A Medley of Potpourri

Sunday, June 9, 2024

Spiral galaxy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Spiral_galaxy

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.

The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope.

Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.

Structure

Tuning-fork-style diagram of the Hubble sequence

Spiral galaxies may consist of several distinct components:

A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components
A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
A bar-shaped distribution of stars
A near-spherical halo of stars, including many in globular clusters
A supermassive black hole at the very middle of the central bulge
A near-spherical dark matter halo

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms

Spiral arms are regions of stars that extend from the center of barred and unbarred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

Bulge

A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106.

Bar

Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies. Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures which typically have a maximum visibility at half the length of the in-plane bar.

Spheroid

The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core.

However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion.

Oldest spiral galaxies

The oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.

A1689B11 is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation. A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang.

BRI 1335-0417 is the most distant known spiral galaxy, as of 2021. The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth.

In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.

Origin of the spiral structure

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:

star formation caused by density waves in the galactic disk of the galaxy.
the stochastic self-propagating star formation model (SSPSF model) – star formation caused by shock waves in the interstellar medium. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.

These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.

Density wave model

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.

Historical theory of Lin and Shu

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.

Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:

As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.

More young stars in spiral arms

Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly, their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.

Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms. As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.

Gravitationally aligned orbits

Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained.

Distribution of stars in spirals

The stars in spirals are distributed in thin disks radial with intensity profiles such that

I (R) = I_{0} e^{- R / h}

with $h$ being the disk scale-length; $I_{0}$ is the central value; it is useful to define: $R_{o p t} = 3.2 h$ as the size of the stellar disk, whose luminosity is

$L_{t o t} = 2 π I_{0} h^{2}$ .

The spiral galaxies light profiles, in terms of the coordinate $R / h$ , do not depend on galaxy luminosity.

Spiral nebula

Drawing of the Whirlpool Galaxy by Rosse in 1845

Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae, due to Lord Rosse, whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the "Whirlpool Galaxy", and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern in Messier 99 and Messier 33 respectively. In 1850 he made the first drawing of Andromeda Galaxy's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula.

The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory. Beginning in 1923, Edwin Hubble observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use.

Milky Way

The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s. Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, which showed that the Milky Way's central bar is larger than what was previously suspected.

Famous examples

Andromeda Galaxy – Barred spiral galaxy in the Local Group
Milky Way – Galaxy containing the Solar System
Pinwheel Galaxy – Galaxy in the constellation Ursa Major
Sunflower Galaxy – Spiral galaxy in the constellation Canes Venatici
Triangulum Galaxy – Spiral galaxy in the constellation Triangulum
Whirlpool Galaxy – Galaxy in the constellation Canes Venatici
Black Eye Galaxy – Spiral galaxy in the constellation Coma Berenices
Malin 1 – Spiral galaxy in the constellation Coma Berenices

Hubble sequence

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Hubble_sequence

The Hubble sequence is a morphological classification scheme for galaxies published by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork. It was invented by John Henry Reynolds and Sir James Jeans.

The tuning fork scheme divided regular galaxies into three broad classes – ellipticals, lenticulars and spirals – based on their visual appearance (originally on photographic plates). A fourth class contains galaxies with an irregular appearance. The Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.

Classes of galaxies

Ellipticals

On the left (in the sense that the sequence is usually drawn) lie the ellipticals. Elliptical galaxies have relatively smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer $n$ representing their degree of ellipticity in the sky. By convention, $n$ is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as   $e = 1 - b / a$   for an ellipse with $a$ the semi-major axis length and $b$ the semi-minor axis length. The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or highly elliptical if viewed edge-on). Observationally, the most flattened "elliptical" galaxies have ellipticities   $e = 0.7$   (denoted E7). However, from studying the light profiles and the ellipticity profiles, rather than just looking at the images, it was realised in the 1960s that the E5–E7 galaxies are probably misclassified lenticular galaxies with large-scale disks seen at various inclinations to our line-of-sight. Observations of the kinematics of early-type galaxies further confirmed this.

Examples of elliptical galaxies: M49, M59, M60, M87, NGC 4125.

