How can globs be so old?

As I understand it, globulars are composed of red giant stars.
They are also the oldest objects in the local galactic area.

To me this doesn't make sense. Red giant stars or giant stars in
general don't last long. They certainly don't live as long as a
typical main-sequence star and our galaxy's globulars are purported to
be ~10 billion years old.

My question is why are they still around? (I'm certainly not
complaining...they are some of the finest deep sky objects visible to
the human eye+telescope. ) Even if some of the giants went
supernova, one would think that the gravitational forces in such a
small confined space would preclude a gas cloud from lasting long
enough to form new stars. So how is it that they exist to this day
with populations, in some cases, in the millions?

Or is this one of the mysteries that we have yet to figure out?

Confused,

Drew

As I understand it, globulars are composed of red giant stars.

Hi Drew:

Where did you hear that? That's not the case. The hallmark of a globular is
that it's composed of old stars, low metallicity Population II stars. There are
no doubt evolved stars, including red giants, in any glob (M15 sports a
planetary nebula, Pease 1, that can be seen in large amateur scopes), but the
main thing is that the Turnoff Point, has moved way on down the main sequence,
way the heck to the right on the HR Diagram, and that the stars still burning
happily are lower mass REDDER ones.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

High MASS stars do burn faster and brighter and burn out more quickly.
Red giants though, are large because they've reached a late stage in
their life span where their atmospheres expand to huge proportions. As
I understand it, the outer part of a red giant's atmosphere would be
considered a vacuum here on earth.
Marty

As I understand it, globulars are composed of red giant stars.
They are also the oldest objects in the local galactic area.

M4 is the closest known globular cluster. At about 6,500 light-years distant,
there are no main sequence stars bright enough to be seen in the modest
apertures most amateurs employ. So, which are the brightest stars? Red giants
and RR Lyrae type variables.

If you look at a color magnitude diagram for a typical globular, one of the
first things you notice is the prominence of the red giant branch. These are
the brightest stars in most globular clusters. Despite the fact that main
sequence stars--mostly red dwarfs--make up the bulk of the population and mass
in globulars, it's the red giants that produce most of the light.

After the red giants, the horizontal branch stars--mostly RR Lyrae
variables--compose the second brightest segment of the cluster population. They
produce about 15% of the light thrown out by most globulars and are A through F
spectral types so, not as red as those on the giant branch. By the way, the
typical brightness of the horizontal branch stars is a pretty good indication
of the degree to which the cluster will be resolved.

The red dwarfs populating the main sequence population within most globulars
are very low mass stars. Cosmic stars are analagous to musical rock stars.
Some--O and B type supergiants--live fast and die young. In other words, these
hot & massive stars consume their nuclear fuel so quickly that they die after
less than 100 million years, the most massive go supernova after just a few
million years. But low mass red dwarfs can stay on the main sequence, calmly
converting hydrogen to helium through fusion, for many billions of years.
Astronomers estimate the oldest globular clusters formed about 12 billion years
ago.

Most stars in globular clusters are both low mass and low metalicity. In other
words, they contain a low percentage of elements heavier than hydrogen and
helium relative to stars like the Sun. Their low metalicity indicates is best
explained if they were among the first stars to be created in the universe. So,
globular clusters are often cited as benchmarks for the era of initial star
formation in the universe.

Regards,




Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

In article , Drew wrote:
As I understand it, globulars are composed of red giant stars.
They are also the oldest objects in the local galactic area.

The *brightest* stars in globulars, the ones you're liable to see with
a telescope, are ones that happen to be passing through their
relatively brief giant phase. But you're right, it wouldn't make sense
if most of the cluster's stars (rather than the most visible ones) were
red giants. Most of present-day globulars' stellar populations are
pretty dim, old, low-mass stars that haven't reached the giant phase yet.

In article ,
wrote:

In article , Drew wrote:
As I understand it, globulars are composed of red giant stars.
They are also the oldest objects in the local galactic area.

