Gaia (spacecraft)

From Wikipedia, the free encyclopedia

Gaia

Artist's impression of the Gaia spacecraft
Mission type	Astrometric observatory
Operator	ESA
COSPAR ID	2013-074A
SATCAT no.	39479
Website	sci.esa.int/gaia/
Mission duration	planned: 5 years; possible extension by one to four years elapsed: 4 years, 10 months and 16 days
Spacecraft properties
Manufacturer	EADS Astrium e2v Technologies
Launch mass	2,029 kg (4,473 lb)
Dry mass	1,392 kg (3,069 lb)
Payload mass	710 kg (1,570 lb)
Dimensions	4.6 m × 2.3 m (15.1 ft × 7.5 ft)
Power	1910 watts
Start of mission
Launch date	19 December 2013, 09:12:14 UTC
Rocket	Soyuz ST-B/Fregat-MT
Launch site	Kourou ELS
Contractor	Arianespace
Orbital parameters
Reference system	Sun–Earth L2
Regime	Lissajous orbit
Periapsis	263,000 km (163,000 mi)
Apoapsis	707,000 km (439,000 mi)
Period	180 days
Epoch	2014
Main telescope
Type	Three-mirror anastigmat
Diameter	1.45 m × 0.5 m (4.8 ft × 1.6 ft)
Collecting area	0.7 m2
Transponders
Band	S Band (TT&C support) X Band (data acquisition)
Bandwidth	few kbit/s down & up (S Band) 3-8 Mbit/s download (X Band)
Instruments
ASTRO: Astrometric instrument BP/RP: Photometric instrument RVS: Radial Velocity Spectrometer
ESA astrophysics insignia for Gaia

Gaia is a space observatory of the European Space Agency (ESA) designed for astrometry: measuring the positions and distances of stars with unprecedented precision. The mission aims to construct by far the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars among others.

The spacecraft will monitor each of its target objects about 70 times over a period of five years to study the precise position and motion of each target. The spacecraft has enough consumables to operate for approximately nine years, if its detectors are not degrading as fast as initially expected. The mission could therefore be extended. The Gaia targets represent approximately 1% of the Milky Way population with all stars brighter than magnitude 20 in a broad photometric band that covers most of the visual range. Additionally, Gaia is expected to detect thousands to tens of thousands of Jupiter-sized exoplanets beyond the Solar System, 500,000 quasars and tens of thousands of new asteroids and comets within the Solar System.

Gaia will create a precise three-dimensional map of astronomical objects throughout the Milky Way and map their motions, which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements will provide the detailed physical properties of all stars observed, characterizing their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to analyze a wide range of important questions related to the origin, structure, and evolutionary history of our galaxy.

Successor to the Hipparcos mission, the telescope is part of ESA's Horizon 2000+ long-term scientific program. Gaia was launched on 19 December 2013 by Arianespace using a Soyuz ST-B/Fregat-MT rocket flying from Kourou in French Guiana. The spacecraft currently operates in a Lissajous orbit around the Sun–Earth L₂ Lagrangian point.

History

The Gaia space telescope has its roots in ESA's Hipparcos mission (1989–1993). Its mission was proposed in October 1993 by Lennart Lindegren (Lund University, Sweden) and Michael Perryman (ESA) in response to a call for proposals for ESA's Horizon Plus long-term scientific programme. It was adopted by ESA's Science Programme Committee as cornerstone mission number 6 on 13 October 2000, and the B2 phase of the project was authorised on 9 February 2006, with EADS Astrium taking responsibility for the hardware. The name "Gaia" was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics. This reflected the optical technique of interferometry that was originally planned for use on the spacecraft. While the working method evolved during studies and the acronym is no longer applicable, the name Gaia remained to provide continuity with the project.

The total cost of the mission is around €740 million (~ $1 billion), including the manufacture, launch and ground operations. Gaia was completed two years behind schedule and 16% above its initial budget, mostly due to the difficulties encountered in polishing Gaia's ten mirrors and assembling and testing the focal plane camera system.

