Transcript Foundation 1 - Discovering Astronomy

Chapter 15
Galaxies
What do you think?
• Do galaxies all have spiral arms?
• Are most of the stars in a spiral galaxy in its
arms?
• Are galaxies isolated objects?
• Are all galaxies moving away from the
Milky Way?
Galaxies seem to take one of four
different appearances
SPIRALS
Galaxies seem to take one of four
different appearances
SPIRALS
Galaxies seem to take one of four
different appearances
SPIRALS
The tightness of a spiral galaxy’s
arms is correlated to the size of
its nuclear bulge
Type Sa
Type Sb
Type Sc
Variety of Spiral Arms
Flocculent spirals
(fleecy)
Grand-design spirals
(highly organized)
We easily see these spiral arms because
they contain numerous bright O and B
stars which illuminate dust in the arms.
However, stars
in total seem to
be evenly
distributed
throughout the
disk.
Self-propagating star formation and
spiral density waves produce spiral arms
Galaxies seem to take one of four
different appearances
BARRED SPIRALS
Galaxies seem to take one of four
different appearances
BARRED SPIRALS
Galaxies seem to take one of four
different appearances
BARRED SPIRALS
Bars of stars run through the nuclear
bulges of barred spiral galaxies
Type SBa
Type SBb
Type SBc
Galaxies seem to take one of four
different appearances
Type E0
Type E3
ELLIPTICALS
Type E7
Galaxies seem to take one of four
different appearances
ELLIPTICALS
Elliptical galaxies display a
variety of sizes and masses
• Giant elliptical galaxies
can be 20 times larger
than the Milky Way
• Dwarf elliptical galaxies
are extremely common
and can contain as few as
a million stars
Galaxies seem to take one of four
different appearances
IRREGULAR
Galaxies seem to take one of four
different appearances
• Spirals
• Barred Spirals
• Ellipticals
• Irregulars
This classification scheme is known as the
Hubble Tuning Fork Scheme
Galaxies are gravitationally bound into
clusters and superclusters
• Galaxies are not scattered randomly throughout the
galaxy
• Galaxies are found in clusters
– The Milky Way is part of the Local Group of about 35
galaxies
• Clusters of galaxies are clustered as well in groups
called superclusters
– Our Local Group is part of the Local Supercluster
• The majority of space is empty - called voids.
The Local Group
Virgo Cluster
Fornax Cluster
Hercules
Cluster
Each dot
represents a
single galaxy
Galaxies in a cluster can collide and combine
NOTE: There is so much space between stars in a galaxy that the
probability of two stars crashing into each other is extremely small.
Colliding galaxies ….
• Do not involve colliding stars
• Might form hot intergalactic gas
• Could initiate rapid star formation - called
STARBURST GALAXIES
• Cause galaxy mergers called “galactic
cannibalism”
Three galaxies,
M81 (big), M82
(medium), and
NGC 3077
(small).
Are they
related to one
another?
Galactic Cannibalism Computer Simulation
Non-luminous material in galactic
halos may account for some dark
matter in the universe
NOTE: No cluster
or supercluster of
galaxies contains
enough visible
matter to provide
enough
gravitational
“glue” to keep
the spinning
clusters bound
together.
The redshifts of remote superclusters
indicate that the universe is expanding
• During the 1920s, Edwin Hubble and Milton
Humanson were studying galaxy spectra
• They found that galaxy spectral lines were
redshifted
• They measured the distance to galaxies by finding
Cepheid variables and using the period-luminosity
relationship
• They found that the most distant galaxies had the
largest redshifts
• This is called the HUBBLE FLOW.
Hubble’s Law
slope = 75 km/s/Mpc
called Hubble’s Constant
Astronomers now measure distances to galaxies
using supernovae and the Tully-Fisher relation
• Standard candle method
– If you know how bright something really is and
compare that to how bright it looks, then the distance
can be determined
• Supernovae
– All Type II a supernovae have the same luminosity
• Tully-Fischer Relation
– The broader the spectral line, the faster it is rotating and
the more luminous the galaxy.
What did you think?
• Do galaxies all have spiral arms?
No, galaxies may be either spiral, barred spiral, elliptical,
or irregular. Only spirals and barred spirals have arms.
• Are most of the stars in a spiral galaxy in its arms?
No, the spiral arms contain only 5% more stars than the
regions between the arms.
• Are galaxies isolated objects?
No, galaxies are grouped in clusters, and clusters are
grouped in superclusters.
• Are all galaxies moving away from the Milky Way?
No, only the galaxies in other superclusters are necessarily
receding from us.
Self-Check
1: Draw and label the tuning fork diagram, showing the principal Hubble
classes for galaxies, and describe the criteria for the assignment of
subclasses.
2: Discuss the differences between flocculent spirals and grand-design spirals.
3: Explain what spiral arms are and how they are sustained.
4: Compare and contrast the contents of the various Hubble classes of galaxies.
5: Describe the distribution of galaxies in space, and distinguish between
regular and irregular clusters of galaxies.
6: Explain the observational basis for the dark matter problem in spiral galaxies
and in clusters of galaxies.
7: Discuss the role of collisions and mergers in our understanding of galaxies.
8: State the Hubble law and describe how the Hubble constant is evaluated and
how it is used to determine the distances of galaxies.