Supernova 1997ff

Rogier Windhorst, Sam Pascarelle,
Arizona State University, STScI, NASA

Larger true-color image of ultraviolet
emission redshifted to visible light.

Early galaxies about 11 billion years
ago tended to be small, bluish, and
peculiarly shaped (more).

Photographed about eight days after it exploded, Supernova 1997ff (SN1997ff) was found by astronomers comparing the northern Hubble Deep Field, a 10-day observation of a tiny region of sky first explored by the Hubble Space Telescope in 1995, with a follow-up observation in 1997. The Deep Field images depicted a diverse myriad of galaxies, including many of the peculiar, small galaxies common in the earliest era of galaxy formation around 11 billion years ago. The supernova was found in a faint and very distant, reddish elliptical galaxy with billions of highly evolved stars, which was apparently brightened by gravitational lensing from closer bright galaxies. It was found to be the farthest and oldest supernova directly observed (Benítez et al, 2002). Although now identified as a very bright Type-Ia supernova, SN1997ff was not seen directly in the Hubble images because its light was buried in the glow of its host galaxy due to their tremendous distance from Earth. It was found when astronomers used special computer software to subtract the light of the galaxy in the Deep Field images taken two years apart (Riess et al, 2001; Gilliland et al, 1999; and Gilliland and Phillips, 1998). SN997ff's extremely high cosmological, photometric redshift (z=1.7 +/-0.1) made it the oldest detected Type-Ia supernova through mid-2002. [See Tycho's Star for the youngest known Type-Ia supernova in the Milky Way observed from Earth.]

Adam Riess, STScI, NASA,
NERSC, LBL

Larger infrared and collage images.

SN 1997ff was found in a small
and peculiar, early irregular
galaxy in the northern Hubble
Deep Field image, at upper right,
(more at SN 1997ff and STScI).

Supernovae of Type I lend themselves well to research into cosmological parameters because they are intrinsically very bright and, hence, can be seen to great distances. These supernovae all have nearly identical intrinsic luminosities, and so comparisons of their actual, observed luminosities with their known intrinsic luminosities allows astronomers to determine their distances (more on their usefulness as "standard candles" (Branch and Tammann, 1992). Moreover, the wavelength distribution of the light from the supernovae indicates how fast they are receding from Sol. Estimating both the distance and recession speed of ancient Type-Ia supernovae allow astronomers to calculate the expansion of the universe, back during an era when matter in the universe was still relatively dense and expansion was still slowing under the influence of gravity and before its later hypothesized, subsequent acceleration from a mysterious repulsive force (more discussion at Tai-Pei Cheng's The Accelerating Universe slide show, usefulness of Type-Ia supernovae from NASA's Observatorium, and NERSC's press release on SN 1997ff).

Adam Riess, STScI, NASA, NERSC, LBL


Larger infrared and collage images.



Type-Ia supernovae (such as SN 1997ff located around
11.3 billion light-years away in the faint reddish galaxy
at left) are useful "standard candles" for their very
distant host galaxies (more from SN 1997ff and STScI).

Record-breaking Supernova 1997ff appeared brighter than it should if the universe had been expanding at a steady rate. Some astronomers hypothesized that a decelerating universe holds galaxies relatively close together and objects in them would have appeared brighter because they would be closer. According to Adam Riess of the Space Telescope Science Institute, the ancient universe may have been slowing down after the Big Bang from the mutual tug of its mass when light left this distant supernova, but by the time of more recent supernovas, the universe had begun accelerating, stretching the expanse between galaxies and making objects in them appear dimmer (more).

Ann Feild, STScI, NASA


Larger illustration and caption.


Ancient Type-Ia supernovae
like SN 1997ff suggest that
the universe once expanded
at a slower rate then
accelerated about 7.5
billion years ago (more).

Supernovae are classified as Type I if their light curves exhibit a sharp peak and then fades away smoothly and gradually. In theory, such supernovae are caused by the detonation of a relatively high-mass white dwarf composed mostly of carbon and oxygen (a stellar remnant whose progenitor star was too low in mass to progress to the core fusion of heavier elements) when infalling matter from the gaseous envelope of a moderately massive, binary companion eventually creates enough gravitational pressure to overcome the electron degeneracy holding up the white dwarf (illustrations of accretion theory). Already more massive than Sol, the white dwarf accretes sufficient additional mass to exceed the critical limit of 1.4 Solar-masses ("Chandrasekhar mass limit"). Such supernovae can also occur when two closely orbiting, however, white dwarfs collide and merge to create a single object that exceeds 1.4 Solar-masses. The spectra of these supernovae are hydrogen-poor relative to the more common Type II supernovae, which is consistent with the presumption the white dwarf progenitors of Type I have already blown off most of their outer layers of hydrogen and helium in planetary nebulae. Moreover, the smooth decline of their light is also believed to result from the gradual decrease in energy available with the radioactive decay of the unstable heavy elements produced in Type-I supernovae.

