Comet Holmes

Lore & Background

Comet Holmes was discovered by Edwin Holmes on November 6, 1892, while he was conducting regular observations of the Andromeda Galaxy (M31). Its discovery was possible because of an increase in its magnitude similar to the 2007 outburst; it brightened to an approximate magnitude of 4 or 5 before fading over several weeks. The discovery was confirmed by Edward Walter Maunder, William Henry Maw, and B. Kidd, with independent discoveries by Thomas David Anderson on November 8 and by Mike Brown and John Ewen Davidson on November 9. First calculations of the elliptical orbit were done independently by Heinrich Kreutz and George Mary Searle, establishing the perihelion date as June 13 and the orbital period as 6.9 years, proving the comet was not a return of Biela's Comet. The comet was observed in 1899 and 1906 but then lost until recovered on July 16, 1964, by Elizabeth Roemer, aided by computer predictions of Brian G. Marsden, and has been observed on every subsequent return.

During its 2007 return, Holmes unexpectedly brightened from magnitude about 17 to about 2.8 in 42 hours, making it visible to the naked eye. The outburst took place from October 23 to 24, 2007. The first person reportedly to notice the change was J. A. Henríquez Santana on Tenerife; minutes later Ramón Naves in Barcelona noticed it at magnitude 7.3. It became easily visible as a bright yellow 'star' in Perseus, and by October 25 it appeared as the third-brightest 'star' in that constellation. Naked-eye observers saw it as star-like until October 26, after which it appeared more comet-like because its orbit took it to near opposition and Earth observers were looking nearly straight down along its tail. The coma expanded from 3.3 arcminutes to over 13 arcminutes, about half the Moon's diameter, and at about 2 AU its true diameter swelled to over 1 million km, about 70% of the Sun's diameter. By November 2007 the coma had dispersed to a volume larger than the Sun, briefly giving it the largest extended atmosphere in the Solar System. The cause of the outburst is not definitely known; possibilities include a collision with a meteoroid or a build-up of gas breaking through the surface, while researchers at the Max Planck Institute suggested a thick, air-tight dust cover and H2O sublimation effects. Models in a 2012 study revealed a recurring outburst pattern roughly every 30 years, with one near-perihelion outburst roughly every 100 years. The comet remained visible in February 2008 at about magnitude +5 in Perseus, having expanded to greater than 2 degrees of arc. An outburst of 3–4 magnitudes occurred in January 2015 but still required a large telescope to be seen.

Reader's Guide

Comet Holmes is significant primarily for its 2007 outburst, which was the largest known outburst by a comet, brightening by a factor of a million and briefly making it the largest object in the Solar System by coma diameter. This event provided a rare opportunity for both professional and amateur astronomers to study a comet undergoing extreme activity. The outburst's cause remains uncertain, with hypotheses including a meteoroid collision, internal gas pressure, or a dust-cover sublimation mechanism, highlighting the complexity of cometary behavior. The comet's history also illustrates the challenges of tracking periodic comets: after its discovery in 1892 and observations in 1899 and 1906, it was lost for decades until recovered in 1964 with the aid of computer predictions. Its recurring outburst pattern, suggested by a 2012 study to occur roughly every 30 years with a larger event near perihelion roughly every 100 years, makes it a target for ongoing study. The 2007 outburst also demonstrated how a comet's orientation relative to Earth can affect its appearance, as observers initially saw it as star-like due to looking along the tail. The comet remains a subject of research, with its dust trail from the 2007 outburst repeatedly converging at the original site.

Did You Know?

• During its 2007 outburst, the comet brightened by a factor of a million in 42 hours, the largest known outburst by a comet.
• Its coma expanded to a diameter greater than that of the Sun, briefly making it the largest object in the Solar System.
• The comet was lost after 1906 and recovered on July 16, 1964, by Elizabeth Roemer using computer predictions by Brian G. Marsden.