Article on Milkyway Galaxy
Transcript of Article on Milkyway Galaxy
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The following are few articles on Milkyway Galaxy
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Lecture 23 1stronomy 1 2
The Milky Way
Galaxy
Our Home Away
From Home
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Lecture 23 2stronomy 1 2
Group of stars are called galaxies
Our star, the Sun, belongs to a system called
The Milky Way Galaxy
The Milky Way can beseen as a band of stars
in the night sky
Galaxies
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Lecture 23 3stronomy 1 2
The Milky Way is basically a disk of stars with a largecentral bulge
The central bulge contains the galactic nucleus
Most of the stars in the Milky Way are contained within the
Galactic Disk
The plane of the Galactic Disk is almost perpendicular to
the plane of our Solar System
The Milky Way Galaxy
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Lecture 23 4stronomy 1 2
Milky Way Dimensions
The central bulge which contains densely packed older
stars is ~ 16,000 light years in radius
The radius of the galactic disk is ~ 50,000 light years
The disk contains mainly young stars as well as
interstellar gas and dust
The thickness of the galactic disk is ~ 2,000 light years
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Lecture 23 5stronomy 1 2
Where is Our Solar System?
Initial attempts to find our solar system's location
were to simply count the numbers of stars in
various regions of the sky
What was found was that there were equal numbersof stars in all regions
Therefore, the solar system is at the center of the
galaxyThis is incorrect!
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Lecture 23 6stronomy 1 2
Where is Our Solar System?
Why is this result incorrect?Assumed that space was a vacuum
However there is a large amount of interstellar dust
This interstellar dust absorbs the light from
the stars and if a star is far enough away, its
light is totally absorbed -
Interstellar dust is also denser towards the
center of the galaxy
Interstellar Extinction
So how did we find our location?
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Lecture 23 7stronomy 1 2
Where is Our Solar System?The globular clusters that surround the Milky
Way were the Key
Within these globular clusters are Cepheid variable
stars
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Lecture 23 8stronomy 1 2
Cepheid Variable Stars
Cepheid Variable stars are stars whoseluminosity varies precisely as a function of time
Typical classical Cepheids pulsate with periods of
a few days to months
Variability comes from a dynamic balance between
gravity and pressure - they have large oscillations
around stability
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Lecture 23 9stronomy 1 2
The usefulness of these stars comes from their
periodluminosity relation
Cepheid Variable Stars
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Lecture 23 10stronomy 1 2
Where is Our Solar System?
Using the measured variation and the measured
apparent luminosity of these Cepheid Variables, it
is possible to deduce the distance to these globularclusters
Then using the known directions and the measured
distances it was seen that the globular clusters were
spherically centered about some point that was not
the Earth
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Lecture 23 11stronomy 1 2
It was found that the Solar system was ~ 30,000
light years from this central point
We are not at the Center
Where is Our Solar System?
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Lecture 23 12stronomy 1 2
Our Galactic Year
It has been determined that our Sun is moving at aspeed of ~200 km/sec
Using this speed and an orbital radius of 28,000 light
years, it takes the Sun 250 million years to make one
complete trip around the center of the galaxy
This time of 250 million years is
called the Galactic Year
Using the time it takes to go around the galaxy
and the age of our Sun, we have gone around
~60 times
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Lecture 23 13stronomy 1 2
Structure of the Milky Way
The general shape is a flat disk, 2000 light yearsthick and 100,000 light years in diameter, with a
central bulge
Is there any internal structure to the Milky Way?
Are the stars spread uniformly or distributed
otherwise?
