Population stars - agreed terms help sensible discussion


The devil is in the detail
Universe - the definitive visual guide Gen Ed Martin Rees DK 2012

"Stellar populations
Stars are broadly classified into groups, called Populations, based on age and chemical content. Population I consists of the youngest stars, which tend to be richer in heavy elements. These elements are principally produced by stars, and Population I stars are created from materials shed by existing stars. In the Milky Way, the majority of Population I stars lie in the galactic disc, where there is an abundance of star-making material.
Population II stars are older, metal-poor stars, existing primarily in the halo, but also in the bulge. Most are found within globular clusters, where all star-making materials have been used up and no new star formation is taking place."

View: https://imgur.com/a/UhlYOBo
Image courtesy https://evolution.calpoly.edu/milky-way-galaxy
"The diagram identifies the primary features of the Milky Way galaxy not counting the massive nearly spherical dark matter halo that has three times the diameter and nearly 30 times the volume of the stellar halo shown in the diagram. The galaxy includes a distribution of stars and intragalactic medium. Population II stars are found in a nearly spherical stellar halo, a central bulge and a thin disk. Population I stars are concentrated in the younger thin disk, but are also found in the bulge. The galactic center is a supermassive black hole. Much smaller black holes are scattered throughout the galaxy. The spiral arms are regions of high density of interstellar gas and dust where giant stars continue to form and live very brief lives before recycling their metal-enriched gas back into the “empty space” of the interstellar medium."

View: https://imgur.com/a/YJjb93v
Globular cluster Messier 22, also known as M22, lies 10,000 light-years away in the constellation of Sagittarius. Image released April 6, 2015.
(Image credit: ESA/Hubble & NASA)
Globular clusters are densely packed collections of ancient stars. Roughly spherical in shape, they contain hundreds of thousands, and sometimes millions, of stars. Studying them helps astronomers estimate the age of the universe or figure out where the center of a galaxy lies.

Oxford Dictionary of Astronomy Ian Ridpath Oxford University Press 2011

"Stellar population
A classification of stars on the basis of certain physical characteristics, such as their location in the Galaxy, the types of orbit they have around it, and their content of heavy elements. Each of these properties is believed to depend on the age of the Galaxy when the star formed, so that Population I, Population II and Population III contain stars formed at progressively earlier periods. Note: heavy elements in this context mean any elements heavier than hydrogen and helium. In astronomy, the heavy elements are often termed metals. Heavy metal star describes a giant with unusual amounts of heavy elements in its spectrum, such as a barium star, or an S star,"

"Population I
Those stars that, like the Sun, lie in the disk of our Galaxy and follow roughly circular orbits around its centre. These have a high content of heavy elements and have probably been formed continuously during the lifetime of the disk. from gas enriched by the debris from supernovae in Population II stars."

"Population II
Those stars that are formed in the halo of our Galaxy and in its central bulge. The stars in the galactic halo, including globular clusters, describe highly elliptical orbits around the Galaxy's centre. Members of Population II have a significantly lower content of heavy elements than stars of Population I. It is believed that Population II stars formed in the first billion years or so of the Galaxy's life, before the formation of the galactic disk."

"Population III
A hypothetical generation of stars, no longer observable, assumed to have been formed before those of Population II. They are presumed to have existed because their supernovae would have been needed to supply the heavy elements observed in Population II stars. The neutron stars or black holes produced by such supernovae might be candidates for the dark matter in the galactic halo."

The State of the Universe Pedro G Ferreira Phoenix 2006

"The Galaxy is made up of different types of stars. Rotating around the disc, there are other stars just like the Sun.. There are stars that contain a lot of heavy elements - the Population I stars (that) contain heavy elements not only because they have reached the stage where they are burning helium to produce carbon, nitrogen and oxygen, but also because they have soaked up heavy elements that were ejected into the interstellar medium by the supernovae of earlier stars. Older (Population II) stars, which contain smaller amounts of heavier elements, can be found in a slight bulge in the centre of the Galaxy. The stars in the bulge are not rotating but are speeding about randomly and quite rapidly, which gives the bulge a spherical appearance. . . . . . . The older Population II stars are thought to have taken part in the initial stages in which the Milky Way was assembled, while the Population I stars are being regenerated in the disc at an almost constant rate. . . . . . . . . . "

