The Milky Way Galaxy (Ch. 25)

- The architecture of the Galaxy -

↳ Made up of stars, gas, and dust. It is a barred spiral galaxy

» spiral arms and a central bar

Bulge

» spherical, in the center contains a central bar

» contains old stars

Disk

» shaped like a frisbee, includes spiral arms

• Parts of disk:

  • Thin Disk > contains youngest starts, new starts form here, denser,

  • Thick Disk > Contains oldish stars, puffier, Stars puff out of the thin disk over the time to create the thick disk

Halo

» spherical, surrounds disk

» contains very old, faint stars

Central Bulge > very center of the galaxy

Global Clusters > groups of very old stars that live above the disk

Mass of the Milky Way

» use the orbital motion of the sun around the center of the Milky Way to measure the mass of the galaxy

• Orbital period = time to complete one full orbit

• Semi major axis = distance to center of milky way

What is the evidence for dark matter that comes from measuring the rotation curves of galaxies?

✩ The accepted theory is that galaxies contain dark matter in a large halo around the galaxy. The dark matter is the unseen matter that speeds up the orbits of stars on the edge of the stellar disk

Dark matter

• Lives in the haloes of galaxies

• A new kind of sub-atomic particle we have been discovered yet

• 95% > milky way

• 5% > visible matter

The Center of the Milky Way

• Sgr A* - our very own supermassive black hole at the center of the Milky Way

  Evidence

  • Radio wavelength image of the center of the universe

  • Astronomers traced the orbits of stars in the central bulge by tracking their orbits they could figure out the mass concentrated at the center of the Milky Way

  • Lots of mass > no light emitted > supermassive black hole

Stellar Populations in the Milky Way

Population I

• bright, blue, in spiral arms within disk

• Age: younger

• Chemical comp: more heavy elements

Population II

• Fainter, redder, In halo and globular clusters

• Age: Older

• Chemical comp: fewer heavy elements

Galaxies (Ch 26)

Types of galaxies

» Spiral galaxies ( 1 million times larger than our solar system )

• The Milky Way is one of them

• parts of the galaxy > Central bulge, halo, disk, spiral arms

• Star-forming » form new stars (contain both Population I and Population II)

• red light = star-forming regions

• blue light = from hot young, massive

• All spiral galaxies rotate around their centers

• some have bars

Hubble system for classifying spiral galaxies > grouped by their morphology or shape, Barred spirals ( SB) and Un-barred spirals (S)

» Elliptical galaxies

• contain almost entirely old starts ( population II )

• reddish old, low mass starts ( very little ongoing star formation )

• no spiral arms

• red and dead

• Size: Giant ellipticals, dwarf ellipticals, and in-between ellipticals

Hubbles Classification > Organized by ellipticity with a spherical shape on the left, more ellipsoidal on the right

» Irregular galaxies

• Dont have regular spiral or ellipsoidal shapes

• Lower mass, and luminosity than spiral galaxies

• disorganized

• Many have significant star formation (both population II and population I)

» Properties of galaxies

Method 1

• Astronomers can measure the mass of a galaxy by measuring the time it takes stars to orbit the center of the galaxy at different distances out

• light from one side will be redshifted light from the other side will be blue-shifted.

• Amount of shift > rotational speed > mass of galaxy

Method 2

• measure the spectrum of the whole galaxy (combines the light of all its stars). Look for doppler shifts due to the rotation of the galaxy.

The Extragalactic Distance Scale

» Standard Candles have known intrinsic luminosity

• Apparent brightness = distance

» Variable Stars

• Get brighter and fainter over time.

• Cepheid variable

• Period= time between brightest phases

• Intrinsic luminosity

How to Measure it?

measure its period, get luminosity from period-luminosity relationships, and calculate its distance. Distance to star = distance to galaxy

» Type 1a supernova

• Happens when a white dwarf in a binary system explodes

• All type 1a have the same intrinsic luminosity.

• They are very bright and hence can be detected from out to very large distances. By catching type 1a going off we can calculate the distance to its host galaxy

The Expanding Universe

Edwin Hubble measured the following for a bunch of galaxies, distance from and their velocity. Used telescope at Mt. wilson

» Hubble’s Law

• The further away a galaxy was the faster it was moving away from us

• v = H x d

• V > velocity , d > distance , H > bubble constant

• Hubbles law works out to large distances

  It means the universe us expanding uniformly at the same rate everywhere in the universe, as the universe expands the space between galaxies increases.

• We are not at the center !!

Active Galaxies, Quasars and Supermassive black holes (Ch 27)

» Quasars ( quasi stellar radio source)

• They look kind of bright blue stars in images (they aren’t stars !)

• Some are bright at radio wavelengths (redshifted)

• Quasars are moving away from us at a high speed ( Hubbles Law ) quasars live at the center of galaxies.

» Supermassive black holes

• Supermassive black holes are the source of the energy admitted by quasars and (and all AGN)

• Quasars are accreting supermassive black holes in the centers of galaxies

• They are one type of a class of objects called Active Galactic Nuclei (AGN), which are galaxies that all have active supermassive black holes in their centers.

• AGN produce abnormal amounts of energy in their central region (nucleus)

• Parts of a quasar:

  • Supermassive black hole

  • Accretion disk

  • Jets

• Quasars as probes of evolution in the universe

• Number of quasars in the universe peaked when the universe was about 2 billion years old

• The universe can change over time!

