Stellar Population

In 1944, Walter Baade categorized groups of stars within the Milky Way into **stellar populations**. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926.

Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters. Two main divisions were defined as **population I** and **population II**, with another newer, hypothetical division called **population III** added in 1978

Among the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important and were possibly related to star formation, observed kinematics, stellar age, and even galaxy evolution in both spiral and elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity

By definition, each population group shows the trend where decreasing metal content indicates increasing age of stars, hence, the first stars in the universe (very low metal content) were deemed population III, old stars (low metallicity) as population II, and recent stars (high metallicity) as population I

The Sun is considered population I, a recent star with a relatively high 1.82% metallicity, Note that astrophysics nomenclature considers any element heavier than helium to be a "metal", including chemical non-metals such as oxygen

## Stellar developement

Observation of stellar spectra has revealed that stars older than the Sun have fewer heavy elements compared with the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar nucleosynthesis

### Formation of the first stars

all matter that created in the Big Bang was mostly hydrogen (75%) and helium (25%), with only a very small fraction consisting of other light elements such as lithium and beryllium

the first stars is very massive population III stars, which their mass was about 1,000 times massive as the sun, hence this, they dies very quickly, and the pair-instability supernova remnant creates the second generation star

Stars too massive to produce pair-instability supernovae would have likely collapsed into black holes through a process known as photodisintegration. Here some matter may have escaped during this process in the form of relativistic jets, and this could have distributed the first metals into the universe

## Chemical classification

### Population I stars

*See also: Population I Star*

Population I star, or metal-rich stars, are young stars with the highest metallicity out of all three populations and are more commonly found in the spiral arms of the Milky Way galaxy. The Sun is an example of a metal-rich star and is considered as an intermediate population I star, while the sun-like Mu Arae is much richer in metals

Also, almost all of well-known stars, including UY Scuti, Stephenson 2-18, V915 Scorpii, Antares, Betelgeuse, Sirius, Vega, and Regulus, are metal-rich population I stars

### Population II stars

*See also: Population II Star*

Population II star, or metal-poor stars, are those with relatively little of the elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate population II stars are common in the bulge near the centre of the Milky Way, whereas population II stars found in the galactic halo are older and thus more metal-deficient, Globular clusters also contain high numbers of population II stars

Scientists have targeted these oldest stars in several different surveys and the Hamburg-ESO survey, originally started for faint quasars, Thus far, they have uncovered and studied in detail about 24 ultra-metal-poor (UMP) stars (such as Sneden's Star, Cayrel's Star, BD +17° 3248, BD -18° 5550, BPS CS29491-0069, BPS CS29497-0004) and six of the oldest stars known to date: HE 0020-1741, HE 0107-5240, HE 0557-4840, HE 1327-2326, HE 1523-0901, HE 2327-5642, SMSS J160540.18-144323.1, SMSS J173823.38-145701.1, SMSS J181609.62-333218.7, Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data

However, in February 2014 the discovery of an even lower-metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 196944 (a red giant star), HD 221170 (a red giant star), HD 222925 (a horizontal branch star), HDE 268835 (a blue-white supergiant star), HDE 269810 (a blue supergiant star), HD 122563 (a red giant star) and HD 140283 (a subgiant star)