An extraordinary sight should be visible in the sky tonight as a giant comet passes in front of a star.

Comet Holmes has put on a spectacular display for amateur astronomers for the past three weeks. Tonight it is expected to shine even brighter as it passes in front of Mirfak, the brightest star in the constellation Perseus.

Comet Holmes appears in our skies every several years, usually as a very dim speck of light. But on October 24 astronomers were astonished to see it explode suddenly into a colossal light, a million times brighter than before, and clear enough to see as a fuzzy dot with the naked eye. Although the icy nucleus of the comet is only 1.9 miles (3km) across, the halo of light around swelled to a ball of 900,000 miles (1,45 million km) in diameter, the largest light in the solar system, bigger than the Sun, which is a mere 1.39 million km diameter.

What caused the comet’s explosion is not certain, but it may have collided with a small asteroid. Alternatively, an eruption of gas, ice and dust from within the heart of the comet may have blown up in the Sun’s heat, although the total amount of material ejected in such an explosion would probably be only about 1 per cent of the total mass of the comet.

Whatever the cause, the comet is unlikely to be destroyed by its eruption. Indeed, it underwent a similar explosion in November 1892, when Edwin Holmes, the amateur English astronomer after whom it was named, first spotted it before it dimmed and fell into obscurity again.

With a small telescope the recent display has looked like a giant jellyfish, the head composed of gas and dust (called the coma) with tentacles of electrically charged particles buffeted by the solar wind. Although the light has grown somewhat diffuse in recent days, tonight the comet is expected to appear much brighter as it passes directly in front of Mirfak. The combination of the star’s light and comet should be visible to the naked eye, even in lightpolluted London, although it will appear brightest away from city lights. Weather permitting, the comet will appear high in the northeast sky in the evening and rise to nearly overhead, where it will remain visible throughout the night. Comet Holmes will continue to shine for the coming days.

Thereby hangs a tail

— Comet Holmes is 150 million miles (241 million km) from Earth, between Mars and Jupiter

— Comets, made of ice and dust, originate in the outer solar system. As they approach the sun they vapourise, developing a bright tail

— They can contain amino acids, one of the building blocks of life. Some scientists believe that comets hitting Earth may have brought essential ingredients of life

— One theory of the Star of Bethlehem is that it was a comet

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