Ngc 2158

[Brian W (Brian)]

Hi all, the other night I was observing M 35 when I noticed a little fuzzy thing that I 'thought' was a globular cluster. When I went in to log it I found out it was the open cluster NGC 2158.

This was puzzling to me so I did a little web searching and found out that in at least one of the earlier catalogs it was categorized as a -globular- cluster.

My question is a simple one; what is the 'scientific' way to discern between an open and a globular cluster?
Brian

[astroron (Ron)]

Most Globular Clusters where formed at or before the formation of the Galaxy so the Stars are all old stars.
the Stars in Globular have a low abundance of the heavier elements than Helium, and as such are the same age as the Galaxy(around 12 billion years give or take a few million)
Most Open Clusters area lot younger, such as the Pleiades which is only about 50 million years old.
Ron

[Brian W (Brian)]

That all agrees with what I found on the later web search. However web search gave the impression that the deciding factor between open and globular was gravitationally bound or not bound?
Brian

[astroron (Ron)]

If open cluster where formed at the formation of the galaxy their stars would no longer be in the position they where then.
I have heard that they have found young Globulars but at the moment the evidence is not conclusive.

[Quark (Trevor)]

Hi Brian,
There are several criteria that discern between open and globular clusters.

Age, Metalicity, density and orbital characteristics are probably the major ones.

Globular star clusters are thought to be the first structure to form when Galaxies first formed. This leads to some interesting defining characteristics.

Being the first structures to form means that their stars formed from primordial material and hence are metal poor, as pointed out by Ron.
Their stars are also the oldest stars in their host galaxy.

With regard to spiral galaxies, globular clusters formed before the law of conservation of angular momentum caused the collapsing proto galaxy to spin down to a spiral shape. This is very important and infers that the globular clusters formed before the spiral arms formed. This has left the globular clusters on great elliptical obits above and below the galactic centre. Following on from this, or more importantly due to this, they don't share in the rotation around the galactic centre that the material within the spiral arms does.

Open clusters formed later in the spiral arms and hence tend to have higher metalicity and have stars at a much greater variety of ages. The densest open cluster is still not as dense as a globular cluster.

That there are mistakes in early catalogs is more an indication of the technology and equipment of the time after all many galaxies were catalogs as spiral nebulae.

That said there have been cases of merged dwarf galaxies being identified as globulars, there is also talk of young globulars, but the current thinking of that is far from being one of consensus. Even these so called young globulars are still very old, I think older than open clusters.

Hope this is of interest.
When doing my degree I wrote an essay on this very subject and although I probably should have dug it up for reference, from memory I think these were the most pertinent points.

Regards
Trevor

[astroron (Ron)]

Thanks Trevor, I knew there would be someone who would give a more comprehensive explanation but mine is in the general ballpark
Regards.

[renormalised (Carl)]

You're pretty much spot on there, Trevor. Another difference is that OC's tend not to be gravitationally bound objects and split apart over time due to their passage through the spiral arms. GC's are gravitationally bound objects, but they can shed stars if those stars exceed the escape velocity of the cluster. GC's are also devoid of gas and dust for the most part whereas the OC's can have gas and dust present.

[Brian W (Brian)]

Yes it was of interest Trevor as yours was also Ron. Digital Universe does a good job of showing how the O.C. and G.C. do occupy different parts of our galaxy. I am hoping that it is also correct to think of G.C.as populated with first (or perhaps 'early') generation stars, hence the low metalicity while O.C. are later generation stars hence the greater metalicity?
Brian

[renormalised (Carl)]

That's correct, Brian. GC's stars are generally the earliest generations of stars, hence they have low to extremely low metals content. The OC's are born of later generations and have higher metals content.

[Quark (Trevor)]

Originally Posted by Brian W

Yes it was of interest Trevor as yours was also Ron. Digital Universe does a good job of showing how the O.C. and G.C. do occupy different parts of our galaxy. I am hoping that it is also correct to think of G.C.as populated with first (or perhaps 'early') generation stars, hence the low metalicity while O.C. are later generation stars hence the greater metalicity?
Brian

Greetings John,

Just a piece of trivia regarding the classification of various populations of stars that may be of interest.

At face value many things in science seem counter intuitive. One would think that the earliest stars would be Population I stars, however that is not how they have been classified.

The metal rich stars of open clusters that formed in the spiral arms from material of previous generations of stars are classified as Population I.

The metal poor stars that formed from primordial material at the very early stages of galaxy formation are classified as Population II stars.

And the very first stars which formed from only hydrogen & helium are classified as Population III stars. These stars, being the first stars, would have only spectral lines of hydrogen & helium

No Population III stars have yet been detected. At that early time, the Universe was much smaller and the density of star forming material would have been very high, consequently Pop III stars were likely to have been high mass stars and hence lived very short lives. That said I have heard professional astronomers say that they expect The James Web Space Telescope, which has been optimized to look at IR wavelengths, to detect Pop II stars.

Regards
Trevor

[jungle11 (Greg)]

Hi guys - this is an interesting thread.

