Transcript The Galaxy Presentation 2011

The Milky Way Galaxy
Overview & History
 Our Galaxy is a collection of stellar and
interstellar matter – stars, gas, dust, neutron
stars, black holes – held together by gravity.
 Our view of the Galaxy….
History of Galactic (& Extragalactic) Astronomy
1610 -Galileo discovered the Milky Way is comprised of many stars
1755 - Immanuel Kant theorized that the galaxy has a planar
structure, some nebulae might actually be entire other galaxies or
island universes
1774 -1781 - Messier catalog compiled including Andromeda
galaxy as M31
1781-1802 - William and Caroline Herschel conducted first “all-sky
survey” and cataloged 5000 nebulae, resolving some into their
individual stars
Later (1845) William Parsons (Lord Rosse), using a 72-inch
telescope, classified the nebulae into featureless ellipticals and
whirlpool-like spiral nebulae
Much later (1888) Dreyer would add to their list to create the
New General Catalog (NGC) and Index Catalog (IC)
1785 - Herschel attempted to determine the shape and size of Galaxy
Assumptions:
All stars have same intrinsic brightness
Star are arranged uniformly throughout the MW
He could see to the edge of the system
then dmax = 100.2(m_max-M+5)
SUN
Herschel could not account well for the effects of dust.
More dust along the disk causes the distribution of stars to drop-off artificially –
objects more than a few kpc from the Sun are obscured by dust.
•Early 1900’s - Kapteyn used stellar parallax to estimate the true size of
the Galaxy  Kapteyn Universe
•10kpc diameter and 2kpc thick with the Sun less than a kpc
from the center (rather heliocentric)
•Tried to estimate Rayleigh scattering due to ISM gas but
determined it to be insignificant (because most obscuration is
due to ISM dust absorption which has a smaller  dependence)
•Shapley (1919) noted that globular clusters are distributed
asymmetrically in the sky and that if one assumes they are distributed
about the center of the galaxy, this implies the Sun in not near the
center
•Estimated distances to GCs using
variable stars and P-M relationship
•Concluded size to be 100kpc
with Sun 15kpc from center
Still wrong…didn’t account for
dust absorption which makes
things look further away
In 1920, the National Academy of Science hosted the Great Debate
concerning the nature of the Spiral Nebulae: were they island universes
outside of the MW?
•Shapley had MW size too big and therefore argued
“NO”, they are part of MW
•Curtis (and many others at that time) believed the
Kapteyn model of a much smaller MW and argued
“YES”, they are separate galaxies beyond the extent of
the MW.
His notes about a variable star
Subsequent claims that the
SMC and LMC are about 3234 kpc away
In 1922-1924 Edwin Hubble resolved the controversy
using the superior 100-inch telescope at Mount Wilson.
He observed Cepheid variables in Andromeda and, using
the P-M relation, determined its distance at 300kpc -well outside of the MW (still off by a factor of 2 due to poor
Cepheid calibrations)
Note the date: 6 Oct 1923
Also in the early 1900’s, Lindblad was doing the first kinematic studies of the MW
•Estimated mass in MW from all stars in Kapteyn’s model
•Determined velocities of GCs to be as high as 250 km/s - much higher than
escape velocity of Kapteyn model
Lindblad (1927) developed first detailed kinematic model of MW
•Spherical component with random motions - HALO
•Flattened component with rotational motion - DISK
•Measured disk to rotate at 200 to 300 km/s near Sun
Oort (1927,1928) developed a complete
theory of Galactic stellar kinematics
In 1944, Baade used the 100-inch Mount Wilson telescope to resolve stars
in the inner regions of nearby spirals and elliptical galaxies.
•Spiral spheroids and Ellipticals contain red giant stars
•Spiral arms in disks contain blue supergiants
•Population I: blue stars and open clusters accompanied by gas and
dust in the disks of spiral galaxies
•Population II: red stars and globular clusters in spheroids and
elliptical galaxies
Plotting stars on HR diagrams
showed that the populations
also differed in age and it was
subsequently determined that
they differed in metallicity:
Pop I young and metal rich
Pop II old and metal poor
Structure of
the Milky
Way
The Milky Way
Components of the Milky
Way
Disk
Population I stars
Open clusters
HII regions
Bulge
Population II stars
Halo
Globular clusters
What MWG
might look like
as seen from
above, based
on recent data
from the
Spitzer Space
Telescope
(infrared)
Globular Clusters in our Galaxy
Shapley realized that
the GCs map out the
true extent of our
galaxy!
Galactic Halo
The hub of the galaxy
is the Galactic Center about 8 kpc from the
Sun
Actual size of the Galaxy and the Sun’s
location not fully determined until 1950’s
The Halo
“senior citizens”; very few newborns
old, red and dim; much smaller than Sun
Far fewer heavy elements than Sun (< 0.02%)
Lack of chem. Enrichment – stars formed early in MWG
history before many supernovae explosions
Very few detectable molecular clouds; virtually gas-free
Lack of gas caused star formation to cease early on
Our neighborhood
- More active than halo; typifies much of galactic disk
- Within 33 l.y. (10 pc) are over 300 stars
- Most are dim, red type M
- A few (Sirius, Vega, Altair, Fomalhaut) are bright, white
stars younger than Sun
- No very massive, short-lived stars (type O or B)
- We are in a quiet “suburb,” but it was not always that way.
