Download PART 3 Galaxies

PART 3 Galaxies
Stars in the “Milky Way”
Our Galaxy, the Milky Way
• A galaxy is a large
collection of billions of
stars
• The galaxy in which the
Sun is located is called
the Milky Way
• From our vantage point
inside the galaxy, the
Milky Way looks like a
band of stars across the
night sky, with dark dust
lanes obscuring the
center of the band.
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The Milky Way is a spiral galaxy approximately 30 kps (100K Ly) across.
The Sun is located around 8 kpc from the center, in one of the spiral arms.
Most of the stars are concentrated in the galactic plane, or in the central bulge at the
center of the galaxy
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Inside the bulge is the nucleus of the galaxy
Surrounding the disk is a roughly spherical distribution of stars called the halo.
Globular clusters are distributed throughout this halo, surrounding the center of the galaxy.
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Two Stellar Populations: Pop I and Pop II
• Stars in neighboring galaxies are segregated
– Younger, blue, “Population I” stars were found mostly in the disks
and spiral arms. Typically less than a few billion years old; Follow
circular orbits in the galactic plane
– Older, red, “Population II” stars were found mostly in the halo and
central bulge; More than 10 billion years old; Follow random
elliptical orbits around the galactic center – not in the plane.
The primary reason that massive O-type stars are not
found in the galactic halo is because they are
a) too massive to be kicked into the halo from the disk.
b) so massive that they settle into the thinner disk.
c) too short-lived to have persisted from halo
formation until today.
d) closer to us in the disk than in the extended halo.
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The Formation of the Milky Way
• Our galaxy likely began 13 billion
years ago as a huge cloud of pure
hydrogen and helium, slowly rotating
and collapsing
• The first stars formed (Pop III?)
within this cloud, burning out
quickly and violently. This added
heavy elements to the cloud
• Population II stars formed next,
capturing some of the heavy
elements and settling into elliptical
orbits around the center of the cloud
• As the collapse continued, a disk
formed, and Population I stars
formed from the ashes of dying Pop I
stars
Compared to stars like the Sun in the disk of the Milky Way, stars that populate the
extended spheroidal halo of the galaxy were born
a) earlier, so have had time to accumulate more heavy elements.
b) later, so have used up their heavy elements.
c) earlier, from more nearly primordial material, so have fewer
heavy elements.
d) later, so have accumulated more heavy elements from previous
generations of stars.
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Mapping the Milky Way’s
spiral arms
• Once this difference between
Population I and II stars was
noted, astronomers could
map our galaxy’s arms
• Population I stars are mostly
bright, blue stars (hot O and
B stars) found in the disk
• By measuring the location of
O and B stars near the Sun,
the first pictures of the Milky
Way’s spiral structure were
produced.
• Dust and gas obscure the
light from more distant stars,
so the map is incomplete.
Galactic Cannibalism
• There are a few observations
that are not explained by this
model
– Some stars follow unusual
orbits in the galaxy
– Not all Pop II stars are the
same age
– Model predicts that the first
stars might not have been very
massive, and should still be
around!
• Galactic cannibalism provides
some answers
– The Milky Way may be
absorbing another galaxy!
– Observations show streams of
stars coming from our galaxy’s
“victim”.
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The Interstellar Medium
The Sombrero Galaxy
• Space is far from empty!
– Clouds of cold gas
– Clouds of dust
• In a galaxy, gravity pulls the dust
into a disk along and within the
galactic plane
• This dust can obscure visible light
from stars and appear to be vast
tracts of empty space
• Fortunately, it doesn’t hide all
wavelengths of light!
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Emission Nebulae
• We frequently see
nebulae (clouds of
interstellar gas and dust)
glowing faintly with a red
or pink color
• Ultraviolet radiation from
nearby hot stars heats the
nebula, causing it to emit
photons
• This is an emission
nebula!
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Reflection Nebulae
• When the cloud of gas
and dust is simply
illuminated by nearby
stars, the light reflects,
creating a reflection
nebula
• Typically glows blue
Dark Nebulae
• Nebulae that are not
illuminated or
heated by nearby
stars are opaque –
they block most of
the visible light
passing through it.
• This is a dark nebula
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Interstellar Reddening
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As starlight passes through a dust
cloud, the dust particles scatter blue
photons, allowing red photons to pass
through easily
The star appears red (reddening) – it
looks older and dimmer (extinction)
than it really is.
If one region of the sky shows nearby stars but no
distant stars or galaxies, our view is probably blocked
by
a) nothing, but directed toward a particularly empty
region of space.
b) an emission nebula of ionized gas.
c) an interstellar gas and dust cloud.
d) a concentration of dark matter.
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Rotation in the Milky Way
• The Milky Way does not rotate like a solid disk!
– Inner parts rotate about the center faster than outer parts
– Similar to the way planets rotate around the Sun
– This is called differential rotation.
• A plot of rotation speed vs. distance is called a rotation curve
• A star is held in orbit around the galactic center by
gravitational forces of all matter inside its orbit!
Rotation in the Milky Way
• The Milky Way does not rotate like a solid disk!
– Inner parts rotate about the center faster than outer parts
– Similar to the way planets rotate around the Sun
– This is called differential rotation.
• A plot of rotation speed vs. distance is called a rotation curve
• A star is held in orbit around the galactic center by
gravitational forces of all matter inside its orbit!
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Rotation in the Milky Way
• The Milky Way does not rotate like a solid disk!
– Inner parts rotate about the center faster than outer parts
– Similar to the way planets rotate around the Sun
– This is called differential rotation.
• A plot of rotation speed vs. distance is called a rotation curve
• A star is held in orbit around the galactic center by
gravitational forces of all matter inside its orbit!
Calculating the Mass of the Galaxy
• The rotational velocity of the Sun
around the center of the galaxy can
be used to estimate the galaxy’s mass
• The combined gravitational effect of
all mass within the Sun’s orbit is
equivalent to one large lump of mass
at the center of the galaxy
• Newton’s Law of Gravitation shows
that the mass of all matter within the
Sun’s orbit is 9×1010 solar masses!
• We can estimate the mass of the
entire galaxy be measuring the
orbital velocity of small satellite
galaxies in orbit around the Milky
Way (2×1012 solar masses)
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If there was no dark Matter in the Milky Way the velocity of
the sun around the Galactic Center would be
a) greater
b) smaller
c) the same
d) impossible to say
The Galactic Center and Edge
• Despite the appearance of
being closely spaced, stars in
the Milky Way are very far
apart
– At the Sun’s distance from the
center, stellar density is around 1
star per 10 cubic parsecs
• Density is much higher at the core
– Exceeds 100,000 stars per cubic
parsec!
• X-ray and gamma ray telescopes
reveal a supermassive black hole at
the Milky Way’s core
– Called Sag A*
– 5 million solar masses!
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