The *brightest* stars in globulars, the ones you're liable to see with
a telescope, are ones that happen to be passing through their
relatively brief giant phase. But you're right, it wouldn't make sense
if most of the cluster's stars (rather than the most visible ones) were
red giants. Most of present-day globulars' stellar populations are
pretty dim, old, low-mass stars that haven't reached the giant phase yet.

Sorry I don't have references handy but IIRC at formation the most
common star mass in GCs is about 0.6 or 0.7 solar masses. There is
active research on how this number changes with time as the more
massive stars loose mass and outlying stars are stripped through
interactions with host galaxies.

A google search for "globular cluster" & IMF ( initial mass function )
returned 2600+ hits. A quick glance at the 1st 2 or 3 pages did not
return any papers about the average star mass, but I've seen this
number ( 0.6 - 0.7 ) recently.

Dark skies,

tom

--
We have discovered a therapy ( NOT a cure )
for the common cold. Play tuba for an hour.

To me this doesn't make sense. Red giant stars or giant stars in
general don't last long. They certainly don't live as long as a
typical main-sequence star and our galaxy's globulars are purported to
be ~10 billion years old.

It is not the current size that determines a star's longevity, but its mass.
The high mass stars are long gone. If you go back to the spectral types, and
could watch a time-elapse movie of a globular cluster (or any other), you
would see the O type stars go first, followed by the B-type and down the
line ("Oh Be A Fine Girl/Guy....) So after a short while, the A-type stars
are the most massive still surviving. Then it gets down F-type stars being
the most massive still surviving. To date the cluster, simply look at the
least massive type that is no longer present. The cluster is then known to
be longer than the lifespan of the least massive type of star (spectral
class --- main sequence) that is no longer present.

However, as each type dies off, they have different death rattles. The most
massive go supernova. But those that are much less massive become red giants
before dying. So you can have a lot of red giants that started out as small,
low-mass main sequence stars but are now near the end. Those that have not
entered their last round are the "... Girl/Guy Kiss Me..." part of the main
sequence and comprise the bulk of the stars in a globular.


Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************


My question is why are they still around? (I'm certainly not
complaining...they are some of the finest deep sky objects visible to
the human eye+telescope. ) Even if some of the giants went
supernova, one would think that the gravitational forces in such a
small confined space would preclude a gas cloud from lasting long
enough to form new stars. So how is it that they exist to this day
with populations, in some cases, in the millions?

Or is this one of the mysteries that we have yet to figure out?

Confused,

Drew

Thanks all for your responses. I forgot that red giants used to be
typical main sequence stars...so since that is the case I guess
globular clusters will continue to dim and finally fade away as the
last remaining M dwarf members slowly fade out?

Makes me wonder how much more impressive they must have been billions
of years ago (not that they aren't impressive now).

-Drew

On Wed, 18 Aug 2004 07:20:09 GMT, Drew
wrote:

To me this doesn't make sense. Red giant stars or giant stars in
general don't last long. They certainly don't live as long as a
typical main-sequence star and our galaxy's globulars are purported to
be ~10 billion years old.

Giant star doesn't mean massive star, it just means that the star has
evolved off the main sequence into the red giant phase. It is giant
in terms of volume, but the density is low. A red giant star is older
than a star of the same mass that is still on the main sequence.
Eventually our Sun will evolve off the main sequence and will become
huge in volume, extending past the orbit of the Earth, but it won't be
any heavier than it is now.

Rod Mollise wrote:


As I understand it, globulars are composed of red giant stars.

Hi Drew:

metallicity

huh? Ive heard of Detroit and Kenarsi but there aint no Metalli City ,
Rodicitie.


















Population II stars. There are
no doubt evolved stars, including red giants, in any glob (M15 sports a
planetary nebula, Pease 1, that can be seen in large amateur scopes), but the
main thing is that the Turnoff Point, has moved way on down the main sequence,
way the heck to the right on the HR Diagram, and that the stars still burning
happily are lower mass REDDER ones.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html