Objectives

The Gaia space mission has the following objectives:

• To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the few ways to achieve this without physical assumptions is through the star's parallax. Ground-based observations would not measure such parallaxes with sufficient precision due to the effects of the atmosphere and instrumental biases. For instance, Cepheid variables are used as standard candles to measure distances to galaxies, but the accuracy in their own distance measurement is poor. Thus, quantities depending on them, such as the speed of expansion of the universe, remain inaccurate. Measuring their distances accurately has a great impact on the understanding of the other galaxies and thus the whole cosmos.
• Observations of the faintest objects will provide a more complete view of the stellar luminosity function. Gaia will observe 1 billion stars and other bodies, representing 1% of such bodies in the Milky Way galaxy. All objects up to a certain magnitude must be measured in order to have unbiased samples.
• To permit a better understanding of the more rapid stages of stellar evolution (such as the classification, frequency, correlations and directly observed attributes of rare fundamental changes and of cyclical changes). This has to be achieved by detailed examination and re-examination of a great number of objects over a long period of operation. Observing a large number of objects in the galaxy is also important to understand the dynamics of our galaxy.
• Measuring the astrometric and kinematic properties of a star is necessary in order to understand the various stellar populations, especially the most distant.

In order to achieve these objectives, Gaia has these goals:

• Determine the position, parallax, and annual proper motion of 1 billion stars with an accuracy of about 20 microarcseconds (µas) at 15 mag, and 200 µas at 20 mag.
• Determine the positions of stars at a magnitude of V = 10 down to a precision of 7 μas—this is equivalent to measuring the position to within the diameter of a hair from 1000 km away—between 12 and 25 μas down to V = 15, and between 100 and 300 μas to V = 20, depending on the colour of the star.
• The distance to about 20 million stars will thus be measured with a precision of 1% or better, and about 200 million distances will be measured to better than 10%. Distances accurate to 10% will be achieved as far away as the Galactic Center, 30,000 light-years away.
• Measure the tangential speed of 40 million stars to a precision of better than 0.5 km/s.
• Derive the atmospheric parameters (effective temperature, line-of-sight interstellar extinction, surface gravity, metallicity) for all stars observed, plus some more detailed chemical abundances for targets brighter than V = 15.
• Measure the orbits and inclinations of a thousand extrasolar planets accurately, determining their true mass using astrometric planet detection methods.
• More precisely measure the bending of starlight by the Sun's gravitational field, predicted by Albert Einstein’s General Theory of Relativity and first detected by Arthur Eddington during a 1919 solar eclipse, and therefore directly observe the structure of spacetime.
• Potential to discover Apohele asteroids with orbits that lie between Earth and the Sun, a region that is difficult for Earth-based telescopes to monitor since this region is only visible in the sky during or near the daytime.
• Detect up to 500,000 quasars.

Spacecraft

Gaia was launched by Arianespace, using a Soyuz ST-B rocket with a Fregat-MT upper stage, from the Ensemble de Lancement Soyouz at Kourou in French Guiana on 19 December 2013 at 09:12 UTC (06:12 local time). The satellite separated from the rocket's upper stage 43 minutes after launch at 09:54 UTC. The craft headed towards the Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth, arriving there 8 January 2014. The L2 point provides the spacecraft with a very stable gravitational and thermal environment. There it uses a Lissajous orbit that avoids blockage of the Sun by the Earth, which would limit the amount of solar energy the satellite could produce through its solar panels, as well as disturb the spacecraft's thermal equilibrium. After launch, a 10-metre-diameter sunshade was deployed. The sunshade always faces the Sun, thus keeping all telescope components cool and powering Gaia using solar panels on its surface.

Scientific instruments

The Gaia payload consists of three main instruments:

1. The astrometry instrument (Astro) precisely determines the positions of stars of magnitude 5.7 to 20 by measuring their angular position. By combining the measurements of any given star over the five-year mission, it will be possible to determine its parallax, and therefore its distance, and its proper motion —the velocity of the star projected on the plane of the sky.
2. The photometric instrument (BP/RP) allows the acquisition of luminosity measurements of stars over the 320–1000 nm spectral band, over the same magnitude 5.7–20. The blue and red photometers (BP/RP) are used to determine stellar properties such as temperature, mass, age and elemental composition. Multi-colour photometry is provided by two low-resolution fused-silica prisms dispersing all the light entering the field of view in the along-scan direction prior to detection. The Blue Photometer (BP) operates in the wavelength range 330–680 nm; the Red Photometer (RP) covers the wavelength range 640–1050 nm.
3. The Radial-Velocity Spectrometer (RVS) is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). The measurements of radial velocities are important to correct for perspective acceleration which is induced by the motion along the line of sight." The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum.

In order to maintain the fine pointing to focus on stars many light years away, there are almost no moving parts. The spacecraft subsystems are mounted on a rigid silicon carbide frame, which provides a stable structure that will not expand or contract due to heat. Attitude control is provided by small cold gas thrusters that can output 1.5 micrograms of nitrogen per second.

The telemetric link with the satellite is about 3 Mbit/s on average, while the total content of the focal plane represents several Gbit/s. Therefore, only a few dozen pixels around each object can be downlinked.

