Digitized Spectra Data
Harvard Woman Astronomers
How the Sun Shines, and neutrinos - A nice historical discussion by John Bahcall about why the sun shines.
The Nearest Brown Dwarf, only 13 ly away
Naming Stars: The International Astronomical Union discussions star names and why you can't buy a star name.
The Sun and stars have absorption spectra because the photosphere is cooler than the core.
All the light we see comes from the photosphere; the core is well hidden below hundreds of thousands of kilometers of solar gas. The absorption lines and the continuum are both formed in the photosphere; the fact that absorption lines are detected shows that the outer part of the photosphere is cooler than the inner part of the photosphere. The photosphere is a few thousand kilometers thick, compared to 700,000 km for the radius of the Sun, so the continuum and the absorption lines are formed in less than the outer 1% of the Sun.
The absorption lines are formed in cool clouds of gas outside the Sun and stars.
The absorption lines are formed in the upper photosphere as continuum from the lower photosphere passes through it.
The chromosphere, seen every day by looking straight at the Sun, has emission lines because it is hotter than the photosphere.
The chromosphere is too transparent to add emission lines to the solar absorption lines. Only when we see it at the edge of the Sun (known as the "limb") do we detect emission lines, because then we see the chromosphere in silhouette against dark sky. We can see the chromosphere and prominences in this way every day with telescopes on Earth that use H-alpha filters or without filters at the times of total solar eclipses.
OBAFGKM uses letters that were assigned arbitrarily to spectral types.
Spectral types were first assigned in order of the strength of hydrogen lines, with A being the strongest hydrogen lines, B being the next strongest hydrogen lines, and so on. Later, it turned out that the order arranged by temperature is different from the order arranged by the strength of hydrogen lines.
The hottest stars have the strongest hydrogen lines.
The hottest stars are types O and B, and so much hydrogen is ionized in their photospheres that the hydrogen lines are weaker than in the somewhat cooler A stars. Similarly, in stars cooler than A stars, the H and K lines of ionized calcium and other lines become stronger and the hydrogen lines become relatively weaker.
The star they studied, called TX Cam, is in the constellation Camelopardalis and lies almost 1000 light years from Earth. Its brightness changes regularly over a period of 80 weeks. It is an example of what is known as a 'Mira' variable, named after the famous variable star Mira which exhibits very regular changes in brightness on a time-scale of about a year.
As the Sun approaches the end of its life in a few billion years it will become a much more violent object than it is now. It will begin to grow quite rapidly until it swells to fill the inner part of our Solar System, swallowing the Earth and the other inner planets. Jupiter and the more distant planets may survive, but they will be subject to a considerable bombardment as the outer layers of the Sun are expelled when it begins to pulsate with a period of about a year. Eventually, much of the Sun's mass will have been thrown back into the depths of interstellar space, from whence it first came, and the Sun will shrink to a shadow of its former glory becoming what astronomers call a white dwarf.
This scenario is in the far distant future for our own star, however it is a reality now for the many thousands of Mira variable stars throughout the Milky Way. Studies of the regions close to these stars show them to be losing their outer layers at a considerable rate, they can shed an Earth mass of material every year. Astronomers can study this mass-loss process in a variety of ways and can deduce much about the mechanism by which the stars expel this material and the nature of the star in its crotchety old age.
Radio astronomers in particular can use a technique known as Very Long Baseline Interferometry (VLBI) to study this process in incredible detail. Diamond and Kemball used the National Science Foundation's Very Long Baseline Array (VLBA) to make their time-lapse movie of the gas being ejected from the surface of the star. They can do this because there are radio beacons sitting in the gas. These beacons, called masers (the radio equivalent of lasers) show up as bright spots in the radio images. The masers arise from one of the gases in the outflow, Silicon Monoxide (SiO), which emits in a narrow frequency band close to 43 GHz.
The images, made with the VLBA at 43 GHz are 500 times more detailed than is possible with the Hubble Space Telescope. The movie can be found on the internet at http://www.jb.man.ac.uk/~pdiamond/txcam44.gif. It shows the complex gas motions that arise close to the star. "They show immensely complex motions which cannot be explained by current theory" Diamond said.
The movie covers a period of 88 weeks, with observations being made every 2 weeks. "The structures that we observe in the outflow suggest that we might be seeing the effects of shock waves passing through the gas," Kemball said. "However, it is difficult to explain why most of the gas is moving away from the star whilst, at the same time, some is falling towards it."
Diamond and Kemball have continued to observe TX Cam with the VLBA. They have data covering another 80 weeks. They hope that, when this is incorporated into their movie, they will be able to understand the nature of this dying star and so cast some light on what awaits our own Sun in its dotage.
Jodrell Bank Observatory forms part of the Department of Physics and Astronomy of the University of Manchester. It operates the MERLIN National Facility, an array of six radio telescopes distributed across central England and funded by the Particle Physics and Astronomy Research Council. Philip Diamond is the Director of MERLIN.
The VLBA is a continent-wide system of ten radio telescope antennas, each 25 meters (82 feet) in diameter and weighing 240 tons. They are distributed across the continental U.S., Hawaii and the U.S. Virgin Islands. Operated from a control center in Socorro, New Mexico, all ten antennas work together as if they were a single telescope more than 5,000 miles in diameter. This allows the VLBA to produce radio images hundreds of times more detailed than the Hubble Space Telescope produces using visible light.
The VLBA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
A high resolution copy of the above image of two movie frames is available here.