White Dwarfs II
Gravity is relentless. It holds us to the surface of Earth, and Earth in orbit around the Sun. It even pulls at light, “stretching” its waves.
An example of this is seen in Sirius B, a white dwarf companion to Sirius, the brightest star in the night sky.
A white dwarf is the dead core of a once-normal star like the Sun. As the star neared the end of its life, the nuclear reactions in its core shut down and its outer layers were blown into space. Gravity then crushed the core so tightly that a blob of its matter the size of a sugar cube would weigh several tons. The core continues to shine, though, because it’s extremely hot.
But the white dwarf’s surface gravity is hundreds of thousands of times stronger than Earth’s, so as the star’s light heads into space, gravity exerts a strong pull on it. The light isn’t slowed down, but it does lose energy, which stretches it out, shifting it to redder wavelengths. This effect is known as gravitational redshift.
The effect can be measured, and when combined with other measurements, it helps determine the star’s size and mass, which in turn reveals its density and the strength of its gravity.
A few years back, astronomers used Hubble Space Telescope to measure this effect on Sirius B. The observations showed that the star is as massive as the Sun, but a bit smaller than Earth — confirming that Sirius B is one of the heavier white dwarfs yet discovered.
More about white dwarfs tomorrow.
Script by Damond Benningfield, Copyright 2011
For more skywatching tips, astronomy news, and much more, read StarDate magazine.