Galaxies
Astronomy 115
First, which of the following is a galaxy?
•
•
•
•
•
•
Open cluster
Globular cluster
Nebula
Interstellar medium (gas and dust)
Supernova remnant
None of these
First, which of the following is a galaxy?
•
•
•
•
•
•
Open cluster
Globular cluster
Nebula
Interstellar medium (gas and dust)
Supernova remnant
None of these
What is a “star cluster”?
• stars formed together
at same time
• stars are at least weakly
gravitationally bound
together
• two types: open
(galactic) and globular
(shown to right)
Image at
http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/2007/1
8/image/a/format/web/results/50/
Open Clusters
• dozens to
thousands of stars
• young stars! only
a few million years
old
• may still be
surrounded by
nebula from which
they formed
• located in the
spiral arms of a
galaxy
• example: Pleiades
• Fate: generally, the
stars drift apart
(not enough
gravity)
Image at
http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2004/20/ima
ge/a/results/50/
More open star clusters
Globular Clusters
• millions to hundreds of millions of
stars
• old! 6 to 13 billion years
• mostly red giants and dwarfs
• stars are clumped closely together,
especially near the center of the
cluster (densely); stars don’t drift
apart
• surround our disk as a halo
Image at
http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular
/1999/26/image/a/results/50/
What is a “nebula”?
• A cloud in space
• Made of gas and dust
– Can have stars inside
• Most of the ones we see are inside our Milky Way
Galaxy
• Different types
Large, massive, bright nebulae
Emission Nebula
•The hot gas is emitting light
Colder, darker nebulae
Dark dust blocking
the hot gas behind
it
Leftovers from an Explosion
Supernova remnant
(smaller, less gas)
So, what is a “galaxy”?
• A large group of stars outside of our own Milky Way
• Made of billions to trillions of stars, held together by
its own gravity, with all different ages of stars
– Also may have gas and dust
• Spiral, or elliptical, or irregular shaped
Galaxy Classification
E0, …, E7
Sa
E0 =
Spherical
Large
nucleus;
tightly
wound arms
E1
Sb
Sc
E7 = Highly
elliptical
E6
Small
nucleus;
loosely
wound arms
Spiral galaxy--Andromeda
NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html
and http://www.noao.edu/image_gallery/html/im0685.html
Elliptical Galaxies
Irregular Galaxies
NASA and NOAO/AURA/NSF Images at
http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/
50/ , http://www.noao.edu/image_gallery/html/im0560.html , and
http://www.noao.edu/image_gallery/html/im0993.html
Irregular Galaxies
Often: result of galaxy
collisions / mergers
Often: Very active star formation
(“Starburst galaxies”)
The Cocoon
Galaxy
NGC 4038/4039
Some: Small (“dwarf galaxies”)
satellites of larger galaxies
(e.g., Magellanic Clouds)
Large
Magellanic
Cloud
Galaxy Diversity
Even seemingly
empty regions
of the sky
contain
thousands of
very faint, very
distant galaxies
Large variety of
galaxy
morphologies:
Spirals
Ellipticals
Irregular
The Hubble Deep Field:
(some
interacting)
10-day exposure on an apparently empty field in the sky
Gas and Dust in Galaxies
Spirals are rich in
gas and dust
Ellipticals are almost
devoid of gas and dust
Galaxies with disk and bulge,
but no dust are termed S0
Barred Spirals
• Some spirals show a
pronounced bar structure
in the center
• They are termed
barred spiral galaxies
• Sequence:
SBa, …, SBc,
analogous to regular
spirals
Our Galaxy: the Milky Way
• has about 200 billion stars, and lots of gas
and dust
• is a barred-spiral (we think)
• about 100,000 light-years wide
• our Sun is halfway to the edge, revolving at
half a million miles per hour around the
center of the Galaxy
• takes our Solar System about 200 million
years to revolve once around our galaxy
The Milky Way
Mapping the Milky Way
How do we know what our Galaxy looks like?
We can see:
• Stars and star clusters – microwaves generated by water from H II
regions (called the MASER technique) traces the Milky Way’s spiral
arms
• Nebulae – infrared light (detected by the Spitzer Space Telescope)
shows the outline of the heat generated by the bar
• Other galaxies (analogous structure as our galaxy)
Do galaxies evolve over time?
Edwin Hubble (whom we’ll hear more about next lecture) in
1926 classified known galaxies according to shape, and
suggested an “evolution” of galaxies from elliptical to spiral
as they aged. The diagram was called a “tuning fork” due to
its shape
No one evolutionary path for galaxies
As detection methods grew more
sophisticated, using the infrared
(Spitzer telescope), radio (Very Long
Baseline Array) and gamma ray
(Compton telescope) portions of the
EM spectrum, the tuning fork is no
longer regarded as containing an
evolutionary sequence – it’s simply a
way of classifying galaxies.
It is true that irregular galaxies seem
to form from galactic collisions, and
that some spiral galaxies lose their
arms to become elliptical (Milky Way +
Andromeda fate), there is no good
model to describe galactic evolution.