Anne’s Picture of the Day: The Pinwheel Galaxy

read more at Anne’s Astronomy News http://annesastronomynews.com/

January 11, 2012

The Pinwheel Galaxy, a Grand Design spiral in Ursa Major

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com)

The Pinwheel Galaxy (also known as Messier 101 or NGC 5457) is a giant asymmetrical spiral galaxy of about 170,000 light-years across (what makes it one of the largest disk galaxies known), located 21 – 27 million light-years away in the constellation of Ursa Major, while moving away from us at approximately 267 kilometers per second.

It is the brightest member of the M101 Group (a group of at least 9 galaxies, with as brightest companions NGC 5474 and NGC 5585), with a luminosity of about 30 billion times that of our Sun, and is estimated to contain at least one trillion stars of which some 100 billion of these stars could be like our Sun in terms of temperature and lifetime.

Although its size exceeds that of our Milky Way its overall mass is similar to our galaxy, with a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.

The Pinwheel Galaxy is also one of the most prominent Grand Design spirals in the sky (a type of spiral galaxy with prominent and well-defined spiral arms, that extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy’s radius).

The galaxy’s tightly-wound spiral arms, laced with dark dust lanes, are sprinkled with brilliant young clusters of hot, blue, newborn stars, and large and extremely bright star-forming regions, of which a total of about 3,000 can be counted, ionized by large numbers of very bright and hot young stars.

Because galaxy’s core is considerably displaced from the center of its thin disk, it is thought that in the recent past (speaking in galactic terms) the Pinwheel underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of the galaxy. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

The gravitational interaction between the Pinwheel and its companion galaxies may also have triggered the formation of its grand design pattern, and probably distorted its companion galaxy NGC 5474.

Four supernovae have been recorded in this galaxy. The first one was SN 1909A in January 1909, the second, a Type II was SN 1951H in September 1951, the third one, also a Type II was SN 1970G in January 1970, and the last one, a Type Ia supernova, was discovered on August 24, 2011.

A supernova is a phenomenon in which a star explodes in the final phase of its life. There are two types: Type I and Type II. The Type I does not show hydrogen in the spectra. Among these, Type Ia are explosions of white dwarf stars. The other group, Type II, are explosions of massive stars (initially more than 8 times the mass of the Sun). The Type II supernovae do show hydrogen in their spectra.

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