NASA's Hubble Space Telescope (HST) has successfully completed the first stage of a cosmic detective hunt for a rare star, called a nova, which erupted half a century ago. HST's needle-in-the-haystack search has provided astronomers with an unprecedented view of the crowded inner region of a globular star cluster located about 70,000 light-years away.
Bruce Margon, Scott Anderson, (University of Washington) and Ronald Downes (University of California at San Diego) used the European Space Agency's Faint Object Camera (FOC) onboard HST to observe the star-sprinkled vicinity of the nova. These images will be used to support follow-on observations which are planned by the HST's Faint Object Spectrograph (FOS) Investigation Definition Team, headed by Richard Harms (Applied Research Corporation). The FOS Team has planned for more than a decade to obtain images and spectra of this region, with the goal of identifying, and then studying in detail, the old nova.
"The results are tremendously encouraging not only with respect to locating the quiescent nova, but also as a preview of the use of HST cameras in crowded, faint regions such as globular clusters," says Margon.
A nova is a sudden stellar eruption where, in just a few days, a star will grow 10,000 times brighter than normal. Over the next few months the outburst fades away and the star returns to its normal light level. A few novae undergo repeated outbursts, typically separated by decades.
For more than two decades astronomers have searched for the nova in M14 which is an extremely rare phenomenon for a globular cluster. However the nova's stellar neighborhood is extremely blurred and confused, as viewed from the ground, due to the high crowding of the stars in the globular cluster. "This is an obvious case where the precision imaging and spectroscopic capabilities of the HST are needed," says Margon.
The FOC images reveal literally hundreds of separate stars in a tiny region of the cluster where only dozens are distinguishable on ground-based images. The FOC image resolves five separate stars in the nova's suspected location, which previously appeared as only one object in ground-based images. Presumably only one of these five stars is the old nova; though it is also possible that the nova is too faint to be seen.
"It is already clear even from this preliminary stage of the analysis that we have learned much," says Margon. "The brightness today of the nova remnant must be considerably less than suggested by the ground-based data, which we now know to have summed at least five separate stars."
The most widely accepted theory today is that all novae are in fact two stars locked in a close orbit by their mutual gravitational attraction. One member of the pair is a normal star fairly similar to our own Sun. The companion is a white dwarf, a dense star at the end of its life which has collapsed to the size of the Earth. Such a compact star has a strong gravitational pull. Hydrogen gas stripped off the outer layers of the normal companion star falls onto the surface of the white dwarf, where it gradually accumulates, squeezed into an ever denser shell on the white dwarf's surface. Eventually the temperature and density of the hydrogen in the shell reaches the point where the gas spontaneously ignites in a thermonuclear explosion — literally an immense hydrogen bomb. This explosive energy release explains both the sudden brightening, and the rapidly ejected shell which reaches speeds as high as thousands of kilometers per second.
Most novae on record have been observed deep within the Milky Way Galaxy. Only twice in astronomical history have novae been observed in globular star clusters, spherical agglomerations of a few hundred thousand stars which are gravitationally bound together. The first of these two known novae was observed in the globular cluster Messier 80 (in the constellation Scorpius) in the year 1860. Since this event ocurred before astronomical photography, the nova was studied only visually, and so no quantitative records remain.
The nova which is now an HST target, was accidentally photographed in 1938 with the 72-inch telescope at the David Dunlap Observatory near Toronto, Canada. However it wasn't until 25 years later that this nova was actually discovered. In 1963, Amelia Wehlau (of David Dunlap Observatory) was studying photographic plates of the globular star cluster Messier 14 (located in the constellation Ophiuchus) obtained over an interval of three decades. The series of plates exposed in 1938 by Helen Sawyer Hogg (who today, at age 85, is still an active researcher) revealed the presence of a bright star where none was previously, nor subsequently, visible.
Apparently the nova was not noticed in 1938, but it probably erupted sometime in May of that year, a few weeks before the serendipitous photograph. The photographic evidence shows that by 1939 the nova had disappeared into the background of thousands of neighboring, brighter stars in the cluster.
In a series of observations over the last few years, Michael Shara of the Space Telescope Science Institute and colleagues obtained sensitive ground-based images and spectra of the area using telescopes in Chile and Hawaii. The researchers have sug- gested that a very faint image near in location to the 1938 event might in fact include the quiescent nova, which is now in its normal, faint state state as a binary star system.
Only a few binaries have been observed in globular star clusters because they are difficult to detect. Binaries may also be hard to detect because they are less numerous in globular star clusters than they are in our own galactic neighborhood. However, if even a small number of binaries exist in globular clusters they can, theoretically, drastically influence the motions of the other stars.
The next step in pinning down the nova's location requires further detailed analysis of the FOC data to identify which (if any) of the candidates is really the old nova. Because images were obtained in two different colors, astronomers will be able to search the dense star field for an object with unusual colors in comparison with its hundreds of neighbors. Such a peculiarity is one signature of many old novae. Although two of the five stellar candidates do seem to have slightly peculiar colors, the oddity is not extreme. Whether this fact alone represents another discovery, for example, that old novae in globular clusters are different from the ordinary kind outside of clusters, remains to be determined.
A further step in this stellar detective hunt requires obtaining the spectra of the stars, to look for the typical spectral signature characteristic of novae. This is a task that would be terribly difficult from the ground, due to the crowding of nearby, unrelated stars, but should be possible with the HST Faint Object Spectrograph. The FOS team plans to perform these observations within the next several months to try and finally uncover this most peculiar star system.