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Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as “stellar nurseries” or “star-forming regions”—collapse and form stars.^[1] As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.^[2]

History

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The first stars were believed to be formed approximately 12-13 billion years ago following the Big Bang. Over intervals of time, stars have fused helium to form a series of chemical elements.

Stellar nurseries

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Interstellar clouds

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Spiral galaxies like the Milky Way contain stars, stellar remnants, and a diffuse interstellar medium (ISM) of gas and dust. The interstellar medium consists of 10⁴ to 10⁶ particles per cm³, and is typically composed of roughly 70% hydrogen, 28% helium, and 1.5% heavier elements by mass. The trace amounts of heavier elements were and are produced within stars via stellar nucleosynthesis and ejected as the stars pass beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae,^[3] where star formation takes place.^[4] In contrast to spiral galaxies, elliptical galaxies lose the cold component^{[definition needed]} of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.^[5]

In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H₂) form, so these nebulae are called molecular clouds.^[4] The Herschel Space Observatory has revealed that filaments, or elongated dense gas structures, are truly ubiquitous in molecular clouds and central to the star formation process. They fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending^{[definition needed]}, and magnetic fields may control the detailed manner in which the filaments are fragmented. Observations of supercritical filaments have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded protostars with outflows.^[jargon][6]

Observations indicate that the coldest clouds tend to form low-mass stars, which are first observed via the infrared light they emit inside the clouds, and then as visible light when the clouds dissipate. Giant molecular clouds, which are generally warmer, produce stars of all masses.^[7] These giant molecular clouds have typical densities of 100 particles per cm³, diameters of 100 light-years (9.5×10¹⁴ km), masses of up to 6 million solar masses (M_☉), or six million times the mass of Earth’s sun.^[8] The average interior temperature is 10 K (−441.7 °F).

About half the total mass of the Milky Way‘s galactic ISM is found in molecular clouds^[9] and the galaxy includes an estimated 6,000 molecular clouds, each with more than 100,000 M_☉.^[10] The nebula nearest to the Sun where massive stars are being formed is the Orion Nebula, 1,300 light-years (1.2×10¹⁶ km) away.^[11] However, lower mass star formation is occurring about 400–450 light-years distant in the ρ Ophiuchi cloud complex.^[12]

A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok. These can form in association with collapsing molecular clouds or possibly independently.^[13] The Bok globules are typically up to a light-year across and contain a few solar masses.^[14] They can be observed as dark clouds silhouetted against bright emission nebulae or background stars. Over half the known Bok globules have been found to contain newly forming stars.^[15]

Cloud collapse

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An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.^[17] If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. The mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.^[4] During cloud collapse dozens to tens of thousands of stars form more or less simultaneously which is observable in so-called embedded clusters. The end product of a core collapse is an open cluster of stars.^[18]

In triggered star formation, one of several events might occur to compress a molecular cloud and initiate its gravitational collapse. Molecular clouds may collide with each other, or a nearby supernova explosion can be a trigger, sending shocked matter into the cloud at very high speeds.^[4] (The resulting new stars may themselves soon produce supernovae, producing self-propagating star formation.) Alternatively, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.^[20] The latter mechanism may be responsible for the formation of globular clusters.^[21]

A supermassive black hole at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole that is accreting infalling matter can become active, emitting a strong wind through a collimated relativistic jet. This can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can also block the formation of new stars in aging galaxies.^[22] However, the radio emissions around the jets may also trigger star formation. Likewise, a weaker jet may trigger star formation when it collides with a cloud.^[23]

As it collapses, a molecular cloud breaks into smaller and smaller pieces in a hierarchical manner, until the fragments reach stellar mass. In each of these fragments, the collapsing gas radiates away the energy gained by the release of gravitational potential energy. As the density increases, the fragments become opaque and are thus less efficient at radiating away their energy. This raises the temperature of the cloud and inhibits further fragmentation. The fragments now condense into rotating spheres of gas that serve as stellar embryos.^[25]

Complicating this picture of a collapsing cloud are the effects of turbulence, macroscopic flows, rotation, magnetic fields and the cloud geometry. Both rotation and magnetic fields can hinder the collapse of a cloud.^[26][27] Turbulence is instrumental in causing fragmentation of the cloud, and on the smallest scales it promotes collapse.^[28]

Protostar

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Main article: Protostar

A protostellar cloud will continue to collapse as long as the gravitational binding energy can be eliminated. This excess energy is primarily lost through radiation. However, the collapsing cloud will eventually become opaque to its own radiation, and the energy must be removed through some other means. The dust within the cloud becomes heated to temperatures of 60–100 K, and these particles radiate at wavelengths in the far infrared where the cloud is transparent. Thus the dust mediates the further collapse of the cloud.^[29]