• (Almost) all galaxies have supermassive black holes in their center

  • How are the mass of the black hole and the mass of the galaxy related?

    If two galaxies merge their supermassive black whole can merge, supermassive black holes can regulate the rate at which new stars form in galaxies. Black hole eats gas > stars are made from gas > if the black hole eats too much gas star formation stops.

 Evolution & Distribution of Galaxies (Ch. 28)

Observations of distant galaxies

» Telescopes are like time machines!!

• Further away in distance is observing the universe as it was further back in time.

» Galaxies life cycle

• Time scales for galaxy evolution are much longer than human life scales so we can’t see individual galaxies ( or stars, or planets ) grow and change.

• we observe galaxies of diff ages and figure out the order of the evolution process.

» Compare galaxies we see today (nearby) to the distant past

• Early galaxies had no identifiable spiral mrs, disks, or bulges. They were more chaotic, and less ordered (clumpier). Extremely blue and smaller than modern galaxies.

• Modern galaxies are more gas-rich, more star-forming

» Galaxy mergers

• when galaxies collide

• very few stars collide during a galaxy merger.

• Galaxy mergers trigger star formation “starburst”

• Multiple spiral galaxies can merge to form a larger elliptical galaxy

• Galaxies merged get “stirred up”, lose their shape get a diff one.

» The distribution of galaxies in space

• Cosmological principle: the universe is the same everywhere. The universe is isotropic and homogenous

• (Isotropic: looks the same in every direction)

• (Homogenous: A large volume of space is essentially the same as any other large volume of space)

• What is our “cosmic address”?

  Earth, solar system, Interstellar neighborhood, Milky way, Local group of galaxies, Virgo cluster, Local supercluster, Observable universe

• Large scale structure of the universe is made up of galaxies

  • Galaxies clump into galaxy groups or clusters

  • Those groups or clusters form superclusters

  • These superclusters form the large scale structure of the universe

» Cosmic web

• Structure of the Universe on the largest scales

• Connected by filaments ( strings of galaxies )

• Regions without galaxies are called voids

• Looks kind of like sponge foam

» Galaxy formation

1. Formed via monolithic collapse of large clouds of gas + dark matter

   • Top-down scenario (kind of like how stars form)

   • Primordial gas + dark matter cloud > collapses under gravity > galaxy forms (bulge + spiral) > Disk galaxy and companion > Smaller galax falls into disk galaxy > bulge inflates with additions of stars + gas

2. Formed through mergers of smaller galaxies (bottom-up scenario)

   • mergers

The Big Bang (Ch. 29)

The age of the universe

» Big Bang Theory

• Theory of cosmology in which the expansion of the universe began with a primordial explosion of space, time, matter, and energy

✩ The universe is currently expanding, we observed this with bubbles law. We can try to rewind to figure out how old the universe is. Due to the expansion of the universe over time, the observable universe has gotten larger

• Rate of Expansion > If we we know how much the observable universe has expanded, we can figure out how long it has been expanding

time= distance/velocity

• the observable universe > a region of space visible to us from Earth beyond the edge of the oberavle universe the time it would take light to reach us is longer than the age of the universe itself

How can the universe be infinite if there was a Big Bang?

» the universe was not all concentrated into one point at the time of the big bang. The observable universe was concentrated into a point.

Timeline of the evolution of the universe

• The early universe was hot and dense. This is because density, pressure, and temperature are related.

  • If you cram the whole universe into a small space it will have high density , high pressure, and high temperature

• As the universe expanded after the big band it cooled down.

Main steps in the timeline of the evolution

• A hot soup ( 0-375,000 years after the Big Bang)

  • The universe was hot and dense plasma, universe is expanding. First things forms such as the first subatomic particles, hydrogen atoms, some helium is created through nuclear fusion

• The Cosmic Microwave Background ( 375,000 years after the big bang)

  • The universe expands enough and cools enough, that there is no longer plasma. There was a time when the universe went from being opaque to being transparent. We see the cosmic microwave background. We observe this glow from the early universe in all directions.

• The Dark Ages ( 375,000-400 million years after the big bang)

  • The universe had expanded/cooled enough to be transparent. No stars or galaxies yet. Nothing was able to shine so it was very dark.

• The first stars formed ( 400 million years after Big Bang)

  • Has cooled long enough for cool clouds of gas to collapse and form the first stars

• The development of galaxies, stars, and planets ( 400 my- 5 billion years after the big bang)

  • First stars formed then galaxies and planets formed. The universe was still expanding bu the expansion was not accelerating (speeding up)

•  The expansion of the universe starts accelerating

  • after it was 5 billion years old, dark energy became the dominant type of matte/energy in the universe

  • Drak energy energy of space

  • expansion is speeding up.

What is the universe really made of?

• What fraction of the matter-energy in the universe is:

  • Dark matter 27%

  • Dark energy 68%

  • “Ordinary” matter 5%

    • us

    • 1/5 is stars

    • 4/5 is hydrogen and helium gas between galaxies

How will the universe end?

1. Big Freeze

   aka heat death of the universe, if the universe keeps expanding forever eventually galaxies and solar systems will drift apart. there won’t be enough gas to form new stars. Black holes will eventually evaporate. Space will be big empty and very cold.

2. Big rip

   The expansion will speed up things will keep drifting apart, and even subatomic particles will get ripped apart.

3. Big crunch

   In this theory the universe will stop expanding at one point n will begin to collapse again. This would be kind of like a reverse big band that would lead to another big bang and the creation of th universe all over again.