Astro Exam 3

Milky Way

Our homme galaxy containing billions of stars, including the Sun

What do we see from Earth?

We see the spiral arm we live inside!

Barred spiral galaxy

A galaxy with a bar-shaped region across its center and spiral arms

DIsk

the flat part containing most of the galaxy’s stars, gas, dust, and spiral arms.

Bulge

The crowded, rounded region near the center of the galaxy

Stellar halo

A large, roughly spherical region surrounding the disk and bulge

Consider the vast distances between stars compared to their sizes. Is it likely that two stars will collide?

. No, the stars are very small compared to the distances between them.

Galaxies contain

all stages of star birth, life, and death ‡

including nebulae ‡

Stars ‡

Supernova Remnants ‡

Molecular Clouds

Molecular cloud

A cold, dense cloud of gas and dust where stars form.

Supernova

A powerful explosion that happens when a massive star dies.

Superbubble

A huge bubble of hot gas created by multiple supernovae.

Galactic fountain

Hot gas rises out of the disk, cools in the halo, and returns.

Star-gas-star cycle

Gas forms stars, stellar material returns to space, and new stars form.

When a star dies and leaves behind a white dwarf or a neutron star, is that core technically made of gas from the galactic recycling cycle?

A. Yes

Where will the galaxy's gas be in 1 trillion years?

C. Locked into white dwarfs and low-mass stars

21 cm radio waves

21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk

Radio emission used by astronomers to map atomic hydrogen gas.

CO radio waves

Radio waves from carbon monoxide that help locate molecular clouds. coldest gas clouds

Radio waves from carbon monoxide (CO) show the locations of molecular clouds.

Infrared light

Wavelengths that reveal dust and stars hidden behind dust clouds.

Infrared light reveals stars whose visible light is blocked by gas clouds

X-rays

High-energy radiation used to reveal very hot gas in the halo.

X-rays are observed from hot gas above and below the Milky Way's disk.

hot gas fills the halo

Gamma rays

Radiation showing where cosmic rays collide with gas atoms.

Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds.

What are spiral arms?

The apparent spiral windmill arms are called spiral arms ‡

They are not solid structures ‡

They are density waves ‡

Regions where stars and gas spend more time in their orbits ‡

Means you end up with a higher density of stars in that area than in surrounding areas ‡

Stars move in and out of these areas ‡

Massive stars are born and die before having enough time to move out of these arms

Density wave

A crowded region of stars and gas that forms spiral arms.

Satellite galaxy

A smaller galaxy orbiting a larger galaxy, like the Magellanic Clouds.

Galaxy merger

The process of two galaxies interacting and combining.

Milky Way is in the process of merging!

Sagittarius Dwarf Galaxy

Stream Lots of other streams of previous mergers!

Disk star orbits

Organized, circular orbits in the same general direction.

Halo and bulge star orbits

Varied orbits with random orientations taking them far from the disk.

Cosmology

the study of the structure and evolution of the universe

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

The study of galaxies is thus intimately connected with cosmology.

The study of the structure, origin, and evolution of the universe

Galaxies can be divided into three categories

Spiral ‡

Elliptical ‡

Irregular.

Spiral Galaxies

• Face-on: Looking directly at the front of the galaxy. You can see the spiral arms.

• Tilted: Looking at the galaxy at an angle.

• Edge-on: Looking at the galaxy from the side. It looks thin, and you can see a dark dust line.

• Here, the central bulge and halo can be seen prominently

• In this barred spiral galaxy, seen face on, the young, bluish-white stars associated with ongoing star formation in the disk can be seen

Why does ongoing star formation lead to a blue-white appearance?

Short-lived blue stars outshine the others.

Lenticular galaxy:

A galaxy with a disk like a spiral but much less dusty gas.

has a disk like a spiral galaxy ‡

but much less dusty gas ‡

intermediate between spiral and elliptical

Elliptical galaxy

All spheroidal component ‡

Virtually no disk component ‡

Red-yellow color indicates an older stellar population

An oval-shaped galaxy with virtually no disk and older, red-yellow stars.

Some are dwarf ellipticals. ‡

Can contain 1 billion to tens of billions of stars. ‡

Similar to globular clusters but 1000 times bigger

Irregular Galaxies

A galaxy with no organized shape and ongoing star formation.

They can contain:

• Young stars.

• Older stars.

• Gas and dust.

• Areas of active star formation.

The slide mentions that their white or bluish-white appearance indicates ongoing star formation.

Hubble Tuning Fork

A classification diagram organizing galaxies by shape, not an evolutionary life cycle.

Galaxy Group vs. Cluster

Groups contain up to a dozen galaxies; clusters contain hundreds to thousands.

Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies). ‡

Clusters!

Local Group

The galaxy group containing the Milky Way, Andromeda, and about 80 other galaxies.

Galaxy Clusters

Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies).

Galaxy Colors

Galaxies tend to fall into two groups, the blue cloud and red sequence.

blue cloud

A grouping of actively star-forming, mostly spiral and irregular galaxies. Blue galaxies usually have young, hot stars and ongoing star formation.

