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Galaxies
Chapter Twenty-Six
Note….
• I posted as a way to help you a list of
topics covered in Exam 2 (they are all in
my class notes).The link is at:
• http://physics.gmu.edu/~mopher/ASTR113
/topics_exam2.html
Guiding Questions
• How did astronomers first discover other galaxies?
• How did astronomers first determine the distances to
galaxies?
• Do all galaxies have spiral arms, like the Milky Way?
• How do modern astronomers tell how far away galaxies
are?
• How do the spectra of galaxies tell astronomers that the
universe is expanding?
• Are galaxies isolated in space, or are they found near
other galaxies?
• What happens when galaxies collide with each other?
• Is dark matter found in galaxies beyond the Milky Way?
• How do astronomers think galaxies formed?
All sort of types of Galaxies….
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spirals
elliptical
irregulares
• Some are of the size of MW but some are 50
times the mass of MW…we also know that only
10% of the typical galaxy mass is in
radiation…the rest is in …dark matter…
• Most galaxies are in Groups and in Clusters
• Remote clusters of galaxies are receding from
us…what is called the Hubble Law
• (this law reveals that the Universe is expanding)
A century ago the astronomers thought
that the entire universe was only thousand
light-year and nothing was beyond
the Milky Way Galaxy!
M31: Andromeda and its
satellites
Look at :
“The Local Group of Galaxies”
http://seds.lpl.arizona.edu/
messier/more/local.html
When galaxies were first discovered, it was not
clear that they lie far beyond the Milky Way
Kant in 1755 suggested that vas collection of stars lie outside of the Milky Way…
What he called “island universes”
In 1845, William Parsons built the largest telescope of the 19th century
1.8 meter in diameter (with no
Photographic equipment)
A modern view of the Sprial Galaxy M51
(its 8.5Mpc away frokm us; has glowing HII regions (sites of star formation))
And one arm extends toward the companion galaxy
Many astronomers of the 19th century disagreed with this notion of
Islands universes-they thought that the “spiral nebulae” were components
of our galaxy
In 1920 two opposing ideas were presented in the National Academy of Sciences
In DC:
Harlow Shapley from Mount Wilson Observatiory (was renowed from determining the
Size of our galaxy)…he thought that the spiral nevulae were relatively small objects
Like the globular clusters that he studied…
Vs
Heber D. Curtis from U. of California Lick Observatory (thought that each spiral nebulae
Were a rotating system much like MW)
…who solved the problem was a young guy
called Edwin Hubble…
In 1929 Hubble took a historic picture of
Andromeda…
he used Cepheids:
Given their period you can get their
luminosity (Period-Luminosity
relationship)
(you also have to determine if it’s a Type I
or Type II Cepheid metal poor or rich;
different period-luminosity relationships
Hubble proved that the spiral nebulae are far
beyond the Milky Way (results presented in
AAS on December 30, 1924)
• Edwin Hubble
used Cepheid
variables to show
that the “nebula”
were actually
immense star
systems far
beyond our
Galaxy
Cepheids were 104LS so
For them to be so dim-they had
To be distant!
Galaxies can be grouped into four major categories:
spirals, barred spirals, ellipticals, and irregulars
Lenticular galaxies are intermediate between spiral
and elliptical galaxies
Spiral Galaxies: They contain young, hot, blue stars and
associated HII regions indicating ongoing star formation
…therefore these galaxies will be rich in
metals…Population I…indeed the visible-light spectrum
of the disk of a spiral galaxy has strong metal absorption
lines….
By contrast there is little star formation in the
central bulges of spiral galaxies…dominated
by Population II that has low metal content…
(central bulges has a yellowish or reddish
color)
…Hubble classification of spirals…
Classification: Sa (smooth broad sprial arms and fat central
bulge); Sb (moderate central size bulge); Sc (well defined spiral
arms and a tiny central bulge)
The difference between Sa, Sb and Sc may be related
To the relative amounts of gas and dust that they contain
4% of the mass of a Sa galaxy is in gas & dust
8% for a Sb
And 25% for Sc
Sc has a greater proportion of its mass involed in star formation
(tiny bulge)
In barred spiral galaxies:
SBa has a large central bulge and thin
tightly wound spiral arms
SBb has a moderate central bulge and
moderately wound spiral arms
SBc has lumpy, loosely wound spiral arms
and tiny central bulge
(the difference might be related to the
amount of gas and dust)
Looking Deeper : Computers in Bars appear to form
naturally in many
Astronomical Research
spiral galaxies-they
(Looking Deeper 26-1)
Outnumber ordinary
Spirals by about two
to one
Why don’t all spiral galaxies have bars?