Diagram of Gaia

Mirrors (M)

• Mirrors of telescope 1 (M1, M2 and M3)
• Mirrors of telescope 2 (M'1, M'2 and M'3)
• mirrors M4, M'4, M5, M6 are not shown

Other components (1–9)

1. Optical bench (silicon carbide torus)
2. Focal plane cooling radiator
3. Focal plane electronics
4. Nitrogen tanks
5. Diffraction grating spectroscope
6. Liquid propellant tanks
7. Star trackers
8. Telecommunication panel and batteries
9. Main propulsion subsystem

(A) Light path of telescope 1

Design of the focal plane and instruments

The design of the Gaia focal plane and instruments. Due to the spacecraft's rotation, images cross the focal plane array right-to-left at 60 arc seconds per second.

1. Incoming light from mirror M3
2. Incoming light from mirror M'3
3. Focal plane, containing the detector for the Astrometric instrument in light blue, Blue Photometer in dark blue, Red Photometer in red, and Radial Velocity Spectrometer in pink.
4. Mirrors M4 and M'4, which combine the two incoming beams of light
5. Mirror M5
6. Mirror M6, which illuminates the focal plane
7. Optics and diffraction grating for the Radial Velocity Spectrometer (RVS)
8. Prisms for the Blue Photometer and Red Photometer (BP and RP)

Measurement principles

Comparison of nominal sizes of apertures of the Gaia (spacecraft) and some notable optical telescopes

Despite its name, Gaia does not actually use interferometry to determine the positions of stars. At the time of the original design, interferometry seemed the best way to achieve the target resolution, but the design later evolved into an imaging telescope. Similar to its predecessor Hipparcos, but with a precision one hundred times better, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle of 106.5° between them. The spacecraft rotates continuously around an axis perpendicular to the two telescopes' lines of sight. The spin axis in turn has a slight precession across the sky, while maintaining the same angle to the Sun. By precisely measuring the relative positions of objects from both observing directions, a rigid system of reference is obtained.

The two key telescope properties are:

• 1.45 × 0.5 m primary mirror for each telescope
• 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a total of 937.8 megapixels (commonly depicted as a gigapixel-class imaging device).

Scanning method

Each celestial object will be observed on average about 70 times during the mission, which is expected to last five years. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time (motion) and lastly, the star's parallax from which distance can be calculated. The radial velocity of the brighter stars is measured by an integrated spectrometer observing the Doppler effect. Because of the physical constraints imposed by the Soyuz spacecraft, Gaia's focal arrays could not be equipped with optimal radiation shielding, and ESA expects their performance to suffer somewhat toward the end of the five-year mission. Ground tests of the CCDs while they were subjected to radiation provided reassurance that the primary mission's objectives can be met.

The expected accuracies of the final catalogue data have been calculated following in-orbit testing, taking into account the issues of stray light, degradation of the optics, and the basic angle instability. The best accuracies for parallax, position, and proper motion are obtained for the brighter observed stars, apparent magnitudes 3-12. The standard deviation for these stars is expected to be 6.7 micro-arc seconds or better. For fainter stars, error levels increase, reaching 26.6 micro-arc seconds error in the parallax for 15th magnitude stars, and several hundred micro-arc seconds for 20th magnitude stars. For comparison, the best parallax error levels from the new Hipparcos reduction are no better than 100 micro-arc seconds, with typical levels several times larger.

Data processing

VST snaps Gaia en route to a billion stars

 

The overall data volume that will be retrieved from the spacecraft during the nominal five-year mission at a compressed data rate of 1 Mbit/s is approximately 60 TB, amounting to about 200 TB of usable uncompressed data on the ground, stored in an InterSystems Caché database. The responsibility of the data processing, partly funded by ESA, is entrusted to a European consortium, the Data Processing and Analysis Consortium (DPAC), which was selected after its proposal to the ESA Announcement of Opportunity released in November 2006. DPAC's funding is provided by the participating countries and has been secured until the production of Gaia's final catalogue scheduled for 2020.

Gaia sends back data for about eight hours every day at about 5 Mbit/s. ESA's three 35-metre-diameter radio dishes of the ESTRACK network in Cebreros, Spain, Malargüe, Argentina and New Norcia, Australia, receive the data.

