Weighing a Dwarf


Sirius A&BAccurately determining the masses of white dwarfs is important to understanding stellar evolution. The Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions. The method used to calculate a white dwarf’s mass relies on a prediction from Einstein’s theory of General Relativity—that light loses energy when it attempts to escape the gravity of a compact star. This effect is known as the gravitational redshift.

The Hubble image above shows Sirius, the brightest star in our nighttime sky. The bright blob in the center is Sirius A. The dim spot at around 7 o’clock is A’s white dwarf companion star Sirius B. The image of Sirius A was overexposed so that dim Sirius B  could be seen. The X-shaped diffraction spikes and concentric rings around Sirius A and the small ring around Sirius B are artifacts produced within Hubble‘s optical and imaging systems. The two stars revolve around each other every 50 years. Sirius is only 8.6 light-years from Earth, making it the fifth closest star system known.

White dwarfs are the leftover remnants of stars similar to the Sun. They have exhausted their nuclear fuel sources and have collapsed down to a very small size. Sirius B is about 10,000 times dimmer than Sirius A. It’s very faint because of its tiny size, only about 12,000 km in diameter. Sirius B’s weaker light makes it a challenge to study, because its light is swamped in the glare of its brighter companion as seen from telescopes on Earth. Hubble‘s Imaging Spectrograph was able to isolate the light from Sirius B and was able to resolve light from Sirius B being stretched to longer, red-shiftedr wavelengths by the white dwarf’s gravity. Based on those measurements, astronomers have calculated Sirius B’s mass at roughly 98 percent that of the Sun. Further analysis of the star’s spectrum showed that its surface temperature is about 25,000 K.

Image Credit: NASA

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