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Astronomy 1021 Study Guide - Final Guide: Stromatolite, Runaway Greenhouse Effect, Terrestrial Planet

Course Code
ASTR 1021
Chris Racknor
Study Guide

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Astronomy 1021 – Final Exam Notes 2016-03-18
Chapter 14 – Our Galaxy
The Milky Way Revealed
The true size and shape of our Milky Way Galaxy are hard to discern
from the way it looks in our night sky. This is because we live inside
the galaxy.
oAlso, a lot of our galaxy’s visible light is hidden from our view
Milky Way Galaxy holds over 100 billion stars
oIt is a spiral galaxy, named because of the spectacular spiral
arms. The spiral arms are part of a fairly flat disk of stars
surrounding a bright central bulge. The entire disk is
surrounded by a dimmer, rounder halo.
oMost of the galaxies bright stars reside in its disk.
oThe most prominent stars in the halo are found in about 200
globular cluster of stars
The entire galaxy is about 100,000 light-years in diameter, but the
disk is only about 1000 light-years thick.
oOur sun is located in the disk about 27000 light-years from
the galactic centre (just more than halfway)
oInterstellar medium (clouds of interstellar gas and dust) fill
the galactic disk, obscuring our view when we try to peer
directly through it.
Each individual star follows its own orbital path around the center of
the galaxy.
oNearly all the stars in the Milky Way galaxy follow on or two
basic orbital patterns.
oThey orbit in roughly circular paths that all go in the same
direction in nearly the same plane.
oIn contrast, stars in the bulge and halo soar high above and
below the disk on randomly oriented orbits.
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 the
localized pull of gravity within the disk itself. A star that’s “too far”
above the disk is pulled back into the disk by gravity.
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oBecause the density of interstellar gas is too low to slow the
star, it flies through the disk until it is “too far” below the disk
on the other side. Gravity then once again pulls it in the other
direction. This process is what the bobbing of the stars is.
The bobbing motions of the disk stars gives the disk its thickness of
about 1000 light-year.
In our galaxy’s disk, the orbital velocities of stars near the edge and
those near the center are about the same. Stars closer to the
center therefore complete each orbit in less time than stars farther
Orbits of Halo and Bulge Stars
Less organized than the orbits of stars in the disk
Individual bulge and halo stars travel around the galactic center on
more or less elliptical paths, but the orientations of these paths are
relatively random.
oNeighbouring halo stars can circle the galactic center in
opposite directions
oThey plunge through the disk at velocities so high that the
disk’s gravity barely alters their trajectories.
Using Stellar Orbits to Measure the Mass of the Galaxy
By measuring the speeds of globular clusters relative to the Sun,
it’s been determined that the sun and its neighbours orbit the Milky
Way at a speed of about 220 km/s. even at this speed though, it
takes the sun about 230 million years to complete one orbit around
the galactic center.
Galactic Recycling
Galactic recycling makes new generations of stars possible, but also
gradually changes the chemical composition of the interstellar
Galactic recycling process has three stages that are called the star-
gas-star cycle.
oStars are born when gravity causes the collapse of molecular
clouds. They shine for millions or billions of years with the
energy produced by nuclear fusion, and die only when they
run out of the fuel. They return much of their material back to
the interstellar medium through supernovae and stellar winds.
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Gas from Dying Stars:
oall stars return much of their original mass to interstellar
space in two basic ways: through stellar winds that blow
throughout their lives and through “death event”.
oLow-mass stars on the MS usually have weaker stellar winds
while theyre on the MS, but they grow stronger and carry
more material into space when they become red giants.
oBy the time a low-mass star like the Sun ends its life with the
ejection of a planetary nebula it has returned almost half its
original mass to the interstellar medium.
oHigh-mass stars have powerful winds and return large
amounts of matter back into the galaxy. The high speed gas
ejected into space by the supernovae when it explodes
sweeps up surrounding interstellar material creating a bubble
of hot, ionized gas (gas in which atoms are mission some of
their electrons). The bubbles are common in the disk of the
galaxy, but are hard to detect.
oSupernovae generate shock waves (waves or pressure that
move faster than the speed of sound). A shock wave sweeps
up surrounding gas as it travels, creating a “wall” of fast-
moving gas on its leading edge.
When we observe a supernova remnant, we are seeing
the aftermath of such a shock wave, which compresses,
heats, and ionizes all the interstellar gas it encounters.
oIn the regions of the galactic disk where many supernovae
have recently exploded, there are giant, elongated bubbles
extending from young clusters of stars to distances of 3000
light-years or more above or below the disk. These probably
are places where the bubbles from many individual
supernovae have merged to make a gian bubble so large that
it cannot be contained within the Milky Way’ disk.
Once the top of a giant bubble breaks out of the disk,
where nearly all of the Milky Way’s gas resides, nothing
remains to slow its expansion except gravity. The result
is a blowout that is similar to a volcaninc eruption.
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