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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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