Launch and orbit

Animation of Gaia trajectory
  Gaia ·   Earth
Top: Polar view; Bottom: Euqtorial view

 

Simplified illustration of Gaia's trajectory and orbit (not to scale)

In October 2013 ESA had to postpone Gaia's original launch date, due to a precautionary replacement of two of Gaia's transponders. These are used to generate timing signals for the downlink of science data. A problem with an identical transponder on a satellite already in orbit motivated their replacement and reverification once incorporated into Gaia. The rescheduled launch window was from 17 December 2013 to 5 January 2014, with Gaia slated for launch on 19 December.

Gaia was successfully launched on 19 December 2013 at 09:12 UTC. About three weeks after launch, on 8 January 2014, it reached its designated orbit around the Sun-Earth L2 Lagrange point (SEL2), about 1.5 million kilometers from Earth.

In 2015 the Pan-STARRS observatory discovered an object orbiting the Earth, which the Minor Planet Center catalogued as object 2015 HP₁₁₆. It was soon found to be an accidental rediscovery of the Gaia spacecraft and the designation was promptly retracted.

Stray light problem

Shortly after launch, ESA revealed that Gaia was suffering from a stray light problem. The problem was initially thought to be due to ice deposits causing some of the light diffracted around the edges of the sunshield and entering the telescope apertures to be reflected towards the focal plane. The actual source of the stray light was later identified as the fibers of the sunshield, protruding beyond the edges of the shield. This results in a "degradation in science performance [which] will be relatively modest and mostly restricted to the faintest of Gaia's one billion stars." Mitigation schemes are being implemented to optimise the mission performance. The degradation is more severe for the RVS spectrograph than for the astrometry measurements. It only affects the fainter stars.

Mission progress

A map of the sky by star density.

The testing and calibration phase, which started while Gaia was en route to SEL2 point, continued until the end of July 2014, three months behind schedule due to unforeseen issues with stray light entering the detector. After the six-month commissioning period, the satellite started its nominal five-year period of scientific operations on 25 July 2014 using a special scanning mode that intensively scanned the region near the ecliptic poles; on 21 August 2014 Gaia began using its normal scanning mode which provides more uniform coverage.

Although it was originally planned to limit Gaia's observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen µas".

On 12 September 2014, Gaia discovered its first supernova in another galaxy. On 3 July 2015, a map of the Milky Way by star density was released, based on data from the spacecraft. As of August 2016, "more than 50 billion focal plane transits, 110 billion photometric observations and 9.4 billion spectroscopic observations have been successfully processed."

Data releases

The Gaia catalogue is released in stages that will contain increasing amounts of information; the early releases will also miss some stars, especially fainter stars located in dense star fields. The first data release, Gaia DR1, based on 14 months of observations made through September 2015, took place on 14 September 2016 and is described in a series of articles published in Astronomy and Astrophysics. The data release includes "positions and … magnitudes for 1.1 billion stars using only Gaia data; positions, parallaxes and proper motions for more than 2 million stars" based on a combination of Gaia and Tycho-2 data for those objects in both catalogues; "light curves and characteristics for about 3000 variable stars; and positions and magnitudes for more than 2000 … extragalactic sources used to define the celestial reference frame". Data from this DR1 release can be accessed at the Gaia archive, as well as through astronomical data centers such as CDS.

Stars and Other Objects in the Second Data Release.

The second data release (DR2), which occurred on 25 April 2018, is based on 22 months of observations made between 25 July 2014 and 23 May 2016. It includes positions, parallaxes and proper motions for about 1.3 billion stars and positions of an additional 300 million stars, red and blue photometric data for about 1.1 billion stars and single colour photometry for an additional 400 million stars, and median radial velocities for about 7 million stars between magnitude 4 and 13. It also contains data for over 14,000 selected Solar System objects. The coordinates in DR2 use the Gaia celestial reference frame (Gaia–CRF2), which is based on observations of 492,006 sources believed to be quasars, and which has been tied to the International Celestial Reference System. Comparison of Gaia–CRF2 with a preliminary version of the forthcoming ICRF3 shows a global agreement of 20 to 30 μas, although individual sources may differ by several mas. Since the data processing procedure links individual Gaia observations with particular sources on the sky, in some cases the association of observations with sources will be different in the second data release. Consequently, DR2 uses different source identification numbers than DR1.

The third data release potentially will include orbital solutions for many binary stars and classifications for spectroscopically "well behaved" objects, as well as improved positions, parallaxes and proper motions. The fourth data release potentially will include variable star classifications, complete Solar System results, and non single-star catalogues. The complete final Gaia catalogue is currently scheduled for 2022, three years after the end of the nominal five-year mission. It would be pushed back if the mission is extended to nine years. The number of releases between DR2 and the final release has not yet been decided.

