Before the star died, its comets, and possibly planets, would have orbited the star in an orderly fashion, much like our own Solar System today. The bright pink glow in the center is the combined ultraviolet and infrared glow of a dusty disk encircling the fading star, most likely kicked up from comets that survived its final death throes. The Helix Nebula is one of the most famous images from Spitzer, showing a dying star unraveling its outer layers out into space.

Comet nuclei range from a few hundred meters to tens of kilometers across, but their tails can stretch for millions of kilometers.Įvidence for comets has even been seen around dying stars. A bigger comet has a bigger surface area, so it gives off more heat and looks brighter in the infrared. In infrared light, however, a comet's brightness depends upon the amount of heat it absorbs from the Sun and re-radiates back into space. Because of that, a large, dark comet can appear to be the same size as a small, highly reflective comet. When we observe a comet in visible light, we see the light that the comet reflects from the Sun. Infrared light is very important for measuring the size of a comet. They are thought to have been formed in the Kuiper belt - a disk of icy worlds and rocky bodies that rings the Solar System at the orbit of Pluto and beyond. Comets that have been seen more than once in human history, like Halley's Comet (which visits the inner Solar System every 75-76 years), are all short-period comets. Can you calculate when it will next be back? How old will you be then?Ĭomets that orbit the Sun at much closer distances are called short-period comets and swing past the Sun more regularly. Halley's Comet last visted the Earth in 1986.

Astronomers estimate that there could be as many as a trillion comets out in the Oort cloud. They trace out large eccentric, or egg shaped orbits rather than circular orbits. Comets that come from the Oort Cloud are long period comets, which only come near the Sun for short periods of time, every few thousand years. The Oort Cloud is a vast, spherical shell of icy bodies left over from the cloud of gas and dust that formed the Sun, which surrounds the Solar System at a distance between 5,000 and 100,000 astronomical units (AU) away. The white dust tail and the blue ion tail could be seen pointing in slightly different directions - the blue ion tail pointing directly away from the Sun, and the white dust tail following the curve of the comet's orbit.Īstronomers believe that comets formed in both the Oort Cloud and the Kuiper Belt. Comet Hale-Bopp was a bright comet that had two very distinct tails. As the name suggests, it is made of ionized gas, which also gets blown away by the Solar wind, and always points directly away from the Sun. If it doesn't break up under the pressure of gases forcing their way out of the nucleus, it could look as bright as the moon, and may even be visible during the day.Ĭomets often form a second tail, called the ion tail, or gas tail. Astronomers hope that Comet ISON, due to swing past the Sun in late 2013, will put on a spectacular show. The comet and its dust tail appear red because they are more than ten times colder than the bright blue stars in the background.Ībout once every ten years, a comet comes with a tail so bright that we can even see it with the naked eye. This dust tail is easily spotted with infrared telescopes, and it traces the comet's curved orbital path.Ĭomet Siding Spring is a great example of this: the dusty tail glows in the infrared, curving along its orbital path. As more gas and dust are lost, the comet then forms a tail that is pushed away from the Sun by the pressure of sunlight. As the nucleus starts to evaporate, a coma, or cloud of this dust and gas, surrounds the nucleus. The solid part of the comet is called the nucleus.

Comet names