Lenticulars

At the centre of the Hubble tuning fork, where the two spiral-galaxy branches and the elliptical branch join, lies an intermediate class of galaxies known as lenticulars and given the symbol S0. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended, disk-like structure. Unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.

When simply looking at a galaxy's image, lenticular galaxies with relatively face-on disks are difficult to distinguish from ellipticals of type E0–E3, making the classification of many such galaxies uncertain. When viewed edge-on, the disk becomes more apparent and prominent dust-lanes are sometimes visible in absorption at optical wavelengths.

At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly flattened "ellipticals" and spirals. Later observations (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage. Missing from the Hubble sequence are the early-type galaxies with intermediate-scale disks, in between the E0 and S0 types, Martha Liller denoted them ES galaxies in 1966.

Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies. The bulge-to-disk flux ratio in lenticular galaxies can take on a range of values, just as it does for each of the spiral galaxy morphological types (Sa, Sb, etc.).

Examples of lenticular galaxies: M85, M86, NGC 1316, NGC 2787, NGC 5866, Centaurus A.

Spirals

The barred spiral galaxy NGC 1300: a type SBbc

On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies. A spiral galaxy consists of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge. Roughly half of all spirals are also observed to have a bar-like structure, with the bar extending from the central bulge, and the arms begin at the ends of the bar. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:

Sa (SBa) – tightly wound, smooth arms; large, bright central bulge
Sb (SBb) – less tightly wound spiral arms than Sa (SBa); somewhat fainter bulge
Sc (SBc) – loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge

Hubble originally described three classes of spiral galaxy. This was extended by Gérard de Vaucouleurs to include a fourth class:

Sd (SBd) – very loosely wound, fragmentary arms; most of the luminosity is in the arms and not the bulge

Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending two lower-case letters to the main galaxy type (for example, Sbc for a galaxy that is intermediate between an Sb and an Sc).

Our own Milky Way is generally classed as Sc or SBc, making it a barred spiral with well-defined arms.

Examples of regular spiral galaxies: (visually) M31 (Andromeda Galaxy), M74, M81, M104 (Sombrero Galaxy), M51a (Whirlpool Galaxy), NGC 300, NGC 772.

Examples of barred spiral galaxies: M91, M95, NGC 1097, NGC 1300, NGC1672, NGC 2536, NGC 2903.

Irregulars

The Large Magellanic Cloud (LMC) – a dwarf irregular galaxy

Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies. Hubble defined two classes of irregular galaxy:

Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters

In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds – two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.

Examples of irregular galaxies: M82, NGC 1427A, Large Magellanic Cloud, Small Magellanic Cloud.

Physical significance

Elliptical and lenticular galaxies are commonly referred to together as "early-type" galaxies, while spirals and irregular galaxies are referred to as "late types". This nomenclature is the source of the common, but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals. In fact, Hubble was clear from the beginning that no such interpretation was implied:

The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution...

The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks; while some lenticular galaxies may have formed this way, others may have accreted their disks around pre-existing spheroids.Some lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation, although the galaxy LEDA 2108986 opens the debate on this.

Shortcomings

A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type). Although not really a shortcoming, since the 1961 Hubble Atlas of Galaxies, the primary criteria used to assign the morphological type (a, b, c, etc.) has been the nature of the spiral arms, rather than the bulge-to-disk flux ratio, and thus a range of flux ratios exist for each morphological type, as with the lenticular galaxies.

Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects. The same galaxy would look very different, if viewed edge-on, as opposed to a face-on or 'broadside' viewpoint. As such, the early-type sequence is poorly represented: The ES galaxies are missing from the Hubble sequence, and the E5–E7 galaxies are actually S0 galaxies. Furthermore, the barred ES and barred S0 galaxies are also absent. Visual classifications are also less reliable for faint or distant galaxies, and the appearance of galaxies can change depending on the wavelength of light in which they are observed.

Nonetheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates.

In June 2019, citizen scientists in the Galaxy Zoo project argued that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported by evidence. Consequently, the scheme may need revision.

