Friday February 27th 1999
Announcements:

Lecture notes:

Class 21

The center of our Galaxy - if we aren't at the center of our galaxy then what is?

Answer is somewhat surprising and we're only getting the final answer over the last 10 years.
Hard to study the center of our galaxy.  Lots of gas and dustblocking the light.

trillion = 1x10¹² optical photons sent out only 1 gets through

Need to study Galactic center where absorption by dust is less.  Redder light or bluer light?  Redder light is better because it is absorbed less.  Infrared is even better.  Also radio and X-rays are good.

Galactic center located in constellation of Sagittarius.  Radio astronomers found a powerful radio source in direction of
galactic center.  Named it Sag A*.  Smaller than a few AU but emits > 5 L_(.) in radio.

People have now studied the motions of the stars and gas near the center - see them moving extremely fast.  Can work out the mass of the object the stars/gas are orbiting.

Like for planets and sun or binary stars.  Use Newton's law to get mass.

See some stars moving at 1500 km/s.  Earth moves around sun at only 29 km/s.  Strong evidence for a massive dark object.

Many people think it is a 2.6x10⁶ M_(.) black hole.

Slides were shown of:
galactic center trail of echo satellite
galactic nucleus at 60 microns
Sag A

Galaxies outside Milky Way.
As described in earlier class people had seen galaxies outside our own for a long time but where not sure what they were -
Messier.

Even great astronomers like Shapley thought the spiral nebulae were just small systems inside the Milky Way  - thought the Milky Way was the whole universe.

Astronomer at Mount Wilson
Edwin Hubble found a Cepheid variable in Andromeda Galaxy and used it to find the distance.  Found it was at a huge distance of about 280 kpc well outside the Milky Way!

This meant the universe was a huge place - our galaxy is just one of many - we are not in the center of things.

1540 - Copernican revolution - Earth not at center
1917 - Shapley revolution - Sun not at center
1923 Hubble revolution - Galaxy not at center

Hubble then went on to gather basic data on galaxies.  Shape size luminosity mass amount of gas and dust.

He then developed a classification system for galaxies = Hubble tuning fork diagram
3 basic types of galaxies
spiral
elliptical
irregular

will work from left to right along tuning fork diagram.

elliptical galaxies
- appear round or elliptical in shape
- range in size
dwarf < 1kpc < 1E6 stars
giant > 100 kpc >1E12 stars

classified according to shape E0-E7
E0=circular E7=most elliptical
projection effects can confuse - ellipsoid viewed edge on looks circular.

mainly old red stars - population II
little gas/dust - little star formation

spiral galaxies - have already talked about Milky Way as an example
contain gas dust and hot bright Pop I stars also bulges and halos
Sa - tighter arms bigger bulge
Sb
Sc - looser arms smaller bulge more young stars and gas

Milky Way is Sb or SBc
Also barred spirals
Galaxy bar = elongated nuclear bulge - spiral arms attached to end of it
SBa
SBb
SBc

irregular galaxies
chaotic shape - large clouds of gas/dust mixed with both old and young stars.
examples LMC and SMC - seen during expedition of Magellan in early 1500s.  Can see with naked eye in Southern Hemisphere.

galaxy census is hard
best estimates - ellipticals more common than spirals and
irregulars are only about 25%

Slides were shown illustrating
Hubble diagram
Sa illustration
Sb illustration
Sc illustration
Spirals - 6 of them
NGC 7217 Sb in Pegasus
NGC 7331 Sb in Pegasus
NGC 5364 Sc in Virgo
NGC 6946 Sc in Cepheus
Whirlool Galaxy Sc I  M51
SBa illustration
Barred spirals - 6 of them
NGC 1398 SBr
M32 E2 elliptical
M84 elliptical
CD elliptical
LMC - irregular galaxy
SMC - irregular galaxy
NGC 4449 irregular