During the collapse, the density of the cloud increases towards the center and thus the middle region becomes optically opaque first. This occurs when the density is about 10⁻¹³ g / cm³. A core region, called the first hydrostatic core, forms where the collapse is essentially halted. It continues to increase in temperature as determined by the virial theorem. The gas falling toward this opaque region collides with it and creates shock waves that further heat the core.^[30]

When the core temperature reaches about 2000 K, the thermal energy dissociates the H₂ molecules.^[30] This is followed by the ionization of the hydrogen and helium atoms. These processes absorb the energy of the contraction, allowing it to continue on timescales comparable to the period of collapse at free fall velocities.^[31] After the density of infalling material has reached about 10⁻⁸ g / cm³, that material is sufficiently transparent to allow energy radiated by the protostar to escape. The combination of convection within the protostar and radiation from its exterior allow the star to contract further.^[30] This continues until the gas is hot enough for the internal pressure to support the protostar against further gravitational collapse—a state called hydrostatic equilibrium. When this accretion phase is nearly complete, the resulting object is known as a protostar.^[4]

Accretion of material onto the protostar continues partially from the newly formed circumstellar disc. When the density and temperature are high enough, deuterium fusion begins, and the outward pressure of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to “rain” onto the protostar. In this stage bipolar jets are produced called Herbig–Haro objects. This is probably the means by which excess angular momentum of the infalling material is expelled, allowing the star to continue to form.

When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered a pre-main-sequence star (PMS star). The energy source of these objects is (gravitational contraction)Kelvin–Helmholtz mechanism, as opposed to hydrogen burning in main sequence stars. The PMS star follows a Hayashi track on the Hertzsprung–Russell (H–R) diagram.^[33] The contraction will proceed until the Hayashi limit is reached, and thereafter contraction will continue on a Kelvin–Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 M_☉ thereafter join the main sequence. For more massive PMS stars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track.^[34]

Finally, hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star’s main sequence phase on the H–R diagram.

The stages of the process are well defined in stars with masses around 1 M_☉ or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars is studied in stellar evolution.

Protostar
Protostar outburst – HOPS 383 (2015).

Observations

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Key elements of star formation are only available by observing in wavelengths other than the optical. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the GMC. Often, these star-forming cocoons known as Bok globules, can be seen in silhouette against bright emission from surrounding gas.^[35] Early stages of a star’s life can be seen in infrared light, which penetrates the dust more easily than visible light.^[36] Observations from the Wide-field Infrared Survey Explorer (WISE) have thus been especially important for unveiling numerous galactic protostars and their parent star clusters.^[37][38] Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.^[39]

The structure of the molecular cloud and the effects of the protostar can be observed in near-IR extinction maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and rotational transitions of CO and other molecules; these last two are observed in the millimeter and submillimeter range. The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, as the extinction caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the Earth’s atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range, atmospheric subtraction techniques must be used.

X-ray observations have proven useful for studying young stars, since X-ray emission from these objects is about 100–100,000 times stronger than X-ray emission from main-sequence stars.^[41] The earliest detections of X-rays from T Tauri stars were made by the Einstein X-ray Observatory.^[42][43] For low-mass stars X-rays are generated by the heating of the stellar corona through magnetic reconnection, while for high-mass O and early B-type stars X-rays are generated through supersonic shocks in the stellar winds. Photons in the soft X-ray energy range covered by the Chandra X-ray Observatory and XMM-Newton may penetrate the interstellar medium with only moderate absorption due to gas, making the X-ray a useful wavelength for seeing the stellar populations within molecular clouds. X-ray emission as evidence of stellar youth makes this band particularly useful for performing censuses of stars in star-forming regions, given that not all young stars have infrared excesses.^[44] X-ray observations have provided near-complete censuses of all stellar-mass objects in the Orion Nebula Cluster and Taurus Molecular Cloud.^[45][46]

The formation of individual stars can only be directly observed in the Milky Way Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.

Initial research indicates star-forming clumps start as giant, dense areas in turbulent gas-rich matter in young galaxies, live about 500 million years, and may migrate to the center of a galaxy, creating the central bulge of a galaxy.^[47]

On February 21, 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.^[48]

In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed – about 180 million years after the Big Bang.^[49]

An article published on October 22, 2019, reported on the detection of 3MM-1, a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds of dust.^[50] At a mass of about 10^10.8 solar masses, it showed a star formation rate about 100 times as high as in the Milky Way.^[51]

Notable pathfinder objects

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• MWC 349 was first discovered in 1978, and is estimated to be only 1,000 years old.
• VLA 1623 – The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years.^[52]
• L1014 – An extremely faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like brown dwarfs or even rogue planets).^[53]
• GCIRS 8* – The youngest known main sequence star in the Galactic Center region, discovered in August 2006. It is estimated to be 3.5 million years old.^[54]

Low mass and high mass star formation

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Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by observation, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 M_☉, however, the mechanism of star formation is not well understood.

Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses.^[57] Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar.^[58][59] Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.

There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks.^[60] Disk accretion in high-mass protostars, similar to their low-mass counterparts, is expected to exhibit bursts of episodic accretion as a result of a gravitationally instability leading to clumpy and in-continuous accretion rates. Recent evidence of accretion bursts in high-mass protostars has indeed been confirmed observationally.^[60][61][62] Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are “seeded” by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.^[63][64]

Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.^[65]

Filamentary nature of star formation

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Recent studies have emphasized the role of filamentary structures in molecular clouds as the initial conditions for star formation. Findings from the Herschel Space Observatory highlight the ubiquitous nature of these filaments in the cold interstellar medium (ISM). The spatial relationship between cores and filaments indicates that the majority of prestellar cores are located within 0.1 pc of supercritical filaments. This supports the hypothesis that filamentary structures act as pathways for the accumulation of gas and dust, leading to core formation.^[66]

Both the core mass function (CMF) and filament line mass function (FLMF) observed in the California GMC follow power-law distributions at the high-mass end, consistent with the Salpeter initial mass function (IMF). Current results strongly support the existence of a connection between the FLMF and the CMF/IMF, demonstrating that this connection holds at the level of an individual cloud, specifically the California GMC.^[66] The FLMF presented is a distribution of local line masses for a complete, homogeneous sample of filaments within the same cloud. It is the local line mass of a filament that defines its ability to fragment at a particular location along its spine, not the average line mass of the filament. This connection is more direct and provides tighter constraints on the origin of the CMF/IMF.^[66]

A stellar active region is a localized, transient volume of a stellar atmosphere in which plages, starspots, faculae, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are bipolar and may be complex if the region contains two or more bipolar groups.

A stellar active region on a star’s surface can form a bright spot which intensifies and grows. An active region may have a coronal portion.

Most stellar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.^[1]

Theory of stellar active regions

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“MOST current literature on solar activity assumes that the planets do not affect it, and that conditions internal to the Sun are primarily responsible for the solar cycle. Bigg¹, however, has shown that the period of Mercury’s orbit appears in the sunspot data, and that the influence of Mercury depends on the phases of Venus, Earth, and Jupiter.”^[2]

“The tidal forces hypothesis for solar cycles has been proposed by Wood (1972) and others. Table 2 below shows the relative tidal forces of the planets on the sun. Jupiter, Venus, Earth and Mercury are called the “tidal planets” because they are the most significant. According to Wood, the especially good alignments of J-V-E with the sun which occur about every 11 years are the cause of the sunspot cycle. He has shown that the sunspot cycle is synchronous with the alignments, and J. Schove’s data for 1500 year of sunspot maxima supports the 11.07 year J-V-E period average.”^[3]

“Both the 11.86 year Jupiter tropical period (time between perihelion’s or closest approaches to the sun and the 9.93 year J-S alignment periods are found in sunspot spectral analysis. Unfortunately direct calculations of the tidal forces of all planets does not support the occurrence of the dominant 11.07 year cycle. Instead, the 11.86 year period of Jupiter’s perihelion dominates the results. This has caused problems for several researchers in this field.”^[3]

Variations in the electron and ion currents to and from the Sun may have a greater effect on active regions than any other.

Spicules

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Main resources: Stars/Sun/Spicules and Spicules

File:Whiskery plasma jets.jpgWhiskery plasma jets, known as spicules, on the sun appear as dark, threadlike structures in this image, acquired at the Goode Solar Telescope in Big Bear, Calif. Credit: T. Samanta, GST & SDO.{{fairuse}}

A spicule is a dynamic jet of about 500 km diameter in the chromosphere of a star. It moves upwards at about 20 km/s from the photosphere. Spicules last for about 15 minutes;^[4] at [a star’s limb] they appear elongated (if seen on the disk, they are known as “mottles” or “fibrils”). They are usually associated with regions of high magnetic flux; their mass flux is about 100 times that of the [stellar wind]. They rise at a rate of 20 km/s (or 72,000 km/h) and can reach several thousand kilometers in height before collapsing and fading away.