Blue cloud:

• Mostly actively star-forming galaxies.

• Often spiral and irregular galaxies.

• Contain young, hot, blue stars.

Red Sequence

A grouping of mostly elliptical galaxies with older stars and little star formation.

• Red galaxies generally have older stars and less ongoing star formation.

• Red sequence:

  • Mostly galaxies with older star populations.

  • Often elliptical galaxies.

  • Have little ongoing star formation.

Standard Candle

An object whose actual luminosity is known or can be determined without measuring distance.

Step 1 Determining Distance: Radar

Determine size of the solar system using radar.

Step 2 Determining Distance: Parralax

Determine the distances of stars out to a few hundred light-years using parallax.

The apparent shift in a star's position as Earth orbits the Sun, used to measure distance.

• Large parallax shift = closer star.

• Small parallax shift = farther star.

Luminosity passing through each sphere is the same. ‡

Area of sphere: ‡ 4pier²

Divide luminosity by area to get brightness.

Thought Question 2 Which kind of stars are best for measuring large distances?

A. High-luminosity stars

Cepheid Variable Star

A luminous star that regularly changes brightness, used as a cosmic standard candle.

Leavitt's Law

The relationship stating that Cepheid variable stars with longer periods have greater luminosities.

Standard Candle

An object whose actual luminosity is known or can be determined without measuring distance.

White-Dwarf Supernova

White-dwarf supernovae can also be used as standard candles.

Type Ia explosion with predictable peak luminosity used to measure vast cosmic distances.

Determining Distance

Step 3

Because the period of Cepheid variable stars tells us their luminosities, we can use them as standard candles with  Leavitt’s Law

Determining Distance

Step 4

• The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years).

The Great Debate

• Before Hubble. some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way.

• The debate remained unsettled until Edwin Hubble finally measured their distances.

• The historical argument over whether spiral nebulae were inside or outside the Milky Way.

Redshift

The shifting of light waves toward longer wavelengths as galaxies move away from us.

Hubble-Lemaître Law

The rule stating that a galaxy's recession velocity equals Hubble's constant times its distance.

Distance and Redshift

Redshift of a galaxy tells us its distance through Hubble's law:

• Distances of the furthest galaxies are measured from their redshifts.

Distance Ladder

A chain of interdependent techniques used by astronomers to measure increasingly larger distances in space. we measure galaxy distances using a chain of interdependent techniques

Cosmological Principle

The principle that the universe looks roughly the same everywhere on very large scales.

Thought Question 3

Your friend leaves your house. She later calls you on her cell phone, saying she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

C. 60 minutes 

Expansion of the Universe

The expansion rate appears to be the same everywhere in space. ‡

The universe has no center and no edge (as far as we can tell)

One example of something that expands but has no center or edge is the surface of a balloon.

Cosmological Principle

The universe looks about the same no matter where you are within it. ‡

Matter is evenly distributed on very large scales in the universe. ‡

It has no center or edges. ‡

The cosmological principle has not been proven beyond a doubt, but it is consistent with all observations to date.

The principle that the universe looks roughly the same everywhere on very large scales.

Thought Question 4

You observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?

D. 14 billion years

Age of the Universe

• Hubble’s constant tells us the age of the universe because it relates the velocity and distances of all galaxies.

• Age = distance/velocity, about 1/H0

Lookback Time

The time light took to travel from a distant object, showing us the past.

• Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time. 

Cosmological Horizon

The time-based boundary marking the limits of our observable universe. The Cosmological Horizon marks the limits of the observable universe.

• It is a horizon in time rather than space. Since looking far away means looking back in time, there must be a limit – the beginning of the universe!

How does expansion affect distance measurements?

A supernova occurs long ago in a distant galaxy.

As time passes, the universe expands and the distance to that galaxy increases.

The galaxy’s distance is therefore much greater when the supernova’s light reaches us in the Milky Way Galaxy.

• Distances between faraway galaxies change while light travels.

• Astronomers think in terms of lookback time rather than distance.

• 
  

• A supernova occurs long ago in a distant galaxy.

• As time passess, the universe expands and the distance to that galaxy increases.

• The galaxy’s distance is therefore much greater when the supernova’s light reaches us in the Milky Way Galaxy.

What does the light from a distant galaxy show us?

B. It shows us what the galaxy looked like when the light left the distant source

Shows what the galaxy looked like when light left.

Observations Through Time

JWST Deep Field ‡

Very distant galaxies appear as they were much earlier in time ‡

Old light ‡

From when galaxies were very young

Life Histories of Galaxies

Observing galaxies at different distances shows us how they age ‡

Far away = young (aka early in Universe) ‡

Close = older (current age of Universe) ‡

We still can't directly observe the earliest galaxies

Far away equals young; close equals older.

