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 or system, fitting the star with the standard spectral and luminosity classification system, and the presence of a protoplanetary disk, 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. [2]
As stars grow older, their luminosity increases at an appreciable rate. [3] 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, because in advanced evolutionary stages of the star, such as the red giant 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, [4] 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, [5] this is an accurate method of determining age.
Various properties of stars can also be used to determine their age. For example, the Eta Carinae 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 within a relatively short period of astronomical time. [6] Very large stars like VY Canis Majoris, one of the largest stars known, together with NML Cygni, VX Sagittarii and Trumpler 27-1 all have radii larger than that of the average orbital radius of Jupiter in the Solar System, thus showing that they are in extremely late evolutionary stages. [7] Betelgeuse in particular is expected to die in a supernova explosion within the next million years. [8]
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 have a characteristic pattern in their lightcurves, the rate of repetition of which is dependent on the luminosity of the star. [9] 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 main sequence phase of the star's life. [10] Additionally, more massive pre-main-sequence stars could be Herbig Ae/Be stars. [11] If a red dwarf star is emitting immense stellar flares and x-rays, 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. [12]
Membership in a star cluster or star system 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. [13] 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. [14]
However, this method does not work for galaxies. 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, [15] even though star production may long since have ceased (see elliptical galaxy). 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. [16]
Along with other factors, the presence of a protoplanetary disk 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. [17] Over time, this disk would coalesce to form planets, with leftover material being deposited into various asteroid belts and other similar locations. However, the presence of pulsar planets complicates this method as a determinant of age.
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. [18] This method has been seen as a more accurate method for the determination of stellar ages than other methods for field stars. [18]
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 stars. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".
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 Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe and include the Sun.
An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
A star is an astronomical object comprising a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.
Star clusters are large groups of stars held together by self-gravitation. 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 clustered groups of stars, generally containing fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.
A variable star is a star whose brightness as seen from Earth changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:
A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.
T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines. T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track, a luminosity–temperature relationship obeyed by infant stars of less than 3 solar masses (M☉) in the pre-main-sequence phase of stellar evolution. It ends when a star of 0.5 M☉ or larger develops a radiative zone, or when a smaller star commences nuclear fusion on the main sequence.
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body or celestial body is a single, tightly bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.
Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. Ground-based, 8-meter-class telescopes like the VLT are also able to measure the TRGB distance within reasonable observation times in the local universe.
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.
The following outline is provided as an overview of and topical guide to astronomy:
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russell diagram which forms a loop towards the blue side of the diagram.
The sun gets brighter about 10% every billion years.