Standard atomic transitions yield visible and
ultraviolet radiation
These wavelengths cannot be used because of
interstellar extinction
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Lecture 23 14stronomy 1 2
The key to determining the structure of the MilkyWay is atomic hydrogenconsisting of an electron
orbiting a nucleus made up of only a proton
Both the proton and the electron have Spin which can
be considered as an inherent angular momentum
The spins of the proton and electron can be aligned
in one of two ways
Parallel Antiparallel
Structure of the Milky Way
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Lecture 23 15stronomy 1 2
The energy for the state where the spins are parallel is higherthan for the state where the spins are anti-parallel
The hydrogen atom, if it is in the spin parallel state, can make
a transition to the spin anti-parallel state releasing energy
When a transition does a occur, energy is released
corresponding to a wavelength of 21 cm, which is in the radio
portion of the electromagnetic spectrum
This radiation can pass through the interstellar medium
unaffected
In 1951 the first radio telescope was built to detect the 21 cm
radiation from the hydrogen transition
Structure of the Milky Way
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Lecture 23 16stronomy 1 2
The key to using this 21 cm radiation is the
Doppler shift
A hypothesis is first formulated as to thestructure of the galaxy
Then predictions are made and then compared to
the experimental data
Structure of the Milky Way
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Lecture 23 17stronomy 1 2
Simplest Hypothesis:
Galaxy is a uniform non-rotating disk
All the radiation will be 21 cm
This is not what is seen
Measure the energy as a
function of direction
Relative intensity as a function
of direction gives the relative
densitiesShould then correlate with known densities as a
function of position
Structure of the Milky Way
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Lecture 23 18stronomy 1 2
Slightly more complicated hypothesis:
Uniform density with galaxy rotating
Radiation will be Doppler
shifted by an amount
dependent on the radial
velocity along the line of sight
Again this is not what is seen
Since the density is uniform, the
energy received will be atuniformly distributed
wavelengths
Structure of the Milky Way
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Lecture 23 19stronomy 1 2
More complicated hypothesis:Galaxy is rotating and has non-uniform density
Radiation is Doppler shifted
by an amount dependentupon the velocity along the
line of sight
This is in fact what is seen
Since Density is non-uniform,
prominent peaks in the energydistribution as a function of
wavelength
Structure of the Milky Way
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Lecture 23 20stronomy 1 2
The Spiral Milky Way Galaxy
From the shifted wavelengths, the components of the
velocities along the line of sight can be determined
From these radialvelocities, the fact that
the Milky Way Galaxy
is a spiral structure was
determined
G l ti St t
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Lecture 23 21stronomy 1 2
This artists conception shows the various parts of our
galaxy, and the position of our Sun
Galactic Structure
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Lecture 23 22stronomy 1 2
The galactic halo and globular clusters formed veryearly
The halo is essentially spherical
All the stars in the halo are very old, and there is
no gas and dustThe galactic disk is where the youngest stars are, as
well as star formation regions
emission nebulae and large clouds of gas and
dustSurrounding the galactic center is the galactic
bulge, which contains a mix of older and younger
stars
Galactic Structure
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Lecture 23 23stronomy 1 2
This infrared view of our galaxy shows much more
detail of the galactic center than the visible-light viewdoes, as infrared is not absorbed as much by gas and
dust
Galactic Structure
G l i S
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Lecture 23 24stronomy 1 2
Stellar orbits in the
disk move on a plane
and in the same
direction
Orbits in the halo
and bulge are much
more random.
Galactic Structure
The Formation of the Milky Way
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Lecture 23 25stronomy 1 2
Any theory of galaxy formation should be able to
account for all the properties below
The Formation of the Milky Way
The Formation of the Milky Way
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Lecture 23 26stronomy 1 2
The formation of the
galaxy is believed to be
similar to theformation of the solar
system, but on a much
larger scale
The Formation of the Milky Way
Galactic Spiral Arms
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Lecture 23 27stronomy 1 2
We have see from measurements of the position and
motion of gas clouds, that the Milky Way has a spiralform
Galactic Spiral Arms
Wh Th S i l A s
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Lecture 23 28stronomy 1 2
Why The Spiral Arms
We have determined that the Milky Way Galaxyhas spiral arms
Other galaxies also have spiral arms,
and other galaxies do not
Some galaxies that have spiral arms,
have arms that are poorly defined
Data show that the stars on the outside edge of
our galaxy are moving at approximately the same
velocity as stars that are closer inward
Why The Spiral Arms
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Lecture 23 29stronomy 1 2
If the spiral arms are an actual part of the
structure of a spiral galaxy, then the spiral armsshould have wound up tightly by now
But they have not! Why not?