“Tendrils of plasma near the surface of the sun emerge from realignments of magnetic fields and [apparently] pump heat into the corona, the sun’s tenuous outer atmosphere.”^[5]

“Spicules undulate like a wind-whipped field of wheat in the chromosphere, the layer of hot gas atop the sun’s surface. These plasma filaments stretch for thousands of kilometers and last for just minutes, shuttling ionized gas into the corona.”^[5]

Thickets “of spicules frequently emerged within minutes after pockets of the local magnetic field reversed course and pointed in the opposite direction from the prevailing field in the area.”^[5]

“Counterpointing magnetic fields create a tension that gets resolved when the fields break and realign”.^[5]

“The magnetic field energy is converted to kinetic and thermal energy. The kinetic energy is in the form of fast plasma motion — jets, or spicules.”^[6]

A “glow from charged iron atoms [occurred in images from NASA’s orbiting Solar Dynamics Observatory] directly over the spicules. That glow […] means the plasma reached roughly 1 million degrees Celsius.”^[6]

Faculae

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Main resources: Stars/Sun/Faculae and Faculae

Def. a bright spot or patch between starspots is called a facula.

Bright spots also occur at the magnetic poles of magnetic stars.

“Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where [starspots] are about to form.”^[7]

Coronal loops

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Main resources: Plasmas/Plasma objects/Coronal loops and Coronal loopsThis movie show the evolution of active region 1520, including coronal loops. Credit: NASA/Goddard Space Flight Center.

The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops project through the chromosphere and transition region, extending high into the corona.

Coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1 MK are generally known as cool loops, those existing at around 1 MK are known as warm loops, and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.^[8]

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and 82% of the total coronal heating energy.^[9] The quiet Sun, although less active than active regions, is awash with dynamic processes and transient events (bright points, nanoflares and jets).^[10] As a general rule, the quiet Sun exists in regions of closed magnetic structures, and active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines. A closed fieldline does not constitute a coronal loop; however, closed flux must be filled with plasma before it can be called a coronal loop.

Nanoflares

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Main resources: Plasmas/Plasma objects/Nanoflares/Stars and Stellar nanoflares

In order to heat a region of very high X-ray emission, over an area 1″ x 1″, a nanoflare of 10¹⁷ J should happen every 20 seconds, and 1000 nanoflares per second should occur in a large active region of 10⁵ x 10⁵ km².

Flickerings, brightenings, small explosions, bright points, flares and mass eruptions are observed very frequently, especially in active regions.

Flares

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Main resources: Plasmas/Plasma objects/Flares and Flares

Def. a violent explosion in a star’s atmosphere is called a flare.

“A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. The brightness increase is across the spectrum, from X rays to radio waves.

“Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.^[11]

BY Draconis variables

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Main resources: Stars/Variables/BY Draconis and BY Draconis variables

BY Draconis variables are main sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other chromospheric activity.^[12] Resultant brightness fluctuations are generally less than 0.5 magnitudes^[12] on timescales equivalent to the star’s rotational period, typically from a fraction of a day to several months. Oddly enough, Procyon the 8th brightest night-time star which is an F5 sub-giant or dwarf has also been classified as a BY Draconis variable.^[13]

Some of these stars may exhibit flares, resulting in additional variations of the UV Ceti type.^[14] Likewise, the spectra of BY Dra variables (particularly in their H and K lines) are similar to RS CVn stars, which are another class of variable stars that have active chromospheres.^[15]

RS Canum Venaticorum variables

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Main resources: Stars/Variables/RS Canum Venaticorum and RS Canum Venaticorum variables

RS Canum Venaticorum variables are a type of variable star. They are close binary stars^[16] having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity. Systems can exhibit variations on timescales of years due to variation in the spot surface coverage fraction, as well as periodic variations which are, in general, close to the orbital period of the binary system. Some systems exhibit variations in luminosity due to their being eclipsing binaries. Typical brightness fluctuation is around 0.2 magnitudes.

UV Ceti variable

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Main resources: Stars/Variables/UV Ceti and UV Ceti variables

The type star goes through fairly extreme changes of brightness: for instance, in 1952, its brightness increased by 75 times in only 20 seconds.

Plages

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Main resources: Plasmas/Plasma objects/Plages and Plages

A plage is a bright region in the chromosphere of [a star], typically found in regions of the chromosphere near [starspots]. The plage regions map closely to the faculae in the photosphere below, but the latter have much smaller spatial scales. Accordingly plage occurs most visibly near a starspot region.

