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Cassiopeia A (Part 1)

Cassiopeia A is also known as Cas A) and it is a supernova remnant (SNR) in the constellation Cassiopeia. The brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way given the width of the Orion Arm it is placed in the next-nearest arm outwards, the Perseus Arm, about 30 degrees from the Galactic anti center. The expanding cloud of material leftover from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective. It has been seen with amateur telescopes down to 234 mm (9.25 in) with filters in wavelengths of visible light.

It is estimated that light from the stellar explosion (supernova) first reached Earth near the decade of the 1690s, from which time there are no definitively corresponding records. Cas A is circumpolar at and above mid-northern latitudes which had extensive records and basic telescopes. Its likely omission in records is probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on 16 August 1680). Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and re-absorbed much of the light released as the inner star collapsed.

Cas A was among the first discrete astronomical radio sources found. Its discovery was reported in 1948 by Martin Ryle and Francis Graham-Smith, astronomers at Cambridge, based on observations with the Long Michelson Interferometer. The optical component was first identified in 1950. Cas A is 3C461 in the Third Cambridge Catalogue of Radio Sources and G111.7-2.1 in the Green Catalog of Supernova Remnants.

Through a series of observations in 2004, the Chandra X-ray Observatory accumulated a million seconds of observations on Cassiopeia A, a remnant of a supernova explosion. Cas A was the first object Chandra observed, and it has continued to probe ever deeper into its structure and composition. These new observations were arranged by Una Hwang of Goddard Space Flight Center.

The three-color image (below, center) shows an outer ring (enhanced in green via the color-coding of the energies) that marks the location of the shock wave generated by the supernova explosion. A large, jet-like structure protruded beyond the shock wave to the upper left. Surprisingly, X-ray spectra show that the jet has a relatively large amount of silicon and a low amount of iron. The cause for this is part of ongoing detailed studies. In addition, enhancing the image to show just the silicon (below, right) reveals a counter-jet to the lower right.

Million-second observation of Cassiopeia A taken by the Chandra X-ray Observatory in 2004. Left: A broadband X-ray image showing the remnant in the Chandra X-ray range of 1.75 - 7.0 keV. Center: An image of the remnant color-coded by energy: Red represents X-rays from 1.78-2.0 keV; Green=4.2-6.4 keV; Blue=6.52-6.95 keV. Right: An image enhanced to emphasize the location of silicon in the remnant. Each image is 8 arcminutes on a side. (Credit: NASA/CXC/GSFC/U. Hwang et al.)

Iron, however, is present in the remnant. The bright blue region just inside the shock wave on the lower left is composed of iron gas. It was somehow ejected in a direction almost perpendicular to the jets.

One curious feature of Cas A is that the central neutron star (visible in the broadband image, below left) is quiet, unlike the pulsars that lie in the center of the Crab nebula and the Vela supernova remnant. A working hypothesis is that the explosion that created Cassiopeia A produced high-speed jets similar to but less energetic than the hypernova jets thought to produce gamma-ray bursts. During the explosion, the neutron star may have developed an extremely strong magnetic field that helped to accelerate the jets. This strong magnetic field later stifled any pulsar wind activity, so the neutron star today resembles other strong-field neutron stars (a.k.a. magnetars) in lacking a pulsar wind nebula.

Other Designations of Cas A

SN 1671, SN 1667, SN 1680, SNR G111.7-02.1, 1ES 2321+58.5, 3C 461, 3C 461.0, 4C 58.40, 8C 2321+585, 1RXS J232325.4+584838, 3FHL J2323.4+5848, 2U 2321+58, 3A 2321+585, 3CR 461, 3U 2321+58, 4U 2321+58, AJG 109, CTB 110, INTREF 1108, [DGW65] 148, PBC J2323.3+5849, 2FGL J2323.4+5849, 3FGL J2323.4+5849, 2FHL J2323.4+5848