An outreach application, Gaia Sky, has been developed to explore the galaxy in three dimensions using Gaia data.

Significant science

In November 2017, scientists led by Davide Massari of the Kapteyn Astronomical Institute, University of Groningen, Netherlands released a paper describing the characterization of proper motion (3D) within the Sculptor dwarf galaxy, and of that galaxy’s trajectory through space and with respect to the Milky Way, using data from Gaia and the Hubble Space Telescope. Massari said, “With the precision achieved we can measure the yearly motion of a star on the sky which corresponds to less than the size of a pinhead on the Moon as seen from Earth.” The data showed that Sculptor orbits the Milky Way in a highly elliptical orbit; it is currently near its closest approach at a distance of about 83.4 kiloparsecs (272,000 ly), but the orbit can take it out to around 222 kiloparsecs (720,000 ly) distant.

In October 2018, Leiden University astronomers were able to determine the orbits of 20 hypervelocity stars from the DR2 dataset. Expecting to find a single star exiting the Milky Way, they instead found seven. More surprisingly, the team found that 13 hypervelocity stars were instead approaching the Milky Way, possibly originating from as-of-yet unknown extragalactic sources. Alternatively, they could be halo stars to this galaxy, and further spectroscopic studies will help determine which scenario is more likely.

Stellar kinematics

From Wikipedia, the free encyclopedia

In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. The subject of stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the measurement of the internal kinematics of more distant galaxies. Measurement of the kinematics of stars in different subcomponents of the Milky Way including the thin disk, the thick disk, the bulge, and the stellar halo provides important information about the formation and evolutionary history of our Galaxy. Kinematic measurements can also identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, which are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center. 

Stellar kinematics is related to but distinct from the subject of stellar dynamics, which involves the theoretical study or modeling of the motions of stars under the influence of gravity. Stellar-dynamical models of systems such as galaxies or star clusters are often compared with or tested against stellar-kinematic data to study their evolutionary history and mass distributions, and to detect the presence of dark matter or supermassive black holes through their gravitational influence on stellar orbits.

Space velocity

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = v_t / d with v_t = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity v_t.)

The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed. This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion. The Sun's motion with respect to the LSR is called the "peculiar solar motion".

The components of space velocity in the Milky Way's Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic Center, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is

(U, V, W) = (11.1, 12.24, 7.25) km/s,

with statistical uncertainty (+0.69−0.75, +0.47−0.47, +0.37−0.36) km/s and systematic uncertainty (1, 2, 0.5) km/s. (Note that V is 7 km/s larger than estimated in 1999 by Dehnen et al.)

Information Obtained from Stellar Kinematic Measurements

Stellar kinematics yields important astrophysical information about stars, and the galaxies in which they reside. Stellar kinematics data combined with astrophysical modeling produces important information about the galactic system as a whole. Measured stellar velocities in the innermost regions of galaxies including the Milky Way have provided evidence that many galaxies host supermassive black holes at their center. In farther out regions of galaxies such as within the galactic halo, velocity measurements of globular clusters orbiting in these halo regions of galaxies provides evidence for dark matter. Both of these cases derive from the key fact that stellar kinematics can be related to the overall potential in which the stars are bound. This means that if accurate stellar kinematics measurements are made for a star or group of stars orbiting in a certain region of a galaxy, the gravitational potential and mass distribution can be inferred given that the gravitational potential in which the star is bound produces its orbit and serves as the impetus for its stellar motion. Examples of using stellar kinematics measurements combined with modeling to construct an astrophysical system are:

