• Our goals for learning

• How did Hubble prove galaxies lie beyond our galaxy?

• How do we observe the life histories of galaxies?

• How did galaxies form?

• Why do galaxies differ?

How did Hubble prove that galaxies lie beyond our galaxy?

The Puzzle of “Spiral Nebulae”

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

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles

Cepheid Variables: A Pulsating Star

The star's atmosphere ionizes, temporarily trapping heat energyThe star temporarily gets larger, increaseing its luminosity

Pulsating Variable Stars

• Pulse period is directly linked to luminosity

• Stars that get very much brighter, take longer to do so.

Cepheid Variables• This means pulse rate can be used to calculate

luminosity -without distance.• Comparing the calculated luminosity, with the

measured apparent brightness allows us to calculate distance to the star

Cepheid Variables on Distance Ladder

Cepheid variables act as standard candles that can measure distances to globular clusters and

nearby galaxies.

Thought Question

Which of these could not be used as a standard candle for finding distance?

A. a G2 main sequence star

B. a massive star supernova

C. a cepheid variable with a measured period

D. a white dwarf supernova

Galaxies and Cosmology• 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 and evolution of the universe

How do we observe the life histories of galaxies?

Deep observations show us very distant galaxies as they were much earlier in time

(Old light from young galaxies)

How did galaxies form?

We still can’t directly observe the earliest galaxies

Our best models for galaxy formation assume:

• Matter originally filled all of space almost uniformly

• Gravity of denser regions pulled in surrounding matter

Denser regions contracted, forming protogalactic clouds

H and He gases in these clouds formed the first stars

These oldest stars form the halo of a galaxy

Supernova explosions from first stars kept much of the gas from forming stars

Leftover gas settled into spinning disk

Conservation of angular momentum

Stars continuously form in disk as galaxy grows older

What have we learned?

How did Hubble prove that galaxies lie far beyond the Milky Way?He measured the distance to the Andromeda galaxy using Cepheid variable stars as standard candles

How do we observe the life histories of galaxies?Deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages

What have we learned?

• How did galaxies form?– Our best models for galaxy formation assume

that gravity made galaxies out of regions of the early universe that were slightly denser than their surroundings

– Galaxies initially form from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk (overly simplified model!)

Why do galaxies differ?

But why do some galaxies end up looking so different?

M87NGC 4414

Spin: Initial angular momentum of protogalactic cloud could determine size of resulting disk

Conditions in Protogalactic Cloud?

Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk

Conditions in Protogalactic Cloud?

We must also consider the effects of collisions

Galaxy-galaxy interactions can create huge distortions.

Galaxies can merge and be cannibalized.

The collisions we observe in nearby galaxies, trigger bursts of star formation

Collisions were much more likely early in time, because galaxies were closer together

Many of the galaxies we see at great distances (and early times) indeed look violently disturbed

The collisions we observe nearby trigger bursts of star formation

Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical

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

• Our Milky Way probably formed by the merger of many smaller protogalaxies in a dark matter “cloud.”

• Several of these are still orbiting the Milky Way as satellite galaxies.

What have we learned?

• Why do galaxies differ?– Some of the differences between galaxies may

arise from the conditions in their protogalactic clouds

– Collisions can play a major role because they can transform two spiral galaxies into an elliptical galaxy

Quasars and other Active Galactic Nuclei

• Our goals for learning• What are Active Galaxies and Quasars?

• What is the power source for quasars and other active galactic nuclei?

• Do supermassive black holes really exist?

What are Active Galaxies and Quasars?

Characteristics of Active Galaxies

• Luminosity can be enormous (>1012 LSun)

• Luminosity can rapidly vary (comes from a space smaller than solar system)

• Emit energy over a wide range of wavelengths (contain matter with wide temperature range)

• Some drive jets of plasma at near light speed

If the center of a galaxy is unusually bright we call it an active galactic nucleus

Quasars are the most luminous examples

Active Nucleus in M87

Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed.

The jets extend over hundreds of millions of light years

What is the power source for quasars and other active galactic nuclei?

An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light

Plasma is accelerated around a small but massive object, then sling-shotted out.

Speed of ejection suggests that a black hole is present

Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars

Jets of material shoot out from the poles of the system. A dense ring (torus) of dust blocks the center. What we see (quasar or radio galaxy) depends on the

viewing angle.

The orbital speeds of gas near the black hole yield its mass.

Supermassive black holes probably exist at the centers of all galaxies.

Normal galactic nuclei do not contain accretion disks.

Material in the accretion disk is an AGN’s source of fuel. Without it, the black hole can only be found by gravitational effects.

Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion MSun

Radio emission from center Swirling gas near center

What About Our Galaxy?

X-ray flares from our galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in.

Orbits of stars at center of Milky Way stars indicate a black hole with mass of 4 million MSun

• Many nearby galaxies – perhaps all of them – have supermassive black holes at their centers

• These black holes seem to be dormant active galactic nuclei

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

Black Holes in Galaxies

Galaxies and Black Holes

• Mass of a galaxy’s central black hole is closely related to mass of its bulge

Galaxies and Black Holes

• Development of central black hole must be somehow related to galaxy evolution

What have we learned?

• What are active galaxies and quasars?– Active galactic nuclei are very bright objects seen in the

centers of some galaxies, quasars are the most luminous

• What is the power source for quasars and other active galactic nuclei?– The only model that adequately explains the

observations holds that supermassive black holes are the power source

What have we learned?

• Do supermassive black holes really exist?– We can see particle jets, accretion disks and the rapid

orbit of stars all around the center of a galaxy.– All of these indicate a tiny super-massive object is there.

The only possible object that fits the evidence is a super-massive back hole.

– Observations of stars and gas clouds orbiting at the centers of galaxies indicate that many galaxies, and perhaps all of them, have supermassive black holes