earth-baseline parallax - useful
in Solar System
Earth-orbit parallax - useful for
nearest stars
Earth-orbit parallax using ground-based
telescopes good for stars within 30 pc (1000 or
so). Tiny volume of Milky Way galaxy. Other
methods later.
Our nearest stellar
neighbors
How Luminous are Stars?
Remember, luminosity of the Sun is
LSun = 4 x1033 erg/s
Luminosity also called “absolute brightness”.
How bright a star appears to us is the “apparent
brightness”, which depends on its luminosity and
distance from us:
apparent brightness α
luminosity / (distance)2
So we can determine luminosity if apparent
brightness and distance are measured:
luminosity  apparent brightness x
(distance)2
Please read about magnitude scale.
Stellar Magnitudes (1)
We measure the apparent magnitude of stars using a
(logarithmic) scale.
A difference of 5 magnitudes = 100 x in brightness.
Astronomers also refer to a star’s absolute magnitude,
which is related to its luminosity.
The visible stars have magnitudes less than about 6.
Larger magnitude = dimmer star.
Smaller magnitude = brighter star.
Brightest star : Sirius, magnitude (V) = -1.5
(Type = A1V)
Stellar Magnitudes (III)
• Apparent magnitude = magnitude we observe by eye, or
measure at the telescope, here on Earth
= dependent on luminosity, and
proportional to 1/distance2
Denoted by lower-case letters, e.g., mV or mB
• Absolute magnitude = apparent magnitude the star
would have if placed at a standard distance (10 pc) from
the Earth
= dependent on luminosity only
Variable Stars (brightness varies periodically) have Different Causes
Intrinsic variables
Luminosity changes periodically,
usually associated with changes
in size (pulsation), and color (spectrum)
time
Periods: hours to weeks, typically
Eclipsing binaries -- example
Binary star seen nearly (not completely) edge-on
Shows changes in the total light due to the
Partial eclipse of one star by another.
Spectral Classes
Strange lettering scheme is a historical
accident.
Spectral Class
Examples
O
B
A
F
G
K
M
Surface Temperature
30,000 K
20,000 K
10,000 K
7000 K
6000 K
4000 K
3000 K
Rigel
Vega,
Sirius
Sun
Betelgeus
Further subdivision: BO - B9, GO - G9, etc. GO
e
hotter than G9. Sun is a G2.
The Hertzsprung-Russell (H-R) Diagram
Red Supergiants
Red Giants
Increasing Mass,
Radius on Main
Sequence
Sun
Main Sequence
White Dwarfs
A star’s position in the H-R diagram depends on its mass and
evolutionary state.
Star Clusters
Open Cluster
Globular Cluster
Comparing with theory, can easily determine cluster age
from H-R diagram.
Star Clusters
Two kinds:
1) Open Clusters
-Example: The Pleiades
-10's to 100's of stars
-Few pc across
-Loose grouping of stars
-Tend to be young (10's to 100's of
millions of years, not billions, but
there are exceptions)
2) Globular Clusters
- few x 10 5 or 10 6 stars
- size about 50 pc
- very tightly packed, roughly
spherical shape
- billions of years old
Clusters are crucial for stellar evolution studies because:
1) All stars in a cluster formed about same time (so about same age)
2) All stars are at about the same distance
3) All stars have same chemical composition
Gas Structures in the ISM
Emission Nebulae or H II Regions
Regions of gas and dust near
stars just formed.
The Hydrogen is almost fully
ionized.
Temperatures near 10,000 K
Sizes about 1-20 pc.
Hot tenuous gas => emission
lines (Kirchhoff's Laws)
Star Formation
Stars form out of molecular gas clouds. Clouds collapse to
form stars (remember, stars are ~1020 x denser than a
molecular cloud).
Probably new molecular clouds form continually out of less
dense gas. Some collapse under their own gravity. Others
may be more stable. Not well understood.
Gravity makes cloud want to
collapse.
Outward gas pressure resists
collapse, like air in a bike pump.
How Long do Stars Live
(as Main Sequence Stars)?
Main Sequence stars fuse H to He in core. Lifetime depends on
mass of H available and rate of fusion. Mass of H in core
depends on mass of star. Fusion rate is related to luminosity
(fusion reactions make the radiation energy).
lifetime
So,
 mass of core
fusion rate

mass of star
luminosity
Because luminosity   (mass) 3,
lifetime
mass

(mass)3
or
1
(mass)2
So if the Sun's lifetime is 10 billion years, a 30 MSun star's lifetime
is only 10 million years. Such massive stars live only "briefly".
Stellar Evolution:
Evolution off the Main Sequence
Main Sequence Lifetimes
Most massive (O and B stars):
millions of years
Stars like the Sun (G stars):
billions of years
Low mass stars (K and M stars): a trillion years!