J. Ostriker and P. Peebles a bar will not develop if a galaxy is
surrounded by a sufficiently massive halo of dark matter …the
difference between barred spirals and ordinary spirals may thus lie in
the amount of dark matter
Elliptical galaxies are nearly devoid of interstellar gas
and dust, and so star formation is severely inhibited
They are mostly composed of old, red, Population II with
little metals
Hubble classified them according to how round or flattened they look
E0: The roundest…and E7 the flattest…be aware that E1 and E2 might actually
Be a flattened disk that we just happen to view face-on….
Giant galaxies in Virgo Cluster
Two giant elliptical galaxies (about 20 times larger than an average
Galaxy)
You also have dwarf elliptical galaxies that are only fraction of the size
Of normal elliptical and contain so few stars that are completely transparent
Tuning Fork Diagram of Hubble:
Hubble first thought that the tuning fork diagram was an
evolutionary sequence -> this is not the case!
Elliptical galaxies have little or no overall rotation, while spiral
and barred spiral have a substantial amount of overall
rotation
A more modern interpretation is that the Hubble tunning fork
diagram is an arrangement of galaxies according to their
overall rotation
Sa and Sba have enough rotation to form a disk altough their
central bulge are still dominant; the galaxies with the greatest
amount of rotation are Sc and SBc
Galaxies that do not fit are the irregular galaxies.
Irr I have many OB associations and HII regions
Irr II had asymmetrical distorted shapes that seem to have
been caused by collisions
Irregular galaxies have ill-defined, asymmetrical shapes
They are often found associated with other galaxies
How far are Galaxies?
Measuring Distances…
we need standard candle an object
that lies in a galaxy and that we know its
luminosity
For a Standard candle you want:
1. Luminous Object (so you can see it at
great distances)
2. Be certain of it’s luminosity
3. Easily identifiable
4. Should be relatively common
For nearby galaxies:
Cepheid (they can be seen out to 30Mpc and
we have Period-Luminosity relationship)
RR Lyrae can be used in the same way but
because they are less luminous they can
only be seen until 100Kpc
For distant stars astronomers are using Type
Ia supernovae (that occurs when a white
dward in a close binary system accretes
enough matter) A Type Ia supernovae can
reach a luminosity of 3x109Ls
Brent Tully and Richard Fisher discover a
new way of determining distances: by the
width of 21-cm emission line of a spiral galaxy
is related to the galaxy luminosity
(Tully-Fisher relation)
[Why: radiation from approaching side of a
rotating galaxy is blueshifted while the one
receding is redshifted. So the broadening is
related to the rotation of the galaxy that is
related to the mass that is related to the
luminosity.]
For elliptical galaxies (that do not rotate)
Marc Davis and George Djorgovski pointed
out a relation between the size of the elliptical
galaxy, the average motion and how the
galaxy brightness will appear distributed: this
is called
fundamental plane
Measuring 2 quantities we can get the actual
size of the galaxy and by the apparent size
get the distance
Astronomers use various techniques to determine
the distances to remote galaxies
Standard candles,
such as Cepheid
variables and the most
luminous supergiants,
globular clusters, H II
regions, and
supernovae in a
galaxy, are used in
estimating intergalactic
distances
The Distance Ladder
• Each technique can be used to callibrate the
other…so a change oin the distance-measuring
techniques for nearby objects can also have
substantial effects on distances to remote galaxies!
Masers
• One distance-measuring technique that has broken free of the distance
ladder uses observations of molecular clouds called masers
• “Maser” is an acronym for “microwave amplification by stimulated emission
of radiation” (nearby luminous stars can stimulate water molecules in a
maser to emit intensely at microwave wavelengths)…this maser technique is
still in the infancy
The more distant a galaxy, the greater its redshift and
the more rapidly it is receding from us!
There is a simple linear
relationship between
the distance from the
Earth to a remote
galaxy and the redshift
of that galaxy (which is
a measure of the speed
with which it is receding
from us)
Redshift
• The redshift of a receding object:
• z = - 0/ 0
The Hubble law is v = H0d
The value of the Hubble constant, H0, is not known with
certainty but is close to 71 km/s/Mpc
• Not all galaxies are moving away from MW; the galaxies
have their own motions relative to one another thanks to
their mutual gravitational attraction-for distant galaxies
the Hubble speed is much greater than any intrinsic
motions that the galaxies might have
• The value of H0? Depends on the determination of the
distance (vary depending on the technique between 40100km/s/Mpc)
• Because the value of H0 is somewhat uncertain usually
• We express the distance in terms of redshift z (that can
be measured very accurately)
• The greater the redshift of a distant galaxy the greater its
distance
• http://bcs.whfreeman.com/universe7e/pages/bcsmain.asp?v=category&s=00090&n=26000&i=26090.01&o=|
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