Artist's impression of the Milky Way (updated - annotated)

During 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 Events January * January 3 – Theodoros Pangalos declares himself dictator in Greece. * January 8 **Abdul-Aziz ibn Saud is crowned King of Hejaz. ** Crown Prince Nguyễn Phúc Vĩnh Thuy ascends the throne, the last monarch of V ...

: Baade noticed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within

globular star clusters A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member ...

. Two main divisions were defined as * Population I and * Population II, with another newer, hypothetical division called * Population III added in 1978; they are often simply abbreviated as Pop. I, Pop. II, and Pop. III. 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 Kinematics is a subfield of physics, developed in classical mechanics, that describes the Motion (physics), motion of points, Physical object, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause ...

, stellar age, and even galaxy evolution in both

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

. 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 1.4% metallicity. Note that astrophysics nomenclature considers any element heavier than helium to be a "metal", including chemical non-metals such as oxygen.

Stellar development

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 Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

Formation of the first stars

Under current cosmological models, all matter created in the

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

was mostly hydrogen (75%) and helium (25%), with only a very tiny fraction consisting of other light elements such as lithium and beryllium. When the universe had cooled sufficiently, the first stars were born as population III stars, without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their nucleosynthetic processes created the first 26 elements (up to

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

in the

periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...

). Many theoretical stellar models show that most high-mass population III stars rapidly exhausted their fuel and likely exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass population III stars should be observable. However, some population III stars might be seen in high-

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

galaxies whose light originated during the earlier history of the universe. Scientists have found evidence of an extremely small ultra metal-poor star, slightly smaller than the Sun, found in a binary system of the spiral arms in the Milky Way. The discovery opens up the possibility of observing even older stars. Stars too massive to produce pair-instability supernovae would have likely collapsed into

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

s through a process known as photodisintegration. Here some matter may have escaped during this process in the form of

relativistic jet An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...

s, and this could have distributed the first metals into the universe.

Formation of the observable stars

The oldest observed stars, known as population II, have very low metallicities; as subsequent generations of stars were born, they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...

manufactured by previous generations. As those stars died, they returned metal-enriched material to the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

via planetary nebulae and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including the Sun, therefore have the highest metal content, and are known as population I stars.

Chemical classification by Baade

Population I stars

Population I, 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 solar-like Mu Arae is much richer in metals. Population I stars usually have regular elliptical orbits of the Galactic Center, with a low relative velocity. It was earlier hypothesized that the high metallicity of population I stars makes them more likely to possess

planetary system A planetary system is a set of gravitationally In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interacti ...

s than the other two populations, because planets, particularly terrestrial planets, are thought to be formed by the

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

of metals. However, observations of the Kepler Space Telescope data have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity—a finding that has implications for theories of gas-giant formation. Between the intermediate population I and the population II stars comes the intermediary disc population.

Population II stars

Population II, 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. A characteristic of population II stars is that despite their lower overall metallicity, they often have a higher ratio of "

alpha elements Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first Letter (alphabet), letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician al ...

" (elements produced by the alpha process, like O and Ne) relative to Fe as compared with population I stars; current theory suggests that this is the result of type II supernovas being more important contributors to the

at the time of their formation, whereas type Ia supernova metal-enrichment came at a later stage in the universe's development. Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg- ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten ultra-metal-poor (UMP) stars (such as Sneden's Star, Cayrel's Star, BD +17° 3248) and three of the oldest stars known to date: HE 0107-5240, HE 1327-2326 and HE 1523-0901.

Caffau's star SDSS J102915+172927 or Caffau's Star is a population II star in the galactic halo, seen in the constellation Leo. It is about 13 billion years old, making it one of the oldest stars in the Galaxy. At the time of its discovery, it had the ...

was identified as the most metal-poor star yet when it was found in 2012 using

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

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 SkyMapper is a fully automated 1.35 m (4.4 ft) wide-angle optical telescope at Siding Spring Observatory in northern New South Wales, Australia. It is one of the telescopes of the Research School of Astronomy and Astrophysics of the ...

astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

) and HD 140283 (a subgiant).

Population III stars

Population III stars are a hypothetical population of extremely massive, luminous and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby population III supernovae. Such stars are likely to have existed in the very early universe (i.e., at high redshift) and may have started the production of

chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

s heavier than hydrogen, which are needed for the later formation of planets and

life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

as we know it. The existence of population III stars is inferred from physical cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe. Their existence may account for the fact that heavy elements – which could not have been created in the

– are observed in

quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...

emission spectra The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...

. They are also thought to be components of faint blue galaxies. These stars likely triggered the universe's period of reionization, a major phase transition of the hydrogen gas composing most of the interstellar medium. Observations of the galaxy UDFy-38135539 suggest that it may have played a role in this reionization process. The

European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...

discovered a bright pocket of early population stars in the very bright galaxy Cosmos Redshift 7 from the reionization period around 800 million years after the Big Bang, at . The rest of the galaxy has some later redder population II stars. Some theories hold that there were two generations of population III stars. NASA-WMAP-first-stars

Current theory is divided on whether the first stars were very massive or not. One possibility is that these stars were much larger than current stars: several hundred

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es, and possibly up to 1,000 solar masses. Such stars would be very short-lived and last only 2–5 million years. Such large stars may have been possible due to the lack of heavy elements and a much warmer

from the Big Bang. Conversely, theories proposed in 2009 and 2011 suggest that the first star groups might have consisted of a massive star surrounded by several smaller stars. The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses () or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy. Analysis of data of extremely low-

population II stars such as HE 0107-5240, which are thought to contain the metals produced by population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses. On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae, which are typically associated with very massive stars, were responsible for their metallic composition. This also explains why there have been no low-mass stars with zero

observed, although models have been constructed for smaller population III stars. Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae) have been proposed as dark matter candidates, but searches for these types of MACHOs through gravitational microlensing have produced negative results. Detection of population III stars is a goal of NASA's

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...

. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate population III stars.

Stellar development

Formation of the first stars

Formation of the observable stars

Chemical classification by Baade

Population I stars

Population II stars

Population III stars

See also

Notes

References

Further reading