“Plages are formed in the inner parts of flux loops emerging from below. … In the early stages of active region growth the appearance of the group is symmetric, while a few days later the f spot may disappear, leaving an extensive plage.”^[17]

“[M]ajor changes in active regions only take place in the following ways:

1. [starspot] formation and break up;
2. flux outflow from [starspots];
3. new flux emergence; and
4. magnetic reconnection.”^[17]

“In general there is no proper motion at all in the plage or the surrounding plagettes except for the latter two.”^[17]

Coronal streamers

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Main resources: Plasmas/Plasma objects/Coronal streamers and Coronal streamers

The interconnections of active regions are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare. Some other features of this kind are helmet streamers—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow solar wind.^[18]

Prominences

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Main resources: Plasmas/Plasma objects/Prominences and Prominences

A prominence is a large, bright feature extending outward from [a star’s] surface, often in a loop shape. Prominences are anchored to [a star’s] surface in the photosphere, and extend outwards into the [star’s] corona. While the corona consists of extremely hot ionized gases, known as plasma, which [does] not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. A prominence forms over timescales of about a day, and stable prominences may persist in the corona for several months. Some prominences break apart and give rise to coronal mass ejections.

A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 kilometres (500,000 mi) long^[19] – roughly the radius of the Sun.

“When a prominence is viewed from a different perspective so that it is against the [star] instead of against space, it appears darker than the surrounding background. This formation is instead called a [stellar] filament.^[19] It is possible for a projection to be both a filament and a prominence. Some prominences are so powerful that they throw out matter from the [star] into space at speeds ranging from 600 km/s to more than 1000 km/s. Other prominences form huge loops or arching columns of glowing gases over [starspots] that can reach heights of hundreds of thousands of kilometres. Prominences may last for a few days or even for a few months.^[20] Flocculi (plural of flocculus) is another term for these filaments, and dark flocculi typically describes the appearance of [stellar] prominences when viewed against the [stellar] disk in certain wavelengths.

Starspots

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Main resources: Stars/Starspots and Starspots

Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.

In 1947, G. E. Kron proposed that starspots were the reason for periodic changes in brightness on red dwarfs.^[1] Since the mid-1990s, starspot observations have been made using increasingly powerful techniques yielding more and more detail: photometry showed starspot growth and decay and showed cyclic behavior similar to the Sun’s; spectroscopy examined the structure of starspot regions by analyzing variations in spectral line splitting due to the Zeeman Effect; Doppler imaging showed differential rotation of spots for several stars and distributions different from the Sun’s; spectral line analysis measured the temperature range of spots and the stellar surfaces. The largest cool starspot ever seen rotating the giant K0 star XX Triangulum (HD 12545) with a temperature of 3,500 K (3,230 °C), together with a warm spot of 4,800 K (4,530 °C).^[1][21]

Stars with sizable sunspots may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. The surface of the star is not uniformly bright, but has darker and brighter areas (like the sun’s solar spots). The star’s chromosphere too may vary in brightness. As the star rotates we observe brightness variations of a few tenths of magnitudes.

Observed starspots have a temperature which is in general 500–2000 Kelvin cooler than the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 magnitudes between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in G-K dwarfs).

The lifetime for a starspot depends on its size.

• For small spots the lifetime is proportional to their size, similar to spots on the Sun.^[22]
• For large spots the sizes depend on the differential rotation of the star, but there are some indications that large spots which give rise to light variations can survive for many years even in stars with differential rotation.^[22]

The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.

Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres.

Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.

Lithiums

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Main resources: Chemicals/Lithiums and Lithiums

A number of lithium emission lines is observed in sunspot umbra. These are “the lithium (I) line doublets from Li(6) and Li(7) at 670.8 nm”.^[23]

“[A] lithium abundance [is] ε_Li = 1.02 ± 0.12¹ … Some evidence for the existence of a small but notable amount of Li⁶ is found.”^[23]

Berylliums

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Main resources: Chemicals/Berylliums and Berylliums

“The isotopes ⁷Be, with a half-life of 53 days, and ¹⁰Be are both cosmogenic nuclides because they are made on a recent timescale in the solar system by spallation, like ¹⁴C. These two radioisotopes of beryllium in the atmosphere track the sun spot cycle and solar activity, since this affects the magnetic field that shields the Earth from cosmic rays. The rate at which the short-lived ⁷Be is transferred from the air to the ground is controlled in part by the weather. ⁷Be decay in the sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment.

Calciums

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Main resources: Chemicals/Calciums and Calciums

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV.^[24]

Coronal mass ejections

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Main resources: Plasmas/Plasma objects/Coronal mass ejections and Coronal mass ejections

Most coronal mass ejections originate from active regions on a star’s surface. Near a stellar maxima a star such as the Sun produces about three CMEs every day, whereas near stellar minima there is about one CME every five days.^[25]

“The magnetic field carried away by the coronal mass ejections (CMEs) is twisted. … [Helicity] is defined asH=∫VA⋅BdV−∫VAp⋅BpdV.