  • Event Type: Supernova remnant, astronomical radio source

  • Spectral class: Type II supernova

  • Date: 1947

  • Constellation: Cassiopeia

  • Right ascension: 23h 23m 24s

  • Declination: +58° 48.9′

  • Epoch: J2000

  • Galactic coordinates: 111.734745°, −02.129570°

  • Distance: 11,000 ly (3.4 kpc)

  • Remnant: Shell

  • Host: Milky Way

  • Progenitor: unknown

  • Progenitor type: unknown

  • Colour (B-V): unknown

  • Notable features: Strongest radio source beyond our solar system

  • Peak apparent magnitude: 6

  • Preceded by: SN 1604

  • Followed by: G1.9+0.3 (unobserved, c. 1868), SN 1885A (next observed)

Calculations working back from the currently observed expansion point to an explosion that would have become visible on Earth around 1667. Astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova on 16 August 1680, when he cataloged a star near its position. Another suggestion from recent cross-disciplinary research is that the supernova was the noonday star observed in 1630, which was thought to have heralded the birth of Charles II, the future monarch of Great Britain. At any rate, no supernova occurring within the Milky Way has been visible to the naked eye from Earth since.

The expansion shell has a temperature of around 30 million K and is expanding at 4000−6000 km/s. Observations of the exploded star through the Hubble telescope have shown that, despite the original belief that the remnants were expanding in a uniform manner, there are high-velocity outlying eject knots moving with transverse velocities of 5,500−14,500 km/s with the highest speeds occurring in two nearly opposing jets. When the view of the expanding star uses colors to differentiate materials of different chemical compositions, it shows that similar materials often remain gathered together in the remnants of the explosion.

Radio source

Cas A had a flux density of 2720 ± 50 Jy at 1 GHz in 1980. Because the supernova remnant is cooling, its flux density is decreasing. At 1 GHz, its flux density is decreasing at a rate of 0.97 ± 0.04 percent per year. This decrease means that, at frequencies below 1 GHz, Cas A is now less intense than Cygnus A. Cas A is still the brightest extrasolar radio source in the sky at frequencies above 1 GHz.

X-ray source

In 1999, the Chandra X-Ray Observatory found CXOU J232327.8+584842, a hot point-like source close to the center of the nebula that is the neutron star remnant left by the explosion.

Although Cas X-1 (or Cas XR-1), the apparent first X-ray source in the constellation Cassiopeia was not detected during the 16 June 1964, Aerobee sounding rocket flight, it was considered as a possible source.[12] Cas A was scanned during another Aerobee rocket flight of 1 October 1964, but no significant X-ray flux above the background was associated with the position. Cas XR-1 was discovered by an Aerobee rocket flight on 25 April 1965, at RA 23h 21m Dec +58° 30′.[15] Cas X-1 is Cas A, a Type II SNR at RA 23h 18m Dec +58° 30′. The designations Cassiopeia X-1, Cas XR-1, Cas X-1 are no longer used, but the X-ray source is Cas A (SNR G111.7-02.1) at 2U 2321+58.

Supernova reflected echo

In 2005 an infrared echo of the Cassiopeia A explosion was observed on nearby gas clouds using Spitzer Space Telescope. The infrared echo was also seen by IRAS and studied with the Infrared Spectrograph. Previously it was suspected that a flare in 1950 from a central pulsar could be responsible for the infrared echo. With the new data, it was concluded that this is unlikely the case and that the infrared echo was caused by thermal emission by dust, which was heated by the radiative output of the supernova during the shock breakout. The infrared echo is accompanied by a scattered light echo. The recorded spectrum of the optical light echo proved the supernova was of Type II supernova, meaning it resulted from the internal collapse and violent explosion of a massive star, most probably a red supergiant with a helium core that had lost almost all of its hydrogen envelope. This was the first observation of the light echo of a supernova whose explosion had not been directly observed which opens up the possibility of studying and reconstructing past astronomical events. In 2011 a study used spectra from different positions of the light echo to confirm that the Cassiopeia A supernova was asymmetric.