• Rotation of the Milky Way's Disc From the proper motions and radial velocities of stars within the Milky way disc one can show that there is differential rotation. When combining these measurements of stars' proper motions and their radial velocities, along with careful modeling, it is possible to obtain a picture of the rotation of the Milky Way disc. The local character of galactic rotation in the solar neighborhood is encapsulated in the Oort constants.
• Structural Components of The Milky Way Using stellar kinematics, astronomers construct models which seek to explain the overall galactic structure in terms of distinct kinematic populations of stars. This is possible because these distinct populations are often located in specific regions of galaxies. For example, within the Milky Way, there are three primary components, each with its own distinct stellar kinematics: the disc, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the Milky Way, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. For the Milky Way, the speed of disk stars is ${\displaystyle \mathrm {V} =220~\mathrm {km} ~\mathrm {s} ^{-1}}$ and an RMS velocity relative to this speed of ${\displaystyle \mathrm {V_{RMS}} =50~\mathrm {km} ~\mathrm {s} ^{-1}}$. For bulge population stars, the velocities are randomly oriented with a larger relative RMS velocity of ${\displaystyle \mathrm {V_{RMS}} =150~\mathrm {km} ~\mathrm {s} ^{-1}}$ and no net circular velocity. The Galactic stellar halo consists of stars with orbits that extend to the outer regions of the galaxy. Some of these stars will continually orbit far from the galactic center, while others are on trajectories which bring them to various distances from the galactic center. These stars have little to no average rotation. Many stars in this group belong to globular clusters which formed long ago and thus have a distinct formation history, which can be inferred from their kinematics and poor metallicities. The halo may be further subdivided into an inner and outer halo, with the inner halo having a net prograde motion with respect to the Milky Way and the outer a net retrograde motion.
• External Galaxies Spectroscopic observations of external galaxies make it possible to characterize the bulk motions of the stars they contain. While these stellar populations in external galaxies are generally not resolved to the level where one can track the motion of individual stars (except for the very nearest galaxies) measurements of the kinematics of the integrated stellar population along the line of sight provides information including the mean velocity and the velocity dispersion which can then be used to infer the distribution of mass within the galaxy. Measurement of the mean velocity as a function of position gives information on the galaxy's rotation, with distinct regions of the galaxy that are redshifted / blueshifted in relation to the galaxy's systemic velocity.
• Mass distributions Through measurement of the kinematics of tracer objects such as globular clusters and the orbits of nearby satellite dwarf galaxies, we can determine the mass distribution of the Milky Way or other galaxies. This is accomplished by combining kinematic measurements with dynamical modeling.

Recent advancements due to Gaia

In 2018 the Gaia data release 2 has yielded an unprecedented number of high quality stellar kinematic measurements as well as stellar parallax measurements which will greatly increase our understanding of the structure of the Milky Way. The Gaia data has also made it possible to determine the proper motions of many objects whose proper motions were previously unknown, including the absolute proper motions of 75 globular clusters orbiting at distances as far as 21 kpc. In addition, the absolute proper motions of nearby dwarf spheroidal galaxies have also been measured, providing multiple tracers of mass for the Milky Way. This increase in accurate measurement of absolute proper motion at such large distances is a major improvement over past surveys, such as those conducted with the Hubble Space Telescope.

Stellar kinematic types

Stars within galaxies may be classified based on their kinematics. For example, the stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I stars with higher metallicity are generally located in the stellar disk while older population II stars are in random orbits with little net rotation. The latter have elliptical orbits that are inclined to the plane of the Milky Way. Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.

There are many additional ways to classify stars based on their measured velocity components, and this provides detailed information about the nature of the star's formation time, its present location, and the general structure of the galaxy. As a star moves in a galaxy, the smoothed out gravitational potential of all the other stars and other mass within the galaxy plays a dominant role in determining the stellar motion. Stellar kinematics can provide insights into the location of where the star formed within the galaxy. Measurements of an individual star's kinematics can identify stars that are peculiar outliers such as a high-velocity star moving much faster than its nearby neighbors.

High-velocity stars

Depending on the definition, a high-velocity star is a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighborhood. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars. High-velocity stars were studied by Jan Oort, who used their kinematic data to predict that high-velocity stars have very little tangential velocity.

Runaway stars

Four runaway stars plowing through regions of dense interstellar gas and creating bright bow waves and trailing tails of glowing gas. The stars in these NASA Hubble Space Telescope images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006

A runaway star is one that is moving through space with an abnormally high velocity relative to the surrounding interstellar medium. The proper motion of a runaway star often points exactly away from a stellar association, of which the star was formerly a member, before it was hurled out.

Mechanisms that may give rise to a runaway star include:

• Gravitational interactions between stars in a stellar system can result in large accelerations of one or more of the involved stars. In some cases, stars may even be ejected. This can occur in seemingly stable star systems of only three stars, as described in studies of the three-body problem in gravitational theory.
• A collision or close encounter between stellar systems, including galaxies, may result in the disruption of both systems, with some of the stars being accelerated to high velocities, or even ejected. A large-scale example is the gravitational interaction between the Milky Way Galaxy and the Large Magellanic Cloud.
• A supernova explosion in a multiple star system can accelerate both the supernova remnant and/or remaining stars to high velocities.

Multiple mechanisms may accelerate the same runaway star. For example, a massive star that was originally ejected due to gravitational interactions with its stellar neighbors may itself go supernova, producing a remnant with a velocity modulated by the supernova kick. If this supernova occurs in the very nearby vicinity of other stars, it is possible that it may produce more runaways in the process.

