Chapter 20 cosmic perspective

About how many galaxies in observable universe?
over 100 billion galaxies
Age of most galaxies around us?
10 billion years
How to study young galaxies?
looking at rgeat distances
Cosmology
study of overall structure and evolution of the universe
Three major types of galaxies
Spiral, Elliptical, irregular
Spiral Galaxy
flat white disks with yellowish bulges at their centers – usually display beautiful spiral arms
Elliptical Galaxy
Redder, more rounded , elongated like football – contain very little cool gas and dust, contain very hot, ionized gas
Irregular Galaxy
appear neither dislike or rounded – blobby star systems – contain young massive stars – megellanic clouds are examples
Why do the colors of galaxies differ?
spiral/irregular galaxies appear white because they ave stars of all different colors and ages, elliptical galaxies have old, reddish stars that produce most their light
Dwarf Galaxies
have as few as 100 million stars
Giant galaxies
more than 1 trillion stars
disk component
flat disk where stars follow orderly, nearly circular orbits around center – contains interstellar medium
How may disk components differ?
molecular, atomic, and ionized gases in the interstellar medium may differ from one to the next
Spheroidal Component
The bulge and Halo – orbits with many inclinations and contain little cool gas or dust
Bulges tend to extend how far?
10,000 Light years
Barred spiral galaxies
spiral galaxies that appear to have a straight bar of stars cutting across the center with spiral arms curling away from the ends of the bar
What do astronomers think our galaxy is?
Barred spiral galaxies because our bulge appears somewhat elongated
Lenticular galaxies
intermediate class between spirals and elliptical (they lack arms) tend to have less cool gas than normal spirals but more than ellipticals
Among large galaxies what percent are spiral or lenticular?
75-85%
elliptical galaxies are sometimes known as?
spheroidal galaxies
Elliptical galaxies lack a significant _______ component.
Disk
Giant Elliptical galaxies
relatively rare and are among the most massive galaxies in the universe
Dwarf Elliptical Galaxies
fewer than a billion stars – often found new larger spiral galaxies
Gas in a giant elliptical galaxy:
Low density, x ray emitting much like the gas in hot bubbles created by supernovae
Lack of cool gas in elliptical galaxy means:
They have little to no star formation (like our own halo)
When were irregular galaxies more common?
when the universe was younger
Hubbles Galaxy Classes
organizes galaxy types into a diagram shaped like a tuning fork
E0 Galaxy
Sphere
Hubble Galaxy Class Elliptical
Designated by E and a number – Elliptical are on the handle at left – larger number/flatter disk
Hubble Galaxy Class Spiral
S for ordinary spiral, SB for barred spirals followed by lowercase a, b, or c – bulge decreases size a-c
S0 (hubble class)
Lenticular Galaxies
Irr (hubble Class)
Irreglar Galaxes
Quantitative galaxy classification
similar to H-R diagram for Stars – measures galaxy luminosity and galaxy color
Blue Cloud
Major group – consists of spiral or irregular galaxies with active star formation
Red Sequence
Consists galaxies that lack active star formation and are redder in color because they have few blue or white stars – most elliptical in shape
Groups
spiral galaxies found in loose collections of up to a few dozen falaxies
Clusters of galaxies
contain hundreds and sometimes thousands of galaxies extending over more than 10 million light years – elliptical
radar ranging
how astronomers measure AU – radio waves are transmited from Earth and bounced of Venus
Standard Candle
light source of a known, standard luminosity
Main-sequence fitting
method of determining distances to different star clusters by comparing brightness to their main sequence stars
Cepheid Variable stars
extremely luminous variable star
Henrietta Leavitt
1912 – discovered Cepheid are closely related to their luminosity (longer the period, more luminous the star)
Period Luminosity Relation
the longer the period, the more luminous the star
Cepheid vary in luminosity because:
they pulsate in size, growing brighter as they grow larger and dimmer as they shrink –
The Great debate
Held in Washington dc April 26 1960 – Shapley ( spiral nebulae were gas clouds internal to milky way) Vs Curtis (Kants island of stars)
What did Hubble discover about Andromeda?
Using the 100 inch telescope at Mt Wilson he saw individual stars – he used Cepheid stars to calculate distance
redshifted
object emitting radiation is moving away from us
Hubble’s Law
the formula that expresses the idea that distant galaxies move away from us faster
Hubble’s constant
H-naught
Two important difficulties when using hubble’s law to measure galactic distances
1 – galaxies do not object the law perfectly – nearly all galaxies experience gravitational tugs from other galaxies
2 – distance as only as accurate as best measurement of hubble’s constant
Does hubble’s law work for galaxies in the local group?
No
Cosmological principle
the idea that the matter in the universe is evenly distributed without a center or edge
Balloon analogy
as the balloon expands, the dogs move apart in the same way galaxies move apart in our expanding universe
lookback time
difference between current age of the universe and the age of the universe when light left the object
spacetime diagram
a way to visualize the relationship bet ween distance, expansion and lookback time
cosmological redshift
as the universe expands photo wavelengths shift to longer, redder wavelengths
cosmological horizon
marks the limits of the observable universe (a boundary in time not space)