Definitions

Whirlpool_Galaxy

Whirlpool Galaxy

The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting grand-design spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. It is one of the most famous spiral galaxies in the sky. The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars. The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.

Discovery

The Whirlpool Galaxy was discovered by Charles Messier on October 13 1773. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain. It was however not until 1845 that the Whirlpool became the first galaxy to be recognized as a spiral. This was achieved by Lord Rosse employing a 72-inch reflecting telescope which he constructed at Birr Castle, Ireland. In 2005 a supernova (SN 2005cs) was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14. Sometimes M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).

Properties

With the recent SN 2005cs derived estimate of 23 Mly distance, and an angular diameter of roughly 11.2′, we can infer that M51's bright circular disk has a radius of about ~38,000 light-years. Its mass is estimated to be 160 billion solar masses. Compared to 100 kly diameter of the Milky Way, M51 has about half its size and mass.

A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral galaxy. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring.

Visual appearance

Located within the constellation Canes Venatici, M51 is easy to find by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, so it is circumpolar for observers located above 43°N latitude and reaches high altitudes throughout the northern hemisphere making it an accessible object, especially from the very early hours in winter through the end of spring season, after which summer solstice somewhat hinders observations.

M51 is visible through binoculars on a dark night, but with modern amateur telescopes this galaxy is truly a sight to behold. It is very forgiving on the instrument, when seen even through a humble 10 cm telescope the basic outlines of M51 and its companion are visible. Under dark skies, and with a moderate eyepiece through a 15 cm telescope, one can detect M51's intrinsic spiral structure. With larger (>30 cm) instruments M51 is simply breathtaking. The various spiral bands are very obvious and several HII regions appear to be visible, and M51 can be seen to be attached to M51B. The shape of the X-formation in the nucleus has often been compared to the Christian cross.

As is usual for galaxies, the true extent of its size can only be gathered from inspecting deep photographs, and very long exposures reveal a large nebula extending beyond the visible circular appearance.

In January 2005 the Hubble Heritage Team constructed a 11477x7965 pixel composite image (shown in the info box above) of M51 using Hubble's ACS instrument, revealing this splendid galaxy and its companion in unprecedented detail.

Spiral structure

The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy NGC 5195.

Star formation

Induced spiral structure in the larger galaxy isn't the only effect of the interaction. Significant compression of hydrogen gas occurs that leads to the development of starbirth regions. In pictures of M51 these show up as the bright blue 'knots' throughout the spiral arms.

Generally speaking, hydrogen gas is the most common component of the interstellar medium (the vast space between stars and planetary systems in galaxies). It exists primarily in its atomic and molecular form, and forms huge clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression (density) waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas, these are dust lanes one so often sees in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further collapse under its own gravitational pull occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.

When this happens, these new-born stars gobble up huge amounts of gas causing them to expand, shine even hotter, and finally sweep away the surrounding layers of dust and gas by increasing efflux of the stellar wind. The gigantic proportions of the clouds out of which they are born means stars seldom, if ever, are created in isolation. Thus regions of several hot young stars emit sufficient light energy that they can be seen in the high resolution pictures of M51 across millions of lightyears distance.

For an example of such a formation in our own galaxy, see M16, the Eagle Nebula.

Companion

Decades ago, it wasn't known for certain if the companion galaxy NGC 5195 was actually a true companion or rather some other galaxy passing at a distance. The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated the reality of the interaction.

Recent simulations bear out that M51's spiral structure was caused by NGC 5195 passing through the main disk of M51 about 500 to 600 million years ago. In this model, NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 Myrs ago until it is where we observe it to be now, slightly behind M51.

Galaxy group information

The Whirlpool Galaxy is the brightest galaxy in the M51 Group, a small group of galaxies that also includes M63 (the Sunflower Galaxy), NGC 5023, and NGC 5229. This small group may actually be a subclump at the southeast end of a large, elongated group that includes the M101 Group and the NGC 5866 Group, although most group identification methods and catalogs identify the three groups as separate entities.

See also

  • NGC 5195 - the companion galaxy to NGC 5194
  • NGC 5457 - another grand-design spiral galaxy

Croatian scientific bibliography: " http://bib.irb.hr/ ", and item or surname of the author: "Spadina"

External links

References

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