![]() Planetary nebulae are quite common, because they are produced by red giant stars late in their lives. They block the light from stars that are far away. Dark nebulaeĭark nebulae do not emit light or reflect light. Reflection nebulae reflect light from nearby stars. This makes them emit light and infra-red radiation. ![]() Usually the gases in an emission nebula are ionized. They are often called H II regions, because it is the ionized hydrogen which makes them glow. Star-forming regions and diffuse nebulaeįour planetary nebulae Emission nebulae / H II regionsĮmission nebulae make their own light. Nebulae can be sorted by what they look like and why we can see them. In the past galaxies and star clusters were also called 'nebulae', but no longer. The nebula is 97% Hydrogen and 3% Helium with trace materials. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. This is the final stage of a low-mass star's life, like Earth's Sun. Other nebulae may form as planetary nebulae. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a neutron star. The supernova event was recorded in the year 1054 and is labelled SN 1054. One of the best examples of this is the Crab nebula, in Taurus. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The formed stars are sometimes known as a young, loose cluster. ![]() These are sites where star formation occurs. The size of these nebulae, known as H II regions, varies depending on the size of the original cloud of gas. As the material contracts, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths. If the anticipated biconical structure is observed, we will follow up with spectroscopic (GALEX grism) observations in Cycle 5.Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. Therefore we propose to image the Red Rectangle in the NUV and FUV with GALEX. If the proposed CO radiative loop excitation mechanism is correct, then UV images of the nebula should also show the Red Rectangle biconical structure. ![]() The biconical nebular structure only appears in the ERE and not in the nebular scattered light. Indeed it is possible that all three spectroscopic anomalies seen in HD 44179 arise from the CO molecule - a very satisfying prospect, and a hypothesis testable by means of UV imaging of the Red Rectangle nebula. An attractive feature of this mechanism is that the CO triplet system decay produces emission in the red part of the spectrum, exactly where the ERE is observed. One possibility is a purely radiative mechanism involving an excitation loop via the 4th Positive bands, various excited triplet terms, and radiative decay in the (spin forbidden) Cameron bands. The complete lack of other emission of comparable excitation (at about 6 eV) implies an noncollisional source. The presence of UV Cameron band emission in HD 44179 is particularly intriguing. Although the ERE is ubiquitous (albeit weak) in many astronomical sources, and a significant contributor to the diffuse galactic background light at optical wavelengths, its carrier has not been identified. In the optical, the biconical Red Rectangle nebula is the strongest known source of Extended Red Emission (ERE), a broad hump of emission from 5000-9000 A. In the UV, HD 44179 shows strong absorption and emission bands of the 4th Positive system of carbon monoxide, and is the only known source of Cameron band emission (near 2000 A) outside the solar system. The post-AGB star HD44179 and the surrounding Red Rectangle nebula exhibit several unusual spectroscopic anomalies. ![]()
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