» Comets Astronomy

Astronomers have at last found definitive evidence that the universe's first dust - the celestial stuff that seeded future generations of stars and planets - was forged in the explosions of massive stars.

The findings, made with NASA's Spitzer Space Telescope, are the most significant clue yet in the longstanding mystery of where the dust in our very young universe came from. Scientists had suspected that exploding stars, or supernovae, were the primary source, but nobody had been able to demonstrate that they can create copious amounts of dust - until now. Spitzer's sensitive infrared detectors have found 10,000 Earth masses worth of dust in the blown-out remains of the well-known supernova remnant Cassiopeia A.

Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe. Developing stars need dust to cool down enough to collapse and ignite, while planets and living creatures consist of the powdery substance. In our nearby universe, dust is pumped out by dying stars like our sun. But back when the universe was young, sun-like stars hadn't been around long enough to die and leave dust.

That's where supernovae come in. These violent explosions occur when the most massive stars in the universe die. Because massive stars don't live very long, theorists reasoned that the very first exploding massive stars could be the suppliers of the unaccounted-for dust. These first stars, called Population III, are the only stars that formed without any dust.

Comets and their Features

Filed under: Comets — admin @ 6:18 am

Comet West Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 kilometers in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
Comet West As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet’s coma of gas and dust.

When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet’s gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

The Sun’s radiation pressure and solar wind accelerate materials away from the comet’s head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.

Comet West

Comet West Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.

Comet Kohoutek

This color photograph of the Comet Kohoutek was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona. They photographed the comet from the Catalina observatory with a 35mm camera on January 11, 1974.

These Hubble Space Telescope images of comet Hyakutake were taken on March 25, 1996, when the comet passed at a distance of almost 15 million kilometers from Earth. These images focus on a very small region near the heart of the comet, the icy, solid nucleus and provide an exceptionally clear view of the near-nucleus region of the comet.

The left image is 3,340 kilometers across and shows that most of the dust is being produced on the sunward-facing hemisphere of the comet. Also at upper left are three small pieces which have broken off the comet and are forming there own tails. Icy regions on the nucleus are activated as they rotate into sunlight, ejecting large amounts of dust in the jets that are faintly visible in this image. Sunlight striking this dust eventually turns it around and "blows" it into the tailward hemisphere.

Comet 1993a Mueller

This is a CCD image of Comet 1993a Mueller, taken on October 6, 1993 with a 288mm f/5.2 Schmidt-Cassegrain telescope. The comet has a coma diameter of 91 centimeters and a fan-shaped tail, up to 2.13 meters long.

Comet West (1975)

Comet West This photograph was taken by amateur astronomer John Laborde on March 9, 1976. This picture shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.

Comet Hale-Bopp

These NASA Hubble Space Telescope pictures of Comet Hale-Bopp show a remarkable "pinwheel" pattern and a blob of free-flying debris near the nucleus. The bright clump of light along the spiral (above the nucleus, which is near the center of the frame) may be a piece of the comet’s icy crust that was ejected into space by a combination of ice evaporation and the comet’s rotation, and which then disintegrated into a bright cloud of particles.

Although the "blob" is about 3.5 times fainter than the brightest portion at the nucleus, the lump appears brighter because it covers a larger area. The debris follows a spiral pattern outward because the solid nucleus is rotating like a lawn sprinkler, completing a single rotation about once per week.

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