Galaxy Formation

Based off of observations ‡

Think large scale structure of the Universe looks approximately like this ‡

Each point of light is a cluster of galaxies

How can we match this? ‡

Our best models for galaxy formation assume: ‡

Matter originally filled all of space almost uniformly. ‡

Gravity of slightly denser regions pulled in surrounding matter.

Denser regions contracted -> protogalactic clouds ‡

H and He gas formed the first stars ‡

Elliptical, spiral, or irregular?

Protogalactic cloud spin effect

Spin: The initial angular momentum of the protogalactic cloud determines the size of the resulting disk. ‡

Low spin = elliptical galaxy ‡

Higher spin = spiral galaxy

Protogalactic cloud density effect

Density: ‡

Higher density clouds may have formed stars faster and ended as elliptical galaxies ‡

Lower density clouds may have formed stars slower, allowing time for much of the gas to settle into a disk

Why do galaxies end up looking so different (e.g., some spiral and some elliptical)?

A. It is probably related to the conditions of the cloud system from which the galaxy formed.

B. It is probably related to interactions between galaxies as they evolve.

Distant Red Ellipticals

Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe.

Life Histories of Galaxies

Collisions were much more likely early in time ‡

galaxies were closer together

Galaxy Interactions

Many of the galaxies we see at great distances look violently disturbed.

Collisions between galaxies are common ‡

Galaxies are not as widely separated compared to their size ‡

The nearby collisions we observe show bursts of star formation

Collision simulations shows that two spiral galaxies can merge to make an elliptical ‡

The final shape and appearance of galaxies is likely largely influenced by collisions and mergers

Evidence of past collisions:

Shells of stars observed around some elliptical galaxies are probably the remains of past collisions

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together. ‡

Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies.

Gas Cycles

Galaxies start as star forming systems, appearing blue in color. ‡

Galaxy mergers create larger, more luminous galaxies with less star formation, appearing red in color.

Star-forming galaxies appear blue.

Starburst galaxies

produce new stars at very high rates ‡ as much as 100 times that of our galaxy

Gas Cycles in Galaxies

Intensity of supernova explosions in starburst galaxies can drive galactic winds ‡

A galactic wind in a small galaxy can drive away most of its gas

Driven by intense supernova explosions in starburst galaxies.

Active Galactic Nucleus (AGN)

If the center of a galaxy is unusually bright, we call it an active galactic nucleus (AGN). ‡ Super bright due to black hole actively accreting material

An unusually bright galaxy center powered by an accreting black hole.

Quasars

are the most luminous examples. ‡

The host galaxy of this quasar is barely visible in the bright glare.

The most luminous AGNs with highly redshifted spectra indicating vast distances.

The highly redshifted spectra of quasars indicate large distances. ‡

Luminosities of some quasars are greater than 1012 LSun ‡

Variability shows that all this energy comes from a region smaller than our solar system.

What can you conclude from the fact that quasars usually have very large redshifts?

A. They are generally very distant.

B. They were more common early in time.

C. Galaxy collisions might turn them on.

D. Nearby galaxies might hold dead quasars.

E. All of the above.

Supermassive Black Hole

Orbital evidence ‡

model matches data for galaxy M106 ‡

suggests a black hole of 36 million MSun

M87 ‡

SMBH imaged by Event Horizon Telescope (EHT) ‡

Mass ~6 billion Msun ‡

Curved streaks ‡

polarization of the microwaves ‡

magnetic fields in the accretion disk

Energy from a Black Hole

Gravitational potential energy of matter falling into black hole turns into kinetic energy. ‡ Friction in an accretion disk turns kinetic energy into thermal energy (heat). ‡

Heat produces thermal radiation (photons). ‡

This process can convert 10 to 40% of E=mc2 into radiation

Gravitational potential turns to kinetic, then thermal energy producing radiation.

Black Holes in Galaxies

Many nearby galaxies — perhaps all of them— have supermassive black holes at their centers. ‡

Many of these black holes seem to be dormant AGN. ‡

All galaxies may have passed through a quasar-like active stage earlier in time

Galaxies and Black Holes

The mass of a galaxy's central black hole is closely related to the mass of its bulge. ‡

The development of the central black hole must be related to galaxy evolution

galaxies with larger bulges have larger black holes.

galaxies with small bulges have small black holes.

Radio galaxies

AGN ‡

Plasma jets emitting in radio

from electrons moving at/near speed of light

The lobes of radio galaxies can extend over hundreds of thousands of light-years

An AGN can shoot out blobs of plasma moving at nearly the speed of light. ‡

This suggests a profound influence on the hot gas surrounding a galaxy.

Here the lobes of radio galaxy NGC 1265 are swept back (upward in the image) because of the motion of the galaxy through intergalactic gas.

Gas Outside of Galaxies

Gas clouds between a quasar and Earth absorb some of the quasar's light. ‡

We can learn about protogalactic clouds by studying the absorption lines they produce in quasar spectra.

As the light from a distant source travels through multiple clouds, the spectra can get very complicated very fast. ‡

This figure shows multiple sources at different distances.

each line in the quasar spectrum that reaches Earth tells us about a unique cloud