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

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?

elliptical galaxies are sometimes known as?
spheroidal galaxies

Elliptical galaxies lack a significant _______ component.

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

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

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

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

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?

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)

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