An example of a related set of runaway stars is the case of AE Aurigae, 53 Arietis and Mu Columbae, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the Sun moves through the Milky Way at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the Orion Nebula about 2 million years ago. Barnard's Loop is believed to be the remnant of the supernova that launched the other stars.

Another example is the X-ray object Vela X-1, where photodigital techniques reveal the presence of a typical supersonic bow shock hyperbola.

Halo stars

Halo stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the center of the Milky Way. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Milky Way. Although their orbital velocities in the Milky Way may be no faster than the Sun's, their different paths result in the high relative velocities.
Typical examples are the halo stars passing through the disk of the Milky Way at steep angles. One of the nearest 45 stars, called Kapteyn's Star, is an example of the high-velocity stars that lie near the Sun. Its observed radial velocity is -245 km/s, and the components of its space velocity are U = 19 km/s, V = -288 km/s, and W = -52 km/s.

Hypervelocity stars

Hypervelocity stars (designated as HVS or HV in stellar catalogues) are stars with velocities that are substantially different from that expected for a star belonging to the normal distribution of stars in a galaxy. Such stars may have velocities so great that they exceed the escape velocity of the galaxy. Ordinary stars in the Milky Way have velocities on the order of 100 km/s, whereas hypervelocity stars (especially those near the center of the Milky Way, which is where most are thought to be produced), have velocities on the order of 1000 km/s. One of the fastest known stars discovered in our Galaxy is the O-class sub-dwarf US 708, moving away from the Milky Way with a total velocity of around 1200 km/s.

The existence of HVSs was first predicted by Jack G. Hills in 1988, and their existence confirmed by Warren Brown, Margaret Geller, Scott Kenyon, and Michael Kurtz in 2005. As of 2008, 10 unbound HVSs were known, one of which was believed to have originated from the Large Magellanic Cloud rather than the Milky Way. Further measurements placed its origin within the Milky Way. Due to uncertainty about the mass distribution within the Milky Way, determining whether a HVS is unbound is difficult; five additional known high-velocity stars may be unbound from the Milky Way and 16 HVSs are thought to be bound. The nearest currently known HVS (HVS2) is about 19 kpc from the Sun.

As of 1 September 2017, there have been roughly 20 observed hypervelocity stars. As most of these were observed in the Northern Hemisphere, the possibility remains that there are HVSs only observable from the Southern Hemisphere.

It is believed that about 1,000 HVSs exist in the Milky Way. Considering that there are around 100 billion stars in the Milky Way, this is a minuscule fraction (~0.000001%). Since the second data release of Gaia (DR2), most high-velocity late-type stars are found to have a high probability of being bound to the Milky Way.

Origin of hypervelocity stars

HVSs are believed to originate by close encounters of binary stars with the supermassive black hole in the center of the Milky Way. One of the two partners is captured by the black hole, whereas the other escapes with high velocity. Also, "captured" does not necessarily mean "swallowed", as the companion to the HVS may enter an orbit around the black hole. However, this can only happen if the binary stars are falling nearly directly toward the black hole from extremely far, otherwise the speed gain would not be sufficiently high.

Supernova-induced HVSs may also be possible, although they are presumably rare. In this scenario, a HVS is ejected from a close binary system as a result of the companion star undergoing a supernova explosion. Ejection velocities up to 770 km/s, as measured from the galactic rest frame, are possible for late-type B-stars. This mechanism can explain the origin of HVSs which are ejected from the galactic disk.

Known HVSs are main-sequence stars with masses a few times that of the Sun. HVSs with smaller masses are also expected and G/K-dwarf HVS candidates have been found.

HVSs that have come into the Milky Way came from the dwarf galaxy Large Magellanic Cloud. When the dwarf galaxy made its closest approach to the center of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some of its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space, due to the slingshot-like effect of the boost.

Some neutron stars are inferred to be traveling with similar speeds. This could be related to HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova explosions, and their extreme speeds are very likely the result of an asymmetric supernova explosion or the loss of their near partner during the supernova explosions that forms them. The neutron star RX J0822-4300, which was measured to move at a record speed of over 1,500 km/s (0.5% of the speed of light) in 2007 by the Chandra X-ray Observatory, is thought to have been produced the first way.

Some kind of supernovas are expected to happen if a white dwarf collides with its nearby partner and consumes the outer matter of this partner. The white dwarf and its nearby partner have very high orbital velocities at this time. The huge mass lost of the white dwarf during the supernova causes the nearby partner to leave at its previous huge orbital speed of several hundred kilometers per second as a hypervelocity star. The supernova remnant of the exploding white dwarf leaves because of its own high orbital speed as a new fast-traveling neutron star. This seems to be the most likely origin of most HVSs and fast-traveling neutron stars.

