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Nebulous explanation

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The planetary nebulae of gas and dust that are formed from red-giant stars are usually far from spherical in shape. Observations of the gas distribution in one red giant caught in the act of transition show why.

Planetary nebulae are glowing remnants of gas, left over from the dying stages of Sun-like stars. Images from the Hubble Space Telescope have provided astronomers with exquisitely detailed images of these beautiful objects. But a long-standing mystery is why most (but not all) planetary nebulae are not spherical, although the distributions of gas and dust in their immediate predecessors are. Now Imai et al.1 (page 829 of this issue), using a ground-based interferometric technique, provide evidence that a well-collimated outflow, or jet, of gas is emanating from a red-giant star that is in transition to becoming a planetary nebula. Their observations also suggest that the jet may be precessing — wobbling around on its axis like a toy top does as it spins. Their findings should lead to a better understanding of how planetary nebulae are shaped.

The Sun is a fairly common type of star, and will remain substantially unchanged over the next several billion years. Nuclear fusion reactions (converting hydrogen into helium) in the core of the Sun provide energy that supports the outer gaseous layers as gravity tries to force them inwards. But after many billions of years, when all the hydrogen in the core of such a star has been converted into helium, the core contracts under gravity, heating up and creating pressure that causes the outer layers of the star to expand and cool. The star becomes a red giant; when the Sun reaches that stage, its outer layers will expand as far as the Earth's orbit.

As the expansion progresses, the outer layers of the star become unstable and the star pulsates, changing its total energy output on a regular basis (the cycle time is usually around a year). The pulsation causes matter to be lost from the star as a very slow 'wind' (with a velocity of about 10 km s−1), into a surrounding shell. These pulsational changes last several tens of thousands of years. Eventually, the core of the star collapses to form a white dwarf with a surface temperature of 30,000 K — hot enough to ionize the gas in the shell that surrounds the dying star. The ionized gas is what we see as a planetary nebula.

This model of the later stages of stellar evolution works just fine until the last part. The slow mass loss is, as far as we can tell, generally spherically symmetrical. So the distribution of any gas that is expelled in this manner should have spherical symmetry. And yet the ionized gas distribution for most planetary nebulae is far from spherical (Fig. 1); many have a flattened, even two-lobed, structure2,3,4,5.

Figure 1: The dying of the light.
figure1

NASA/ESA

These Hubble Space Telescope images show planetary nebulae NGC 6826 (left) and OH 231.8 (right) — the gaseous remnants surrounding dying stars. Gas expelled from the stars during their red-giant phase forms a spherical shell around the stellar core. But most planetary nebulae are not spherical. New observations by Imai et al.1 of the planetary nebula W43A reveal a strong, precessing jet of gas escaping from its core. This may explain the loss of spherical symmetry; moreover, the motion of the jet suggests that W43A has been caught exactly at the point of its transition from red giant to planetary nebula.

During the red-giant stage, molecules form in the gas that has been expelled into the circumstellar shell. The presence of molecules such as hydroxyl, silicon monoxide and water has been established from their maser emission6 (the microwave analogue of laser emission). The study of maser emission at radio wavelengths is a useful tool for two reasons. First, the gas motion in the shell can be determined from the Doppler-shifted frequency of emission. And second, the masers are so bright that a ground-based technique, 'very long baseline interferometry' (VLBI), can be used to measure their position accurately. VLBI combines the measurements of several telescopes positioned over a wide area. In the case of water masers, their position can be established to better than 6 × 10−8 degrees using VLBI. This technique has become routine since the opening in 1992 of the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA), a dedicated VLBI instrument of ten radio antennae extending from Hawaii to the Virgin Islands.

As the central stellar core becomes hot enough to ionize the surrounding gas, the molecules in the gas are expected to be destroyed, so their maser action should cease. But Miranda et al.7 have found water and hydroxyl masers in a very young planetary nebula, and Likkel et al.8 discovered water masers in W43A, the nebula observed by Imai and colleagues1.

W43A is an intriguing object, thought to be in transition from a red giant to a planetary nebula. Imai et al. have used the VLBA to determine the distribution of water masers in the nebula with a precision 200 times greater than that of optical observations by the Hubble Space Telescope. Surprisingly, they found that the gas traced by the maser emission was not spherical in distribution, but instead forms rather narrow, oppositely directed jets. The gas in the jets is moving at 150 km s−1 — although that is not particularly high for gas jets, it does lead to an estimated age for the jet of only around 30 years. This strongly suggests that the star has indeed been caught in the act of transition to a planetary nebula.

Imai et al. also draw another significant conclusion: the masers do not lie in a straight line but on a curve, suggesting that the source of the outflow might be precessing. This is perhaps the most exciting part of their observations. There are several possible explanations for such a precessing jet — perhaps a close binary companion9, or strong magnetic fields around the star10.

The molecular jet seen by Imai et al. suggests a mechanism for producing the non-spherical morphologies seen in most planetary nebulae, and further observations of young planetary or transition objects promise to finally solve the mystery. The next steps, necessarily a collaborative effort between theorists and observers, will be to investigate what causes the jet, to confirm its precession and to determine how the precession occurs.

References

  1. 1

    Imai, H., Obara, K., Diamond, P. J., Omodaka, T. & Sasao, T. Nature 417, 829–831 (2002).

  2. 2

    Balick, B. Astrophys. J. 94, 671–678 (1987).

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    Kwok, S. in Asymmetrical Planetary Nebulae II: From Origins to Microstructures (eds Kaster, J. H., Soker, N. & Rappaport, S. A.) 199, 9–16 (2000).

  4. 4

    Terzian, Y. & Hajian, A. R. in Asymmetrical Planetary Nebulae II: From Origins to Microstructures (eds Kaster, J. H., Soker, N. & Rappaport, S. A.) 199, 33–40 (2000).

  5. 5

    Balick, B. in Asymmetrical Planetary Nebulae II: From Origins to Microstructures (eds Kaster, J. H., Soker, N. & Rappaport, S. A.) 199, 41–48 (2000).

  6. 6

    Reid, M. J. & Moran, J. M. Annu. Rev. Astron. Astrophys. 19, 231–276 (1981).

  7. 7

    Miranda, L. F., Gómez, Y., Anglada, G. & Torrelles, J. M. Nature 414, 284–286 (2001).

  8. 8

    Likkel, L., Morris, M. & Maddalena, R. J. Astron. Astrophys. 256, 581–594 (1992).

  9. 9

    Morris, M. Publ. Astron. Soc. Pacif. 99, 1115–1122 (1987).

  10. 10

    Garcia-Segura, G. Astrophys. J. 489, L189–L192 (1997).

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Correspondence to Mark Claussen.

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