Europeans detect stellar cannibalism

Oct. 1, 2000
During a recent observation program with the European Southern Observatory (ESO) Very Large Telescope at the Paranal Observatory (Chile), Italian astronomers received a faint light signal from NGC 1316, a giant elliptical galaxy in the Fornax cluster of galaxies.

ASTRONOMY

During a recent observation program with the European Southern Observatory (ESO) Very Large Telescope at the Paranal Observatory (Chile), Italian astronomers received a faint light signal from NGC 1316, a giant elliptical galaxy in the Fornax cluster of galaxies. The light, which traveled 70 million light years, carried the message of a series of violent thermonuclear explosions during the age of the dinosaurs. According to the scientists, these events were caused by "stellar cannibalism" in a binary system.

In the Milky Way galaxy, a stellar outburst such as this is called a stellar nova, or just nova. These events occur relatively frequently, and roughly every second or third year one is bright enough to be visible to the naked eye. The most common nova explosion occurs in a binary stellar system in which a white dwarf—a very dense, hot, compact star with a mass comparable to that of the Sun and a size similar to that of the Earth—takes captive hydrogen from a cooler and larger red dwarf star. The collected hydrogen on the surface of the white dwarf star heats up, igniting a thermonuclear reaction and explosion.

Click here to enlarge image

Composite photo shows the central area of NGC 1316 based on charge-coupled-device (CCD) exposures obtained with the 8.2-m Very Large Telescope (called ANTU) and the FORS-1 (Focal Reducer Spectrograph) multimode instrument through blue, green-yellow, and red filters, respectively. The "pyramids" above and below the bright center of the galaxy and the vertical lines at some of the brighter stars are caused by overexposure or CCD bleeding. The field measures 6.8 x 6.8 arc min2 (0.2 arc sec per pixel). The image quality of this composite is about 0.9 arc sec. North is up. East is to the left.

The release of such a large quantity of energy produces about a million times the brightness of the binary system within a few hours. After reaching maximum brightness within days or weeks, energy levels taper off as the hydrogen supply exhausts into space. The material is ejected at about 1000 km/s and may later be visible as an expanding shell emitting gas. The tremendous flash of light translates to the release in just a few weeks of about as much energy as the Sun produces in 10,000 years. The nova star will finally die of "old age" when the cool companion has been completely cannibalized.

The giant dusty galaxy NGC 1316 is of special interest in connection with current attempts to establish an accurate distance scale in the universe. Twenty years ago, NGC 1316 was the host of two supernovae of type 1a, a class of objects widely used as a "cosmological standard candle" to determine the distance to very distant galaxies. A precise measurement of the distance to NGC 1316 may therefore provide an independent calibration of the intrinsic brightness of these supernovae. The novae in NGC 1316 are faint, magnitude 24, and decreasing toward magnitude 25-26—nearly 100 million times fainter than what can be seen with naked eye.

These results clearly show that the new generation of 8-m-class telescopes such as the Very Large Telescope, can improve the efficiency of this type of astronomical investigation by a factor of 10 or more.

Roland Roux

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