… Helicity [(H)] is a quantitative measure of the chiral properties of the structures observed in the solar atmosphere.”^[26]

Evolution

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Main resources: Stars/Sun/Sunspots/Evolution and Sunspot evolution

“The region (McMath 12510) arose on the back side of the Sun and first was visible on August 30, 1973, displaying a normal amount of plage. … During its second transit the spot (now Mt. Wilson 12510, McMath 12542) was the largest on the disk but already naked … The spot returned a third time (Mt. Wilson 19281, McMath 12585), greatly reduced in size.”^[27]

Solar spicules

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Main resources: Stars/Sun/Spicules and Solar spicules

There are about 300,000 active spicules at any one time on the Sun’s chromosphere, amounting to about 1% of the Sun’s surface.^[4] At any one time there are around 60,000 to 70,000 active spicules on the Sun; an individual spicule typically reaches 3,000-10,000 km altitude above the photosphere.^[28]

Solar microflares

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Main resources: Plasmas/Plasma objects/Microflares/Sun and Solar microflares

Ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual [solar] micro-flares as small brightenings in extreme ultraviolet light.^[29]

Coronal arcades

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Main resources: Plasmas/Plasma objects/Coronal arcades and Coronal arcades

Def. a close collection of loops in a cylindrical structure is called an arcade.

The TRACE image at right “is from near flare maximum (11:00 UT) and has a width of 230,000 km […] how in the world can the footpoints of the arcade have such a clearly-organized pattern whose scale greatly exceeds the known scales of the largest convective scales known in the photosphere?”^[30]

“The most obvious coronal signatures of CMEs in the low corona are the arcades of bright loops that develop after the CME material has erupted […] nearly all (92%) EIT post-eruptive arcades from 1997 – 2002 were associated with LASCO CMEs […] The activity associated with halo CMEs includes the formation of dimming regions, long-lived loop arcades, flaring active regions, large-scale coronal waves and filament eruptions”.^[31]

Helmet streamers

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Main resources: Plasmas/Plasma objects/Helmet streamers and Helmet streamers

Helmet streamers are bright loop-like structures which develop over active regions on the sun. They are closed magnetic loops which connect regions of opposite magnetic polarity. Electrons are captured in these loops, and cause them to glow very brightly. The solar wind elongates these loops to pointy tips. They far extend above most prominences into the corona, and can be readily observed during a solar eclipse. Helmet streamers are usually confined to the “streamer belt” in the mid latitudes, and their distribution follows the movement of active regions during the solar cycle. Small blobs of plasma, or “plasmoids” are sometimes released from the tips of helmet streamers, and this is one source of the slow component of the solar wind. In contrast, formations with open magnetic field lines are called coronal holes, and these are darker and are a source of the fast solar wind. Helmet streamers can also create coronal mass ejections if a large volume of plasma becomes disconnected near the tip of the streamer.

Solar flares

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Main resources: Plasmas/Plasma objects/Flares/Sun and Solar flares

Balfour Stewart recorded a super flare on the evening of 28 August 1859 and the morning of 2 September 1859, at the Kew Observatory, and presented his findings in a paper presented to the Royal Society on 21 November 1861.^[32][33] He noted that while “magnetic disturbances of unusual violence and very wide extent” were recorded in various places around the world, the Kew Observatory had the benefit of self-recording magnetographs,^[34] which allowed “the means of obtaining a continuous photographic register of the state of the three elements of the earth’s magnetic force—namely, the declination, and the horizontal and vertical intensity.”

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV.^[24] “The coronal temperature was 4000000°.”^[24] “The December 18, 1956, flare appears to have been a violent condensation of material from a dense coronal cloud above an active region.”^[24]

“The first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. This line results from the formation of deuterium via the union of a neutron and proton; in a solar flare the neutrons appear as secondaries from interactions of high-energy ions accelerated in the flare process. These first gamma-ray line observations were from OSO-3 [and] OSO-7

Fairly large fluxes of neutrons have been observed during solar flares such as that of November 12, 1960, with a flux of 30-70 neutrons per cm^-2 s^-1.^[35]

The Bastille Day Flare or Bastille Day Event was a powerful solar flare on July 14, 2000, occurring near the peak of the solar maximum in solar cycle 23.^[36][37] NOAA Active region 9077 produced an X5.7-class flare, which caused an S3 radiation storm on Earth fifteen minutes later as energetic protons bombarded the ionosphere.^[36][38] It was the biggest solar radiation event since 1989.^[38] The proton event was four times more intense than any previously recorded since the launches of SOHO in 1995 and ACE in 1997.^[36] The flare was followed by a full-halo coronal mass ejection^[36] and a geomagnetic super storm on July 15-16. The extreme level, G5, was peaked in late hours of July 15.

“The Bastille Day event was observed by Voyager I and Voyager II,^[39] thus it is the farthest out observed solar storm.”

