Astronomy Picture of the Day 6-29-03

Explanation: It is still not known why the Sun's light is missing some colors. Shown above are all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device. The above spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our yellow-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light. The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below. Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun. Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth. Today, the majority of spectral absorption lines have been identified - but not all.

The Next Supernova
Astronomers think Rho Cassiopeiae might be the next star to go supernova.
by Vanessa Thomas

This illustration shows what Rho Cassiopeiae might look like from the surface of a nearby planet.
David A. Aguilar / CfA

In the summer of 2000, amateur and professional astronomers began noticing a transformation in the naked-eye variable star Rho Cassiopeiae. By fall, the star had dimmed by more than one full visual magnitude, and its yellowish-white color was changing to a deep orange.

An international research team headed by Alex Lobel of the Harvard-Smithsonian Center for Astrophysics had been monitoring Rho Cassiopeiae since 1993 and knew something big was happening. After analyzing their observations, the astronomers realized that the star had cooled significantly and ejected more material from its atmosphere than during any other similar stellar eruption observed. Their findings are reported in the February 1 issue of the Astrophysical Journal.

The first sign of trouble came when Rho Cassiopeiae brightened briefly. The astronomers believe this was due to gases falling inward, which got compressed and heated. But then a shock wave began expanding the star's outer atmosphere — which usually has a radius between 400 and 500 times that of the sun — to twice its normal size. The star dimmed as the shock wave carried as much as 10,000 Earth masses of atmospheric material away from the star. In just a few months, this expanding shell of gas cooled from 7,000 to 4,000 kelvins (about 12,000 to 7,000 degrees Fahrenheit).

Astronomers have used the 4.2-meter William Herschel Telescope to monitor Rho Cassiopeiae.
Nik Szymanek and Ian King

About 10,000 light-years away in Cassiopeia, Rho Cassiopeiae is only one of seven known yellow "hypergiant" stars, which are extremely luminous and believed to be near the end of their lives. Additional observations over the past two years show that the star's atmosphere remains unstable, and Lobel and his colleagues suspect that another event like the one witnessed in 2000 could trigger a supernova explosion.

"Rho Cassiopeiae could end up in a supernova explosion at any time as it has almost consumed the nuclear fuel at its core," says team member Garik Israelian of Spain. "It is perhaps the best candidate for a supernova in our galaxy."

Similar displays by Rho Cassiopeiae were observed in 1946 and 1893. The timing of the most recent event suggests the star experiences such episodes of mass loss about every 50 years or so. If this is the case, it's possible that the star has already become a supernova, but we have yet to witness the explosion.

"Given the large distance it is possible that Rho Cassiopeiae has already exploded and become a black hole or a neutron star," Israelian suggests. If the 50-year period is typical, it would experience 200 such events over 10,000 years, he points out. "In each event it will lose 0.1 times the mass of the sun and therefore 20 solar masses will be lost in 10,000 years! Very likely Rho Cassiopeiae does not exist anymore."

Here is a chart of northern circumpolar constellations. Note Cassiopeia in the lower left.

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