- Hot X-Ray emitting gas coming from nearby in all
directions. Surrounding hot gas is region of cooler gas (100
light years distant)
- We and all our stellar neighbors live inside a hot bubble!
- The existence of this LOCAL BUBBLE means that a
number of supernovae must have detonated within our
stellar neighborhood over past several million years!
Hot Star Hangouts
- Hot, massive stars live fast and die young; never get far
from the molecular cloud from which they were formed.
Therefore, they are found in clusters close to molecular
clouds
- IONIZATION NEBULAE (colorful, wispy glowing gas)
found throughout galactic disk, particularly in spiral arms
Also called EMISSION NEBULAE or H II (abbrev. For
ionized Hydrogen)
- Energy that powers H II comes from nearby hot stars that
irradiate them with UV photons, which ionize and excite
the atoms causing them to emit light
- The Orion Nebula is the most famous example (p. 555)
Differential Rotation
Everything in the Galaxy orbits around
the Galactic center
Material closer to the center travels on
faster orbits (takes less time to make
one full orbit)
Similar to the way the planets orbit the
Sun
Orbital periods at different distances
from GC tell us the distribution of mass
in the Galaxy
The Milky Way in motion
- Halo - Chaos of stars soaring on randomly oriented orbits
- Disk – organized “stately” rotation
- All stars in disk orbit in same direction like a merry-goround
- Individual stars bob up and down as they orbit
- The up and down motions spread the disk to a width of
about 1,000 l.y.
- Diameter of the disk is much, much larger – 100,000 l.y.
- Orbits of stars in halo and disk are much less organized
- Arcturus – 4th brightest star in sky is a halo star
- Sun’s orbital path is called the SOLAR CIRCLE
- Its radius is 28,000 from the galactic center
- By measuring the speed of globular clusters relative to
the Sun, it has been determined that the Sun (and its
neighbors) orbit the center of the Galaxy at a speed of 220
km/sec
- It takes the Sun 230 million years to do one orbit
- The last time the Sun was on this side of the Galaxy,
dinosaurs ruled the Earth
Slight Aside on Determining Distances
 We get distances to
nearby planets from radar
ranging.
 That sets the scale for the
whole solar system (1 AU).
 Given 1 AU plus stellar
parallax, we find distances to
“nearby” stars.
Use these nearby stars,
with known Distances,
Fluxes and Luminosities, to
calibrate Luminosity classes
in HR diagram.
Then spectral class + Flux yields Luminosity + Distance for farther stars
(Spectroscopic Parallax).
Cepheids (variable stars) use P-M relation to determined distances to nearby
galaxies
Henrietta Leavitt & the Cepheid P-L
Relationship
see original paper: here
Light curve of a Cepheid variable
Large & Small Magellaic Clouds
Period versus magnitude of Cepheids in SMC
1923 - Hubble
Measures
Distance to M 31
using Cepheid
Variables
Discovery of
Cepheids in M 31
Debate
OVER!
100-inch Hooker Telescope, Mt. Wilson
Edwin Hubble
The HR Diagram: Spectroscopic “Parallax”
Example:
1) Determine Temperature from color
or spectral type.
Main Sequence
2) Determine Luminosity based on
Main Sequence position.
3) Compare Luminosity with Flux
(apparent brightness).
4) Use inverse square law to
determine distance.
Flux =
Luminosity
4d2
The HR Diagram: Luminosity & Spectroscopic Parallax
What if the star doesn’t happen to lie on the Main
Sequence - maybe it is a red giant or white dwarf???
We determine the star’s Luminosity Class based on its
spectral line widths:
A star
Spectral lines
get broader
when the
stellar gas is at
higher
densities indicates
smaller star.
Supergiant
A star
Giant
A star Dwarf
(Main Sequence)
Wavelength

The HR Diagram: Luminosity Class
Bright Supergiants
Supergiants
Bright Giants
Giants
Sub-giants
Main-Sequence (Dwarfs)
Distribution of Gas
To understand star formation on a
galactic scale, we need to know how the
interstellar gas is distributed
Radial distribution?
Degree of confinement to the disk?
Constant thickness?
Is galactic plane truly flat or bumpy?
“Dust Happened”
At visible wavelengths, the center of our galaxy suffers ~ 30 mag of
extinction by dust!! Even with big modern telescopes, we cannot see very
far in the plane of our galaxy at visible wavelengths
It is the hot, young massive stars
that trace out the spiral structure
Galaxy Evolution Explorer (GALEX)
sees UV light
Distribution of mass in the Milky Way Galaxy
- ROTATION CURVE – plots rotational velocity vs. distance
from center
- Merry-go-round – outer horses make larger circles and
thus travel faster than inner horses. Opposite is true for
Solar System rotation curve – inner planets move at faster
speeds than outer planets (because most of SS mass in the
Sun)
- Rotation curve for MWG is mysterious
- Most of mass of MWG is in the halo
- Halo might outweigh all of the disks stars by a factor of 10however, only a small number of halo stars?