In 2013, astronomers detected phosphorus in Cassiopeia A, which confirmed that this element is produced in supernovae through supernova nucleosynthesis. The phosphorus-to-iron ratio in material from the supernova remnant could be up to 100 times higher than in the Milky Way in general.


Cassiopeia A: First Light

Cassiopeia A
Cassiopeia A (X-ray Image)

Cas A is the remnant of a star that exploded about 300 years ago. The X-ray image shows an expanding shell of hot gas produced by the explosion. This gaseous shell is about 10 light-years in diameter and has a temperature of about 50 million degrees.

  • Credit: NASA/CXC/SAO

  • Category: Supernovas & Supernova Remnants

  • Coordinates (J2000): RA 23h 23m 26s | Dec +58° 8´ 53.4"

  • Constellation: Cassiopeia

  • Observation Dates: August 19, 1999

  • Observation Time: 1 hour

  • Obs. IDs: 214

  • Color Code: Intensity

  • Instrument: ACIS

  • Also Known As: Cas A

  • Distance Estimate: 11,000 light-years

  • Release Date: August 26, 1999

Cassiopeia A (Optical Image)
Cassiopeia A (Optical Image)
Cassiopeia A (Infrared Image)
Cassiopeia A (Infrared Image)
Cassiopeia A (Radio Image)
Cassiopeia A (Radio Image)

Cassiopeia A: Chandra Maps Vital Elements in Supernovas

Cassiopeia A
Cassiopeia A

Chandra X-ray image of the supernovas remnant Cassiopeia A (Cas A). The red, green, and blue regions in this Chandra X-ray image of the supernovas remnant Cassiopeia A show where the intensity of low, medium, and high-energy X-rays, respectively, is greatest. The red material on the left outer edge is enriched in iron, whereas the bright greenish-white region on the lower left is enriched in silicon and sulfur. In the blue region on the right edge, low and medium energy X-rays have been filtered out by a cloud of dust and gas in the remnant.

  • Credit: NASA/CXC/SAO/Rutgers/J.Hughes

  • Observation Dates: August 19, 1999

  • Observation Time: 2 hours

  • Color Code: Intensity: Blue=high-energy; Green=medium energy; Red=low energy

  • Release Date: December 21, 1999

Cassiopeia A: Elemental Image Of Exploded Star

Cassiopeia A
Cassiopeia A

A new 14 hour Chandra observation of the supernova remnant Cassiopeia A has given the best map yet of heavy elements ejected in a supernova explosion. Upper left: Broadband X-ray image of Cassiopeia A (Cas A) Upper right: Image made by X-rays from silicon ions. Lower left: Image made by X-rays from calcium ions. Lower right: Image made by X-rays from iron ions. All images are 8.5 arc minutes on a side (corresponding to 28.2 light-years at a distance of 11,000 light-years).

These images are designed to show the distribution of some of the elements ejected in the explosion that produced Cas A. The elements are part of a gas that has a temperature of about 50 million degrees Celsius. The colors represent the intensity of X-rays, with yellow the most intense, then red, purple, and green.

The broadband image, which shows all the X rays detected from Cas A, is more symmetric than the others. This could be due to the presence of X-rays from synchrotron radiation by extremely high-energy particles spiraling in the magnetic field of the remnant, or to shock waves traveling through material puffed off thousands of years before the supernova.

The silicon image shows a bright, broad jet breaking out of the upper left side of the remnant, and faint streamers in an opposite direction. This jet could be due to an asymmetry in the explosion.

The calcium image is similar to the silicon image but less bright and clumpier. The iron image shows significant differences from other images. Since iron is the heaviest element shown, these maps support the suggestion that the layers of the star were overturned either before or during the explosion.

Cassiopeia A (Broadband)
Cassiopeia A (Broadband)
Cassiopeia A (Silicon)
Cassiopeia A (Silicon)
Cassiopeia A (Calcium)
Cassiopeia A (Calcium)