Saturday, June 8, 2024

Cosmic Background Explorer

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cosmic_Background_Explorer

Cosmic Background Explorer
Spacecraft properties
Artist's concept of the COBE spacecraft
Names	Explorer 66

Mission type	Cosmic microwave background Astronomy
Operator	NASA
COSPAR ID	1989-089A
SATCAT no.	20322
Website	lambda.gsfc.nasa.gov/product/cobe
Mission duration	6 months (planned) 4 years, 1 month and 4 days (achieved)

Spacecraft	Explorer LXVI
Spacecraft type	Cosmic Background Explorer
Bus	COBE
Manufacturer	Goddard Space Flight Center
Launch mass	2,206 kg (4,863 lb)
Dry mass	1,408 kg (3,104 lb)
Dimensions	5.49 × 2.44 m (18.0 × 8.0 ft)
Power	750 watts

Start of mission
Launch date	18 November 1989, 14:34 UTC
Rocket	Delta 5920-8 (Delta 189)
Launch site	Vandenberg, SLC-2W
Contractor	Douglas Aircraft Company
Entered service	18 November 1989

End of mission
Deactivated	23 December 1993

Orbital parameters
Reference system	Geocentric orbit
Regime	Sun-synchronous orbit
Perigee altitude	900 km (560 mi)
Apogee altitude	900 km (560 mi)
Inclination	99.00°
Period	103.00 minutes
Instruments
Differential Microwave Radiometer (DMR) Diffuse Infrared Background Experiment (DIRBE) Far-InfraRed Absolute Spectrophotometer (FIRAS)
Cosmic Background Explorer mission patch Explorer program ← AMPTE-CCE (Explorer 65) Extreme Ultraviolet Explorer (Explorer 67) →

The Cosmic Background Explorer (COBE /ˈkoʊbi/ KOH-bee), also referred to as Explorer 66, was a NASA satellite dedicated to cosmology, which operated from 1989 to 1993. Its goals were to investigate the cosmic microwave background radiation (CMB or CMBR) of the universe and provide measurements that would help shape our understanding of the cosmos.

COBE's measurements provided two key pieces of evidence that supported the Big Bang theory of the universe: that the CMB has a near-perfect black-body spectrum, and that it has very faint anisotropies. Two of COBE's principal investigators, George F. Smoot and John C. Mather, received the Nobel Prize in Physics in 2006 for their work on the project. According to the Nobel Prize committee, "the COBE project can also be regarded as the starting point for cosmology as a precision science".

COBE was the second cosmic microwave background satellite, following RELIKT-1, and was followed by two more advanced spacecraft: the Wilkinson Microwave Anisotropy Probe (WMAP) operated from 2001 to 2010 and the Planck spacecraft from 2009 to 2013.

Mission

The purpose of the Cosmic Background Explorer (COBE) mission was to take precise measurements of the diffuse radiation between 1 micrometre and 1 cm (0.39 in) over the whole celestial sphere. The following quantities were measured: (1) the spectrum of the 3 K radiation over the range 100 micrometres to 1 cm (0.39 in) (2) the anisotropy of this radiation from 3 to 10 mm (0.39 in); and, (3) the spectrum and angular distribution of diffuse infrared background radiation at wavelengths from 1 to 300 micrometres.

History

In 1974, NASA issued an Announcement of Opportunity for astronomical missions that would use a small- or medium-sized Explorer spacecraft. Out of the 121 proposals received, three dealt with studying the cosmological background radiation. Though these proposals lost out to the Infrared Astronomical Satellite (IRAS), their strength made NASA further explore the idea. In 1976, NASA formed a committee of members from each of 1974's three proposal teams to put together their ideas for such a satellite. A year later, this committee suggested a polar-orbiting satellite called COBE to be launched by either a Delta 5920-8 launch vehicle or the Space Shuttle. It would contain the following instruments:

Instruments
Instrument	Acronym	Description	Principal Investigator
Differential Microwave Radiometer	DMR	Microwave instrument that would map variations (or anisotropies) in the Cosmic microwave background (CMB) radiation	George F. Smoot
Diffuse Infrared Background Experiment	DIRBE	Multiwavelength infrared detector used to map dust emission	Michael G. Hauser
Far-InfraRed Absolute Spectrophotometer	FIRAS	Spectrophotometer used to measure the spectrum of the CMB	John C. Mather

Launch of the COBE spacecraft on 18 November 1989.

