Various methods and tools are involved in stellar age estimation, an attempt to identify within reasonable degrees of confidence what the age of a star is. These methods include stellar evolutionary models, membership in a given

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely cl ...

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...

, fitting the star with the standard spectral and luminosity classification system, and the presence of a

protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...

, among others. Nearly all of the methods of determining age require knowledge of the mass of the star, which can be known through various methods. No individual method can provide accurate results for all types of stars.

Luminosity increase and the Hertzsprung–Russell diagram

As stars grow older, their luminosity increases at an appreciable rate. Given the mass of the star, one can use this rate of increase in luminosity in order to determine the age of the star. This method only works for calculating stellar age on the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

, because in advanced evolutionary stages of the star, such as the

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 o ...

stage, the standard relationship for the determination of age no longer holds. However, when one can observe a red giant star with a known mass, one can calculate the main-sequence lifetime, and thus the minimum age of star is known given that it is in an advanced stage of its evolution. As the star spends only about 1% of its total lifetime as a red giant, this is an accurate method of determining age.

Stellar age estimation of stars

Various properties of stars can also be used to determine their age. For example, the

Eta Carinae Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in th ...

system is emitting large quantities of gas and dust. These enormous outbursts can be used to infer that the star system is nearing the end of its life, and will explode as a

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

within a relatively short period of astronomical time. Very large stars like

VY Canis Majoris VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star from the Solar System in the slightly southern constellation of Canis Major. It is o ...

, one of the largest stars known, together with NML Cygni,

VX Sagittarii VX Sagittarii is an extreme asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius (constellation), Sagittarius. It is a pulsating variable star with an unusually large magni ...

and Trumpler 27-1 all have radii larger than that of the average orbital radius of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

, thus showing that they are in extremely late evolutionary stages.

Betelgeuse Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of O ...

in particular is expected to die in a supernova explosion within the next million years. As well as the scenarios of supermassive stars violently casting off their outer layers before their deaths, other examples can be found of the properties of stars which illustrate their age. For example,

Cepheid variables A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

have a characteristic pattern in their

lightcurve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular freq ...

s, the rate of repetition of which is dependent on the luminosity of the star. Since Cepheid variables are a relatively short evolutionary stage in the lifecycle of stars, and knowing the mass of the star allows for the star to be tracked in its evolutionary path, one can estimate the age of the Cepheid variable. Exceptional stellar properties which allow for an estimation of age are not confined to advanced evolutionary stages. When a roughly solar-mass star exhibits T Tauri variability, astronomers can locate the age of the star as being before the beginning of the

phase of the star's life. Additionally, more massive pre-main-sequence stars could be

Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young () star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines ...

s. If a

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. ...

star is emitting immense

stellar flare The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star. Computer scientists and mathematicians often voc ...

s and

x-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...

, the star can be calculated to be in an early stage of its main-sequence lifetime, after which it will become less variable and become stable.

Membership in a star cluster or system

Membership in a

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or ''galaxy'', although, broadly speaking ...

permits an assignment of rough ages to a large number of stars present within. When one can determine the age of stars through other methods, such as the ones listed above, one can identify the age of all of the bodies in a system. This is especially useful in clusters of stars which exhibit a large amount of variety in their stellar masses, evolutionary stages, and classifications. While not ''entirely'' independent of the properties of the stars in the cluster, system, or other reasonably-sized association of stars, an astronomer would only need a representative sample of stars to determine the age of the cluster, rather than painstakingly finding the age of every star in the cluster through other properties. In addition, knowing the age of one member of a star system can help determine the age of that system. In a star system, stars almost always form at the same time as each other, and given the age of one star, the age of all of the others can be known. However, this method does not work for

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...

. These units are much larger, and are not merely a one-off creation of stars which allows their age to be determined in this fashion. The creation of stars in a galaxy takes place over billions of years, even though star production may long since have ceased (see

elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

). The oldest stars in a galaxy can only set a minimum age for the galaxy (when star formation began) but by no means determine the actual age.

Presence of a protoplanetary disk

Along with other factors, the presence of a

sets a maximum limit on the age of stars. Stars with protoplanetary disks are typically young, having moved onto the main sequence only a relatively short time ago. Over time, this disk would coalesce to form

planets A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a youn ...

, with leftover material being deposited into various

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...

s and other similar locations. However, the presence of pulsar planets complicates this method as a determinant of age.

Gyrochronology

Gyro-chronology is a method used to determine the age of field stars by measuring their rotation rate, and then comparing this rate with the rotation rate of the Sun, which serves as a precalibrated clock for this measurement. This method has been seen as a more accurate method for the determination of stellar ages than other methods for field stars.

References

External links

Red Dwarf Stars from UniverseTodayWhat is a Binary Star? from UniverseTodayReview Article:Protoplanetary Disks and their Evolution from Astrobites
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