Weighing Sirius B

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

An Ultracompact Dwarf Galaxy

M60-UCD1_by_HSTM60-UCD1 is an ultracompact dwarf galaxy. Half of its stellar mass is in the central sphere only 160 light years in diameter. It is probably the densest known galaxy with over one hundred stars per cubic light-year, and it is the smallest and least massive galaxy known to host a central black hole. It is also the most massive ultracompact dwarf galaxy known. It is also one of the oldest galaxies in the universe.

Image Credit: NASA

A Red Dwarf

Proxima_CentauriA red dwarf is a small and relatively cool star on the main sequence. Red dwarfs range in mass from about 7 to roughly 50 percent the mass of the Sun and have surface temperatures of less than 4,000 K. They are by far the most common type of star in the Milky Way, but because they are dim in the visible light spectrum, individual red dwarfs cannot easily be observed. Indeed, not a single one can be seen by the naked eye from Earth. According to some estimates, three-fourths of the stars in the Milky Way are red dwarfs—as is our nearest neighbor.

Proxima Centauri  is a red dwarf about 4.24 light-years from the Sun and is the nearest known star to the Sun. Its distance to the second- and third-nearest stars, which form the bright binary Alpha Centauri, is sufficiently close (about .24 light-year) that it is very likely part of a triple star system with Alpha Centauri A and B, but its orbital period in that system may be greater than 500,000 years.

Image Credit: NASA

A Lopsided Galaxy

A galactic maelstromThis is Messier 96, a spiral galaxy a bit more than 35 million light-years away in the constellation of Leo (The Lion). It is roughly the same mass and size as the Milky Way, but unlike our more or less symmetrical galaxy, M96 is lopsided. Its dust and gas are unevenly spread throughout its weak spiral arms, and its core is not exactly at the apparent galactic center. Its arms are also asymmetrical, perhaps because of the gravitational pull of other galaxies within the same group as Messier 96.

Image Credit: ESA / NASA