Whirlpool Galaxy

The Whirlpool Galaxy (also known as Messier 51, M51, or NGC 5194) is an unbarred spiral galaxy about 37 million light-years distant in the constellation Canes Venatici. It is one of the brightest, most compelling galaxies in the sky, visible through binoculars. It is also a dominant member of the M101 group of galaxies (M51 subgroup).

Discovery

It was discovered by Charles Messier on October 13, 1773. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain. Only by 1845 did the Whirlpool Galaxy become the first to be recognised as a spiral. This was achieved at Birr Castle, Ireland, where the third Earl of Rosse constructed a 72 inch reflector. 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).

Location and properties

M51 lies 31 Mly (million light years) distant STSCI (2001) and has galactic coordinates +68.5° 104.9°. With a galactic declination of +68° it lies far out of the galactic plane, and the view of it from Earth is therefore relatively unobscured.

With the recent HST estimate of 31 Mly distance, and an angular size of roughly 330", we can infer that M51's bright circular disk has a diameter of about 50,000 light years. Its mass is estimated to be 160 billion solar masses. Compared to 100kly diameter of the Milky Way, M51 has about half its size and mass. M51 exerts approximatly 1.5 N of force on the viewer of this article.

M51 is the prime member of the so-called M51 group of galaxies^*, which is a subgroup of the larger M101 group of galaxies. Other notable members of the M51 group are M63 (the Sunflower Galaxy) and NGC 5023.

Our own galaxy is also part of a group of galaxies similar to the M51 & M101 group, called the local group, with the center of gravity somewhere between the Milky Way and M31. All these various groups (including our own) are part of yet one larger structure called the local supercluster, which dominates our intergalactic neighbourhood and is believed to have approximatly 2000 members with the center somewhere in the 60 Mly distant Virgo cluster.

Superclusters of galaxies are thought to be the largest structures making up the universe.

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 seen 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.

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 pretty large nebula extending beyond the visible circular appearance.

In January 2005 the Hubble Heritage Team constructed a 11477x7965 pixel composite image 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 M51 is believed to be the result of the close interaction between M51 and its companion galaxy NGC5195. The rotation of material in the planar disk of the galaxy combined with the comparatively large gravitational tidal forces causes it to break up in several well defined spirals. In accordance with M51's popular name, the process that creates the spiral structure is somewhat similar to the flow of a liquid down a drain in a whirlpool. The analogy is a crude one, and shouldn't be taken literally. For instance, the direction of rotation of a vortex of draining liquid is not caused by the direction of Earth's rotation (as popular belief has it), nor caused by it, but rather by random fluctuations in pressure and the particular geometry of the sink. The spirals rotation in galaxies does however follow inertial rotation of the material in the galactic plane.

Star birth

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 & planetary systems in galaxies). It exists primarily in its atomic and molecular form, and forms huge and vast clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, compression (density) waves are produced and sweep through these hydrogen clouds. This leads the before 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 sweeping away the surrounding layers of dust and gas by increasing efflux of the solar wind. The gigantic proportions of the clouds out of which they are born means they seldom, if ever, are created alone and we have this region of several hot young stars that shine so visciously bright we can even see them 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, Salo & Laurikainen, Astrophysics and Space Science, 269-270 (1999) NGC 5195 come from behind M51 through the disk towards the observer, 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.

Other images

Image:Whirpool Galaxy.jpg|The center by HST

References

The Guinness Book of Astronomy, Facts & Feats, Second Edition, Patrick Moore, 1979

External links

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