“On May 17, 2012 an M-class flare exploded from the sun. The eruption also shot out a burst of solar particles traveling at nearly the speed of light that reached Earth about 20 minutes after the light from the flare. An M-class flare is considered a “moderate” flare, at least ten times less powerful than the largest X-class flares, but the particles sent out on May 17 were so fast and energetic that when they collided with atoms in Earth’s atmosphere, they caused a shower of particles to cascade down toward Earth’s surface. The shower created what’s called a ground level enhancement (GLE).”^[40]

“[O]n Saturday, May 5, … a large sunspot rotated into view on the left side of the sun. … [J]ust before [Active Region 1476] disappeared over the right side of the sun, it … erupted with an M-class flare.”^[40]

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons.”^[41] The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare.^[41] The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.^[41] Line-broadening is due to “the velocity of the positronium.”^[41] “The width of the annihilation line is also consistent … with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 10⁵ K. … The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 10¹² H cm^-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. … positrons produced by ³He interactions form higher in the solar atmosphere … all observations are consistent with densities > 10¹² H cm^-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures.”^[41]

“[N]eutrino flux increases noted in Homestake results [coincide] with major solar flares [14].”^[42]

“The correlation between a great solar flare and Homestake neutrino enhancement was tested in 1991. Six major flares occurred from May 25 to June 15 including the great June 4 flare associated with a coronal mass ejection and production of the strongest interplanetary shock wave ever recorded (later detected from spacecraft at 34, 35, 48, and 53 AU) [15]. It also caused the largest and most persistent (several months) signal ever detected by terrestrial cosmic ray neutron monitors in 30 years of operation [16]. The Homestake exposure (June 1–7) measured a mean ³⁷Ar production rate of 3.2 ± 1.5 atoms/day (≈19 ³⁷Ar atoms produced in 6 days) [13]; about 5 times the rate of ≈ 0.65 day ⁻¹ for the preceding and following runs, > 6 times the long term mean of ≈ 0.5 day⁻¹ and > 2 1/2 times the highest rates recorded in ∼ 25 operating years.”^[42]

Solar Moreton waves

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Main resources: Stars/Chromospheres/Tsunamis and Chromospheric tsunamisFile:MoretonWaveAnimation200612.gifThis is an animation of a Moreton wave which occurred on the Sun at December 6, 2006. Credit: National Solar Observatory (NSO)/AURA/NSF and USAF Research Laboratory.

A Moreton wave is the chromospheric signature of a large-scale solar coronal shock wave. Described as a kind of solar ‘tsunami‘,^[43] they are generated by solar flares^[44][45][46].

The 1995 launch of the Solar and Heliospheric Observatory led to observation of coronal waves, which cause Moreton waves. (SOHO’s EIT instrument discovered another, different wave type called ‘EIT waves’.)^[47] The reality of Moreton waves (aka fast-mode MHD waves) has also been confirmed by the two STEREO spacecraft. They observed a 100,000-km-high wave of hot plasma and magnetism, moving at 250 km/second, in conjunction with a big coronal mass ejection in February 2009.^[48][49]

Moreton waves propagate at a speed of usually 500–1500 km/s. Yutaka Uchida interpreted Moreton waves as MHD fast mode shock waves propagating in the corona.^[50] He links them to type II radio bursts, which are radio wave discharges created when coronal mass ejections accelerate shocks.^[51]

Moreton waves can be observed primarily in the Hα band.^[52]

Sunspots

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Main resources: Stars/Sun/Sunspots and SunspotsFile:Sunspot vtt1.jpgA sunspot is a depression on the Sun’s face that is slightly cooler and less luminous than the rest of the Sun. Credit: Vacuum Tower Telescope, NSO, NOAO.

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles. Although they are at temperatures of roughly 3,000–4,500 K (2,727–4,227 °C), the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (49,710 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope.^[53] They may also travel at relative speeds (“proper motions”) of a few hundred m/s when they first emerge onto the solar photosphere.

Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loops (prominences) and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.^[1]

Solar active region AR 10030 contained a group of sunspots including the largest one partially included in the image at the right. It is a planet-sized sunspot showing for the first time the dark cores of the filaments extending into the sunspot. These filaments are thousands of km long by about 100 km wide. The image is recorded on July 15, 2002, using the Swedish Solar Telescope (SST).

Solar CMEs

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Main resources: Plasmas/Plasma objects/Coronal mass ejections/Sun and Solar coronal mass ejectionsFile:LASCO20011001.gifA coronal mass ejection in time-lapse imagery is obtained with the LASCO instrument. The Sun (center) is obscured by the coronagraph’s mask. (September 30 – October 1, 2001). Credit: SOHO (ESA & NASA).

