Can a comet be larger than the Sun?

Yes. Comet tails can easily extend 10s of millions of miles in length, and in some rare super-comets, over 100 million miles. The nuclear regions is perhaps only 10-50 kilometers across, but can produce a 'coma' larger than the diameter of the Sun. The above telescopic view of Comet Hale-Bopp in 1999 shows a coma size that is equal to the diameter of the Sun!

Recently the Ulysses space craft detected the tail plasma from Comet Hyakutake at a distance of 500 million miles! The above photo is of Comet Hale-Bopp taken at the ESA Southern Observatory. A portion of their Press Release follows:

The most recent image of Comet Hale-Bopp was obtained at ESO on June 18, 1999, with the SUSI2 instrument at the ESO 3.5-m New Technology Telescope (NTT) at La Silla. This observation was somewhat difficult, as the comet was very low in the sky at the beginning of the twilight. At that time, Comet Hale-Bopp was located in the southern constellation Dorado (The Goldfish).

It was about 1295 million km (8.66 AU) distant from both the Earth and the Sun, i.e. at about as far away as planet Saturn. The photo shows that there was still a substantial cloud of dust -- a "coma" -- around the comet's icy nucleus, even at this very large distance

This photo has been contrast-enhanced to show the truly enormous size of the comet's coma, over 3 arcmin across. The optical reflection from the bright star to the lower left was also enhanced by this process.

At the distance of the comet, the part that is visible in this photo is no less than 1.1 million km across, or nearly 10 times larger than Saturn!

No other comets have ever been found to have such a large coma at this large distance [1]. There is now little structure in the coma and no "jets" are seen any more. Much of this coma consists of dust that was ejected from the nucleus when it was closer to the Sun. However, it is likely that dust is still lost from the nucleus into the coma, perhaps as a result of a continuing outflow of certain gases (in particular CO and CO2), albeit at a lower level than before.

The maximum extent of the coma observed around Hale-Bopp when it was near the Earth and the Sun in early 1997, was 2-3 million km, or only a few times more than now; the uncertainty arises from the different observational methods used. Comae of other comets rarely ever become larger than a few hundred thousand kilometres.

There is little doubt that the exceptional dimensions of the coma around Hale-Bopp is a direct consequence of its unusually large nucleus. Although it never became possible to measure its size accurately, a variety of observational methods points towards a diameter in the 40 - 70 km range. The nuclei of most other comets are at most a few km across. With its larger surface, more material is released -- this provides a natural explanation of the unusual coma of Hale-Bopp.


Return to Ask the Astronomer