Why The Spiral Arms
Why The Spiral Arms
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Lecture 23 30stronomy 1 2
The spiral arms are not a physical reality in
the normal senseWhat the Milky Way has is a pattern of
spiralness that is moving through the galaxy -
like waves in water
There are density waves moving through the stars
and interstellar dust, temporarily piling up
material
These density waves are moving at a rate of ~300 km/s
The interstellar medium can only support waves
up to 10 km/s
Why The Spiral Arms
Why The Spiral Arms
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Lecture 23 31stronomy 1 2
Shock waves develop in front of the leading edge
of the density waveThe interstellar medium can not move out of the
way fast enough
Matter is compressed to highdensities, densities that are
high enough for star formation
to begin
The spiral arms will thereforecontain young, hot stars, while
the region between the arms
contains older stars
Why The Spiral Arms
Why The Spiral Arms
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Lecture 23 32stronomy 1 2
The origin of the spiral arms is not yet understood
Why The Spiral Arms
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Lecture 23 33stronomy 1 2
Precessing Orbits
A star's movement through the galaxy is affectedby the other stars in the galaxy
The paths of the stars are not circular,
but are elliptical
These ellipses appear to precess
The elliptical orbits of the stars are notindependent of each other, but are correlated
Rotation Rates
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Lecture 23 34stronomy 1 2
Rotation RatesNot all parts of the galaxy are rotating at the
same rate Differential RotationThe measurement of stars
velocities show that the
velocities increase rather
sharply, like that of a rigidbody, and then continue to
increase with a slow rise
This behavior for the orbital velocities cannot be
accounted for in a straight forward mannerIt was expected that the rotation velocities would
decrease like Keplerian orbits as a function of
distance from center
G l ti M
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Lecture 23 35stronomy 1 2
The orbital speed of an object depends only on theamount of mass between it and the galactic center
Galactic Mass
Galactic Mass
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Lecture 23 36stronomy 1 2
Once all the galaxy is within an orbit, the velocity
should diminish with distance, as the dashed curve
shows
It doesnt
More than twice the mass of the galaxy would have to
be outside the visible part to reproduce the observed
curve.
Galactic Mass
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Lecture 23 37stronomy 1 2
What could this dark matter be?
It is dark at all wavelengths, not just the visible
Stellar-mass black holes?
Probably no way enough of them could have beencreated
Brown dwarfs, faint white dwarfs, and red dwarfs?
Currently the best star-like option
Weird subatomic particles?
Could be, although no direct evidence so far
Galactic Mass
Galactic Mass
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Lecture 23 38stronomy 1 2
A Hubble search for red dwarfs turned up too few to
account for dark matterIf enough existed, they should have been detected
Ga act c ass
Galactic Mass
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Lecture 23 39stronomy 1 2
The bending of spacetime can allow a large mass to act as a gravitational
lens
Observation of such events suggests that low-mass white dwarfs couldaccount for as much as 20% of the mass needed
The rest is still a mystery.
Center of Milky Way Galaxy
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Lecture 23 40stronomy 1 2
y y y
However we can use radio and infrared wavelengths
The center of ourgalaxy is in the
general direction of
the constellation
Sagittarius
The Galaxy's center is difficult to
observe using visible light becauseof interstellar extinction
Center of Milky Way Galaxy
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Lecture 23 41stronomy 1 2
These images in infrared, radio, and X-ray offer a
different view of the galactic center
y y y
Center of Milky Way Galaxy
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Lecture 23 42stronomy 1 2
Center of Milky Way Galaxy
The center of the MilkyWay is very bright in
the radio wavelengths
The brightest infrared
source comes from
Sagittarius A and within
this is a very bright radiosource Sgr A* which
marks the center of the
galaxy
Center of Milky Way Galaxy
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Lecture 23 43stronomy 1 2
The galactic center appears to have
A stellar density a million times higher than near
Earth
A ring of molecular gas 400 pc across
Strong magnetic fields
A rotating ring or disk of matter a few parsecs
across
A strong X-ray source at the center
Center of Milky Way Galaxy
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Lecture 23 44stronomy 1 2
Apparently, there is an enormous black hole at the
center of the galaxy, which is the source of thesephenomena
An accretion disk surrounding the black hole emits
enormous amounts of radiation
Observations on three stars that areorbiting the core region at a distance
ranging from ~1000 A.U. to ~2700
A.U. seem to show them orbiting a
common pointMeasurements indicate that the mass is roughly
3.7 million solar masses a black hole whose size
would extend out to the orbit of Mars
Center of Milky Way Galaxy
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Lecture 23 45stronomy 1 2
Center of Milky Way Galaxy
There are other interestingobjects at the center of the
Milky Way
Observations have been
made of the remnant of asupernova explosion that
had occurred near the Sgr
A* and that may have at one
time fed the black hole
There is still much to learned about the center of the
Milky Way
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The
Milky
WayGalaxy
Th fi d i i f h
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The first description of the
formation of the Galaxy was
published by the Germanphilosopher Immanuel Kant
(1724-1804) in his 1755
book, theAllgemeine Natur-
geschichte und Theorie des
Himmels. The graphic inour book shows the same
basic idea.
