Download Galaxies

Galaxies
(And a bit about distances)
This image
shows galaxy
M 100 in
which the
Hubble Space
Telescope
detected
Cepheid
variables.
Cepheid variable stars
• As their period is related to their luminosity,
Cepheid variable stars are reliable standard
candles.
– If you know the period of
the star’s variation, you can
use the period–luminosity
relation to learn its absolute
magnitude.
– By comparing its absolute
and apparent magnitudes,
you can find its distance.
Other ways to measure distance: The Distance Ladder
Supernovae as “Standard Candles”
Edwin Hubble discovered a relationship
between a galaxy’s redshift (an indicator
of speed) and its distance.
Like runners that begin at the crack of a
gunshot, the fastest move the farthest
along the track.
Unlike runners, these galaxies aren’t
really moving on their own…the redshift
observed is called cosmological redshift
and is due to the expansion of space!
(But this idea came along after Hubble’s
pioneering discovery.)
Telescopes as Time Machines
• When you look at a more distant
galaxy, you look back into the past
by an amount called the look-back
time.
– This is the time in years equal to the distance
to the galaxy in light-years.
Galaxy Types: Spirals
Spiral Galaxies
• Spiral arms and contain gas and dust,
although some have very little. Their hot,
bright and young stars make the arms very
luminous, blue in color and easy to see.
• Among spiral galaxies, about two-thirds
are barred spirals.
Galaxy Types: Barred Spirals
Galaxy Types: Ellipticals
Elliptical Galaxies
These old galaxies have no disk, no
spiral arms, and almost no gas and dust.
Star formation has ceased.
Elliptical galaxies range from huge giants
to small dwarfs.
Irregular galaxies may be fragments left over by the
merger of larger galaxies.
The Selection Effect
The table above suggests there are more spirals, but in reality,
elliptical galaxies are more abundant and irregulars make up
about 25 percent of all galaxies. The luminous young stars of
spirals make it much easier to notice them than other galaxy
types.
Still Much to Learn…
Recent studies in the deep universe have revealed blue elliptical
and “green pea” galaxies.
The Origin and Evolution of Galaxies
Astronomers now suspect that most elliptical
galaxies are formed by the merger of at
least two or more galaxies.
Evidence supports that they are often found
near the center of galaxy clusters.
Interacting galaxies can distort
each other with tides—
producing tidal tails, rings or
shells of stars.
– They may trigger new star formation
episodes
– They may create spiral arms in galaxies
previously without them.
– Large galaxies can even absorb smaller
galaxies by merger or acts of
cannabalism.
Evidence left inside galaxies reveals that
they have suffered past interactions and
mergers.
– The Milky Way is a cannibal
galaxy—snacking on the
Magellanic Clouds (and other
galaxies) as they orbit it.
– Its tides are pulling apart
the Sagittarius and the
Canis Major Dwarf galaxies.
– This produces great streamers
of stars wrapped around the
Milky Way.
– Almost certainly, our galaxy
has dined on other small
galaxies in the past.
M82: A Starburst Galaxy
Peculiar Galaxies
Beautiful ring galaxies and other peculiar
features are bull’s-eyes and artifacts left
behind by high-speed collisions.
Centaurus A – A Galaxy Merger?
The Local Group of galaxies
Bubble and void structure
Seyfert Galaxies: A type of AGN
• In 1943, astronomer Carl Seyfert conducted a study of
spiral galaxies. He noted that about two percent of
spirals have small, highly luminous nuclei in their
bulges.
• Many of these galaxies have powerful radio sources at
their centers. This indicates the presence of a
supermassive black hole as the power source.
These are now called
AGN: Active Galactic Nuclei
Quasars: Quasi- Stellar Objects
• Quasars are the most luminous AGN.
• They often look like a star, due to their great distance
(the rest of their galaxy structure can’t be seen).
• Many quasars reveal more structure when viewed in
radio wavelengths.
Quasars Through Time
• Quasars represent the earliest observed
stages of galaxies. They are the farthest
visible objects. The most distant ones we
see represent a time
when the universe was
very young, less than a
billion years old.
Quasars: Mystery Solved
Quasars exhibit strange spectra, showing emission lines
in places where no known elements could create them.
In 1963, astronomer Maarten Schmidt calculated that
if hydrogen Balmer lines were redshifted by z = 0.158,
they would fit the observed lines in 3C 273’s spectrum.
Seeing far away Galaxies: Gravitational Lensing