Partial list of HVSs

As of 2014, twenty HVS were known.

• HVS 1 – (SDSS J090744.99+024506.8) (a.k.a. The Outcast Star) – the first hypervelocity star to be discovered
• HVS 2 – (SDSS J093320.86+441705.4 or US 708)
• HVS 3 – (HE 0437-5439) – possibly from the Large Magellanic Cloud
• HVS 4 – (SDSS J091301.00+305120.0)
• HVS 5 – (SDSS J091759.42+672238.7)
• HVS 6 – (SDSS J110557.45+093439.5)
• HVS 7 – (SDSS J113312.12+010824.9)
• HVS 8 – (SDSS J094214.04+200322.1)
• HVS 9 – (SDSS J102137.08-005234.8)
• HVS 10 – (SDSS J120337.85+180250.4)
• TYC 8840-1782-1

Kinematic groups

A set of stars with similar space motion and ages is known as a kinematic group. These are stars that could share a common origin, such as the evaporation of an open cluster, the remains of a star forming region, or collections of overlapping star formation bursts at differing time periods in adjacent regions. Most stars are born within molecular clouds known as stellar nurseries. The stars formed within such a cloud compose gravitationally bound open clusters containing dozens to thousands of members with similar ages and compositions. These clusters dissociate with time. Groups of young stars that escape a cluster, or are no longer bound to each other, form stellar associations. As these stars age and disperse, their association is no longer readily apparent and they become moving groups of stars.

Astronomers are able to determine if stars are members of a kinematic group because they share the same age, metallicity, and kinematics (radial velocity and proper motion). As the stars in a moving group formed in proximity and at nearly the same time from the same gas cloud, although later disrupted by tidal forces, they share similar characteristics.

Stellar associations

A stellar association is a very loose star cluster, whose stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.

Stellar associations were first discovered by the Armenian astronomer Viktor Ambartsumian in 1947. The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.

Types

Infrared ESO's VISTA view of a stellar nursery in Monoceros.

Viktor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars. A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae. The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

OB associations

Carina OB1, a large OB association.

Young associations will contain 10 to 100 massive stars of spectral class O and B, and are known as OB associations. In addition, these associations also contain hundreds or thousands of low- and intermediate-mass stars. Association members are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in the Milky Way were formed in OB associations. O-class stars are short-lived, and will expire as supernovae after roughly one million years. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within ten million years. (Compare this to the current age of the Sun at about five billion years.)

The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun. The nearest OB association is the Scorpius–Centaurus Association, located about 400 light-years from the Sun.

OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light-years in diameter.

T associations

Young stellar groups can contain a number of infant T Tauri stars that are still in the process of entering the main sequence. These sparse populations of up to a thousand T Tauri stars are known as T associations. The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun. Other examples of T associations include the R Corona Australis T association, the Lupus T association, the Chamaeleon T association and the Velorum T association. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. Group members have the same age and origin, the same chemical composition, and the same amplitude and direction in their vector of velocity.

R associations

Associations of stars that illuminate reflection nebulae are called R associations, a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms. An example of an R association is Monoceros R2, located 830 ± 50 parsecs from the Sun.

Moving groups

If the remnants of a stellar association drift through the Milky Way as a somewhat coherent assemblage, then they are termed a moving group or kinematic group. Moving groups can be old, such as the HR 1614 moving group at two billion years, or young, such as the AB Dor Moving Group at only 120 million years.

Moving groups were studied intensely by Olin Eggen in the 1960s. A list of the nearest young moving groups has been compiled by López-Santiago et al. The closest is the Ursa Major Moving Group which includes all of the stars in the Plough/Big Dipper asterism except for α Ursae Majoris and η Ursae Majoris. This is sufficiently close that the Sun lies in its outer fringes, without being part of the group. Hence, although members are concentrated at declinations near 60° N, some outliers are as far away across the sky as Triangulum Australe at 70° S.

Stellar streams

A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces.

Known kinematic groups

Some nearby kinematic groups include:

• Local Association (Pleiades moving group)
• AB Doradus moving group
• Alpha Persei moving cluster
• Beta Pictoris moving group
• Castor moving group
• Corona Australis association
• Eta Chamaeleontis cluster
• Hercules stream
• Hyades Stream
• IC 2391 supercluster (Argus Association)
• Kapteyn group
• MBM 12 association
• Tucana-Horologium Association
• TW Hydrae association
• Ursa Major Moving Group
• Wolf 630 moving group
• Zeta Herculis moving group