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Because the Sun in situated in the plane of the Milky
Way, as we scan around the sky, we see a band oflight. Perpendicular to the plane of the Milky Way
there are many fewer stars. Some constellations
have lots of star clusters, like Sagittarius, Scutum,
Scorpius, and Cygnus. Other constellations such as
Coma Berenices cover the North Galactic Pole. Wesee few Galactic star clusters here but many external
galaxies.
Wm. Herschel (1738-
1822) and his sister
Caroline (1750-1848)
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William Herschel's 1785 model of the Galaxy placed us
close to the center of flattened system of stars. Buthe did not know about the effect of interstellar dust
on his star gauges.
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In the early part of the 20thcentury J. C. Kapteyn (1851-1922)
produced a very similar model of the Galaxy to that of
Herschel. He too did not take into account the effect of
interstellar dust.
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Meanwhile, a very important tool for Galactic astronomy
was being exploited by a young astronomer from Missouri.In 1912 the Harvard astronomer Henrietta Leavitt (1868-1921)
discovered that the brighter Cepheid variable stars in the
Large Magellanic Cloud had longer periods than the Cepheids
with shorter periods. This is the famous period-luminosity
law.
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Since the stars in the Large Magellanic Cloud are all
at approximately the same distance from us, arelationship between their apparent magnitudes and
periods implied a relationship between their intrinsic
luminosities (i.e. absolute magnitudes) and periods.
Harlow Shapley (1885-1972) noticed that most of the
globular clusters in the sky were situated in the
constellations Scorpius and Sagittarius. He wondered:
Is the center of the Galaxy the same as the center
of the globular cluster system?
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Shapley determined the distances to a number of globular
clusters using the period-luminosity law for Cepheids.He noted that most globular clusters had linear diameters
of about 25 pc. He could then use their angular diameters
to get approximate distance for clusters whose stars were
too faint to study individually. He discovered that
the center of the globular cluster system was situated inSagittarius at a distance of some 50,000 light years.
However, he did not take extinction by interstellar dust
into account. Modern determinations of the distance
to the center of the Galaxy place it at a distance of about
25,000 light-years, or about 8000 parsecs.
After Copernicus
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After Copernicus
moved the Earth from
the center of the solar
system, Shapley moved
the Sun from the center
of the Galaxy.
Looking ahead, it appears
that no matter what direction
you look, distant galaxies
are receding from us.
Is our Galaxy at the centerof the universe?
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Essentially all of thegas, and all of the
bright blue stars are
found in the plane of
the Galaxy.
B i d i id
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Because we are situated inside
the Milky Way Galaxy, andoptical light is extinguished by
interstellar dust so much, it
is difficult to get a picture of
our galaxy. But we feel
confident that the side-onand face-on views must be
similar to these two other
galaxies.
The star-gas-star cycle of the Galaxy
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A spectrum of the Orion Nebula reveals many emissionlines.
Just like the planets
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Just like the planets
in the solar system,stars further out in
the Galaxy orbit the
Galactic center more
slowly. The Sun is
moving 220 km/sectoward Cygnus and
orbits the center every
240 million years.
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Unlike the solar system, however, the circular speeds around
the center do not decrease nicely in accord with Kepler's
Third Law.