While on Main Sequence, stellar core has H -> He fusion, by p-p
chain in stars like Sun or less massive. In more massive stars,
“CNO cycle” becomes more important.
Final States of a Star
1. White Dwarf
If initial star mass < 8-12 Msun .
2. Neutron Star
If initial mass > 12 MSun and < 25 ? MSun .
3. Black Hole
If initial mass > 25 ? MSun .
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
- All H converted to He in core.
- Core too cool for He burning. Contracts.
Heats up.
- H burns in hot, dense shell around core:
"H-shell burning phase".
- Tremendous energy produced. Star must
expand.
- Star now a "Red Giant". Diameter ~ 1 AU!
- Phase lasts ~ 109 years for 1 MSun star.
- Example: Arcturus
Red Giant
Helium Runs out in Core
- All He -> C. Not hot enough
-for C fusion.
-
- Core shrinks and heats up, as
-does H-burning shell.
-
- Get new helium burning shell
(inside H burning shell).
- High rate of burning, star
expands, luminosity way up.
- Called ''Red Supergiant'' (or
Asymptotic Giant Branch) phase.
- Only ~106 years for 1 MSun star.
Red Supergiant
"Planetary Nebulae"
- Core continues to contract. Never hot
enough for C fusion.
- He shell dense, fusion becomes unstable
=> “He shell flashes”.
- Whole star pulsates more and more violently.
- Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun ejected.
- Shells appear as a nebula around star, called “Planetary Nebula”
(awful, historical name, nothing to do with planets).
White Dwarfs
- Dead core of low-mass star after
Planetary Nebula thrown off.
- Mass: few tenths of a MSun
- Radius: about REarth
- Density: 106 g/cm3! (a cubic
cm of it would weigh a ton on
Earth).
-
-
- Composition: C, O.
- White dwarfs slowly cool to
oblivion. No fusion.
Evolution of Stars > 12 MSun
Low mass stars never got
past this structure:
Eventual state of > 12 MSun star
Higher mass stars fuse heavier
elements.
Result is "onion" structure with
many shells of fusion-produced
elements. Heaviest element
made is iron. Strong winds.
They evolve more rapidly.
Example: 20 MSun star lives
"only" ~107 years.
Neutron Stars
If star has mass 12-25 MSun , remnant of supernova expected to be a
tightly packed ball of neutrons.
Diameter: 10 km only!
Mass: 1.4 - 3(?) MSun
Density: 1014 g / cm3 !
Rotation rate: few to many times
per second!!!
Magnetic field: 1010 x typical bar
A neutron star over the Sandias?
magnet!
Please read about observable neutron stars: pulsars.
Black Holes
If core with about 3 MSun or more collapses, not even neutron
pressure can stop it (total mass of star about 25 MSun ?).
Core collapses to a point, a "singularity".
Gravity is so strong that not even light can escape.
RS for a 3 MSun object is 9 km.
Event horizon: imaginary sphere around object, with radius RS .
Event horizon
Anything crossing the event horizon,
including light, is trapped
RS
Effects around Black Holes
1) Enormous tidal forces.
2) Gravitational redshift. Example, blue
light emitted just outside event horizon
may appear red to distant observer.
3) Time dilation. Clock just outside
event horizon appears to run slow to a
distant observer. At event horizon, clock
appears to stop.
Do Black Holes Really Exist? Good
Candidate: Cygnus X-1
- Binary system: 30 MSun star with unseen companion.
- Binary orbit => companion > 7 MSun.
- X-rays => million degree gas falling into black hole.
The Three Main Structural Components of the Milky Way
1. Disk
- 30 kpc diameter
- contains young and old stars, gas, dust. Has spiral structure
- vertical thickness roughly 100 pc - 2 kpc (depending on component.
Most gas and dust in thinner layer, most stars in thicker layer)
2. Halo
- at least 30 kpc across
- contains globular clusters, old stars,
little gas and dust, much "dark matter"
- roughly spherical
3. Bulge
- About 4 kpc across
- old stars, some gas, dust
- central black hole of 3 x 106 solar masses
- spherical
New distance unit: the parsec (pc).
Using Earth-orbit parallax, if a star has a parallactic angle of 1",
it is 1 pc away.
If the angle is 0.5", the distance is 2 pc.
1
Distance (pc) = Parallactic angle (arcsec)
Closest star to Sun is Proxima Centauri. Parallactic angle is 0.7”, so
distance is 1.3 pc.
1 pc = 3.3 light years
= 3.1 x 1018 cm
= 206,000 AU
1 kiloparsec (kpc) = 1000 pc
1 Megaparsec (Mpc) = 10 6 pc
Stellar Orbits
Halo: stars and globular clusters swarm around center of Milky Way. Very
elliptical orbits with random orientations. They also cross the disk.
