galaxies Flashcards
(26 cards)
The Universe in 1920
Astronomers were aware of different
types of nebulae.
* Some, such as the Orion Nebula,
were bright enough to be studied,
and known to be in the Milky Way.
* Others, the ‘spiral nebulae,’ are
fainter and harder to resolve in detail.
the great debate
Is the Galaxy the whole Universe?
Problem hinges on finding distances:
* How big is the Milky Way?
* How distant are the Spiral Nebulae?
‘Island Universe’ Hypothesis
Nebulae more distant than our
Galaxy is big
‘Nebular’ Hypothesis
Nebulae fit comfortably within the
Galaxy
The Shapley-Curtis Debate
1920 Debate on The Scale of the Universe
Sponsor: National Academy of the Sciences
Harlow Shapley: (Harvard)
* Defended his model for the Galaxy and the
“conventional” Nebular Hypothesis.
Heber Curtis: (Lick Observatory)
* Defended the “radical” Island Universe
Hypothesis.
No clear ‘winner’ since distances were still
unknown.
RR Lyrae Stars
Found in old clusters, Galactic bulge & halo
* Shapley used to measure distances to
globular clusters
* All have roughly the same luminosity
Distance Limit: ~1 Mpc
Cepheid Variable Stars
Supergiants found only in young clusters (in spiral galaxies)
* Period-Luminosity relationship calibrated in 1913
Distance Limit: ~30-40 Mpc
The Realm of the Galaxies
We call the Milky Way “The Galaxy”
Spiral “Nebulae” are Spiral Galaxies:
* Stellar systems like the Milky Way
* Typical sizes are tens of kpc
* Typical distances are in Megaparsecs
(Mpc)
The Universe suddenly became
a much bigger place.
The key to showing that spiral objects seen in the sky
were really separate galaxies was
measuring the period of variable stars in the objects.
All bright galaxies fall into three
broad classes according to their
shape
spiral galaxies
ellepical galaxies
irregular galaxies
Type S: Ordinary spirals
Structure:
* halo – stars, little gas or dust
* disk – stars, gas, and dust
* bulge
* spiral arms – hot young stars,
emission nebulae
Common features :
* Mix of young & old stars
* Rotation
* Lots of gas & dust
Type S: Ordinary spirals
Classified by relative strength of the central bulge & tightness of
the spiral arms
Sa, Sb, and Sc
Sa: strong bulge & tight, indistinct arms
* Sb: intermediate type
* Sc: small bulge & loose, well-defined arms
Type SB: Barred Spirals
Parallel group to the ordinary spirals:
* Equal numbers of barred and ordinary spirals.
Strong central stellar bar:
* rotates as a unit (solid body rotation)
* arms emerge from the ends of the bar
Type E: Elliptical galaxies
Show little internal structure
* No disks, spiral arms, or dust lanes
* Brightest stars are red (Pop II)
* Little gas or dust
* No systematic rotation
Classified by degree of apparent flatness:
* E0 is round
* E7 is very elongated (3:1 aspect ratio
A galaxy populated by mostly red stars probably
had blue stars that are not present anymore
Type S0: Lenticular
Disk shaped with central bulge,
but no spiral arms:
* consist of old stars
* little or no gas, and no star
formation
* significant dust lanes
May possess a central ba
In Hubble’s system, an intermediately wound
barred spiral galaxy would be classified as
SBb
What characterizes an S0 (lenticular) galaxy?
disk, bulge, no spiral arms
irregular galazies
Galaxies that don’t fit in the spiral
or elliptical categories
Properties
* Lower masses and luminosities
than spirals
* Chaotic structure
* Moderate rotation
* Many hot, blue stars
* Lots of gas & dus
The Magellanic Clouds
Two irregular, satellite galaxies of the
Milky Way
* visible from the Southern
Hemisphere
* named for the first European
expedition to observe them
* SMC is tidally stretched by the
Milky Way; 6 times longer than it is
wide
* LMC is now classified as a
disrupted spira
Relative Stellar & Gas Content
Spirals:
Range is ~10-20% gas
* On-going star formation in the disks
* Mix of Pop I and Pop II stars
Relative Stellar & Gas Content
Ellipticals:
Very little or no gas or dust
* Star formation ended billions of years ago
* See only old Pop II stars
Relative Stellar & Gas Content
irregulars
Can range up to 90% gas content
* Often a great deal of on-going star
formation
* Dominated by young Pop I stars
