The Hubble Space Telescope has produced a image of Sirius A and its faint companion Sirius B, two stars in our night sky. At the center of the image is the brightest star in the night sky, Sirius A.To the bottom left we can see the tiny dot of Sirius B, which would’ve gone undetected without its overexposure. The cross-shaped diffraction spikes and concentric rings around Sirius A,and the small ring around Sirius B are artifacts produced by the telescope’s imaging system.
The two stars are in a binary star system, meaning they revolve around each other in an elliptical orbit every 50 years. Sirius A is located 8.6 light-years away from Earth and is the fifth closest star system known. Sirius B is a white dwarf star, a small remnant of a star similar to our sun that has exhausted its nuclear fuel sources and collapsed down to a fraction of its original size. It is 10,000 times fainter than Sirius A and its feeble light makes it very hard to observe from Earth.
Fortunately for us, the Hubble Space Telescope has come to the rescue! The telescope’s Space Telescope Imaging Spectrograph (STIS) was able to capture enough light from Sirius B to disperse it into a spectrum. This allowed astronomers to measure the light being stretched by the white dwarf’s powerful gravitational pull and calculate its mass at 98 percent that of the sun. Further analysis of Sirius B’s spectrum also revealed its surface temperature to be around 44,900 degrees Fahrenheit (25,200 degrees Kelvin).
Accurately determining the mass of white dwarfs is essential for understanding stellar evolution and a variety of other cosmological phenomena. White dwarfs are the remnants of stars that have exhausted all their nuclear fuel, and are incredibly dense objects. They are also the source of Type Ia supernova explosions, which are used because of their immense brightness to measure the distance to distant galaxies and the expansion rate of the universe. Furthermore, measurements based on Type Ia supernovae are fundamental to understanding “dark energy”, a mysterious repulsive force that drives the universe apart.
The method of accurately determining the mass of a white dwarf relies on one of the key predictions of Einstein’s theory of General Relativity: that light loses energy when it attempts to escape the gravitational pull of a compact star. This effect is known as gravitational redshift of light. To measure this effect, Oct. 15, 2003, astronomers use the Hubble Space Telescope’s Wide Field Planetary Camera 2 to capture detailed images of white dwarfs. From these images, they can measure the position of Sirius B, and use that information to point the STIS (Space Telescope Imaging Spectrograph) instrument exactly on the white dwarf, allowing them to make precise measurements of its gravitational redshift and mass.
The importance of accurately determining the mass of white dwarfs cannot be overstated. Not only does this enable us to better understand stellar evolution (as our own sun will eventually become a white dwarf), but it is also essential for elaborating our understanding of dark energy and for measuring the expansion rate of the universe. Without an accurate measurement of a white dwarf’s mass, we would not be able to make precise observations about our universe and its history. Thus, white dwarf mass determinations remain one of the key pieces in unlocking cosmological mysteries.