Final Units of Astronomy

Components of our Galaxy

Disk, bulge, and halo

Disk

flat, contains spiral arms which are home to bright/young stars

Bulge

Bright, central, thicker region, older stars, cigar shape

Halo

Spherical region around disk, very few stars, globular clusters

Interstellar medium

dusty gas clouds that obscure our view of the galaxy because they absorb so much light

Does dust hide most of galaxy

Yes

How big is Milky Way and is there anything similar

Milky Way is very large, closest galaxy to that size is Andromeda

Do several small galaxies orbit our galaxy?

Yes, large and small magellanic clouds

Disk star orbits

move in a nearly circular orbit around the center

Why do disk stars bob up and down

gravity

What does the bobbing cause

disk thickness

Halo Star orbits

randomly oriented, elongated orbits

Do halo stars pass through disk very fast?

Yes, the disk gravity barely changes their path

Does the randomness cause puffiness in the halo?

yes

Bulge star orbits

Some are like halo orbits and some are like disk orbits; it's very hard to measure

What shape do the stars orbiting the bulge give it

a cigar shape

How to find mass within an orbit

Use Newton's version of Kepler's Third Law. Need the orbital speed and radius of the star

Does the halo contain visible stars or gas?

Almost none

Galactic Recycling

Milky Way constantly recycles gas between stars and the interstellar medium

Stages of Galactic Recycling

Atomic clouds, molecular clouds, star formation, nuclear fusion in stars, returning gas in supernovae, hot bubbles

Atomic clouds

Most iSM gas is cool atomic hydrogen

What do atomic clouds emit

radio waves, these help astronomers map them

What do atomic clouds contain

dust grains which absorb visible light and is the reason we can't see across the disk

When do atomic clouds form

They form as hit gas cools, allowing electrons to join with protons

Molecular clouds

Come about when atomic clouds cool/contract

How do you detect molecular clouds?

Via the CO emission lines

What are molecular clouds mostly made of

H2

Are moiecular clouds dense

yes, very

Star formation

Gravity collapses molecular clouds and forms a star cluster in the cloud

What do massive newborn stars emit

UV rays, those rays also erode the cloud

How much of a cloud of gas becomes stars

Just a fraction

Nuclear Fusion in Stars

Nuclear fusion happens in stars and they're stars

Gas returned by a low mass star

Returns gas to interstellar space through winds and planetary nebulae

How strong are the winds of a low-mass star during life

Weak winds; strong winds as a red giant though

Gas returned by High-mass stars

explode as supernovae and eject huge amounts of gas and heavy elements

How strong are winds from a high-mass star

Very strong

Supernovae and Stellar winds create

Hot bubble of ionized gas and shock fronts (wall of gas moving faster than sound)

Hot bubbles

Help mix heavy elements across the galaxy and enrich our galaxy

Many supernovae in one region merge their bubbles and make

giant bubbles that burst out of the disk

Where are molecular clouds

central plane of disk

Where is atomic hydrogen

fills the disk

Where are hot bubbles

in disk/halo

Where do stars form

only in molecular clouds

Regions with newborn hot stars show

ionization nebulae, reflection nebulae, and dark dust clouds

Spiral Arms

Full of massive stars, ionization nebulae, gas/dust,

What are spiral arms in galaxies?

Density waves

What is one result of the density waves in spiral arms?

Compression of gas leading to star formation

Why do blue stars remain near the spiral arms?

They die fast

Why do low mass stars spread around the disk of a galaxy?

They live long

Where do we find ionization nebulae

In the disk because there's star formation there

What is a density wave

slow-moving ripple of high-density

How do stars and gas move through a density wave

Like cars in traffic jam

What causes spiral arms

Compression waves reproducing around the disk (density waves)

Spiral arms appear what color compared to bulge

Blue

Halo stars

Formed first (old), random orbits, no new stars

Disk stars

all ages, circular orbits, ongoing star formation

Why are halo stars so old

Halo has hot gas, bad environment for the molecular clouds and it formed before supernovae enriched the galaxy

Simple version of galaxy forming

1. Giant gas cloud to begin with

2. Halo stars start to form as gravity causes cloud to contract

3. Remaining gas settled into a spinning disk

4. Stars continuously form in the disk as the galaxy grows older

More realistic version

the galaxy gets assembled from a lot of smaller clumps of gas and stars. Little galaxies get merged together over time. The halo stars are very old, but many come from small galaxies accreted earlier

Evidence halo stars are from other galaxies

streams of halo stars = remnants of old dwarf galaxies

Is the halo a smooth sphere of stars?

No many are in "tidal streams"

Can tidal streams destroy smaller galaxies near Milky Way?

Yes, we can destroy small

What brought the universe more heavy elements over time?

The star-gas-star cycle

Black hole at center of Milky Way?

Yes

How do we know black hole at center of our universe?

Infrared images, radio emissions, x-ray flares, and stars orbiting

A galaxy's age, distance, and age of the universe are all closely related

Truth

Disk component

stars of all ages, many gas clouds

Spheroidal component

bulge and halo, old stars, few gas clouds

red-yellow color in galaxy

older star population

blue-white color in galaxy

ongoing star formation

What shape do elliptical galaxies typically have?

Round or oval

What type of stars are primarily found in elliptical galaxies?

Red old stars

Do elliptical galaxies contain a lot of cool gas?

No, they contain little cool gas

Is there new star formation in elliptical galaxies?

No, there is no new star formation

What is a characteristic feature of elliptical galaxies regarding their structure?

They have no arms

Can elliptical galaxies vary in size?

Yes, they can be huge or tiny

Lenticular Galaxy

has a disk like spiral galaxy but much less dusty gas

Is lenticular intermediate between spiral and elliptical?

Yes

What type of galaxy is not spiral or elliptical?

Irregular galaxy

What is a characteristic shape of an irregular galaxy?

No clear shape

What is a common size characteristic of irregular galaxies?

Often small

What color are irregular galaxies typically described as?

White and dusty

What type of activity is often found in irregular galaxies?

Active star formation

When were irregular galaxies more common in the universe?

In the early universe

Barred spirals

Spiral galaxy with straight bar of stars across the center

The Hubble "Tuning Fork"

Qualitative way of classifying galaxies

Does The Hubble "Tuning Fork" answer the question of why galaxies have different shapes?

No

What is the scale of the The Hubble "Tuning Fork"

Spheroidal dominant to disk dominant

Blue Cloud

spirals and irregulars: lots of cold gas and new star formation

Red sequence

ellipticals and very luminous at the top

Where are spiral galaxies often found

in isolation or in groups of galaxies (group being a couple dozen of galaxies)

Where are elliptical galaxies found

Much more common in huge clusters of galaxies (hundreds to thousands of galaxies packed into a volume not much larger than distance between Andromeda and Milky Way)

Why distance of galaxies matters

Allows us to determine the size, age, and expansion rate of the universe

Distance Ladder

a method used in astronomy where greater and greater distances are determined using many different measuring techniques that overlap to establish a sequence of increasing distances.

standard candle

an easily recognizable astronomical object whose absolute brightness is confidently known

Cepheid Variables

A variable star that brightens and dims regularly, or pulses, and whose distance can be determined from its period of pulsation (longer period = brighter)

Hubble's Law

The observation that the farther away a galaxy is, the faster it is moving away. As we look back in time galaxies should be closer together and more dense

How did Hubble say galaxies are shifting?

Redshifting

Hubble's constant

tells us the age of the universe because it relates velocities and distances of all galaxies

How did Galaxies form?

Our best models for galaxy formation assume that gravity made galaxies out of regions in the early universe that were slightly denser than their surroundings.