Your book gives estimates of the mass of the Galaxy from
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Your book gives estimates of the mass of the Galaxy from
1 to 4 X 1011solar masses. A more recent determination
of the mass of the Galaxy is even larger, 1.5-4.0 X 1012
solar masses. This is on the basis of stars of known
absolute magnitude called horizontal branch stars. An
analysis of 1000 of them out in the Galaxy's halo shows
that the visible Galaxy we know that gives off most of thelight is embedded in a halo of invisible Dark Matter.
What could this dark matter be? Relic particles from the
Big Bang? Lots of 3 solar mass black holes? This is
one of the biggest mysteries in modern astronomy.
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Different Stellar Populations
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Gas is confined to the Galactic plane, with a thickness
of only 100 pc. This is where the most recent stars
are being formed.
Perturbations of stars' motion by giant molecular
clouds and star clusters has elongated the orbits of
other stars over time.
Stars formed in the halo can have highly elliptical
orbits around the Galactic Center. These orbits are
not confined to the plane of the Galaxy.
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Globular clusters were formed when the Galaxy was young.They have ages up to 10 to 13 billions years. Also, the
oldest white dwarfs in the plane are now sort of orange
in color. We can estimate that they may be 9 to 10 billion
years old. This is how we can get an estimate of the age
of the Galaxy.
Evidence for the spiral structure of the Galaxy comes
f b i i f h Al
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from nearby associations of hot, new stars. Also
from observations of neutral hydrogen gas.
Spiral structure of
our Galaxy as
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our Galaxy, as
determined fromionized hydrogen
regions
Georgelin & Georgelin 1976
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Our Galaxy, the Milky Way
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In the night sky,the Milky Way
appears as afaint band oflight.
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Dusty gas cloudsobscure our viewbecause they absorbvisible light.
This is theinterstellar mediumthat makes new star
systems.
It comprises clouds
of hydrogen gas(atomic & molecular)and dust.
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All-Sky View of the Milky Way
MILKY WAY
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TOP (artists
conception)
SIDE
Size of the Milky Way (side view)
Di t 100 000 li ht
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Diameter ~ 100,000 light years Thickness ~ 1,000 light years (flatter than a CD !)
Distance from Sun to center ~ 30,000 light years
About 100 billion stars in total.
Stellar components of the Milky Way
1 Di k t ti thi ll ti f t & d t
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1. Disk: rotating, thin collection of stars, gas & dust.2. Halo: tenuous outer sphere of stars & globular
clusters, and very little gas.
3. Bulge: spherical concentration of stars near the center
If we could view the Milky Way from abovethe disk, we would see its spiral arms
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Another spiral galaxy seen edge-on
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The Sombrero Galaxy in optical and IR light
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Closest large spiral galaxy
M31 the Andromeda Galaxy
8X the diameter of the Full Moon!
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8X the diameter of the Full Moon!
Stellar Orbits: Stars in the Galactic Disk
Disk stars all orbit in the same direction of rotation
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Disk stars all orbit in the same direction of rotation,with a small amount of vertical (up-and-down) motion. Rotation due to angular momentum from the galaxys formation. Vertical motion due to gravitational attraction of the disk stars.
Stellar Orbits: Galactic Halo & Bulge
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Stars in the halo& bulge also
orbit the centerof the galaxy.
But their orbitshave randomorientations, w/o
any overallsense of rotation.
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How do we measure the mass of the Galaxy?
Suns orbital motion
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Sun s orbital motion(radius & velocity) tellus the mass inside
Suns orbit:~1.0 x 1011 Msun.
(i.e. 100 billion stars )
Cannot measure themass outside of theSuns orbit in thisfashion (we use othermethods).
Galactic Recycling
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Galactic Recycling
How does our galaxy recycle gas into stars?
Where do stars tend to form in our galaxy?
Whats the Milky Way got to do with us?
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It holds onto the gas and allows new stars to formfrom recycled (and enriched) material
How does our galaxy form stars?
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How does our galaxy form stars?
Recycles gas from old stars into new stars.
With each cycle, more heavy elements are
made by nuclear fusion in stars.
Star-gas-star cycle
Star-gas-star
cycle
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cycle Recycles gas
from old starsinto new stars.
With each cycle,more heavyelements are
made by nuclearfusion in stars.
Summary of Galactic Recycling
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Summary of Galactic Recycling
Stars make new heavy elements by fusion.
