Leonid meteors. Optical time-exposure image of Leonid meteors (green/white streaks) against a starfield. North is at top. Meteors, or shooting stars, are tiny dust particles which enter the Earth's atmosphere at high speeds. They are heated by air resistance, making them visible as streaks of light. The Leonid shower occurs annually for about 2 days around 17th November, when the Earth crosses the debris produced by the comet Tempel- Tuttle (55P). The constellation Orion is at centre. The star Sirius (Alpha Canis Majoris), the brightest star in the sky, is at lower left. Photographed in 2001.