ASTRO: NGC 6791

Third attempt to post this.
Rick
_________________

NGC 6791 is an often overlooked star cluster in Lyra. Everyone goes
there for M57 the ring nebula and sometimes for its globular, M56, but
few ever stop by to see this cluster. Too bad as it is quite
interesting and a big problem for astronomers. It is a rather large but
distant cluster at 13,300 light years give or take a few hundred. It
shows no real hint of resolution on my 10" scope. It is just a grainy ball.

Normally star clusters are thought to form from a cold molecular cloud.
As the first stars form the really big ones shine in ultraviolet light
and this lights up the Hydrogen by causing it to glow its characteristic
pink color but they also start to disrupt star formation by heating the
dust and gas to where it is too hot to condense to form stars then
blowing it away before more can form. This tends to limit the size of
most clusters to a few hundred stars. But this cluster contains several
thousand stars. It could be that it just didn't form the massive stars
needed to disrupt star formation and thus more stars formed. We see
several other large clusters. This one though is very old. Most
clusters are torn apart by the tidal forces of our galaxy and only last
a few hundred million years if that. Our sun is thought to have formed
in such a cluster that has since so broken up.

This cluster though has a star population that is 8 billion years old
and shows no sign of being torn apart. But this isn't a mystery. Its
location and mass are such that the tidal forces haven't been able to
overcome the gravitational force of the thousands of stars to tear it
apart. Several other such clusters are known.

What is really odd though is that instead of all the stars being about
the same 8 billion years old it also contains a smaller population of 6
billion year old stars and another of 4 billion years. Adding to the
confusion the 6 billion year old stars are white dwarfs, that is, dead
stars that are just cooling embers today. This means this population
was made up of stars all of which were more massive than our sun. Rather
than a mix of masses that you'd expect. I know of no other cluster
that can claim this and how it came to be is a big mystery.

But this isn't the only mystery. For that we must go back to the
original stars in our universe. After the big bang, the universe
consisted mostly of hydrogen and helium. A very few other elements,
mostly lithium, existed in such small amounts as to be insignificant. As
the first stars aged they, like today's stars, first turned hydrogen
into helium for energy then helium into carbon. Really massive stars
then turned the carbon into heavier elements all the way up to iron.
When these massive stars died in super nova explosions they spread these
elements through the universe where they were picked up by newly forming
stars. So this next generation of stars had more of these heavier
elements than did the first. Astronomers call these heavier elements
"metals" even though many (like carbon and oxygen) certainly aren't what
a chemist would call a metal. Each succeeding generation had more of
these metals than the previous. So by the metal content of a star you
should be able to get a rough idea of when it formed. Since some areas
of our galaxy had more metals than others this is only a rough guide and
there are far better ways to determine the age of a star. Still the two
should have some agreement.

NGC 6791 is a huge exception to this. Its 8 billion year old stars
contain far more of these metals than our much younger sun. No one can
explain this as yet. So how this cluster got so "dirty" is a huge
puzzle. Stars forming three different times and having far too many
metals makes this a highly studied cluster but so far the answers have
been slow in coming.

For more on this and Hubble's view of the very core of the cluster (it
can't see the whole thing with its narrow vision) see:
http://hubblesite.org/newscenter/arc.../2008/25/full/

A few distant galaxies show as fuzzy stars or fuzzy patches in my image.

14" LX200R @ f/10, L=5x10' RGB=1x10', STL-11000XM, Paramount ME
Reproduced at 1.5" per pixel rather than my normal 1" as resolution
wasn't needed for this one.

Rick
--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Very good image and accompanying text. An open cluster with several periods
of star formation sounds strange indeed.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
ster.com...
Third attempt to post this.
Rick
_________________

NGC 6791 is an often overlooked star cluster in Lyra. Everyone goes
there for M57 the ring nebula and sometimes for its globular, M56, but
few ever stop by to see this cluster. Too bad as it is quite
interesting and a big problem for astronomers. It is a rather large but
distant cluster at 13,300 light years give or take a few hundred. It
shows no real hint of resolution on my 10" scope. It is just a grainy
ball.

Normally star clusters are thought to form from a cold molecular cloud.
As the first stars form the really big ones shine in ultraviolet light
and this lights up the Hydrogen by causing it to glow its characteristic
pink color but they also start to disrupt star formation by heating the
dust and gas to where it is too hot to condense to form stars then
blowing it away before more can form. This tends to limit the size of
most clusters to a few hundred stars. But this cluster contains several
thousand stars. It could be that it just didn't form the massive stars
needed to disrupt star formation and thus more stars formed. We see
several other large clusters. This one though is very old. Most
clusters are torn apart by the tidal forces of our galaxy and only last
a few hundred million years if that. Our sun is thought to have formed
in such a cluster that has since so broken up.

This cluster though has a star population that is 8 billion years old
and shows no sign of being torn apart. But this isn't a mystery. Its
location and mass are such that the tidal forces haven't been able to
overcome the gravitational force of the thousands of stars to tear it
apart. Several other such clusters are known.

What is really odd though is that instead of all the stars being about
the same 8 billion years old it also contains a smaller population of 6
billion year old stars and another of 4 billion years. Adding to the
confusion the 6 billion year old stars are white dwarfs, that is, dead
stars that are just cooling embers today. This means this population
was made up of stars all of which were more massive than our sun. Rather
than a mix of masses that you'd expect. I know of no other cluster
that can claim this and how it came to be is a big mystery.

But this isn't the only mystery. For that we must go back to the
original stars in our universe. After the big bang, the universe
consisted mostly of hydrogen and helium. A very few other elements,
mostly lithium, existed in such small amounts as to be insignificant. As
the first stars aged they, like today's stars, first turned hydrogen
into helium for energy then helium into carbon. Really massive stars
then turned the carbon into heavier elements all the way up to iron.
When these massive stars died in super nova explosions they spread these
elements through the universe where they were picked up by newly forming
stars. So this next generation of stars had more of these heavier
elements than did the first. Astronomers call these heavier elements
"metals" even though many (like carbon and oxygen) certainly aren't what
a chemist would call a metal. Each succeeding generation had more of
these metals than the previous. So by the metal content of a star you
should be able to get a rough idea of when it formed. Since some areas
of our galaxy had more metals than others this is only a rough guide and
there are far better ways to determine the age of a star. Still the two
should have some agreement.

NGC 6791 is a huge exception to this. Its 8 billion year old stars
contain far more of these metals than our much younger sun. No one can
explain this as yet. So how this cluster got so "dirty" is a huge
puzzle. Stars forming three different times and having far too many
metals makes this a highly studied cluster but so far the answers have
been slow in coming.

For more on this and Hubble's view of the very core of the cluster (it
can't see the whole thing with its narrow vision) see:
http://hubblesite.org/newscenter/arc.../2008/25/full/

A few distant galaxies show as fuzzy stars or fuzzy patches in my image.

14" LX200R @ f/10, L=5x10' RGB=1x10', STL-11000XM, Paramount ME
Reproduced at 1.5" per pixel rather than my normal 1" as resolution
wasn't needed for this one.

Rick
--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".