Astronomy 2

Overview of the Universe

Spring 2007

11. Lectures on External Galaxies.

Joe Miller

Are the “spiral nebulae” extragalactic? Or, do we live in the only galaxy in the universe? The age-old astronomy problem: distance.

April 1920: the Curtis-Shapely debate

Shapely (Harvard): Argued that spiral nebulae were inside our galaxy for several reasons

• Galaxy was huge (he didn’t know about dust).

• van Maanen’s observations showed that one spiral nebula, M 101, could be observed to rotate. It it were outside our galaxy, it would have to be turning faster than the speed of light.

• Spiral nebulae were never seen in the Milky Way: the “zone of avoidance.” Therefore, their distribution acknowledged the geometry of our galaxy, and they must be part of our galaxy.

• A nova, S Andromeda, was observed in the Andromeda Nebula. If this nebula were outside our galaxy, the nova would have to be incredibly bright, much brighter than known novae.

Curtis-Shapely debate (cont.)

Curtis (Lick): Argued that the spiral nebulae were other galaxies, well outside our own galaxy:• van Maanen must have made errors in his measurements of M101, and

it wasn’t rotating.• The zone of avoidance must be an artifact of something in our galaxy-

perhaps dust extinction- blocking our view of galaxies in the Milky Way.

• S Andromeda was not a normal nova. It was something else, something much brighter.

• Slipher’s observations of several spiral nebulae showed that they had large red Doppler shifts, indicating they were moving away from us at very high speeds, speeds too high to be a part of our galaxy.

Shapely had the strongest arguments, but Curtis was right!

Edwin Hubble completely settled the controversy.

He found a Cepheid variable star in M 31, the Andromeda galaxy. The period-luminosity relationship for Cepheids defines a close relationship between the period of pulsation and the absolute magnitude of the star. Sine the apparent magnitude can be measured directly, determining the period of a Cepheid variable gives a precise measurement of its distance. Using present-day data, it is about 700,000 pc from us. It can be seen with the naked eye.

The universe is teeming with galaxies

M 31: nearest example of a large galaxy.

Has all the same components that are found in our galaxy: globular star clusters, halo and disk stars, young star clusters, dust, gas, etc.

Literally billions of galaxies can be recorded on images with large modern telescopes. The vast majority are much further away than M 31, and once again measuring distances is a major problem.

The Realm of the Galaxies

The variety is enormous, and our understanding of what is going on is a

work in progress!

Hubble’s “tuning fork” diagram.

Distances to galaxies

Measuring distance to galaxies:

1) Out to 10 million pc (~30 million with HST): Cepheids, O and B stars, supergiants.

2) Tully-Fisher relationship: (100 million pc)There is a relationship between the line width of a galaxy in neutral hydrogen radiation, which measures rotation speed of a galaxy, which is a measure of its mass, and the luminosity of the galaxy. This works for spirals.

3) supernovae: billions of pc (appear to be quite good).

4) redshifts- a great discovery!

The expanding universe: Hubble’s great discovery

The farther away a galaxy is from us, the faster it is moving away from us. Since motion away from us produces a Doppler shift toward the red, it is equivalent to say that the redshifts of galaxies increase with distance.

The Hubble diagram:

This does not mean we are at the center of the universe. The entire universe is expanding, so the distance between all galaxies is increasing (except ones close enough to be bound together by gravity). If we lived in another distant galaxy, we would see all galaxies moving away from us.

The Hubble Constant

The Hubble constant H0 is the slope of the line in the Hubble diagram. It is the rate of expansion, and the subscript “0” means the present value of the constant. The universe could change its rate of expansion.

One current value for the Hubble constant is around

H0 = 73 km/sec/Mpc

with an uncertainty of plus or minus about 5 km/sec/Mpc. This means that outward velocities of galaxies increase 73 km/sec for every Mpc (million pc) of increasing distance.

Using redshifts to derive distances:

Since the Hubble constant H0 is the slope of the line in the Hubble diagram, we can write

Example:

If v = 6500 km/sec, then 6500/65 = 100 Mpc. An object with a redshift corresponding to a velocity of 6500 km/sec would be 100 million pc away.

c(Δλλ

) =v=H0d, where d is in Mpc. Therefore

d=vH0

Rotation of our galaxy allows mass determination

Masses of galaxies- a dark mystery. Dark matter.

It now appears that there is about 10 times as much dark matter as ordinary matter! What it is remains a mystery, but many think it is some kind of as yet undiscovered elementary particle.

Galaxies often come in groups, ranging from a few members to 1000’s of galaxies.

The Local Group- a small cluster

The building up of galaxies through merging

Additional evidence for dark matter:

Gravitational lensing

Once again, the amount of dark matter needed to make these lenses work is about 10 times the visible matter. The dark matter is not distributed exactly the same as the visible light.

Radio astronomy: the discovery of radio galaxies, active galaxies, and quasars.

Finally, a radio star is found!

Surprising result: this star is traveling away from us at roughly 16% of the speed of light!!!

Major discovery: 3C 273 is found to vary significantly in light output in days! It can’t be more than a few light days in size.

Extreme energy generation from a very compact region or new physics to explain

the redshifts.