An explosive limb flare occurred above 30,000 km in the corona of the Sun.^[54] “So the aftermath of the flare explosion, usually visible in disk pictures as extensive Hα brightening, but hidden from us in this case, was seen by the ionosphere as an intense flux of ionizing radiation from the coronal cloud created by the explosion.”^[54] “The November 20, 1960, event is very similar to that of February 10, 1956, which was observed at Sacramento Peak. A bright ball appears above the surface, grows in size and Hα brightness, and explodes upward and outward.”^[54] “The great breadth and intensity of the Hα emission from the suspended ball at 2013 U.T. testify to the large amount of energy stored there, as no corresponding macroscopic motion was observed until the explosion at 2023 U.T.”^[54] “[T]he great energy of the preflare cloud was released into the corona by the explosion of 2023 U.T., and Hα radiation disappeared by 2035 U.T.”^[54]

“On 16 June 1972, the Naval Research Laboratory‘s coronagraph aboard OSO-7 tracked a huge coronal cloud moving outward from the Sun.”^[55]

“Many CMEs have also been observed to be unassociated with any obvious solar surface activity … The frequency of occurrence of CMEs observed in white light tends to follow the solar cycle in both phase and amplitude, which varies by an order of magnitude over the cycle … The latitude distribution of the central position angles of CMEs tends to cluster about the equator around solar minimum but broadens over all latitudes near solar maximum. … Many CMEs viewed at the solar limb also appear to arise from large-scale, pre-existing coronal streamers which often overlie active regions … Many energetic CMEs actually involve the disruption (“blowout”) of such a structure, which can increase in brightness and size for days before erupting as a CME … A streamer is a bright (dense) structure containing closed and open fields, which help guide denser, outward-flowing solar wind material. … The absence of solar surface activity with observed CME activity is not a new observation … the CME originated high enough up in the corona such that no surface signatures were evident. … about a third of the CMEs were “stealth”, having no distinct surface association, and tending to be slow, i.e., < 300 km s^–1. Faint coronal changes could be detected in about half of the stealth CMEs … Based on the highest mass (10¹⁴ kg) and speed (∼ 3500 km s^–1) observed one can estimate a maximum kinetic energy of ∼ 6 x 10³⁴ erg. Assuming that only a fraction of the stored energy is released in a single episode and that the CME derives all of its energy from a single active region, we can set a limit of ∼ 10³⁶ erg for the maximum free energy available in a solar active region. This is consistent with the size and magnetic field strengths in solar active regions”^[31].

Solar ejections

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Main resources: Plasmas/Plasma objects/Coronal ejections/Sun and Solar coronal ejections

Magnetic clouds represent about one third of ejecta observed by satellites at Earth. Other types of ejecta are multiple-magnetic cloud events (a single structure with multiple subclouds distinguishable)^[56][57] and complex ejecta, which can be the result of the interaction of multiple CMEs.

Active region designations

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Main resources: Stars/Sun/Active regions/Designations and Solar active region designations

“75% of the naked sunspots represented the return of large dominant p spots which had been part of large active regions during previous rotations.”^[27]

Greenwich numbering

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Date	Kodaikanal number	Greenwich group number
1/29/68-2/2/68	13105	21482
2/20-26/69	13483	21894
3/21/69	13510	21936
8/2-3/69	13621	22064
8/2/69	13625	22068
9/26/69	13640	22086
10/8-10/69	13683	22138
11/2/69	13696	22152
11/24/69	13713	22176
12/26/69	13743	22210
1/17/70	13776	22247?
1/25-30/70	13778	22251
1/26/70	13783	22255
1/29/70	13784	22261
2/9-12/70	13791	22272
2/7/70	13792	22274
2/21-25/70	13811	22291
4/9-13/70	13859	22349?
4/24-25/70	13870	22362
4/25/70	13875	22370?
5/7-8/70	13881	22379
5/15-16/70	13891	22392
5/30/70	13901	22411
6/13-16/70	13916	22433
6/27-30/70	13932	22448
6/30/70-7/1/70	13937	22454
8/6/70	13973	22495
8/24/70	13980	22508
9/27-29/70	14021	22556
11/13-14/70	14064	22608
12/1/70	14108	22664
1/21-5/71	14120	22679
1/31/71-2/3/71	14128	22686
2/16/71	14144	22710
3/21/71	14175	22738
4/11/71	14184	22755

In a 1904 article, Maunder was to describe the storm as a “very intense and long-continued disturbance”, which in total, lasted between November 11 and 26. He pointed out that this synchronised “with the entire passage across the visible disc” of sunspot group 885 (Greenwich numbering).^[59] This group originally had formed on the disc on October 20, passed off at the west limb on October 28, passed again east-west between November 12–25, and returned at the east limb on December 10, before finally disappearing on the disc on December 20.^[60]