Friday, July 15, 2022

Could the coma of a comet expand so much to become bigger than the Sun?

Surprisingly yes, and this is what happened to Comet Holmes during the 2007 passage.

Holmes is a comet that orbits the Sun every 6.9 years, discovered in 1892 by Edwin Holmes, from which it then took its name.

The comet never aroused particular interest until the passage of 2007: between 23th and 24th October of that year the comet in fact underwent an exceptional change in brightness, passing from the 17th apparent magnitude to an apparent magnitude of +2.5, thus becoming visible to the naked eye.

This change was due to a sudden increase in the activity of the cometary nucleus. During these days some fragments were also observed detaching from the comet.

Following this outburst, Holmes was carefully studied by astronomers. The observations made it possible to establish that the nucleus has a diameter of about 3.5 km.

Meanwhile, the diameter of the comet's coma continued to grow. Observations conducted in mid-November 2007 revealed something impressive: the coma had a diameter of 1.4 million km. For a short time, Comet Holmes had a larger atmosphere than the solar one!

Following the passage to perihelion, the activity of the nucleus decreased and with it the extension of the coma, which then completely disappeared when Holmes moved away from the Sun.

Even today it is not known what triggered the intense activity of the nucleus that led Holmes to become so bright. It also isn't

clear why this only happened during the 2007 passage and not in all the others.

To solve some of these questions, the Spitzer Telescope studied the Holmes nucleus in the infrared. Some very particular morphological structures have been discovered and never observed on other comets, but further studies and observations will be needed to explain what happened in 2007.

Credit: Jean-Charles Cuillandre, Giovanni Anselmi, Hawaiian Starlight.

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