Archive for category Astronomy
The G-Type Stars
Posted by in Astronomy on July 12, 2011
The temperatures of B stars, although less than that of 0 stars, average more than 20,000 degrees, and the degree of excitation of the atoms in their atmospheres which determines the temperature is slightly less than that of the O-stars. High as these temperatures are they are but surface temperatures. The temperature at the center of an average star has been estimated to be around 40,000,000 degrees. Other well-known blue-white stars of B-type are Bellatrix and Saiph, diagonally opposite each other in the constellation of Orion, and Zeta Tauri, the more southerly of the two stars that mark the tips of the horn of Taurus. Three of the stars in the Southern Cross, Acrux at the top of the Cross, and Beta and Delta, which mark the ends of the cross-arm, are all bluish-white, B-type, or helium stars.
In later types of stars helium lines disappear. The A-type stars are the white stars, often called the hydrogen stars because the lines of this element predominate in their spectra. Their surface temperatures are about 12,000 degrees Absolute. Sirius and Vega are the most prominent members of this type, although to many observers both of these stars appear to be bluish-white rather than white. This may be because of their great brilliance and the scintillation of their light. In type F the lines of calcium become more prominent than those of hydrogen, the surface temperatures average between 7,500 and 6,500 degrees Absolute, and the stars are pale yellow in color. Two well-known stars of this type are Canopus and Procyon.
The G-type stars are also called the solar stars because our own sun is a star of this type. These are the yellow stars whose temperatures are around 5,500-6,000 degrees Absolute. In their spectra the lines of calcium are still prominent, and many other metallic lines appear, while the hydrogen lines are no longer prominent. Among the stars of this type will be found Capella and Alpa Centauri, our nearest neighbor among the first magnitude stars. Read the rest of this entry »
Sailing the Solar System
Posted by in Astronomy on July 12, 2011
People think it s cool that I’m working on a proposal for a major new NASA Venus mission. But when they ask when it will launch and I answer, “If we’re lucky, perhaps around 2020,” they suddenly appear less enthusiastic, like I’m describing a crazy pipe dream.
Maybe you have to be a little bit crazy to do this for a living. At the very least, you need to be comfortable with delayed gratification. Interplanetary spacecraft take a lot of time and work before any metal is cut, and most of them go nowhere. We spend years designing, proposing, and planning missions that never get selected for funding and so never make it anywhere near a launch pad.
Because there is no wiggle room in planetary orbits, these projects, once selected, cannot slip schedule without consequence. There is precious little chance of fixing any mistake discovered after launch. If you make it to launch, you get to enjoy moments of intense anxiety as something you’ve poured years of your life into sits on the pad on a stack of rockets loaded with enough fuel to blow it into shrapnel.
At a time when jet travel has shrunk our home planet to within a day’s travel time, planetary exploration requires timescales, and levels of patience, more characteristic of an earlier era of oceanic exploration.
Charles Darwin was the naturalist on board the HMS Beagle from 1831 to 1836. His 5-year mission was to explore the strange new worlds of South America and the Pacific, seeking out and cataloguing new life. His discoveries and insights on this voyage repositioned humanity within the web of life on Earth. Now, less than two centuries later, we’re exploring other planets, searching for other sites of Darwinian evolution. Read the rest of this entry »