- Much of the halo’s mass is in the form of DARK MATTER
- If the more distant stars orbit faster – shouldn’t the spiral
arms look like a tightly- wound coil?? (pp. 560-562)
Spiral Structure
 Many external galaxies
show spiral structure
 Hard to “see”
morphology of MW
(since we are in it!)
 Use other galaxy’s
properties to determine
the nature of the WM
and conclude we are in
a spiral galaxy
Rotation Curve of
MWG - v is almost
constant or slightly
increasing with
distance from center!
Most other galaxies
have similar rotations
curves.
Dilemma!! This requires that
mass is distributed far out in
these galaxies, but images seem
to show that stars aren’t doing
this! - DARK MATTER!
In other galaxies, HII regions and OB associations trace
out spiral arms.
• Using spectroscopic
parallax, we can place the
nearby O and B stars at
their proper distances.
• They appear to
delineate spiral arms.
• Since O and B stars are
young objects, spiral arms
are associated with star
formation.
•Problem: Can’t see very
far in the optical…
Large scale surveys of CO in the Galaxy identify many Giant
Molecular Clouds. If spiral arms are associated with star formation,
then they must also be traced out by the locations of GMCs.
•GMC positions interior to Sun’s orbit in
Galaxy have some ambiguity
•Bits and pieces of arms
•Less distance ambiguity outside of Solar
orbit, and better evidence of arm-like
morphology
The Galactic Center
 Inner 500pc of Galaxy
 Extinction makes optical
studies impossible - use
radio or IR
 Observe ionized gas,
line emission, dust, star
clusters
 Resolution greatly
improved with VLA and
VLBI observatories, plus
sensitive IR arrays
- The center of the MWG lies in the constellation
Sagittarius.
- Interstellar dust prevents us from seeing how bright this
region actually is
- The Milky Way is transparent to long-wavelength
radiation (infrared, radio) so those types of radiation are
used to study the galactic center.
- Swirling clouds of gas and a cluster of several million
stars
- Bright radio emission traces out magnetic fields that
thread this region
- In the core of it sits an even brighter radio emission
called Sagittarius A (SgrA)
- The motions of gas and stars in SgrA indicate that it contains
a few million solar masses in a region no bigger than 3 lightyears.
- Astronomers suspect SgrA is a black hole weighing 2.5
million solar masses; this is like no other observed black hole.
- Other black hole candidates like Cygnus X-1 accrete matter
from their companions and radiate brightly in X-Rays. SgrA
radiates only very faint X-Rays
- Many other observed galaxies have strong X-ray emissions
from their centers
- Explanation #1: possible that SgrA contains a huge black
hole that has “run out of gas” to accrete
- Explanation #2: possible that it might be consuming the
energy contained in the accreting matter before it has a
chance to escape as radiation
- The nature of SgrA remains mysterious
Galactic Center
Optical vs Radio observations
Galactic Center:
75pc
150pc
Radio Schematic
•Radio emission shows
bent arc of gas,
filamentary structure
•Also seen in IR
•Thermal and synchrotron
radiation
•X-ray emission (produced
when electrons from
filaments collide with colder
gas cloud) gives gas
temperatures of T=107 to
108 K
•Could result from past SN
explosions
Star Formation in the Galactic Center
•Molecular material in inner 200pc relatively hot and
dense: 104 per cm3 and 70 K
•High velocity dispersion (50 km/s) of molecules
•Mass: 108 Msun
•High density helps star formation but high temps don’t
•SF rate ~ 1Msun/year
Supermassive Black Hole in GC
Radio image (80 pc
across) shows
feature SgrA and
radio filaments
Radio image (10
pc across) shows
feature known as
SgrA* - thought to
be position of
SMBH
Investigate IR
stellar motions in
region about 1pc
across (~few ly)
to estimate BH
mass
•Measure proper motions of stars in GC
•Adaptive optics at Keck improved groundbased resolution to 0.5” in IR (stellar positions
measured to 0.002”)
•90 stars identified and proper motions (largest
at 1400 km/s!) centered about SgrA* to within
0.1”
•Velocities consistent with Keplarian motion (all
mass at center)
•M = 2.6 +/- 0.2 x 106 Msun
Curvature of the paths near SgrA* constrain the
volume of the mass to ~ Schwarzchild radius (few x
106 km), supporting SMBH theory.
Additional evidence - x-ray emission
•Chandra X-ray image of Sgr A*
showing nucleus and several thousand
other X-ray sources.
•During 2-week observation period,
several X-ray flares occurred.
•Rapidity of flares indicates they
originate near the Schwarzchild radius
of the BH.
•Even during the flares, X-ray emission
from the nucleus is relatively weak.
Suggests that Sgr A* is a starved black
hole, possibly because explosive
events in the past have cleared much
of the gas from around it.