NASA accepted the proposal provided that the costs be kept under US$30 million, excluding launcher and data analysis. Due to cost overruns in the Explorer program due to IRAS, work on constructing the satellite at Goddard Space Flight Center (GSFC) did not begin until 1981. To save costs, the infrared detectors and liquid helium dewar on COBE would be similar to those used on Infrared Astronomical Satellite (IRAS).

COBE was originally planned to be launched on a Space Shuttle mission STS-82-B in 1988 from Vandenberg Air Force Base, but the Challenger explosion delayed this plan when the Shuttles were grounded. NASA prevented COBE's engineers from going to other space companies to launch COBE, and eventually a redesigned COBE was placed into Sun-synchronous orbit on 18 November 1989 aboard a Delta launch vehicle.

On 23 April 1992, COBE scientists announced at the APS April Meeting in Washington, D.C. the finding of the "primordial seeds" (CMBE anisotropy) in data from the DMR instrument; until then the other instruments were "unable to see the template." The following day The New York Times ran the story on the front page, explaining the finding as "the first evidence revealing how an initially smooth cosmos evolved into today's panorama of stars, galaxies and gigantic clusters of galaxies."

The Nobel Prize in Physics for 2006 was jointly awarded to John C. Mather, NASA Goddard Space Flight Center, and George F. Smoot, University of California, Berkeley, "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation".

Spacecraft

COBE was an Explorer class satellite, with technology borrowed heavily from IRAS, but with some unique characteristics.

The need to control and measure all the sources of systematic errors required a rigorous and integrated design. COBE would have to operate for a minimum of 6 months, and constrain the amount of radio interference from the ground, COBE and other satellites as well as radiative interference from the Earth, Sun and Moon. The instruments required temperature stability and to maintain gain, and a high level of cleanliness to reduce entry of stray light and thermal emission from particulates.

The need to control systematic error in the measurement of the CMB anisotropy and measuring the zodiacal cloud at different elongation angles for subsequent modeling required that the satellite rotate at a 0.8 rpm spin rate. The spin axis is also tilted back from the orbital velocity vector as a precaution against possible deposits of residual atmospheric gas on the optics as well against the infrared glow that would result from fast neutral particles hitting its surfaces at extremely high speed.

In order to meet the twin demands of slow rotation and three-axis attitude control, a sophisticated pair of yaw angular momentum wheels were employed with their axis oriented along the spin axis . These wheels were used to carry an angular momentum opposite that of the entire spacecraft in order to create a zero net angular momentum system.

The orbit would prove to be determined based on the specifics of the spacecraft's mission. The overriding considerations were the need for full sky coverage, the need to eliminate stray radiation from the instruments and the need to maintain thermal stability of the dewar and the instruments. A circular Sun-synchronous orbit satisfied all these requirements. A 900 km (560 mi) altitude orbit with a 99° inclination was chosen as it fit within the capabilities of either a Space Shuttle (with an auxiliary propulsion on COBE) or a Delta launch vehicle. This altitude was a good compromise between Earth's radiation and the charged particle in Earth's radiation belts at higher altitudes. An ascending node at 18:00 was chosen to allow COBE to follow the boundary between sunlight and darkness on Earth throughout the year.

The orbit combined with the spin axis made it possible to keep the Earth and the Sun continually below the plane of the shield, allowing a full sky scan every six months.

The last two important parts pertaining to the COBE mission were the dewar and Sun-Earth shield. The dewar was a 650 L (140 imp gal; 170 US gal) superfluid helium cryostat designed to keep the FIRAS and DIRBE instruments cooled during the duration of the mission. It was based on the same design as one used on IRAS and was able to vent helium along the spin axis near the communication arrays. The conical Sun-Earth shield protected the instruments from direct solar and Earth based radiation as well as radio interference from Earth and the COBE's transmitting antenna. Its multilayer insulating blankets provided thermal isolation for the dewar.

In January 1994, engineering operations concluded and the operation of the spacecraft was transferred to Wallops Flight Facility (WFF) for use as a test satellite.

