Comet Holmes tops the solar system, now larger than Sun

William Atkins
Thursday, 15 November 2007 20:36

Science - Space

Comet 17P/Holmes, which has been growing larger since October 23, 2007, is now larger than the Sun. Although the Sun is now the second largest body in the solar system it still is much more massive than the tenuous comet.         

On November 9, 2007, the comet was measured by a team of Hawaiian astronomers to be about 869,900 miles (1.4 million kilometers) in diameter. The Sun has been measured with a diameter of about 864,900 miles (1.392 million kilometers).

The mass of the Sun is approximately 4.385 x 10³⁰ pounds (1.9891 ×10³⁰ kilograms). Comet Holmes has only a tiny fraction of the mass when compared to the Sun, and is losing mass as material is ejected from its nucleus.

The astronomers taking the measurements included Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii (UH) Institute for Astronomy. They used observations from a wide-field camera on the 3.6-meter Canada-France-Hawaii Telescope (CFHT). Other astronomers involved in the UH program to study the comet include Bin Yang, Nuno Peixinho and David Jewitt.

The present eruption of comet Holmes was first reported on October 24, 2007, and has continued to expand at a fairly constant rate of 1,100 miles per hour (0.5 kilometers per second). The comet, once unable to be seen by small telescopes, is now about half a million times brighter than before the eruption began. This gigantic eruption of the comet is produced by dust being ejected from a tiny solid nucleus made of ice and rock. The nucleus is only about 2.2 miles (3.6 kilometers) in diameter.

Comparison (side-by-side) photographs of the two objects, Comet Homes and the Sun, are shown at University of Hawaii astronomer Dave Jewitt’s webpage: http://www.ifa.hawaii.edu/faculty/jewitt/holmes.html.

The Sun will eventually retain its status as largest body in the solar system because Comet Holmes will eventually collide with the Sun, a planet, or another solar system body; be ejected out of the solar system by one of the massive bodies of the solar system; or simply run out of material to eject from its nucleus and die away.

The gigantic debris cloud of Comet Holmes is easily visible with the naked eye in the night sky. Its nucleus is visible with the use of an average telescope. It appears within the constellation Perseus.

November 19th is being targeted as an excellent opportunity to watch the comet because on that night it will fly in front of the star Mirfak (alpha Persei), the brightest star in the constellation Perseus, and appear to engulf it with its large dusty cloud. [It will not "really" do that because the comet is much closer to the Earth than the star, but only appear so due to our vantage point here on the Earth.]

A good sky map of the event is found at: http://spaceweather.com/images2007/15nov07/skymap_north_holmes.gif?PHPSESSID=1hl10agp2hmr67a6opitfhi524.