Dying stars expel gas and new elements, producinghot bubbles of gas (~106 K). These emit X-rays.
This hot gas cools, allowing atomic hydrogen cloudsto form (~100-10,000 K). This hydrogen emits at
21-cm wavelength emission line. Further cooling permits molecules (CO, etc) to form,
making molecular clouds (~30 K). CO emits anemission line spectrum at 3 mm.
Gravity forms new stars (and planets) in molecularclouds. Process starts over.
GasCools
Effect of low-mass stars on the interstellar medium
L t
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Low-mass starseject gas throughtheir (very small)
stellar winds andmass loss duringthe planetarynebula phase.
Overall, these havemuch less effect on
the ISM than highmass stars.
Effect of high-mass stars on the interstellar medium
During their lives
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During their lives,high-mass starshave strong stellar
winds that blowbubbles of hot gas.
High mass stars die
as supernovae,injecting heavyelements into theinterstellar medium.
Have a very strongeffect on the ISM.
10 light-years
Supernova remnants: Xrays Supernovat
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premnants arefilled with hot
gas (~106
K),which emitthermal
radiation atmostly X-raywavelengths.
20 light years
How does light tell us the
Review: Learning from Light
How does light tell us what
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How does light tell us thetemperatures of dense objects?We can determine temperature
from the (continuous) spectrumof thermal radiation.
How does light tell us whatthings are made of?Every kind of atom, ion, and
molecule produces a uniqueset of spectral lines, seen inemission or absorption spectra.
Supernova remnants
The gas of the supernova remnant expands and cools.
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Begins to emit visible light, mostly emission line spectra.
130 light years
Supernova remnants
The gas of the supernova remnant expands and cools.Begins to emit visible light as emission line spectra
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Begins to emit visible light as emission line spectra. These spectra show heavy elements (O, Ne, N, S) made
by the star, which are distributed back into the ISM.
SN superbubbles Multiple supernovae
can create hugeb bbl f h t
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pbubbles of hot gas,which blow out ofthe galactic disk.
Gas clouds coolingin the halo can rain
back down onto thedisk.
These collisions
may trigger futurestar formation.
Atomic hydrogen in the ISM
As the hot gas cools electrons combine with protons
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As the hot gas cools, electrons combine with protonsto form clouds of atomic hydrogen (H).
Atomic hydrogen produces an emission line at 21cmwavelength (in the radio) called Hydrogen hyperfinestructure or the spin flip transition. Can use this tomap the spatial distribution.
Molecular hydrogen in the ISM
Atomic hydrogen clouds slowly contract & cool further. Once they get cold & dense enough, the single H atoms
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Once they get cold & dense enough, the single H atomscombine to form molecular hydrogen (H2) clouds.
Optical image
Molecular clouds
Composition:M l H
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Mostly H2 About 28% He About 1% CO Many other molecules.
Unlike atomic hydrogen (H),molecular hydrogen (H
2
)is very hard to detect, as itemits very weak radiation.
Detect molecular clouds from3-mm emission line of CO(a trace constituent bymass).
Molecular clouds collapse due to gravity to form new
stars, thereby completing the star-gas-star cycle.
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Star formation in molecular clouds
Young massive
stars can erodethe birth clouds,
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the birth clouds,preventingfurther star
formation.
Only a smallfraction of gas
in molecularclouds formsinto stars.
Summary of Galactic Recycling
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Stars make new heavy elements by fusion.
Dying stars expel gas and new elements, producinghot bubbles of gas (~106 K). These emit Xrays.
This hot gas cools, allowing atomic hydrogen cloudsto form (~100-10,000 K). This hydrogen emits at
21-cm wavelength emission line. Further cooling permits molecules (CO, etc) to form,
making molecular clouds (~30 K). CO emits anemission line spectrum at 3 mm.
Gravity forms new stars (and planets) in molecularclouds. Process starts over.
GasCools
QUESTION: Where will our
Galaxys gas be in 1 trillion yearsf ?
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from now?
A. Blown out of galaxyB. Still recycling just like now
C. Locked into white dwarfs and low-
mass stars
QUESTION: Where will our
Galaxys gas be in 1 trillion yearsf ?
