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?

By observing galaxies at great distances. Because light takes time to travel, the farther away a galaxy is, the further back in time we are seeing it—allowing astronomers to study galaxies as they were when the universe was younger.

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 disklike 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

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

elliptical galaxies are sometimes known as?

spheroidal galaxies

Elliptical galaxies lack a significant _______ component.

Disk

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

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 of galaxies

spiral galaxies found in loose collections of up to a few dozen galaxies

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

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

Hubble's Law

the formula that expresses the idea that distant galaxies move away from us faster

v = H0​×d

Hubble's constant

H-naught = H0

Two important difficulties when using hubble's law to measure galactic distances

1 - Galaxies do not obey the law perfectly - nearly all galaxies experience gravitational tugs from other galaxies

2 - Distance as accurate as the best measurement of Hubble's constant

3 - Does not work for galaxies in the local group because their motion is dominated by gravity, not cosmic expansion

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)

What does the “Blue Cloud” vs. “Red Sequence” distinction refer to?

Blue Cloud = galaxies with active star formation (spirals/irregulars);
Red Sequence = old, red, non-star-forming galaxies (ellipticals).

What does the Hubble Tuning Fork classify?

Galaxy morphologies: ellipticals (E), spirals (Sa–Sc), barred spirals (SBa–SBc), and lenticulars (S0).

What is the cosmic distance ladder?

A series of distance-measuring techniques: radar → parallax → main sequence fitting → Cepheids → white dwarf supernovae.

What makes white dwarf supernovae useful as standard candles?

They always explode at the same mass (1.4 M☉), so their peak luminosity is consistent.

What is Leavitt’s Law (Period-Luminosity Relation)?

The longer the period of a Cepheid variable, the more luminous it is.

What is lookback time?

The difference between the current age of the universe and the age when light left an object.

What is the cosmological horizon?

The maximum distance from which light has had time to reach us—marks the edge of the observable universe.

What does a cosmological redshift tell us?

It shows how much the universe has expanded since the light left a distant galaxy.