Monday March 29th 1999
Announcements:

Lecture notes:

Class 31
Constellations

We have covered most of the major constellations of the northern sky now - just 4 or 5 more to finish off.

Will teach 4 new constellations today.  First 3 are in the same part of the sky Virgo Corvus and Leo.

Virgo - to find Arc to Arcturus and then keep going along this
arc until you come to another bright star.  This will be the star Spica (1000 L_(.) 190 ly) in Virgo. Virgin - looking at Bootes the Herdsman. Spica is her diamond
Virgo cluster is in this constellation.  Nearest large cluster of galaxies - 18 Mpc.

Then move over a little bit to see four stars in a quadrangle shape.  This is Corvus = Crow.  Supposedly looking over at
Virgin's diamond wanting to snatch it.

Then go above Virgo and Corvus to see Leo.  The Lion.  Planets move through it because it is one of the constellations on the
ecliptic.  There are 12 major constellations along the ecliptic.
These form the zodiac.

Finally in another part of the sky we have Perseus.
Above Andromeda and to the left of Cassopeia. Andromeda - daughter of Cepheus/Cassopeia.  Andromeda got chained
to a rock Perseus saved and married her they flew off on Pegasus.  Contains Algol - binary star (2.5 day period)  Means
demon star in Arabic.  Dims for about 5 hours every 2.5 days.

Back to cosmology.
Olbers' Paradox - dark sky --> finite age Universe and Universe birth event.
Hubble expansion of Universe -> v=H₀D.  Light is stretched to longer wavelengths as it travels through space.  Space itself is
expanding!

These facts raise some questions that we will address.

Does the Universe have an "edge"  No.  Since the Universe is expanding - one might think that it must be expanding into
something.  So it must have an edge but remember.... the Big Bang was not just an explosion in some space that was already
there - the Big Bang made the Universe and made space itself.

We can make a rough analogy to explain why the Universe does not have an edge.

Remember the expanding balloon from the last class.   We can think of this as the 2D analogy for our Universe.  An ant living
on this balloon would see the Hubble expansion but note that the surface of the balloon has no edge (also no center).

Scientists think our Universe is similar to this but in higher dimensions - no edge and no center.  The analogy has
shortcomings.  We see balloon expanding into 3rd dimension of space - by the analogy we might think the 3D Universe is
expanding into some 4th dimension.  This is not the case as far as we know.  Need to know Einstein's general relativity to
understand properly.

How old is the Universe?
There is one way to roughly estimate this using the Hubble Law.
v=H₀D
H₀=70 km/s / Mpc
units cancel and H₀=2.3 x10 ^-18 1/s

In the Big Bang all galaxies -- and all of space itself was compressed into a point.  Now using the Hubble Law we can
roughly work out when 2 galaxies that are currently distance D apart were touching.

Time = D/v = 1/H₀
so the galaxies were touching at time 1/H₀.  This is the age of
the Universe.  About 4.3 x10¹⁷ s or 14 billion years.
Has a large error 10-20 billion years is the possible range of age.

Notice this is true for any 2 arbitrary galaxies.  Doesn't depend on initial value of D.

Video "Creation of the Universe" was shown.
Nice view of Milky Way.  Home to about 100 x10⁶ stars.
Local Group - Virgo Super Cluster - 100 x10⁶ ly in size.
Scattering of Virgo Super Cluster by expansion of Universe.

Expansion of Universe predicted by general relativity.
Einstein rejected expansion.
1929 Hubble finds expansion
1931 Einstein/Hubble meet.

Not expanding into something.  Space itself is expanding. Expansion creates the space.  Balloon analogy.  No center of the
expansion - can't find it.

Slides shown of:
Distance indicators
Our place in Universe
Galaxy redshifts
Expanding Universe raisin bread
Age of Universe
Big Bang