Instruments

Differential Microwave Radiometers (DMR)

The Differential Microwave Radiometer (DMR) investigation uses three differential radiometers to map the sky at 31.4, 53, and 90 GHz. The radiometers are distributed around the outer surface of the cryostat. Each radiometer employs a pair of horn antennas viewing at 30° from the spin axis of the spacecraft, measuring the differential temperature between points in the sky separated by 60°. At each frequency there are two channels for dual polarization measurements for improved sensitivity and for reliability. Each radiometer is a microwave receiver whose input is switched rapidly between the two horn antennas, obtaining the difference in brightness of two fields of view 7° in diameter located 60° apart and 30° from the axis of the spacecraft. High sensitivity is achieved by temperature stabilization (at 300 K for 31.4 GHz and at 140 K for 53 and 90 GHz), by spacecraft spin, and by the ability to integrate over the entire year. Sensitivity to large-scale anisotropies is about 3E-5 K. The instrument weighs 120 kg (260 lb), uses 114 watts, and has a data rate of 500 bit/s.

Diffuse Infrared Background Experiment (DIRBE)

The Diffuse Infrared Background Experiment (DIRBE) consists of a cryogenically cooled (to 2 K) multiband radiometer used to investigate diffuse infrared radiation from 1 to 300 micrometres. The instrument measures the absolute flux in 10 wavelength bands with a 1° field of view pointed 30° off the spin axis. Detectors (photoconductors) and filters for the 8 to 100 micrometre channels are the same as for the IRAS mission. Bolometers are used for the longest wavelength channel (120 to 300 micrometres). The telescope is a well baffled, off-axis, Gregorian flux collector with re-imaging. The instrument weighs approximately 34 kg (75 lb), uses 100 W and has a data rate of 1700 bit/s.

Far Infrared Absolute Spectrophotometer (FIRAS)

The Far Infrared Absolute Spectrophotometer (FIRAS) is a cryogenically cooled polarizing Michelson interferometer used as a Fourier transform spectrometer. The instrument points along the spin axis and has a 7° field of view. This device measures the spectrum to a precision of 1/1000 of the peak flux at 1.7 mm (0.067 in) for each 7° field of view on the sky (over the range 0.1 to 10 mm (0.39 in)). The FIRAS uses a special flared trumpet horn flux collector having very low sidelobe levels and an external calibrator covering the entire beam; precise temperature regulation and calibration are required. The instrument has a differential input to compare the sky with an internal reference at 3 K. This feature provides immunity from systematic errors in the spectrometer, and contributes significantly to the ability to detect small deviations from a blackbody spectrum. The instrument weighs 60 kg (130 lb), uses 84 watts and has a data rate of 1200 bit/s.

Scientific findings

The science mission was conducted by the three instruments detailed previously: DIRBE, FIRAS and DMR. The instruments overlapped in wavelength coverage, providing consistency check on measurements in the regions of spectral overlap and assistance in discriminating signals from our galaxy, Solar System and CMB.

COBE's instruments would fulfill each of their objectives as well as making observations that would have implications outside COBE's initial scope.

Black-body curve of CMB

Data from COBE showed a perfect fit between the black body curve predicted by big bang theory and that observed in the microwave background.

Comparison of CMB results from COBE, WMAP and Planck - 21 March 2013.

During the 15-year-long period between the proposal and launch of COBE, there were two significant astronomical developments:

First, in 1981, two teams of astronomers, one led by David Wilkinson of Princeton University and the other by Francesco Melchiorri of the University of Florence, simultaneously announced that they detected a quadrupole distribution of CMB using balloon-borne instruments. This finding would have been the detection of the black-body distribution of CMB that FIRAS on COBE was to measure. In particular, the Florence group claimed a detection of intermediate angular scale anisotropies at the level 100 microkelvins in agreement with later measurements made by the BOOMERanG experiment. However, a number of other experiments attempted to duplicate their results and were unable to do so.
Second, in 1987 a Japanese-American team led by Andrew E. Lange and Paul Richards of University of California, Berkeley and Toshio Matsumoto of Nagoya University made an announcement that CMB was not that of a true black body. In a sounding rocket experiment, they detected an excess brightness at 0.5 and 0.7 mm (0.028 in) wavelengths.

With these developments serving as a backdrop to COBE's mission, scientists eagerly awaited results from FIRAS. The results of FIRAS were startling in that they showed a perfect fit of the CMB and the theoretical curve for a black body at a temperature of 2.7 K, in contrast to the Berkeley-Nagoya results.