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from now?
A. Blown out of galaxyB. Still recycling just like now
C. Locked into white dwarfs and low-
mass stars
Galactic recycling is an imperfect process.
More and more gas gets locked up intolow-mass stars and white dwarfs, which neverreturn their material to the interstellar medium.
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We observe star-gas-star cycle operating in the Milky Waysdisk using many different wavelengths of light.
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Infrared light reveals stars whose visible light isblocked by clouds of gas & dust.
Infrared
Visible
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X-rays are observed from hot gas above andbelow the Milky Ways disk.
X-rays
Radio (21cm)
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21-cm radio waves emitted by cold atomic hydrogenshow where gas has cooled and settled into disk.
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3-mm radio waves from carbon monoxide (CO) showlocations of molecular clouds.
Radio (3 mm)
Far IR
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Long-wavelength infrared emission shows whereyoung stars have heated dust grains.
Far-IR(dust)
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Gamma rays show where cosmic rays from supernovae
collide with atomic nuclei in gas clouds
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Where do stars tend to form inour galaxy?
Much of the starformation in diskgalaxies happensin the spiral arms
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in the spiral arms.
Whirlpool Galaxy
Ionization NebulaeBlue (massive) starsDusty Gas Clouds
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Ionization nebulae
Regions of ionized gas
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Found around short-lived
high-mass stars and signifyactive star formation.
The blue light of the
massive stars is scatteredby nearby dust clouds.
The nebulae tend to appearreddish, b/c of strongemission lines at thesewavelengths.
Reflection nebulaeare
dusty gas clouds whichscatter the light fromstars
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stars.
Why do reflection nebulaelook bluer than the nearby
stars? For the same reasonour sky is blue, and sunsetsare red. Blue light is preferentially
scattered by gas moleculesand small dust particles.
Spiral arms are waves ofstar formation
1. Gas clouds getsq ee ed as the
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squeezed as theymove into spiral arms
2. Squeezing of cloudstriggers star
formation.
3. Young stars flow outof spiral arms.
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How did our galaxy form?
Halo: No ionization nebulae, no blue stars
no star formation
(and hence no recycling)
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Disk: Ionization nebulae, blue stars star formationMilky Ways star formation rate is about 1 MSun/yr.
Halo Stars:0.02-0.2% heavy elements (O, Fe),
only old stars
Halo starsformed first,
then stopped.
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Disk Stars:2% heavy elements,stars of all ages
Disk starsformed later, &
keep on forming.
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Our galaxy probably formed from a giant gas cloud
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Halo stars formed first as gravity caused cloud to contract
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Remaining gas settled into spinning diskdue to conservation of angular momentum
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Stars continuously form in disk as galaxy grows older
The collapsing cloud model explains the age, chemical, andorbital differences between halo and disk stars.
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More detailed studies: Halo stars formed inclumps that later merged (galactic cannibalism).
The halo generally contains only old low
What do we learn about the
galaxys history from its halo stars?
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The halo generally contains only old, low-
mass stars with a much smaller proportionof heavy elements than stars in the disk.
Thus, halo stars must have formed early in
the galaxys history, before the gas settledinto a disk.
How did our galaxy form?
The galaxy probably beganas a huge blob of gas calleda protogalactic cloud.
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p g
Gravity caused the cloud toshrink in size, andconservation of angularmomentum caused the gas
to form the spinning disk ofour galaxy.
Stars in the halo formedbefore the gas finished
collapsing into the disk.
The black hole at the
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The black hole at thecenter of the Galaxy
Review: What is a black hole?
A black hole is a place where gravity is so powerful thatnothing can ever escape from it, not even light.(Therefore, out of contact with the rest of the Universe.)
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The event horizonthe surface of the BH,
where escape velocity is
the speed of light (c)
Escape speed > c
Escape speed < c
Size of event horizon = Schwarzschild radius = 2GM/c2
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Schwarzschild radius of a 1MSunblack hole ~ 3 km
Galactic center in IR light
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Galactic center in IR light Galactic center in radio
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Galactic center in radio
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Strange radio sources ingalactic center
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Stars at galactic centerStrange radio sources ingalactic center
Stellar orbits size and mass black hole
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