Melissa Weiss, CXC, NASA




Larger illustration.




Supernova 1997ff may
have begun as a
white dwarf that
accreted too much
mass from a close
companion star
and exploded as a
Type-Ia supernova
(more).

In the accretion scenario, the white dwarf accretes mass from its companion relatively rapidly. Moreover, any "nova outbursts" that occur on the white dwarf are relatively weak and eject little matter, so that the white dwarf grows in mass. (This is different from the mechanism of a "nova" in which the white dwarf doesn't reach the Chandrasekhar limit and collapse, but merely ignites nuclear fusion in the matter that has accreted on its surface) When the accretion has raised the white dwarf's mass to the critical mass of about 1.4 solar masses, the density and temperature in the star's center become so severe that carbon and oxygen start fusing ("burning") explosively. Within roughly one second, the burning front moves all the way to the surface, making the entire white dwarf into one huge nuclear fireball (more illustrated discussion of novae versus supernovae). A thermonuclear shockwave races through the supernova's expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of billions of stars. The entire star explodes and destroys itself, without leaving a compact central object. All of the star's matter -- namely, the products of the nuclear burning (iron, nickel, silicon, magnesium, and other heavy elements) plus unburned carbon and oxygen -- are ejected into space at speeds ranging from about 6,000 to 8,000 miles/second (20 to 30 million miles/hour). The supernova explosion and the sudden dispersion of its gravitational mass presumably flings its companion star away at high velocity. Unlike supernovae of Type II, the matter ejected in Type-I supernovae consists almost entirely of the heavier elements (spectrum of some elements in Tycho's SNR), as there is very little hydrogen left on white dwarfs. While the tremendous increase in luminosity is given by energy liberated by the explosion, its gradually fading light is fueled by radioactive cobalt decaying into iron.

John P. Hughes, Parviz Ghavamian,
Cara Rakowski, CXC, NASA



Larger x-ray image.



Supernova 1997ff was a
Type-Ia supernova like
DEM L71 in the nearby
Large Magellanic Cloud
(more at CXC and APOD).

In contrast to Type-Ia supernovae, Type Ib and Type Ic do not exhibit silicon lines and are even less understood than Type Ia. Types Ib and Ic are believed to correspond to stars ending their lives (as Type-II supernovae), but such stars would have lost their hydrogen before, and so their hydrogen lines don't appear on their spectra (more). Type Ib supernovae result from a high mass star that has blown off much of its outer hydrogen and helium shells and so resemble most closely Type Ia supernovae. They are somewhat dimmer as much of the light is absorbed by the surrounding nebula of material that the star has just recently thrown off, and no helium seen in their spectra. Type Ic supernovae are produced by high-mass stars that have blown off much of their outer hydrogen layers while still retaining significant helium layers, and so they are similar to Type Ib except that helium is seen in their spectrum.

Supernova 1997ff lies around 11.3 +/-0.2 billion light-years (ly) from Sol, which means that the progenitor star that evolved into the exploding white dwarf was even older (Riess et al, 2001; Gilliland et al, 1999; and Gilliland and Phillips, 1998). It is located in the northeast corner (12:36:44.18:+62:12:44.8, J2000 and ICRS 2000.0) of Constellation Ursa Major, the Great Bear, which also encompasses the Big Dipper or Plow (Plough) -- northeast of Megrez (Delta Ursae Majoris), northwest of Alioth (Epsilon Ursae Majoris) and Alcor (80 Ursae Majoris), southwest of Thuban (Alpha Draconis), south of Kappa Draconis, and southeast of Giausar (Lamda Draconis). The observed colors and temporal behavior of this supernova matched that of a typical SN Ia (Riess et al, 2001).Unfortunately, it has never been visible with the naked eye. Useful catalogue numbers and designations for this supernova remnant include: SN 1997ff.