One question from a newb

I understand what you mean about metalicity. I always thought the first stars in the galaxy that were light on metals were supposed to be short lived ( millions of years) - this makes it sound like they are the longest lived in globs. Is this wrong?

[astroron (Ron)]

Population Two stars such as those in Globulars are very light in metals so they are very long lived, as Trevor mentions population Three stars where massive stars,upto 150 solar masses or more but had only Hydrogen and Helium.
These stars have not yet been found, but could be found in the future with the James Webb Telescope
All today's stars have their metals due to past Supernovae

[renormalised (Carl)]

The length of time a star lives for is dependent on mass...the more massive a star, the hotter and brighter it is, the shorter its lifespan. Metalicity will effect how a star evolves but not to the same extent as the initial mass of a star. Metalicity effects the opacity of the gases, temperature and other characteristics. Usually, stars with low metalicities have a higher surface temperature for a given spectral type than "normal" stars (those with similar metalicity to the Sun). For example, a G2V class star in a glob', because of its low metalicity, will have a surface temp of around 6100-6200K, as compared to the Sun's 5800K. Funnily enough, the two stars will have roughly the same luminosity because the glob star's low metalicity effects its size, which is slightly smaller than the Sun (around .85-.9 solar radii).

In general, the lifespan of most stars is equivalent to 1/M^2.5...so knowing the Sun has a lifetime of about 10billion years, you can get an approximate age for most stars by multiplying the answer you get by the Sun's lifespan, thereby giving you the star's lifespan. For example, take Sirius, it's mass is 2 times the Sun's. So plug that into the equation and you get... 1/2^2.5 (x 10^10) = 1.8 billion years. For a typical 5 solar mass star (B0) you get 179 million years. For you typical K class dwarf (K0-3, .85M), you get 15 billion years.

Where metalicity does effect the initial mass of a star is during their formation. The less metals are in a gas cloud, the more transparent its gases are, the hotter and higher its mass will be for any given star that forms out of it. There's a property of a protostellar gas cloud called the Jeans Mass, which is very dependent on mass temperature and metalicity, amongst other factors. Very early star formation, which basically occured in almost pure Hydrogen and Helium, precluded the formation of small (smaller than 10 or so solar masses) stars as the clouds couldn't become dense enough and divide up into smaller units. These pure H/He protostellar clouds had high Jeans Masses, so the stars that formed out of them were large and very bright, as a consequence. Because of the absence of metals in the gases, many of these stars (called Population III stars) grew to extreme sizes...250-1000 solar masses. So you can imagine how bright some of these stars were. Probably upto 100-500 million times brighter than the Sun!!!...especially for those stars of early spectral type (O to A class). Some of the biggest and brightest probably burnt themselves out in less than a million years. A typical O class Pop III star, because of the absence of metals, would've had a surface temp of around 70-80,000K and was most likely 200-400 solar radii in size. Also, the absence of metals effected the end products of nuclear burning in their cores. Because the gases in these stars were nowhere near as opaque as in later stars, hence the density of the gases was substantially less in the core regions and nuclear burning could only get as far as Oxygen before the stars went hyper/supernova. Metals such as Sulphur, Silicon, Magnesium and Iron came from later stars, once the metalicity of the ISM reached a certain level to be able to support the growth of higher metal stars.

I hope that's helped answer some of your questions

[Quark (Trevor)]

Hi Greg,
The evolutionary path that any star will likely follow is determined by the mass that the star achieved when it formed. Whether the star had high or low metalicity is really a reflection more on when and where it formed.

High mass stars live fast, die young while low mass stars chill out, do nothing in a hurry and live long lives.

No doubt the density of the material available for star birth at the time the globs formed is a major reason for the size and density of these clusters.

The number of stellar fossils found in globs bears witness to the age of these clusters. Black holes, Pulsars or Neutron stars being the fossils from the high mass stars and White dwarfs being the upper end of the low mass star fossils. The truly low mass stars still happily on the main sequence as the Universe is not yet old enough for them to have moved off of it.

Regards
Trevor

[jungle11 (Greg)]

Whew!
So these pop III stars were the first stars, man that's silly.
Considering they are proberly the ones I was reading about and were supermassive which is why I thought the first stars were short lived.
Which is why the pop II stars are still fusing away cause they have a more reasonalbe mass, and the proposed blackholes in globs are relics of Pop III stars?
So the JWT will look for these pop III stars in the distant universe way back at the beginning, right?

Sorry, Im a bit slow but I get there in the end!

[astroron (Ron)]

So the JWT will look for these pop III stars in the distant universe way back at the beginning, right?
Yes

[renormalised (Carl)]

The Pop III stars I mentioned formed even before most of the globs. They were the very first stars to form out of the gases that went into making up the galaxies. They most likely formed in dense clusters of stars buried within small protogalaxies at the very earliest stages of the formation of large galaxies like our own, even before. The black holes and other relics in globs are pretty much from Pop II stars. No glob ever found is that devoid of metals that its stars could be classed as Pop III. Pop III stars have essentially 0% metals....maybe 1 lithium or beryllium atom per 100 trillion H/He atoms at the very most.