Bulge: similar to halo.
Disk: rotates.
Spiral Structure of Disk
Spiral arms best traced by:
Young stars and clusters
Emission Nebulae
Atomic gas
Molecular Clouds
(old stars to a lesser extent)
Disk not empty between arms,
just less material there.
90% of Matter in Milky Way is Dark Matter
Gives off no detectable radiation. Evidence is from rotation curve:
10
Rotation
Velocity
(AU/yr) 5
Solar System Rotation Curve: when
almost all mass at center, velocity
decreases with radius ("Keplerian")
1
1
10
20
30
R (AU)
observed curve
Milky Way
Rotation
Curve
Curve if Milky
Way ended
where radiating
matter pretty
much runs out.
The Variety of Galaxy Morphologies
Galaxy Classification
Hubble’s 1924 "tuning fork diagram"
bulge less prominent,
arms more loosely
wrapped
increasing apparent
flatness
disk and large
bulge, but no
spiral
Spirals
Ellipticals
barred unbarred
SBa-SBc Sa-Sc
E0 - E7
Irregulars
Irr I
"misshapen
spirals"
Irr II
truly
irregular
I
r
r
A further distinction for ellipticals and irregulars:
Giant
1010 - 1013 stars
10's of kpc across
Dwarf Elliptical NGC 205
Spiral M31
Dwarf Elliptical M32
vs.
Dwarf
106 - 108 stars
few kpc across
How Far Away are Galaxies?
For "nearby" (out to 20 Mpc or so) galaxies, use a very bright class of
variable star called a "Cepheid".
luminosity
average
luminosity
time (days or weeks)
Cepheid star in
galaxy M100
with Hubble.
Brightness
varies over a
few weeks.
(average)
From Cepheids in Milky Way star clusters (with known
distances), it was found that period (days to weeks) is
related to average luminosity.
So measure period of Cepheid in nearby galaxy, this gives
star's average luminosity. Measure average apparent
brightness. Now can determine distance to star and galaxy.
Has been used to find distances to galaxies up to 25 Mpc.
Structures of Galaxies
Groups
A few to a few dozen
galaxies bound together by
their combined gravity.
No regular structure to them.
The Milky Way is part of the Local Group of
about 30 galaxies, including Andromeda.
About 20 dwarfs in Local Group found since
2004, most with Sloan Digital Sky Survey in NM!
Clusters
Larger structures typically containing thousands of galaxies.
Center of Virgo Cluster of about 2500 galaxies
Center of the Hercules Cluster
Galaxies orbit in groups or clusters just like stars in a stellar cluster.
Most galaxies are in groups or clusters.
Superclusters
Recognizable structures containing clusters and groups. 10,000's of
galaxies.
50 Mpc
The Local Supercluster consists of the Virgo Cluster, the Local Group
and many other groups.
Galaxy Interactions and Mergers
Galaxies sometimes come near each other, especially in groups and clusters.
Large tidal force can
draw stars and gas
out of them => tidal
tails in spirals.
Galaxy shapes can
become badly
distorted.
Schematic of galaxy formation
Subsequent mergers of large galaxies also important for galaxy evolution. Large
galaxies continue to swallow small ones today: “galactic cannibalism”.
Cosmology: The Study of the Universe as a Whole
Given no evidence of further structure, assume:
The Cosmological Principle
On the largest scales, the universe is roughly homogeneous (same at all
places) and isotropic (same in all directions). Laws of physics same.
Hubble's Law might suggest that everything is expanding away
from us, putting us at center of expansion. Is this necessarily true?
(assumes
H0 = 65
km/sec/Mpc)
So if the CP is correct, there is no center, and no edge to the Universe!
Best evidence for CP comes from Cosmic Microwave Background
Radiation (later).
The Big Bang
All galaxies moving away from each other. If twice as far away from
us, then moving twice as fast (Hubble's Law). So, reversing the
Hubble expansion, all separations go back to zero. How long ago?
H0 gives rate of expansion. Assume H0 = 75 km / sec / Mpc. So galaxy at
100 Mpc from us moves away at 7500 km/sec. How long did it take to
move 100 Mpc from us?
time =
=
=
distance
velocity
100 Mpc
7500 km/sec
13 billion years
(Experts note that this time is just
1
).
H0
The faster the expansion (the greater H0), the shorter the time to get to
the present separation.
Big Bang: we assume that at time zero, all separations were infinitely
small. Universe then expanded in all directions. Galaxies formed as
expansion continued.
If all distances increase, so do wavelengths of photons as they travel and
time goes on.
When we record a photon from a distant source, its wavelength will be
longer. This is like the Doppler Shift, but it is not due to relative motion of
source and receiver. This is correct way to think of redshifts of galaxies.
