The Dog Star, Sirius, and its Tiny Companion
About This Image
Caption
This Hubble Space Telescope image shows Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Astronomers overexposed the image of Sirius A [at center] so that the dim Sirius B [tiny dot at lower left] could be seen. The cross-shaped diffraction spikes and concentric rings around Sirius A, and the small ring around Sirius B, are artifacts produced within the telescope's imaging system. The two stars revolve around each other every 50 years. Sirius A, only 8.6 light-years from Earth, is the fifth closest star system known.
Sirius B, a white dwarf, is very faint because of its tiny size, only 7,500 miles in diameter. 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 fainter than Sirius A. The white dwarf's feeble 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. However, using the keen eye of Hubble's Space Telescope Imaging Spectrograph (STIS), astronomers have now been able to isolate the light from Sirius B and disperse it into a spectrum. STIS measured light from Sirius B being stretched to longer, redder wavelengths due to the white dwarf's powerful gravitational pull. Based on those measurements, astronomers have calculated Sirius B's mass at 98 percent that of the sun. Analysis of the white dwarf's spectrum also has allowed astronomers to refine the estimate for its surface temperature to about 44,900 degrees Fahrenheit (25,200 degrees Kelvin).
Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. The sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions, which are used because of their brightness to measure the distance to distant galaxies and the expansion rate of the universe. Measurements based on Type Ia supernovae are fundamental to understanding "dark energy," a dominant repulsive force stretching the universe apart. Also, the method used to determine the white dwarf's mass relies on one of the key predictions of 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 of the light.
This image was taken Oct. 15, 2003, with Hubble's Wide Field Planetary Camera 2. Based on detailed measurements of the position of Sirius B in this image, astronomers were then able to point the STIS instrument exactly on the white dwarf and make the measurements to determine its gravitational redshift and mass.
Credits
NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)Keywords
| About The Object | |
|---|---|
| Object Name | Sirius A, the Dog Star, and Sirius B |
| Object Description | Binary Star System |
| R.A. Position | 06h 45m 8.91s |
| Dec. Position | -16° 42' 57.99" |
| Constellation | Canis Major |
| Distance | 8.6 light-years away (2.6 parsecs) |
| Dimensions | The projected separation of Sirius A and Sirius B in this image is 6".10, which at the distance of Sirius is 16.1 Astronomical Units (AU). The semimajor axis of the relative orbit is 7".48, or 19.7 AU. |
| About The Data | |
| Data Description | The Hubble image was created from HST data from proposal 9964: H. Bond (STScI), M. Barstow and M. Burleigh (University of Leicester), J. Holberg (University of Arizona), and E. Nelan (STScI). I. Hubeny (University of Arizona) and D. Koester (University of Kiel, Germany) are also on the science team. |
| Instrument | HST>WFPC2 and HST>STIS |
| Exposure Dates | October 2003, Exposure Time: 11.2 min (WFPC2), and February 2004, Exposure Time: 15.8 min (STIS) |
| Filters | WFPC2: F1042M, STIS: G430L and G750M |
| About The Image | |
| Compass Image |  |
| About The Object | |
|---|---|
| Object Name | A name or catalog number that astronomers use to identify an astronomical object. |
| Object Description | The type of astronomical object. |
| R.A. Position | Right ascension – analogous to longitude – is one component of an object's position. |
| Dec. Position | Declination – analogous to latitude – is one component of an object's position. |
| Constellation | One of 88 recognized regions of the celestial sphere in which the object appears. |
| Distance | The physical distance from Earth to the astronomical object. Distances within our solar system are usually measured in Astronomical Units (AU). Distances between stars are usually measured in light-years. Interstellar distances can also be measured in parsecs. |
| Dimensions | The physical size of the object or the apparent angle it subtends on the sky. |
| About The Data | |
| Data Description |
|
| Instrument | The science instrument used to produce the data. |
| Exposure Dates | The date(s) that the telescope made its observations and the total exposure time. |
| Filters | The camera filters that were used in the science observations. |
| About The Image | |
| Image Credit | The primary individuals and institutions responsible for the content. |
| Publication Date | The date and time the release content became public. |
| Color Info | A brief description of the methods used to convert telescope data into the color image being presented. |
| Orientation | The rotation of the image on the sky with respect to the north pole of the celestial sphere. |