FIRAS measurements were made by measuring the spectral difference between a 7° patch of the sky against an internal black body. The interferometer in FIRAS covered between 2- and 95-cm−1 in two bands separated at 20-cm−1. There are two scan lengths (short and long) and two scan speeds (fast and slow) for a total of four different scan modes. The data were collected over a ten-month period.

Intrinsic anisotropy of CMB

The DMR was able to spend four years mapping the detectable anisotropy of cosmic background radiation as it was the only instrument not dependent on the dewar's supply of helium to keep it cooled. This operation was able to create full sky maps of the CMB by subtracting out galactic emissions and dipole at various frequencies. The cosmic microwave background fluctuations are extremely faint, only one part in 100,000 compared to the 2.73 K average temperature of the radiation field. The cosmic microwave background radiation is a remnant of the Big Bang and the fluctuations are the imprint of density contrast in the early universe. The density ripples are believed to have produced structure formation as observed in the universe today: clusters of galaxies and vast regions devoid of galaxies.

Detecting early galaxies

DIRBE also detected 10 new far-IR emitting galaxies in the region not surveyed by IRAS as well as nine other candidates in the weak far-IR that may be spiral galaxies. Galaxies that were detected at the 140 and 240 μm were also able to provide information on very cold dust (VCD). At these wavelengths, the mass and temperature of VCD can be derived. When these data were joined with 60 and 100 μm data taken from IRAS, it was found that the far-infrared luminosity arises from cold (≈17–22 K) dust associated with diffuse H I region cirrus clouds, 15-30% from cold (≈19 K) dust associated with molecular gas, and less than 10% from warm (≈29 K) dust in the extended low-density H II regions.

DIRBE

On top of the findings DIRBE had on galaxies, it also made two other significant contributions to science. The DIRBE instrument was able to conduct studies on interplanetary dust (IPD) and determine if its origin was from asteroid or cometary particles. The DIRBE data collected at 12, 25, 50 and 100 μm were able to conclude that grains of asteroidal origin populate the IPD bands and the smooth IPD cloud.

The second contribution DIRBE made was a model of the Galactic disk as seen edge-on from our position. According to the model, if the Sun is 8.6 kpc from the Galactic Center, then it is 15.6% above the midplane of the disk, which has a radial and vertical scale lengths of 2.64 and 0.333 kpc, respectively, and is warped in a way consistent with the HI layer. There is also no indication of a thick disk.

To create this model, the IPD had to be subtracted out of the DIRBE data. It was found that this cloud, which as seen from Earth is Zodiacal light, was not centered on the Sun, as previously thought, but on a place in space a few million kilometers away. This is due to the gravitation influence of Saturn and Jupiter.

Cosmological implications

In addition to the science results detailed in the last section, there are numerous cosmological questions left unanswered by COBE's results. A direct measurement of the extragalactic background light (EBL) can also provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emissions by dust.

By looking at the results from DIRBE and FIRAS in the 140 to 5000 μm we can detect that the integrated EBL intensity is ≈16 nW/(m²·sr). This is consistent with the energy released during nucleosynthesis and constitutes about 20–50% of the total energy released in the formation of helium and metals throughout the history of the universe. Attributed only to nuclear sources, this intensity implies that more than 5–15% of the baryonic mass density implied by big bang nucleosynthesis analysis has been processed in stars to helium and heavier elements.

There were also significant implications into star formation. COBE observations provide important constraints on the cosmic star formation rate, and help us calculate the EBL spectrum for various star formation histories. Observation made by COBE require that star formation rate at redshifts of z ≈ 1.5 to be larger than that inferred from UV-optical observations by a factor of 2. This excess stellar energy must be mainly generated by massive stars in yet - undetected dust enshrouded galaxies or extremely dusty star forming regions in observed galaxies. The exact star formation history cannot unambiguously be resolved by COBE and further observations must be made in the future.

On 30 June 2001, NASA launched a follow-up mission to COBE led by DMR Deputy Principal Investigator Charles L. Bennett. The Wilkinson Microwave Anisotropy Probe has clarified and expanded upon COBE's accomplishments. Following WMAP, the European Space Agency's probe, Planck has continued to increase the resolution at which the background has been mapped.

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