Expert: Tom Whiting - 12/13/2008

Question
Hi Mr. Whiting, can you help me figure out which of the following belongs in the two Catogories of DISK STARS YELLOW and HALO STARS GREEN:

1.Global clusters
2.high-mass star
3.The sun
4.oldest stars
5.youngest stars
6.Stars that orbit ib nearly the same plane
7.Stars that orbits can be inckined at any angle
8.stars with the smallest abundance of heavy metals

Also can you help me sort the following in the two catogories of: SPIRAL GALAXIES and ELLIPTICAL GALAXIES

1.are rare in the central regions of galxy clusters
2.Have significant on-going star formation
3.Contains abundant clouds of cool gas and dust
4.are more reddish in color
5.Have a flattened disk of stars
6.Contains primarily old, low-mass stars
7.Contains many bright, hot stars

Answer
Hi Justin,
Yes, in reverse order, the older elliptical galaxies have little
dust, are older, (read that-redder because far more stars have evolved to the Red Giant Stage) and are mainly composed of the older Population II stars deficient in heavy metals. (To an astronomer, a "heavy metal" is any element above the Hydrogen-Helium 1 and 2...so a heavy metal is defined as lithium #3 to Uranium # 92).
And this Population II also applies to the Spiral galaxies' outer
haloes, as that is what formed up first, as the spiral evolved to a flat disk, but the flat disk has lots of dust and gas...read that lots of new star formation, and lots of Population I stars like the sun...lots of 'heavy metals'. (But only up to 3% today....97% of our Universe is still hydrogen and helium - so 13.7 billion years since
the Big Bang is starting to sound like a relatively very short time span as compared to the total expected lifespan of the entire Universe of hundreds of trillions of years).  

So your question 2, Spiral galaxies are numbers 1, 2, 3, 5, and 7.
Obviously Ellipticals are 4 and 6.

On your question #1, the old Population II stars formed up first,
in the halo and core, in the Globular star clusters, have the oldest
stars, stars that can be inclined at any angle, and the stars with
the smallest abundance of heavy metals.

The spiral disk stars are the sun, orbit in nearly the same plane
of the spiral disk, and are the youngest Population I stars.
The high mass stars are difficult...most are in the newer Population
I, like the star Eta Carinae, Rho Cassiopeiae, Tau Canis Majoris,
but this does not preclude other high mass stars in the halo too.
Although they would have long since gone supernovae, so I guess they
want spiral disk stars are the higher mass stars, by today's standards.  
I hope that hits everything.
Clear Skies,
Tom Whiting
Erie, PA