The Whirlpool Galaxy (M51) is a spiral galaxy 23 million light-years from Earth in the constellation Canes Venatici (the Hunting Dogs).
The galaxy's massive center, the bright ball of light in the center of the photographs below, is about 80 light-years across and has a brightness of about 100 million suns. Astronomers estimate that it is about 400 million years old and has a mass 40 million times larger than our Sun. The concentration of stars is about 5,000 times higher than in our solar neighborhood, the Milky Way Galaxy. We would see a continuously bright sky if we lived near the bright center.
(credits to Hubble Space Telescope facility)
Surrounding the center is a much older stellar population that covers a region of about 1,500 light-years in diameter and is at least 8 billion years old, and may be as old as the Universe itself, about 13 billion years.
Further away, there is a "necklace" of very young star-forming regions, clusters of infant stars, younger than 10 million years, which are about 700 light-years away from the center. Normally, young stars are found thousands of light-years away.
Astronomers believe that stars in the central region were formed when a dwarf companion galaxy - which is shown in the top photograph - passed close to it, about 400 million years ago, stirring up dust and material for new star birth. The close encounter has been felt for a long time and is believed to be responsible also for the unusually high star formation activity in the bright necklace of young stars.
NASA's Hubble Space Telescope returned in 1994 the most detailed images ever of supernova 1994I which is in the "Whirlpool Galaxy" (M51).
The view in this picture encompasses the inner region of the galaxy's grand spiral disk, which extends all the way to the bright nucleus.
An arrow points to the location of the supernova, which lies approximately 2,000 light-years from the nucleus. The supernova appears to be superposed on a diffuse background of starlight. The Hubble Space Telescope was also used to measure the spectrum of the supernova in the ultraviolet light, which can be used to analyze the chemical composition and the motion of the gas ejected in the explosion.
A supernova is a violent stellar explosion which destroys a star, while ejecting the products of nuclear burning into the gas between stars. The energy for some supernova explosions comes from the collapse of a massive star to a compact neutron star, with the mass of the Sun, but the size of a city. Elements out of which the Earth is formed had their origin in ancient supernova explosions in our own Milky Way Galaxy.
This supernova was discovered on April 2, 1994 by amateur astronomers and has been the target of investigations by astronomers using ground-based optical and radio telescopes and NASA's International Ultraviolet Explorer satellite. Because a supernova explosion is a billion times as bright as a star like the Sun, they can be seen to great distances and may prove useful in charting the size of the universe. These previous observations show that SN 1994I is a very unusual supernova, called "Type Ic," for which very few examples have been studied carefully. The ultraviolet observations made with HST will help astronomers understand what type of stellar explosion led to supernova 1994I.
Observations of SN 1994I with the Hubble Space Telescope allow astronomers to probe the chemical composition of the debris and to learn more about the type of star that exploded.
The HST image was taken with the Wide Field Planetary Camera on May 12, 1994.
Credit: Robert P. Kirshner/Harvard-Smithsonian Center forAstrophysics, NASA