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15. Galaxies
Swan Leavitt’s Study of Cepheid Variables Leads to Measurement of Galactic Distances
The
parallax method of distance measurement only works for “near” stars. A special type of pulsating stars,
Cepheid variables, has an absolute brightness correlated to its pulsation period, which can be used to
determine distance. Much of the analysis of Cepheid variable stars was performed by American
Henrietta Swan Leavitt in 1912.
Hubble Proves Other Galaxies Exist
Using the 100-inch telescope at Mount Wilson, Edwin Hubble
was able to resolve individual stars of “nebulae,” and using Cepheid variable distance data, proved that
other galaxies exist in 1925.
Zwicky Detects Dark Matter
The “discovery” of dark matter has been a long process. Fritz Zwicky
(Swiss, MIT) conjectured its existence based on galactic rotation curves. Vera Rubin continued the work
in the 1970s.
In 2005, a galaxy made almost entirely of dark matter was unveiled.
Definition of a galaxy
A galaxy is an isolated grouping of billions of stars, held together by its own
gravity
The Milky Way Galaxy
The galaxy we live in is called the Milky Way (because is looks like a milky
band of light when seen edge-wise from Earth). It’s also called Our Galaxy. ur Sun is situated toward
the edge of our Galaxy, which contains some 200 billion stars.
Map of the Milky Way
The Local Group
100,000 light years from edge to edge
Our Galaxy is a member of a group of galaxies, called the Local Group. he closest
“big” galaxy to us is the Andromeda Galaxy, 2.65 million light years away.
Super Cluster
Even the clusters of galaxies are lumped together in associations, called super
clusters. Our super cluster is called the Local Supercluster.
The Universe
Everything that exists is in the Universe. The Universe has an infinite number of
galaxies, and an infinite number of stars and planets. It has no limit, no edge, no boundaries. But it
does have an age—13.7 billion years.
How do we measure the distance to galaxies?
Pulsating stars, such as Cepheid variables, cycle with
a fixed relation between period and luminosity. By comparing how bright a variable star appears from
Earth, to its actual luminosity, we can calculate the distance.
Three Major Classification of Galaxies
Galaxy, Elliptical Galaxy, Irregular Galaxy
Hubble classified galaxies according to their shape: Spiral
Parts of a Spiral Galaxy
Flattened galactic disk, containing spiral arms. Central galactic bulge,
containing a dense nucleus of many stars. Extended spherical halo of faint, old stars.
Spiral Arms Spiral arms contain a large amount of gas and dust (15% of disk mass), with many hot,
young stars (looking blue). New star formation in on-going. The spiral arms spin, as stars orbit around
the galactic nucleus. The origin of spiral arms is based on density wave theory.
Barred Spirals
Similar to Spiral Galaxies are Barred Spirals, having an elongated central region (bar).
The Milky Way has a “weak” bar.
Elliptical Galaxies
Elliptical galaxies are spherical or oblong shaped, with no real structure. Typical
stars in elliptical galaxies are old, cool, low-mass stars, making them look reddish. There is little gas and
dust, and little formation of new stars. Elliptical galaxies range greatly in size, from ten million solar
masses to one hundred times the mass of the Milky Way.
Irregular Galaxies
Unusual-looking galaxies (that resemble neither spiral nor elliptical galaxies) are
called irregular galaxies. Irregular galaxies have much gas and dust, where new stars are being formed,
but they don’t have spiral arms. The Milky Way has two nearby satellite galaxies, which are irregular
galaxies—the Large and Small Magellanic Clouds
Hubble Tuning-Fork Diagram
S = spiral galaxies -- these are flattened objects with spiral structure
(arms), usually a central bulge + dust are present as well. SB = barred-spiral galaxies -- are similar to
spirals, but with the presence of a bar in the central regions. E = elliptical galaxies -- galaxies with
smooth light distributions, biaxial profiles. Round to oval in shape. Irr = irregular galaxies -- galaxies
that did not have the regular structure of the above groups
Cause of Galaxy Types
Rotation affects the shape of galaxies, with spirals rotating faster than
ellipticals. Evidence indicates that merging (colliding) spiral galaxies form an elliptical galaxy (when little
gas and dust remains), but form a large spiral, when considerable gas and dust does remain.
Galaxy Collisions and Mergers
Galaxy collisions are common, but the individual stars are generally
unaffected by the collision. Existing gas and dust can cause a burst of star formation—called star burst
galaxies.
Measuring the Mass of a Galaxy
We can determine the velocity of individual stars in a galaxy by
measuring the Doppler shift of the light. And since the stars are orbiting the galaxy, we can use this
motion to calculate the total mass encircled within the orbit.
Dark Matter
When measuring the mass of a galaxy, we expect the mass to decrease further from the
center (since there are fewer stars), but the high velocity of the stars shows that there is a lot of matter
in the galaxy that we can’t see (not stars). This is dark matter. The dark matter comprises about 90%
of the galaxy’s mass. Dark matter is made of…
Active Galaxies
Galaxies that emit great amounts of energy from a small core are known as active
galaxies. Active galaxies that emit much energy at radio wavelengths are radio galaxies. Very distant,
extremely bright, radio-energy emitters are quasars (quasi-stellar radio source). The power source of
active galaxies is likely due to energy interactions with super-massive black holes.
Our Galaxy, The Milky Way
The Milky Way is a (barred) spiral, 100,000 light years in diameter,
containing some 200 billion stars (though composed primarily of dark matter), and is approximately 13
billion years old (based on the age of the oldest stars). The Sun is located near the outer edge, and is
orbiting the galactic core at 220 km per second.
Milky Way in the Night Sky
When we look at the Milky Way at night, it looks like a hazy band of
light. The haze is the light from billions of stars. Since the galaxy is flat, like a pizza, with the Sun
toward the edge, we see most of the stars arranged in a band of light.
Star Clusters within Our Galaxy
Open Clusters contain a few hundred stars, characterized by young,
hot, blue stars (Population I stars)—located in the galaxy’s spiral arms. Globular Clusters contain a few
thousand to several million stars, characterized by old, cool, red stars (Population II stars)—located in
the galaxy’s halo.
Nebulae
A nebula is a visible region of gas. Hot gas glows red—an emission nebula. Cool gas can
scatter light, scattering more blue light than other colors—a reflection nebula. Dense gas can block light
coming from behind it—a dark nebula.
How Will Our Galaxy End???
As stars die, some become white dwarfs and then black dwarfs. Other
stars become supernovae, making matter available to form new stars. But eventually, most of the matter
will end up as old, cold, dead stars, and black holes.
Questions to be Answered:
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What is a galaxy?
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How do we measure the distance to galaxies?
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What are the characteristics of the major galaxy types?
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How can the shape of galaxies change?
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What is dark matter, and how do we know it’s there?
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How do active galaxies produce their abundant energy?
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What is the shape, parts, structure, and size of our Galaxy?
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When we look at the Milky Way in the night sky, what are we seeing?
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What are the differences between the two major kinds of star clusters?
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How do the three types of nebulae look?
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How will the Milky Way end up?