"Astronomers have hypothesized that there is another type of star, Population III, that do not have any heavy elements. These stars would have been born very early on in the evolution of the Universe from mild irregularities in the distribution of gas and matter. Population III stars would have been very massive and would have collapsed very rapidly due to their own tremendous gravitational pull. They would have been born early, been very big, and died young, making them very difficult to see today. In fact, there has been no unambiguous observation of a Population III star, and it is quite possible that one has ever been seen, although stars with incredibly low abundances of heavy metals are constantly being discovered. So, although we have been able to build up a rich and consistent picture of how the elements of the periodic table were created, we are still missing key observational evidence. for the first step in the life cycle of stars. We still do not know how the very first stars were born and what they really look like."

View: https://imgur.com/a/UhlYOBo

An Introduction to the Sun and Stars Ed S F Green and M H Jones OU 2007

"Metallicities and stellar populations
The effect of nucleosynthesis is to increase the metallicity of the Inter Stellar Medium (ISM) as time passes. The observed metallicity of a star provides an indication of the level of enrichment in heavy elements that had occurred in the ISM prior to the formatioon of that star." . . . . . . . . .
"Young stars tend to have high metallicities (since they are composed of older ISM which has had the time to accrue heavier elements) whereas older stars tend to have low metallicities. Astronomers categorize stars into classes called 'Populations' which correspond approximately with age. Roughly speaking, Population IFavourite shuttle stars are those that are young or of moderate age, and these stars are found to have metallicities of up to 2-3%. Population II stars appear to be intermediate age and old stars, and typically have metallicities of less than 0.8%."
"The picture that emerges from the study of stellar populations is that there are some regions of the Galaxy, such as the halo, where star formation stopped a long time ago. The metallicity of stars (Population II) that we see in those regions corresponds to the level of enrichment of the ISM at that time. There are also other regions, such as the disc of the Galaxy, where star formation is recent or even on-going and the metallicity of stars in these regions (Population I) has been enhanced by the greater degree of cosmic recycling that has taken place."
"So even the oldest stars that have been observed in our Galaxy were formed from material that had already been enriched by a previous generation of stars. No stars have ever been observed that correspond to a primordial composition."
Population III.

All About Space Issue 95 October 2019 Tracking down the first stars by Lee Cavendish

"We expected (the first stars to be) very massive - up to 100 to 1000 times as massive as our Sun - which would imply they should be incredibly large, incredibly blue . . . . . . . . . and have lifetimes of just a few million years, and definitely much shorter than stars which are Sun-like and live for ten billion years or so."
"In other words, the (first stars) were stellar behemoths, soaking up the untainted supply of hydrogen and helium as they burnt at unbearable temperatures. These behemoths have had a huge influence on the Universe as we know it now, with two main contributions: one being the introduction into the 'reionisation' era in a cosmological sense, and the other being the creation of the very first metals - 'metals' in an astronomical sense are any elements other than hydrogen or helium." . . . . . . . . .
"At the cores of these early stars, hydrogen and helium were being converted into carbon, oxygen, sodium, neon, magnesium and so on by the process of nuclear fusion, until the stars reached an extremely violent end (one scenario) being core-collapse supernovae, pair0instability supernovae and black holes. . . . . . . There are many different types of supernova . . . the two just mentioned both occur in the most massive stars. When a star reaches the end of its life, the star enters an internal struggle to balance out the gravitational collapse of the star and its radiation output. . . . . . . . . . When these stars undergo supernovae, they form and release heavier elements into their surrounding areas, mixing with the so-called interstellar medium . . . . . . after time, this newly metal-enriched gas will once again undergo gravitational collapse and form the next generation of now metal-enriched stars, or Population II stars. . . . . . . Population III stars could not form planets because there were no heavy elements."

WORK IN PROGRESS Last edit Monday 12th July 20.20 BST.
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