The Cosmic Microwave Background Radiation (CMBR)
A prediction of Big Bang theory in 1940's. "Leftover" radiation from
early, hot universe, uniformly filling space (i.e. isotropic,
homogeneous). Predicted to have perfect black-body spectrum.
Photons stretched as they travel and universe expands, but spectrum
always black-body. Wien's Law: temperature decreases as wavelength
of brightest emission increases => was predicted to be ~ 3 K now.
The Cosmic Microwave Background Radiation (CMBR)
A prediction of Big Bang theory in 1940's. "Leftover" radiation from
early, hot universe, uniformly filling space (i.e. isotropic,
homogeneous). Predicted to have perfect black-body spectrum.
Photons stretched as they travel and universe expands, but spectrum
always black-body. Wien's Law: temperature decreases as wavelength
of brightest emission increases => was predicted to be ~ 3 K now.
That the CMBR comes to us from every direction is best evidence
that Big Bang happened everywhere in the universe. That the
temperature is so constant in every direction is best evidence for
homogeneity on large scales.
IF the Big Bang happened at one point in 3-d space:
Later, galaxies form and fly
apart. But radiation from Big
Bang streams freely at speed
of light! Wouldn't see it now.
The Early Universe
The First Matter
At the earliest moments, the universe is thought to have been
dominated by high-energy, high-temperature radiation. Photons had
enough energy to form particle-antiparticle pairs. Why? E=mc2.
pair
production
annihilation
Primordial Nucleosynthesis
Hot and dense universe => fusion reactions.
At time 100-1000 sec (T = 109 - 3 x 108 K), helium formed.
Stopped when universe too cool. Predicted end result: 75% hydrogen,
25% helium.
Oldest stars' atmospheres (unaffected by stellar nucleosynthesis)
confirm Big Bang prediction of 25% helium.
Other Measurements Support this Model of the Universe
In 1920's, Hubble used Cepheids to find distances to galaxies. Showed
that redshift or recessional velocity is proportional to distance:
V = H0 x D
velocity (km / sec)
(Hubble's Law)
Distance (Mpc)
Hubble's Constant (km / sec / Mpc)
Or graphically. . .
Current estimate:
H0 = 65 -75 km/sec/Mpc
If H0 = 70 km/sec/Mpc, a
galaxy at 1 Mpc moves
away from us at 70 km/sec,
etc.
The Expansion of the Universe Seems to be Accelerating
The gravity of matter should retard the expansion. But a new distance
indicator shows that the expansion rate was slower in the past!
Redshift (fractional shift in
wavelength of spectral
lines)
Type I supernovae: from ones in nearby
galaxies, know luminosity. In distant galaxies,
determine apparent brightness. Thus
determine distance. Works for more
than 3000 Mpc. From redshifts, they are
not expanding as quickly from each
other as galaxies are now.
Taking this into account, best age estimate of
Universe is 13.8 Gyrs.
H0 was
smaller in
past (i.e.
for distant
galaxies)
Current Theories : Universe is 70% + “Dark Energy” !!
Supported by –
CMBR measurements
Supernovae Ia measurements
Statistical studies of
Galaxy Clusters
Successes of the Big Bang Theory
1) It explains the expansion of the universe.
2) It predicted the cosmic microwave background radiation, its
uniformity, its current temperature, and its black-body spectrum.
3) It predicted the correct helium abundance (and lack of other
primordial elements).
Is There Life Elsewhere in the Universe?
Requirements for Life:
- Elements to build cells (C, H, O most important)
- Energy source to make chemical reactions happen to fuel
metabolism
- Liquid medium – water allows organic (C-based) molecules to
dissolve and be carried into cells for metabolic reactions
These conditions occurred on Earth, but it's not clear whether life was
then inevitable. Also, how unique are they?
Life Elsewhere in the Milky Way:
To address question of whether other intelligent life exists in the Milky
Way, Frank Drake formulated the Drake Equation.
The equation is usually written:
N = R* • fp • ne • fl • fi • fc • L
Where,
N = The number of civilizations in The Milky Way Galaxy whose electromagnetic
emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs
of their existence into space.
L = The length of time such civilizations release detectable signals into space.
Allen Telescope Array – radio SETI
Optical SETI
[Harvard U.]
SETI League
(Amateur Radio Astronomers)
Extrasolar Planets
More than 300 discovered.
Main technique: detect Doppler Shift due to wobble of
star caused by unseen planet. Biased – easier to detect
heavier (Jupiter-class) planets.
Second technique: detect eclipse (transit) of planet –
Kepler mission.
Third technique: detect wobble in star's position in sky
due to unseen planet (astrometric method).
Fourth technique: direct imaging of planet.
Fifth technique: microlensing
Future missions hope to detect
Terrestrial planets
Astrometric method
http://planetquest.jpl.nasa.gov/science/finding_planets.cfm