Cosmic Perspective Chapter 19 Our Galaxy

How many stars does our galaxy hold?

100 billion stars

What does our galaxy look like?

It is a spiral galaxy, with spiral arms, a flat disk surrounding a bulge, a halo and globular clusters of stars

Spiral galaxy

galaxies that look like flat white disks with yellowish bulges at their centers

Spiral Arms

Bright, prominent arms in a spiral pattern

Disk

Contains an interstellar medium with cool gas and dust, many stars are found here

Bulge

central portion of galaxy that is roughly spherical, goes above and below the plane of galactic disk

Halo

spherical region surrounding the disk of a spiral galaxy

globular clusters

a spherically shaped cluster of up to a million or more stars , found in halos and have very old stars

How many light years is our galaxy?

100,000 light years in diameter

How many light years thick is the disk?

1,000 light years

Where is our sun in the galaxy?

27,000 light years from the center

interstellar gas and dust is known as

Interstellar Medium

Which galaxies orbit ours?

Small Magellanic Cloud and Large Magellanic Cloud,

About how many stars are in these galaxies?

one billion to a few billion stars

What is the orbit of disk stars?

Like a merry go round - bobbing up and down as they orbit

In our suns vicinity how long does an orbit take?

More than 200 million years

How long is each "bob"?

Few tens of millions of years

What is the orbit of Halo and Bulge stars?

elliptical paths but random, swooping

Speed of sun and neighbors orbiting?

220 kilometers per second (800,000 km/hr)

Dark matter

Does not give off any light, majority of mass in Galaxy

How is gas recycled in our galaxy?

Through the Star-gas-star cycle

Star-gas-star cycle

Hot bubbles - Atomic hydrogen clouds - molecular clouds - star formation - nuclear fusion in stars - returning gas

How do low mass stars return gas?

Stellar winds

How do high mass stars return gas?

supernovae

Bubble

Hot, ionized gas (gas where some atoms are missing some of their electrons) around exploding star

shock fronts

from supernova - abrupt, high-gas-pressure walls that move faster than the speed at which sound can travel through interstellar space

supernova remnant

the structure resulting from the explosion of a star in a supernova

synchrotron radiation

radio waves from fast electrons

local bubble

cavity in the interstellar medium (ISM) in the Orion Arm of the Milky Way

Cosmic rays

made of electrons, protons and atomic nuclei that zip through interstellar space at close to the speed of light

Superbubble

individual bubbles combining

blowout

hot gas erupts from the disk spreading out and shooting upward into galactic halo

Galactic Fountain

Supernova explosions in a young star cluster blow a super bubble in galactic disk - when its thicker than disk, hot gas can blowout into the halo, how gas in bubble cools and forms cloud, cool clouds then rain back down onto galactic disk

atomic hydrogen gas

cooler gas - hydrogen atoms that remain neutral rather than ionized

Atomic hydrogen gas is mapped by what waves?

Radio observations

What is the 21 centimeter line?

A specific radio wavelength (21 cm) emitted by atomic hydrogen atoms that is used by astronomers to map atomic hydrogen gas in the galaxy, even through dust clouds.

atomic hydrogen is found where?

Large, tenuous clouds of warm (10,000 k) atomic hydrogen and smaller, denser clouds of cool (100k) atomic hydrogen

interstellar dust

tiny, solid particles that form in the winds of red giant stars and resemble smoke particles

molecular cloud

coldest , densest collections of gas and birthplace of stars

giant molecular clouds

contain millions of solar masses of gas - only a few degrees above absolute zero

ionization nebulae

colorful wispy blobs of lowing gas near hot stars

Why do the nebulae glow?

Because ultraviolet light from nearby hot stars excites the atoms in the nebula. This ionizes the atoms or raises their electrons to higher energy levels—when the electrons fall back down, they emit visible light, causing the nebula to glow.

reflection nebulae

bluer in color - starlight reflected from dust grains

Spiral arms are full of?

newly forming stars

model of spiral density waves

spirals arms are places in a galaxy's disk where stars and gas clouds get more densely packed

Stars are created more readily in spiral arms why?

gravity bunches interstellar gas clouds more tightly than anywhere else in the disk - collisions between gas clouds compress gas inside, increasing the strength of gravity and triggering star formation

What is the disk population (Population I) in our galaxy?

A group of stars found in the galactic disk that includes both young and old stars. These stars have relatively high amounts of heavy elements (about 2%), meaning they formed from gas enriched by earlier generations of stars.

What is the spheroidal population (Population II) in our galaxy?

Stars found in the halo and bulge of the galaxy. These stars are old, low in mass, and have very low amounts of heavy elements (as little as 0.02%), meaning they formed early in the galaxy’s history—before much chemical enrichment occurred.

protogalactic cloud

hydrogen and helium gas

How did our galaxy form?

protogalactic cloud containing only H and He, halo stars begin to form as the cloud collapses, conservation of angular momentum ensures that the remaining gas flattens into a spinning disk, billions of years later, the star gas star cycle supports ongoing star formation within disk

Galactic Center

4 million solar masses packed into a region just larger than our solar system - most likely a black hole

What is the orbital velocity of the Sun around the galaxy, and how do we use it to calculate mass?

220 km/s; Using the orbital velocity law:
M= v²r/G with M=10^11 M⊙​

What is the typical orbital period of the Sun around the Milky Way?

Over 200 million years.

What are the typical temperatures of gas in the star-gas-star cycle?

• Hot bubbles: ~10610^6106 K

• Atomic hydrogen: ~10210^2102–10410^4104 K

• Molecular clouds: ~30 K

What is the chemical composition of molecular clouds?

Mostly H₂, ~28% He, ~1% CO, with trace molecules.

Why do reflection nebulae appear blue?

Scattering of shorter-wavelength light—same reason the sky is blue.

What do halo stars tell us about the Milky Way’s formation?

They are old and metal-poor, indicating the halo formed early from clumps that later merged.

What evidence shows the galaxy formed from mergers?

Stellar streams—trails of stars from smaller galaxies being incorporated.

What is the mass of the black hole at the Milky Way’s center, and how do we know?

~4 million M☉, inferred from orbits of stars near Sagittarius A*.