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AST201H1 Study Guide - Electromagnetic Spectrum, Nucleosynthesis, Accelerating Expansion Of The Universe

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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Chapter 19 Our Galaxy
19.1 The Milky Way Revealed
What does our galaxy look like?
Spiral galaxy: a galaxy that looks like a flat, white disk with a yellowish bulge at its
centre. The
disk is filled with cool gas and dust, interspersed with hotter ionized gas,
usually displays spiral arms.
Spiral arms: the bright, prominent arms, usually in a spiral pattern, found in spiral
Disk: the portion of a spiral galaxy that looks like a disk and contains an
interstellar medium
with cool gas and dust; stars of many ages are found in the disk
Bulge: the central portion of a spiral galaxy that is roughly spherical and bulges
above and
below the plane of the galactic disk
Halo: the spherical region surrounding the disk of a spiral galaxy
Globular clusters: spherically shaped clusters of up to a million or more stars;
globular clusters
are found primarily in the halos of galaxies and contain only very old
the entire galaxy is about 100,000 ly in diameter and the disk is 1000ly thick
our Sun is located in the disk about 28,000 ly from the galactic centre
How do stars orbit in our galaxy?
Stars in the disk orbit in roughly circular paths that all go in the same direction in
nearly the same plane

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Stars in the bulge and halo soar high above and below the disk on randomly oriented
Orbits of Disk Stars
Individual stars bob up and down through the disk as they orbit
The general orbit of a star around the galaxy arises from its gravitational
attraction toward the galactic centre, while the bobbing arises from the
localized pull of gravity within the disk itself
Orbits of Halo and Bulge Stars
Individual bulge and halo stars travel around the galactic centre on more or
less elliptical paths, but the orientations of these paths are relatively random
These swooping orbits explain why the bulge and halo are much puffier than
the disk
Halo and bulge stars soar to heights above the disk far greater than
the up-and-down bobbing of disk stars
The great difference between orbits in the disk and orbits in the bulge and
halo indicate that disk stars and halo stars have very different origins
Stellar Orbits and the Mass of the Galaxy
The orbital motion of the Sun and other stars gives us a way to determine the
mass of the galaxy
Newtons law of gravity determines how quickly objects orbit one another
Newtons version of Keplers third law allows us to determine the mass of a
relatively large object when we know the period and average distance of a
much smaller object in orbit around it
For example, we can use the Suns orbital velocity and its distance from the
galactic centre to determine the mass of our galaxy lying within the suns orbit
19.2 Galactic Recycling
The birth of the Sun and the planets could not have occurred without galactic
recycling that takes place within the disk of the galaxy and its interstellar
It gradually changes the chemical composition of the interstellar medium
The newly created elements mix with other interstellar gas and thereby
become incorporated into new generations of stars
How is gas recycled in our galaxy?

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The galactic recycling process proceeds in several stages, making up the star-
gas-star cycle
Molecular clouds star formationnuclear fusion in starsreturning gashot
bubbles atomic-hydrogen clouds molecular clouds star formation
nuclear fusion in starsetc
Stars are born when gravity causes the collapse of molecular clouds
They die when theyve exhausted their fuel for fusion
They ultimately return much of their material back to the interstellar medium
during their lives through stellar winds, and at death through a planetary
nebula in the case of low-mass stars or through supernova in the case of
high-mass stars
Gas from Dying Stars
All stars return much of their original mass to interstellar space in two basic
1.Stellar winds that blow throughout their lives
2.Death events such as planetary nebulae and supernovae
The high-speed gas ejected into space by supernovae or powerful stellar
winds sweeps up surrounding interstellar material, excavating a bubble of hot,
ionized gas around the exploding star
Superbubbles and Fountains
Superbubble: arise when many individual bubbles combine to form a much
larger bubble
Galactic fountain: when a superbubble grows too large for the Milky Way and
spills out, creating a massive blowout
The gravity of the galactic disk slows and eventually halts the rise of the gas
from a blowout
The ejected gas starts to cool and form clouds
Gravity then causes these clouds to rain back down into the disk, where their
contents mix with the gas in that region of the galaxy
Cooling and Cloud Formation
Atomic hydrogen gas: cool gas in the galaxy that hydrogen atoms remain
neutral rather than being ionized
after the gas that makes the bubbles cool, it becomes part of this widespread
atomic hydrogen gas in the galaxy
it emits a spectral line with a wavelength of 21 centimetres
Atomic hydrogen gas tends to be found in two distinct forms:
1.Tenuous clouds of warm atomic hydrogen
2.Smaller, denser clouds of cool atomic hydrogen
Gravity slowly draws blobs of this gas together into clouds
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